Climate control of incubators related to growth and thermoregulation of newborn infants

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CLIMATE CONTROL OF INCUBATORS

RELATED TO

GROWTH AND THERMOREGULATION

OF NEWBORN INFANTS

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Climate control of incubators related to

growth and thermoregulation of newborn infants

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CLIMATE CONTROL OF INCUBATORS

RELATED TO

GROWTH AND THERMOREGULATION

OF NEWBORN INFANTS

PROEFSCHRIFT

ter verkrijging van de graad van doctor aan de Technische Universiteit Delft,

op gezag van de Rector Magnificus, prof.dr. J.M. Dirken, in het openbaar te verdedigen ten overstaan van een commissie, aangewezen door het College van Dekanen op donderdag 7 mei 1987 te 14.00 uur door

HUIBERT JACOB DANE

geboren te 's-Gravenhage natuurkundig ingenieur

Gebotekst Zoetermeer / 1987

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Dit proefschrift is goedgekeurd door de promotoren prof.ir. B.P.Th. Veltman

prof.dr. H.K.A. Visser

All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior written permission of the author, H.J. Dane, Delft University of Technology, Dept. of Applied Physics, P.O. Box 5046,2600 GA Delft, The Netherlands.

CIP-DATA KONINKLIJKE BIBLIOTHEEK, DEN HAAG Dane, Huibert Jacob

Climate control of incubators related to growth and thermoregulation of newborn infants / Huibert Jacob Dane. - [S.l. : s.n.] (Zoetcrmeer: Gebotekst). - 111.

Thesis Delft. - With ref. - With summary in Dutch ISBN 90-9001634-1

SISO 605.2 UDC 612.65.014.4(043.3)

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And, if two lie side by side, they keep each other warm; but how can one keep warm by himself?

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1 INTRODUCTION

A newborn infant needs his mother to feed him and to care for him. If he is born preterm, special care may be necessary because some vital organs, biochemical or enzyme systems may not have developed sufficiently, or because the growth of the fetus may have been disturbed, with the result that the infant is unlikely to survive undamaged without special protection. An infant is called preterm if it is born after a gestation of less than 37 weeks. Depending on culture and on geographic location, either intimate thermal contact with the mother or thermal insulation has been a major form of protection for many centuries. Since about 150 years incubators are being used for this purpose.

An incubator is a small (0,5 x 0,5 x 1 m3) cabinet, figure 1.1, with walls that are transparent

to light, to easily observe the infant. Inside the incubator an artificial climate is maintained, which usually differs from the local environment with respect to temperature, humidity and/or oxygen concentration.

Marx (1968) records the historical development of incubators. Perhaps motivated by the severe Russian winter, the first one has been developed at the imperial foundling hospital in St. Petersburg in 1835 — the days of Pushkin and Gogol — by an unknown craftsman at the suggestion of Von Ruehl, physician-in-ordinary to Czarina Feodorovna, wife of Czar Paul I. By 1850 some 40 of these incubators are used in the Moscow foundling hospital and at the end of the 19th century various modified versions are applied in Germany, France and England.

After an initial period, during which incubators are applied on an intuitive base, accumulated experience leads to systematic investigations as to how the temperature inside the incubator affects the survival of the infant. Around 1900 Budin finds that the mortality of infants with a bodyweight of less than 2 kg decreases from 98% to 23% if the rectal temperature is maintained higher than 35 °C instead of lower than 32 °C. As Hey and O'Connell (1970) report: "He kept all his infants clothed and covered with light blankets, but came to the conclusion that this was not in itself enough for the smaller infants. He therefore extended and popularised the idea of nursing the more vulnerable infants fully clothed in specially constructed incubators. Clinical experience led him to recommend an air temperature of 30 °C in these incubators for the smallest (1 kg) infants and a temperature of 25 °C for most of the other small infants at risk".

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f-vS;

Warmewanne des Moskauer Findelhuuses.

W = Doppelwiod mlt WirmwuMrfüllmig. - S ■=■ Scbleienus Moussdin. - d =

Draht-bugeo znm Halten dejsetben. — a s Eingim-Oeffuuua;. - b = Ablassliahn.

%r

Ahb ir.' tnkubator 7310 (mit Fahrgesiell und Aggregat 701V}.

Zum lïinlegcn des Kindts ist die irontsettige Scheibe heruntergeklappt

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STELLINGEN

behorende bij het proefschrift "Climate control of incubators related to growth and thermoregulation of newborn infants" 1) De klimaatregeling van thans gangbare couveuses behoeft geen verbetering.

2) Ter vereenvoudiging verdient het aanbeveling om de sensor voor het meten van de buikhuid-temperatuur af te schaffen en de couveuse van een dubbele wand te voorzien.

3) Hoewel het begrip relatieve vochtigheid nuttig is bij het drogen van de was, zou het rondom de couveuse niet gebruikt moeten worden, ten gunste van dauwpuntstemperatuur of damp-spanning.

4) Het zwak-parameter criterium is een efficiënt hulpmiddel bij het selecteren van de orde van een model.

5) Het kiezen van een criterium op grond van een criterium op grond van ..., illustreert de instinc tieve en intui'tieve wortels van intelligent gedrag.

6) Het is onjuist om het vastleggen van door simulatie gegenereerde getallen als "meten" aan te duiden, omdat dan geen onderscheid blijft tussen een zelf gecreëerde en een door Moeder Natuur gegeven werkelijkheid.

7) In het wetenschappelijk onderwijs dient meer systematische aandacht besteed te worden aan de vraag, welk ander dan een reeds gekozen model consistent zou kunnen zijn met de beschik bare waarnemingen en welk nieuw experiment daaromtrent opheldering zou kunnen verschaf fen.

8) Effectief communiceren, dat wil zeggen eerlijk, duidelijk, met respect en terzake, is van door slaggevend belang voor de vruchtbaarheid en het plezier waarmee mensen samenwerken en con flicten hanteren.

9) Er moet in ons land nog als een paard worden gewerkt om, wanneer het aardgas op is, van de wind te kunnen leven.

.0) Het uitzenden van godsdienstige overpeinzingen via radio 4 veronderstelt ten onrechte dat het woord de luisteraar als muziek in de oren klinkt.

.1) Een rechtvaardiging van politieke beslissingen door middel van een commissie van deskundigen, vertoont sterke overeenkomst met een aan douaniers gegeven verklaring, dat de na een vakan tie uit Frankrijk gesmokkelde wijn door kenners is uitgezocht.

Delft, 7 mei 1987 H.J. Dane

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Since Budin's pioneering work, many other investigations have been made into the clinical and physiological effects of climate control, and as a result several authors have published guidelines for the practical use of incubators: temperature settings for a particular infant, given its weight, age and gestational age, the length of time between conception and birth.

Hey (1971) thoroughly discusses the design, construction and application of incubators from a medical point of view. His extensive review deals with climate control, optimum warmth, oxygen supply, humidity, cleanliness, noise, light and cost. Although it is his personal belief that most healthy infants can be nursed quite adequately in an open cot because that is simpler and less expensive, incubators provide a convenient means of controlling environmental warmth, humidity and oxygen, and of tailoring these needs to the child's requirements.

Bell (1983) compares the merits of incubators with those of overhead radiant heating panels. The major advantage of the latter is the open access they provide to infants who require procedures like endotracheal intubation etc., the major disadvantage is the increase of evaporative heat loss.

While in the Third World their number may amount to some 25%, in Western Europe and Northern America today about 7% of all live born children have less than 2,3 kg body weight and one third of these is born "small for gestational age", which means smaller than the 10lh percentile on a growth chart (Usher and McLean, 1969). Five percent of all live born children are born preterm (Visser, 1982). Many of these require intensive care, and therefore are nursed in incubators. According to Hey (1971) the principal indication for incubator care is to provide an environment suitable to keep the infant under continuous observation. But, as Bell (1983) has noted, many nurseries are kept at temperatures below 25 °C for the comfort of the nursing staff. Thus special precautions must be taken to create a favorable thermal environment. Since 1967 the University Hospital Rotterdam/Sophia Children's Hospital has its intensive care unit temperature controlled at 30 °C. Given the difference between the climate required around the infant and in the nursery, a sufficiently large gradient must be created across some thermal barrier. This may be achieved by increasing the thermal resistance of the barrier, by increasing the heatflow through it, or by a combination of both methods. The first is simple and cheap (a blanket), the second requires a thermostat to adjust the heatflow corresponding to the temperature of the nursery. The last one was and still is practiced by kangaroos and other animals, as well as in several human cultures.

From an engineering point of view an incubator is one subsystem among other subsystems, like, for instance, the feeding system or the diagnostic systems, in the more general system of medical care for newborn infants. The design of this subsystem and the evaluation of occasionally proposed modifications requires an adequate physical analysis of the process to be

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controlled, and of the disturbances whose influence has to be minimized. Current analyses of incubator and infant mostly deal with only one or two thermal aspects, for instance convective heat transfer only, or convection and radiation. Furthermore, their approaches of the process are invariably static, although various authors have measured variations of temperature and heat transfer as a function of time. So there is a need for a complete analysis of the thermal processes inside an incubator, including the dynamics of thermoregulation in newborn infants.

The aim of this thesis is 1) to present such an analysis, 2) to translate existing medical and physiological knowledge in terms of applied mathematics and physics, and 3) to investigate the dynamic behavior of the thermoregulatory system in newborn infants by applying existing engineering methods for the analysis of dynamical systems and heat transfer. The central question to be answered is: If the growth rate of a newborn infant appears to be reduced, how, and to what extent, may climatological factors have caused such a reduction? To be more specific, how does an infant respond to variations of the incubator temperature? To answer these questions a simple mathematical model of thermoregulation in an infant is presented. The parameters of this model are estimated from in vivo measurements, using a specially designed calorimetric system.

The outline of the thesis is as follows: in chapter 2 a survey of the literature is presented, the chapter contains sections on medical criteria of clinical management, on growth of infants, on thermoregulation and on climate control of incubators. Chapter 3 discusses the climate control of an incubator in detail, and in chapter 4 a simple mathematical model of the thermoregulation is formulated. The instrumentation and the numerical procedures to estimate parameters of the model are described in chapter 5, while chapter 6 presents some measurements and results. Finally chapter 7 contains the conclusions.

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2 LITERATURE

2.1 Medical criteria of clinical management

Around 1930 Blackfan and Yaglou study the effects of temperature and humidity on growth and development of premature infants. They "wonder whether the rate of growth and mortality rate would be affected favorably in an environment in which the interrelated air conditions (ventilation, temperature and humidity) were properly adjusted and accurately controlled". At a constant feeding regime with a daily energetic input of 552 kJ per kg bodyweight after the first week of life, corresponding to 6,4 W/kg, they study growth under various conditions. Their observations "support the belief that both the requirements of food and growth depend to a significant extent on the temperature and humidity to which the organism is exposed". They define the environmental temperature as optimal when "the body is able to maintain thermal equilibrium with its environment, with the least conscious sensation to the subject or with least physiologic demand on the heat regulation mechanism". Note that it is the difficulty for an infant to report his conscious sensations to a doctor or a nurse which forms the basis for their investigation. According to Blackfan and Yaglou the optimal temperature varies inversely with body weight between 24 and 38 °C, and the optimal humidity should be 50 - 75% in terms of relative humidity or 16 - 18 °C in terms of dewpoint temperature.

Unfortunately, humidity has been specified as relative humidity in many publications eversince, without mentioning a reference temperature, causing much difficulties in comparing the results of different studies.

With respect to the mortality rate in climate-conditioned nurseries as compared with unconditioned ones, Blackfan and Yaglou note: "A significant lowering of the mortality rate can be observed in infants of the low weightgroup. The reduction in the gross mortality rate ranges from about 29% among the two lowest weight groups to about 12% in the two highest weight groups. The general net mortality for all weight and age groups was 7% in the conditioned nurseries as compared with 28,9% in the unconditioned nursery".

Mortality is not the only medical criterion for the quality of clinical management. Blackfan and Yaglou also study the influence of air conditioning on digestive disorders. They state that "in an environment with a relative humidity of between 50 and 65% symptoms referable to the

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gastro-intestinal tract were less frequent, the disturbance was of shorter duration and the attack subsided more rapidly. In comparison with the group of patients kept in low humidities and with patients in the unconditioned nursery the reaction was attended by far less consequence".

In their 1933 paper Blackfan and Yaglou report about 7 years of pioneering work. A delay of several decades is not unusual between a fundamental discovery and its practical application: nearly thirty years later, shortly after the first satellite has been launched successfully, Silverman (1959) is "encouraged to think that the rules of evidence that have applied in the laboratory are now increasingly consulted on ward rounds". He has studied survival rates as a function of ambient (relative) humidity. His results "lend support to the long held view that survival rates of premature infants can be improved by reducing their heat loss". However, "they do not permit us to give an opinion on whether the bodytemperature of the premature infant should be maintained at the intra-uterine level", because "we do not know whether survival is the best criterion for judging the ideal level of temperature of new born infants".

So it is not only the problem to determine the optimal temperature and humidity for a particular infant with respect to one criterion or another, but to have a criterion to select criteria. This looks like the beginning of an infinite series of criteria to select criteria. Silverman suggests that there is a "best" criterion, but we do not know the selection rules for such a criterion. As the optimal bodytemperature of premature infants, Silverman proposes the one which is associated with the highest rate of intact survival. Apparently humidity too affects survival, and Silverman, Agate and Fertig (1963) study the possible non-thermal effect of atmospheric humidity. They obtain results which "are consistent with the suggestion that the only important acute effect of placing newly born premature infants in atmospheres of high humidity is that of reducing their heat loss", and they state: "when the discrepancy in body temperature of infants in moderate and high humidity environments was compensated, the clinical course in the first days of life was essentially the same". In a comment on the generally adopted custom to maintain a permanent mist of water vapor inside the incubator they declare that "there is no convincing evidence of a beneficial nonthermal effect of high humidity". Of course the water balance of the infant changes a little bit, but the side-effects of bacterial growth should not be neglected, and "the amount of water saved by reducing insensible loss is not enough to provide any appreciable additional urine volume, and therefore high humidity environments are probably not critical in body water economy".

Instead of environmental variables one can measure variables of the infant itself. Buetow and Klein (1964) investigate the effect on survival rates of maintaining the skin temperature of newborn preterm infants at a level of 36,0 ± 0,2 °C. Compared with maintaining the ambient temperature between 31 and 32 °C an increase in neonatal survival rates from 46 to 58% was observed. Similar results are reported by Day et al (1964), who maintained a skin temperature of 36 °C by means of a radiant heater. Whereas in an environment of 32 °C only 63% survived, maintaining a sufficiently high skin temperature enabled 77% of the infants to survive.

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Survival percentage is a criterion which is necessarily defined over a group of infants. Incidence of diseases is a criterion that can be applied to an individual infant, but it needs an observation time of at least several days. Briick, Parmelee and Briick (1962) attempt to find a suitable criterion equivalent to the criteria "pleasantness" or "thermal comfort" in adults. They find that "the evaluation of the length of time spent in quiet sleep during observation periods of two to four hours might furnish such a criterion". Buetow and Klein (1964), referring to Briick, Parmelee and Briick (1962) report that "recent studies indicate a significant reduction in the percentage of time spent in quiet sleep when the environmental temperature of prematures fell below the neutral temperature zone".

Keeping the infants clothed will diminish their sensitivity to lower environmental temperatures: "A naked infant soon cries and becomes active when exposed to a cold environment. In surroundings cold enough to increase heat production by 50% few naked infants sleep more than a few minutes a time. In contrast, almost all the swaddled infants exposed to an environment cold enough to provide a comparable increase in heat production continued to sleep quietly": Hey and O'Connell (1970).

Although naked infants are easier to observe and to access, which is important if they need intensive care, Hey and O'Connell (1970) note that there is little convincing evidence that the cult for nursing many infants completely naked has improved on the standard of nursing care that was pioneered by Budin, Blackfan and Yaglou. From a technological point of view, thermal insulation is just as effective as climate control, and much simpler to accomplish. Nowadays it is a general clinical practice to keep naked only those infants who require continuous monitoring.

Since the times of Budin the impact of diseases has been fought successfully and survival rates have improved drastically. In the western affluent society rate of growth is now accepted as a major medical criterion of clinical management, in particular if an infant is undergrown at birth. In discussing some of the consequences of intra-uterine growth retardation, Lafeber, Jones and Rolph (1979) note that "available clinical data suggests that if growth restriction occurs after 26 weeks, at which time much neurogenesis and some myelination has occurred, the brain mass is reduced but there is postnatal compensation probably to produce a normal sized brain. Growth restriction before 26 weeks causes permanently stunted brain growth and neurological damage". Clinical efforts are often aimed at compensation of the intra-uterine growth retardation, because a speedy "catch-up" growth is considered likely to improve an infant's well-being even at later age.

According to Visser (1978,1982) and Sauer (1982) the growth rate of undergrown neonates should be stimulated by proper feedings to approximate the intra-uterine growth rate of normal infants as published by Usher and McLean (1969).

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2.2 Growth of infants

In a general sense, growth of a living organism can be described as an increase of number and size of the "units" which constitute the organism. Furthermore the functional differentiation between various "units" increases, as well as the complexity of the interaction and coordination. These phenomenae can be observed irrespective of the nature of a particular organism, whether it is a single cell, an organ, a complete animal, or a social community.

Growth can be seen as the development of autonomy or self-reference of an organism. It is possible to occur in a safe environment which provides challenge and reward (Toynbee, 1972) with an intensity adapted to the strength of the organism, resulting in excitement (Perls, 1969) and pleasure: "Biologically, pleasure is closely tied to the phenomenon of growth, which is an important aspect of the ongoing process of life. We grow by incorporating the environment into our beings both physically and psychologically" (Lowen, 1970). Growth is a luxury and a necessity at the same time. A luxury in the sense that an excess of food is needed above the minimum which is required for the maintenance of a physiological balance in the body. Growth is a necessity in that it is the only way to become a mature human being. Perls (1969) characterizes maturing as the transcendence from environmental support to self-support. Periods of gradual development are alternated rather suddenly by characteristic transition phases. The contradictory aspects of growth are also reflected in Maslow's (1954) hierarchy of needs, where biological security and food are primary needs and self-actualization is ranked as the highest need. Under sufficiently favourable environmental conditions, that is if the primary needs are sufficiently satisfied, growth becomes possible. Apart from food and physical protection, a safe attachment to a motherfigure appears to be an important need that must be satisfied (Bowlby, 1969). From this point of view an incubator only partly fulfills the need for a proper environment.

Growth appears to be governed by some internal references in a still almost unknown way. No single measure can characterize growth; weight, length, shape, mobility, whatever variable one selects, its variation will represent only part of the many changes that happen during growth. Much work has been done to develop standards of what can be considered "normal growth", for instance the well known Lubchenko growth charts or the recent graphs of weight and length versus time by Usher and McLean (1969). For the sake of simplicity bodyweight is often used as a physical variable to measure growth. However in the first two weeks after birth bodyweight alone is a rather poor measure: a relatively large amount of interstitial water is lost, which the synthesis of new tissue can not compensate for. In a normal pregnancy the bodyweight increases from 1 kg at a postconceptual age of 28 weeks to 3 kg at 38 weeks. In this period a major part of the brain does develop.

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As Sauer (1982) has shown, an energy balance can be associated with growth. The increase of energy stored in the body is equal to the input of energy in food minus the loss in faeces and urine and the energy used to maintain the physiological balance of the body.

E + E = E _ — E - E (2 n

cmp syn f excr m \*"i)

M = Mr- M (O 2~>

cmp I excr K^'^-J

With respect to some convenient reference the energy content of the food may be calculated from the heat of combustion of each of its components. The energy content of the newly formed tissue can be decomposed into the energy of the components Ecmp and the energy Esyn to

synthesize the components into living tissue. From the total energy Ef of the consumed food a

small part, Eexcr is not absorbed by the digestive system. The remainder is either oxidized or

stored into new tissue. The energy resulting from oxidation is used to perform internal work and is finally removed as heat. A small part is used for synthesis of new tissue. Under optimal conditions some 3 Watt per kg is required for maintenance, like heart and lung function, brain-metabolism etc., or some 250 kJ per day per kg bodyweight. Since it takes approximately 12 kJ to build one gram of new tissue (Hommes 1975, 1980), Sauer (1982), another 3 W/kg is needed to promote growth equal to the intra-uterine accretion of 17 g per kg per day. If this amount of energy is not available growth rate will be reduced.

Two factors may cause such a reduction. The amount of unabsorbed energy may increase due to some pathological condition. Perhaps more frequently an increased amount of energy is used for maintenance, or control of bodytemperature. Since it appears impossible to increase the energy intake above 6 - 7 W/kg, increased energy expenditure to control body temperature will reduce growth. Thus it is important to control the thermal environment of the infant appropriately.

Because metabolic and physiological processes always produce some waste heat, in the thermal balance of the body the heat loss to the environment must be positive, whatever the environmental temperature may be. Furthermore, a human infant is a warm blooded being, attempting to maintain its bodytemperature at a particular level in a varying climate. Glass, Silverman and Sinclair (1968) study the influence of a cool environment on growth: "Studies on experimental animals suggest that temperature acclimation and growth in the post-natal period can be modified by the external environment". Their studies are undertaken "to determine whether similar effects could be detected beyond the first week of life in small human infants maintained in two frequently recommended thermal environments", namely a mean skin temperature of 35 and 36,5 °C respectively. The birth weight of the infants is between 1 and 2 kg, their gestational age is between 30 and 37 weeks, while they are studied after the first week of life. The infants receive a daily standard amount of food, corresponding to an energy input of = 5,5 Watt per kg bodyweight. The results indicate that cold resistance, (the ability to

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prevent the deep bodytemperature from falling when exposed to a low environmental temperature), is greater after the two weeks in the slightly cooler environment, growth is slightly faster in the warmer of the two conditions. In a sequel to their investigation, Glass, Silverman and Sinclair (1969) study "the differential milk requirements to equalize growth in infants raised in warm and in subthermoneutral conditions". Whereas in a cool environment the daily increase of bodyweight dropped from 1,51% to 1,17% if no additional food was given to the infants, 33 kJ per kg bodyweight, which corresponds to 0,62 Watt for an infant of 1,6 kg, could completely compensate for the additional heat loss and restore the growth to 1,55% per day again: "the additional milk given to the infants raised under subthermoneutral conditions, appeared to be sufficient to permit them to grow at approximately the same rate as matched controls raised under thermoneutral conditions but fed less milk".

Here one clearly sees the interaction between food, growth, thermoregulation and climate control. Therefore our attention changes from growth to thermoregulation.

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2.3 Thermoregulation

2.3.1 General

The temperature of the human body is approximately constant, despite variations in temperature, humidity and air velocity of the environment, and despite a varying level of activity. This homeostasis is the result of a thermoregulatory control system which balances heat production and heat loss. Machle and Hatch (1947), Hardy (1961), Benzinger (1964), Bligh (1966), Hammel (1968), Hensel (1972, 1973) and Cabanac (1975) have written extensive reviews of the physiology of this system.

disturbances

controller A

_V

Figure 2.1 Basic structure of a control system.

Just like in any other control system two major elements (figure 2.1) can be recognized here: the controlled part, which is the body itself, and the controller, which is largely nervous. The controller can be divided into three subsystems: a sensory, a processing and an effector system. The sensory system consists of a large number of temperature-sensitive neurons, some acting as cold-, other as warm receptors. They are mainly located in the brain (hypothalamus), the spinal cord, the skin and upper part of the respiratory tract. Various types do exist: the firing rate of a central thermoreceptor responds to a constant temperature level (with respect to a certain threshold), receptors in the skin also respond to a temperature variation with time (proportional plus rate sensitivity). Skin receptors are unevenly distributed over the body-surface, the facial area having a relatively high density. In adults the average depth of thermoreceptors in the skin is 0,15 - 0,17 mm for cold receptors and 0,3 - 0,6 mm for warm receptors (Hensel, 1972).

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Their rate-dependent output is negligible if the temperature changes less than 0,1 °C per minute, and otherwise disappears in a matter of seconds, a period apparently too short to be significant in regulating body temperature (Cabanac, 1975).

Very little is known about the way in which the central nervous system processes the various signals it receives from the thermoreceptors. A vital role is played by the hypothalamus, but the interaction of central and peripheral signals is not clear yet. Some measurements support the assumption that both signals are added (Benzinger, 1959; Hammel, 1968) whereas from other investigations it appears that they are multiplied. However, according to Cabanac (1975) the difference between additive and multiplicative models usually is obvious only for extreme temperatures. When an interaction is examined within the usual temperature range it is most often impossible to assess whether it is multiplicative or additive, at least as far as internal and mean skin temperature are concerned. Between species the influence of internal sensors increases with bodyweight: skin to core about 3:1 in the rat, 1:1 in the cat, 1:4 in the dog and between 1:3 and 1:7 in adult man. Perhaps it is not pure coincidence that these numbers approximately reflect the ratio of surface area to bodyweight.

The processing system has a static non-linear characteristic without integral- or derivative control action. The effector system consists of an increasing metabolic heat production, an increasing or decreasing peripheral blood flow and an increasing evaporative heat loss. All these processes are involved in other regulatory systems as well, like respiration or voluntary muscle control systems. As Hardy (1972) has pointed out, sweating is unique in that it is completely dedicated to the thermoregulatory system, and it receives only minor signals from other systems like, for instance, the salt balance.

Hemingway (1963) reviews shivering and non-shivering thermogenesis. Shivering is controlled by the spinal and supra-spinal motor system, whereas the other effector mechanisms are mediated by the sympathetic nervous system. Cooling of the spinal cord increases the excitability of motor neurons, and thereby causes shivering. Non-shivering thermogenesis is a very effective way to produce heat through the oxidation of brown adipose fatty tissues. Normally it occurs in newborn infants as a substitute for shivering, and adults can use it as an additive source of heat production. On average it amounts to about 1% of the total body weight, enough to maintain the cold-induced increase in heat production of a 3 kg infant for more than 3 days (Heim, 1981).

Regional differences exist in the vasomotor control of peripheral blood flow, at least three different zones can be distinguished: the extremities (hands, feet, lips, nose), the trunk plus proximal limbs, and the head. In the first zone vasoconstriction is the result of increased sympathetic tone and vasodilation of decreased tone. In the second zone, vasodilation is the result of an active process, acting via the sweat glands. The third zone shows a negligible constrictive response to cold, but a dilation coupled to sweating when heated. Passive heating of the body produces a blood flow pattern different from heating by active work, the main increase

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in blood flow occurring in zone 1 in the first case, and in zones 2 and 3 in the second situation. Passive hearing also causes sweating, first in the lower extremities if the whole body is heated. A rising temperature due to active work causes sweating in zones 2 and 3.

Local heating or cooling of a particular part of the body results in a local response next to the central one. Local control systems are coupled to the central system in a hierarchical structure.

Wyndham (1973) reviews temperature regulation during exercise under heat stress. Prolonged exposure to hot environments gives rise to acclimatization. Although resting in the heat is associated with a limited adjustment, bodily work within the limits of tolerance is necessary for full development of acclimatization.

Hellon and Townsend (1982) survey the understanding of the changes in the thermoregulation system which occur during fever (as an indication of illness fever is recognized already in Deuteronomy 28 verse 22). Fever raises the conceptual "central setpoint" and also causes the sensitivity of central warm receptors to decrease, and of cold receptors to increase although the "closed-loop gain" of the system appears to remain unchanged (Cabanac, 1975). Effects on peripheral receptors are unknown. Rowell (1983) reviews the cardiovascular aspects of human thermoregulation. In the control of skin blood flow, blood pressure regulation and muscle demands appear to have precedence over core temperature, unless the latter rises beyond tolerable limits; then temperature regulation fails.

The digestion of a meal causes the production of a certain amount of heat, this phenomenon is called specific dynamic action. The amount of heat might be related to the protein content of the food, and it is added to the cold induced heat production.

During sleep a lowering of the bodytemperature occurs after a vasodilation of the extremities, which can be described as caused by the displacement of a "setpoint". In adults REM sleep is associated with a depression of the sweating activity (Shapiro et al 1974). Stern (1978) and Heim (1981) review the thermoregulation of newborn infants.

2.3.2 Newborn infants

Newborn infants have only a modest capacity for sweating and heat production, which depends upon gestational age and degree of maturation. Considerable variations do occur in their ability to stabilize bodytemperature, some infants may even temporarily loose their thermoregulatory capacity. Shortly after birth "lability of body temperature is a wellknown characteristic of premature infants" as Day, Curtis and Kelly write in their, by now classical, 1943 paper. They study premature infants with body weight between 1,4 and 2,9 kg at ages from 4 to 53 days. They find that "healthy premature infants do not suffer, as a group, from a defect in circulatory adjustments to different air temperatures. They do have an adequate production of sweat, a large surface area and a poor layer of insulating subcutaneous fat for protection against heat loss". In his famous 1961 paper Briick however concludes that "the thermoregulatory processes begin

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just as quickly in premature as in full term infants, but heat production in the former is insufficient to counterbalance heat loss". Briick explicitly formulates thermoregulation as a control system subject to disturbances, but does not attempt to model it. The development of this control system after birth is studied by Adams et al (1964), who sequentially expose premature infants to neutral (32 - 34 °C), low (21 - 23 "C) and high (36 - 38 °C) ambient temperatures. At low temperatures oxygen consumption increases by some 60% even in infants under 24 hours of age. At high temperature the mean oxygen consumption increases 18% in infants aged younger than 7'/2 hours, whereas in older infants there was no significant increase, although the respiratory rate increased slightly. Infants between 6 and 12 days show a faster response of oxygen consumption to rewarming after being cooled than younger ones.

In studying a control system an interesting question is to determine what variables can be considered as input, that is, which stimuli control heat production. Adamsons, Gandy and James (1965) try to find these in the fhermoregulatory system. They want "to elucidate which thermal factors govern the metabolic rate in the neonatal period", and study 50 healthy fullterm infants. Is the metabolic heat production a function of the net thermal exchange between body surface and environment, i.e. a function of heat flow, or is it a function of one or several absolute temperatures of the body? They correlate oxygen consumption with rectal temperature and with skin temperature (measured at the inferior abdominal wall) in a cold and a warm environment. They conclude "that the rate of oxygen consumption of the newborn infant is predominantly a function of the temperature gradient between bodysurface and the environment, rather than absolute values of either deep body or surface temperature". If heat production were controlled by heat flow, a flow sensor would be necessary, or a sensor to measure the temperature difference between skin and environment. Although a sensor for the environmental temperature has not been located, it is conceivable, for instance in the nose. The equivalent of a heat flow sensor could be the result of temperature sensors at varying depth under the skin. Some thermoreceptive areas, such as the facial skin and the membranes of the upper respiratory tract seem to play an important role in the fine regulation of heat production (Heim, 1981).

The exposure of mature, normally grown human newborns to environmental temperature of 23 °C is associated with a large increase in heat production, with muscular activity, crying, peripheral vasoconstriction, and probably chemical thermogenesis within the highly metabolic brown adipose tissue. These mechanisms allow the healthy newborn to maintain the colon temperature at approximately 36,5 °C when acutely faced with an environment of 23 °C after the second or third day of life. How infants born small for gestational age respond to mild cold-exposure is measured by Lees, Younger and Babson (1966). When the infants were exposed to 27 °C environmental temperature: "all but two showed important falls in colon temperature. Although mature undergrown infants were more successful in preventing a fall in colon temperature than premature, their performance mostly fell far short of successful homeothermy". They suggest that "the thermal requirements of the undergrown infant are closer

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to the premature infants of similar weight than to the larger infant of similar gestation". It is not possible to decide from their data whether the limited homeothermy is caused by a low closed loop gain of the thermoregulatory control system, or by the limitations to increase the heat production sufficiently.

The relation between environmental temperature and oxygen consumption is also measured by Hey (1969). He finds a minimum value of 5 - 7 ml/kg min at environmental temperatures between 35 and 38 °C. In a cooler environment the oxygen consumption increases to at most 2,5 times the minimum value, and the increase was 0,6 to 1,3 ml/kg min per °C of environmental temperature. Occasionally however, low birthweight infants who are able to increase their heat production normally during the first few days of life, may loose this ability for a number of days (Hey and Katz, 1969a).

The sweating response of new born infants to a warm environment is measured by Foster, Hey and Katz (1969). "No sweating could be detected in healthy infants of less than 210 days postconceptual age in an environment of 36 - 37 °C, even when rectal temperature rose as high as 37,8 °C. Direct measurement of heat flow showed, however, that the infants were able to control heat loss from the hand, even at this early age, presumably by changing skin blood flow. The infants were also capable of increasing their heat production in cool surroundings". Immaturity of the sweat glands can account for the "lack of any response to thermal stimuli in premature babies, but not for the modest thermal response obtained in babies at term. Functional maturation appears to depend on intact central innervation, and is marginally hastened by postnatal factors". In a subsequent paper Hey and Katz (1969b) report: "Sweating was never seen in quiet resting infants, unless environmental temperature was above 34 °C and rectal temperature also above 36 °C. In Day's important study of thermoregulation in the premature infant, published in 1943, environmental temperature was not taken above 34 °C which explains why no consistent thermal sweat response was detected. In most of the newborn infants studied here a high evaporative water loss was only obtained when rectal temperature had risen nearly 1 °C above normal. These infants appeared to differ from adults in allowing rectal temperature to rise significantly before bringing the sweat mechanism into play". Again it is almost impossible to decide whether the limited sweating response is caused by a high setpoint or by a low gain of the feedback loop.

Several results quoted in this chapter can be expressed quantitatively, and their meaning becomes even more clear when the numbers can be interpreted as values of parameters in a mathematical model.

2.3.3 Models of thermoregulation

Because the thermoregulation system has been studied from various points of view, different kinds of models have been developed to describe its behavior in part or as a whole: Pharmaco-chemical models to characterize the effect of drugs, pyrogens and the like, and the response of

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thermoregulation to phenomena as dehydration: changes in the concentration of ions that normally belong to the body. Physical models, in the form of an analog electrical network or a set of mathematical equations have been constructed to examine how a temperature profile results from the distribution of heat through convection and conduction in living tissue. Here the basic problem to be solved is the transient state heat conduction equation, with internal heat generation and non-uniform tissue properties. Neuronal models have been made to elucidate the highly complex neuronal network and its interactions in terms of excitation and inhibition. Due to the complexity of this network, "a neuronal wiring diagram can be nothing but an absurd simplification, lacking adequate experimental evidence" (Bligh, 1976), and therefore the controller in the thermoregulation system has to be described by a black box relation. Thanks to the knowledge of its anatomy, the controlled part of the thermoregulation system can be modelled with a considerable amount of detail. The essential ingredients are the number of compartments, their relative spatial arrangement, their thermal properties and geometrical form, and the topography of arteries and veins.

The availability of electronic computers has influenced the form of the mathematical models. Machle and Hatch (1947) and Pennes (1948) solve the equations in their models analytically. MacDonald and Wyndham (1950) are the first to employ an analog electrical network with resistors and capacitors, while Wissler (1964) applies a digital computer to solve the partial differential equations of his model.

Fan, Hsu and Hwang (1971) have written a review on mathematical models of the human thermal system, upon which Iberall (1972) and Shitzer (1973) have published a comment and an addendum respectively. Shitzer (1972) has published a compendium of research on mathematical models of thermoregulation. Hardy (1972) discusses mathematical and other models, Hwang and Konz (1977) classify the literature on engineering models of the human thermoregulation system, in a sequel to the article of Fan, Hsu and Hwang. In what follows we shall briefly highlight some of the more prominent models, classified according to the computational methods employed: analytical, analog simulation, or digital simulation, each category arranged chronologically.

Shortly before the second world war is the beginning of the analytical method of modelling the thermoregulation system. Noting that "physiology must start with physics" Burton (1934, 1935) is the first to apply the theory of heat transfer to the study of metabolism. He relates heat production and transfer in the body to size, weight and age, and computes weighting factors for the determination of the mean body temperature from rectal and skin temperatures. He also estimates the specific heat of the living human body to be between 2,9 and 3,8 kJ/kgK.

Machle and Hatch (1947) are interested in the upper level of tolerance to heat, in the nature, course and duration of acclimatization, and in the evaluation of thermal stress from known ambient conditions. Aware that their knowledge does not permit a fully descriptive mathematical

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expression for thermal relationships and effects, they modestly develop a first order differential equation, representing the whole body by a single compartment. The temperature of the compartment is a weighted sum of rectal and mean skin temperature and the model can be used to predict the rate of equilibration of the skin temperature to a new (lower) environmental temperature.

Pennes (1948) proposes a model for the human forearm assuming cylindrical geometry, "the use of elliptical coordinates would involve a degree of complexity which is probably not justified by the underlying physiological complications". With uniform tissue properties and metabolic rate, and no axial heat-flow he solves a partial differential equation to obtain the steady state radial temperature distribution, and the relation between skin and environmental temperature in terms of Bessel functions. Insertion of a thin thermocouple into the forearm furnishes the data to support his model.

Wissler (1961) extends Pennes' work and calculates the steady-state temperature distribution of the entire body, composed of 6 cylinders: head, torso, two arms and two legs. The model does not include a controller. The interdependence among the cylinders is provided through a central blood pool, heart and lungs, where the six venous streams mix. Countercurrent heat exchange does occur between arteries and veins in all elements except the trunk. Each part of the model has its specific local metabolic heat production and blood flow. Wissler obtains an analytical solution, which he subsequently programmes on an IBM 650 computer to calculate steady-state radial temperature profiles of an adult man under various conditions of work-load and environmental temperature.

Two years later Wissler (1963) presents the dynamic extension of his static 1961 model. Analytically he solves the transient heat transfer equations, using finite Hankel transforms and machine computation of eigenvalues. The model is slightly more complex, in that each part of the body is covered by an insulating layer of fat and skin. Because of the amount of computational work required to calculate, for instance, a step response, Wissler codes his final equations on an IBM 709 computer. He also mentions the problem of how to choose the data to be used in his model, it is almost impossible to measure in vivo the local metabolic rate and local blood flow throughout the model. Nevertheless he presents a comparison of his model with experimental results of a transient work load.

Ho and Fan (1975) study the effect of clothing on the steady state temperature distribution of the human thermal system. They expand Wissler's (1963) model with two additional layers, one for still air and one for clothing, and solve the equations analytically.

Mitchell and Myers (1968) formulate an analytical model for the countercurrent heat exchange mechanism, and determine non-dimensional parameters that govern heat transfer. The results of their analysis indicate that there is not a significant countercurrent effect in the arm of a man. The largest probable effect would reduce heat loss from the arm by 5% at most.

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electrical network. They take into consideration the limitations of Machle and Hatch's single compartment model under non-equilibrium conditions, and choose a model with two compartments, a core surrounded by a shell, with fixed thermal capacities in a 2:1 ratio. Two feedback loops control heat production, sweating and heat transfer between core and skin. In a footnote the authors mention that N. Wiener's book on cybernetics introduced the concept of feedback to them. They validate the model through measurement of evaporative water loss, rectal and mean skin temperature under various conditions of ambient temperature and work load.

Wyndham and Atkins (1960) propose the first simulation on an analog computer. They treat the body as a single cylinder of three layers, a core composed of internal organs, a middle layer of muscle, fat and subcutaneous tissue and a thin (0,5 mm) skin layer. By allowing the thermal conductivity to vary as a function of core and skin temperature they simulate the effect of peripheral circulation. An equivalent electrical network with three resistors and two capacitors represents a single layer of the cylinder. They do not present experiments to provide numerical values of the parameters in the model. Atkins (1963) further develops this model. He approximates the body by a set of six cylinders, head, trunk, arms and legs, and assumes every cylinder to consist of four concentric and homogeneous layers: core, muscle (except in the head), tissue and skin, a structure which is similar to Wissler's (1961, 1963) model. Temperature receptors in the heart-lung compartment and in the skin send signals to the controller, although he does not mention the equations.

Brown (1963, 1966) uses a single cylinder of four layers, a core, a muscle layer, a subcutaneous one and the skin, with thermal capacities in a 2:2:1:1 ratio. Implemented by the switching of relays, variation of the thermal conductance simulates convective heat transfer through blood flow between the layers. A proportional controller measures core and skin temperature deviations from respective setpoints. In the 1966 version he adds derivative controller action. Cyclic thermal stress during two 70 min. periods of a single sinusoidal variation of the ambient temperature provides the data for this model. It is applied to assess the thermal load of aircraft pilots when wearing anti-exposure suits. Crosbie, Hardy and Fessenden (1963) solve the partial differential equation of a single cylinder on an analog computer. Constrained by the limited size of their machine they approximate the cylinder by three compartments in a 4:2:1 ratio. A core where the basic metabolic heat is produced, a muscle layer being the source of increased heat production through shivering and exercise, and a skin layer where heat is lost to the environment. They assume the controlled variable to be a weighted mean temperature of the compartments, proportional to the total heat content of the body. Their controller has proportional action for metabolic heat production and proportional plus derivative action for sweating and tissue conductance. The transient responses of a nude man to step changes of exercise and of environmental temperature provide the data to test the model.

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Smith and James (1964) describe the body with six segments, interconnected by the heart-lung system. Each segment is composed of three concentric cylinders. However, because of the limited size of their computer, they contract the model to include only one segment and the heart-lung system, and state that this proved satisfactory for most problems they have studied. In the controller part of their model, skin temperature controls cutaneous blood flow and sweating through two positive feedback loops, with the hypothalamic temperature as a multiplier to determine the loop gain. They test the model by comparing the heart rate during periodic work loads.

Atkins and Wyndham (1969) use a single cylinder, four layer model to determine which are the predominant forms of control and how they interact with one another. Both muscle and tissue layers are represented by two thermal capacities instead of one. Static nonlinear function generators simulate the controller. From a comparison between the analogue and experimental results for a resting subject at different ambient conditions they conclude that no complex control with derivative action is necessary, although it may be likely for working subjects. Central temperature is the most important in controlling sweat rate, while skin temperature does not play an important part, and blood flow control is also initiated mainly by the core temperature.

The Stolwijk and Hardy (1966) study is more extensively documented than the other models. Using a three-layered cylinder to represent the trunk and two two-layered cylinders to represent the head and the extremities, they record all assumptions necessary to obtain the parameters in the equations. A proportional controller is driven by the product of deviations from the head core and an average muscle and skin temperature. However, the head core temperature determines whether the major effector response is that for increasing heat loss or for increasing heat production. This switching action is considered an essential part of the model. A small local action is added to the central response, and an integrating controller action has been included to account for the passive vasodilation due to the effects of local work induced muscle anoxia. The use of a central bloodpool as a center of convective heat transfer also substantially improves the dynamic behavior. According to Stolwijk and Hardy their studies seemed to screen out several concepts of temperature regulation as they apply to man. The model is capable to describe a variety of experiments, including exposure to periodically varying air temperature, experimental fever induction, shivering and the ice-cream experiments of Benzinger, Kitzinger and Pratt (1963), without resorting to their conclusions.

Wissler (1964) reports the first simulation on a digital (CDC 1604) computer. He feels that his model "contains as much information as the currently available experimental data warrant". His purpose is "to study the characteristics of the model in order to determine what kind of experiments might be useful in determining those parameters which cannot be measured directly". He divides the body into 15 regions, the head, the thorax, the abdomen, and the

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proximal, medial and distal segments of the arms and legs. Each of the segments is subdivided into 15 concentric radial sections, thereby allowing considerable freedom in the assignment of physical properties such as thermal conductivity and rate of blood flow to the capillaries. Wissler applies a finite-difference technique, with a time step of 5 seconds to solve his equations. The most striking difference with respect to his 1961 analytical results is that the central abdominal temperature computed numerically falls more rapidly during the early resting period after 40 minutes exercise than the corresponding temperature using the analytical procedure. This is because the metabolic heat generated in the trunk, was concentrated in the abdomen in the digital model but distributed uniformly throughout the entire trunk in the analytical computation.

Of course it is interesting to find out whether it is really necessary to use such a relatively large, over 200, number of compartments. From a comparison between his model and a 14 compartment modification (Kuznetz, 1968) of the Stolwijk-Hardy (1966) model, Wissler (1970) concludes "that the use of simple models is probably justified in those cases which involve pronounced vasodilation and small differences between central and surface temperatures". As might be expected from the number of compartments, transient fluctuations in temperatures are more pronounced in his model.

With the aim to prevent hyperthermia of astronauts, Stolwijk (1971) further improves his 1966 model and presents a carefully documented computer program, written in FORTRAN. He uses a total of 25 compartments, six cylinders with four concentric layers and a central blood compartment. The method by which the difference equations are integrated is a simple one step prediction, with a time step of 1 minute, and a check is made whether the temperature in any compartment increases more than 0,1 °C. If the predicted increase is larger, the length of the time step is reduced accordingly. Although the coefficients of the controller are based on steady-state relationships, the model is tested against dynamical experiments. Young male subjects dressed in shorts were quickly transferred between two ambient temperatures, for instance 43 and 17 °C, or they performed heavy periodic exercise. Stolwijk attributes some discrepancies between his model and experimental results during a cold exposure to the small number of radial nodal points in his model.

Gagge (1973) applies the same concepts and principles as Stolwijk (1971) to construct a simplified version with only two compartments, a core surrounded by a skin layer. A peculiar detail is that the relative size of the compartments, their sum being constant, depends on the blood flow to the skin. Vasoconstriction leads to an increased size of the skin compartment, up to 40% of the whole body, while under neutral and vasodilated conditions the skin has only 5

-10% of the total thermal capacity. The blood flow to the skin is a static nonlinear function of core and mean skin temperature. According to Gagge the model can be used with reasonable accuracy for exercise and work.

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Through modelling a single extremity, Kitney (1974) has concluded that heat flux at the skinsurface has to be regarded as an input to the controller.

Miller and Seagrave (1974) derive a model of human thermoregulation during inmersion in a water bath with a varying temperature. They use 9 segments to describe a body with one arm and one leg. The head, thorax and abdomen have three layers each, and the three cylinders representing an extremity have two layers, so a total of 21 compartments. However, no central blood pool is incorporated in the model. The controller influences blood flow only, and has an additive combination of core and mean skin temperature, plus a small non-linear derivative action of the latter. The FORTRAN routine runs on a IBM 360-65, and the differential equations are integrated using Hamming's predictor-corrector method or a modified Euler method. From tests of the model, the authors conclude that there is some question about the meaning of the core temperature as measured in the esophagus, because it does not correspond to the average thorax temperature, nor is it an accurate indicator of the average temperature of the blood returning to the heart.

Gordon and Roemer (1975a) compare finite difference calculations of the transient temperature distribution in a single cylinder of living tissue, with an analytical solution in which metabolic heat production blood flow and arterial blood temperature vary with time. They study the effects of node spacing and integration time step on the accuracy of the calculation. No significant numerical errors occur if ten unequally spaced nodes are utilized with a one minute integration interval. In a subsequent paper (1975b) Gordon and Roemer extend their conclusion to include the performance of the Stolwijk (1971) model. It appears that for severe cold exposure (ambient temperature 5 °C) a ten node unequally spaced formulation of the passive systems is necessary to generate acceptably (1 °C) small numerical errors. Under less severe conditions four nodes are enough.

Gordon, Roemer and Horvath (1976) describe a model of the human temperature regulatory system which has a detailed (154 nodes) formulation of the passive system and a control system especially designed for cold exposure. They note that "previous models have been primarily restricted to neutral or warm exposures, and have been limited in the availability of experimental evidence against which comparisons of model predictions could be made. Stolwijk's (1971) model, for example, was developed primarily for investigating heat stress situations". Gordon et al propose a controller which utilizes head core temperature, mean skin temperature, and mean skin heat flux as input signals to control metabolic heat production, skin blood flow and extremity muscle blood flow. They claim that "one of the successful characteristics of the control system, and in particular the heat flux controller is the effective simulation of the initial overshoot of the metabolic response". This overshoot has been reported by Benzinger et al (1963) and Brengelmann (1967), who tried to explain it as a result of a nonlinear derivative action of the controller. Gordon et al test their model against experimental data obtained under severe cold stress. From a large number of simulations Gordon et al conclude that skin heat flux

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provides an important input to the temperature regulatory system.

Osman and Afify (1984) describe a comprehensive thermal model of the normal women's breast. Using a finite element technique with 1380 compartments the surface temperature distribution of the breast is computed. The results show good agreement with thermographic measurements.

Mekjavic and Morrison (1985) investigate shivering. While other stimulus-response relations predicting shivering are based on static core and skin temperatures, they develop a model which' incorporates both static and dynamic nonlinear characteristics of thermoreceptors. They use data from a series of cold-water immersions to estimate some parameters in their model.

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2.4 Climate control of incubators

2.4.1 Heat transfer

Four variables determine the thermal climate inside an incubator: (mean) air temperature, (mean) wall temperature, humidity and ventilation. Usually the rate of ventilation is fixed by the construction of the incubator, and the clinician has to manipulate the remaining variables.

Air temperature alone is not a proper indicator of the climate around the infant, unless the heat transfer through radiation and evaporation is either closely proportional to convection, or can be neglected because it is small. In incubators without a fan to circulate the air, so with natural convection only, the temperature of the wall is halfway between the temperature of the air inside the incubator and the room temperature. In a usual ward the wall may thus be some 8 °C cooler than the incubator air. In an incubator with forced convection the difference is less, about 4 °C, due to the better thermal contact between incubator-air and wall. Hey and Mount (1966, 1967) describe a simple hinging radiant shield, placed over the infant and heated by the air in the incubator. The shield effectively reduces radiant loss and renders it proportional to convective loss.

Today the room temperature in neonatal intensive care units is not as low as in the 1960's, and a minimal level of 25 °C is quite normal. So the effect of making a forced convection incubator double-walled is less dramatic. Marks and coworkers (1981) compare single- with double-walled incubators, their temperature set at 33 °C in a room of 25 °C. On the single wall they measure the temperature of the inner surface to be 31 °C, whereas on the double wall they find 32 °C. From a study in eight premature infants using the Air Shields C100 Double Wall incubator with and without the inner wall, Bell (1983) concludes that as long as the doors are closed, a double walled incubator offers no advantage over a single-walled one when an abdominal skin temperature sensor is applied in the temperature control system. Then the lower surface temperature of a single wall is compensated by a higher air temperature. However, if the doors are opened when room temperature is 12 - 15 °C below incubator temperature the larger thermal capacity and higher temperature of the double wall reduce the rapid increase of heat loss from the infant: the operative temperature falls only 1,7 °C instead of 4,5 °C in a single walled incubator.

Not only temperature differences determine heat flow, the transfer coefficient and effective surface area also do. By changing its posture a infant may change its effective radiant surface area from 0,48 to 0,70 times its total surface area, concludes Wheldon (1982) from measurements with a manikin in a standard (Vickers) forced convection incubator. She measures a value of 5,4 W/m2K for the convective heat transfer coefficient, but if the infant

takes a foetal rather than a relaxed position the transfer coefficient is only 4,0 W/m2K, relative

to the total surface area of the infant.

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around an infant. The convective heat transfer coefficient appears to increase by 25% when the fan motor is switched on. Without discussing the idea in detail, Clark suggests to apply this effect to obtain a fine control of convective heat loss.

Normally the evaporative heat loss equals about one third of the resting metabolic rate. However, in the first week of their life, infants born after a gestation of less than 29 weeks may evaporate more than their basal metabolic rate if they are nursed naked in unhumidified incubators (Harpin and Rutter, 1985). Since in some countries safety regulations do not allow the air temperature to exceed 36,9 °C, the only way to maintain a normal body temperature without impregnating the skin with a water resistant fat like vernix, is to increase the absolute humidity inside the incubator. According to Harpin and Rutter (1985) the humidification should be stopped after four to seven days to minimize the risk of bacterial infection.

Physically the evaporation of water from the skin is linked to the convective heat transfer, because both processes are determined by the flow conditions in the boundary layer near the skin. Reduction of the convective transfer coefficient will also reduce evaporation. This phenomenon has been demonstrated by Fanaroff and coworkers (1972). Placing a hood over the infant inside an incubator reduces evaporation by 25%. It is not clear whether this is the result of a decreased transfer coefficient only, or whether a local accumulation of water vapor under the hood has occurred also.

Using a photographic technique, Stothers and Warner (1984) estimate 10% of the total body surface area to be in skin to skin contact, and thus not to take part in heat transfer. Depending upon the position of the infant, the area involved in conductive transfer to the mattress varies between 5 and 20% of the total body surface. Due to the low thermal conductance of the mattress the total conductive heat loss is only a few percent of the total transfer. The fractional area involved in radiative and convective heat loss appears from their studies to be 79% in thermal comfort and 69% in a cool environment, values differing slightly from the ones given by Wheldon, probably due to the different experimental technique.

Once it is known what precisely constitutes a proper climate for a particular infant, the next thing is to maintain it.

2,42 The thermostat

Usually only a single thermometer gives some indication of the actual climate, but the heat transfer from the infant differs from that of a thermometer: By the end of the fifties it becomes known that the temperature of a mercury bulb is not a very good indicator of the climate the infant is exposed to. It appears "impossible to predict the equilibrium body temperature of a small newborn infant placed in an incubator of a known ambient temperature" (Agate and Silverman, 1963). A Bedford and Warner globe thermometer could overcome the problems of the mercury bulb, but its location in the incubator would be a difficult compromise between

Climate control of incubators related to growth and thermoregulation of newborn infants

CLIMATE CONTROL OF INCUBATORS

RELATED TO

GROWTH AND THERMOREGULATION

OF NEWBORN INFANTS

CLIMATE CONTROL OF INCUBATORS

RELATED TO

GROWTH AND THERMOREGULATION

OF NEWBORN INFANTS

CONTENTS

1

INTRODUCTION

%r

STELLINGEN

2

LITERATURE

2.1 Medical criteria of clinical management

2.2 Growth of infants

2.3 Thermoregulation

disturbances

controller A

V

2.4 Climate control of incubators

_V