This program is an approximation model for the heat balance of a naked infant in an incubator. It shows the influence of the different types of body heat loss and demonstrates how the Dräger Caleo, Isolette C2000 and Isolette 8000 Incubators can maintain thermal stability of infants. This program also shows the importance of humidity to achieve heat balance for premature infants.

The importance of providing humidity during incubator therapy is clearly demonstrated as is the high efficiency of the Dräger Incubators when compared with the ability of other types of incubators in supplying the required humidity.

Caution:
This program is an approximation model only. It is intended to demonstrate the effects of incubator settings and is for educational purposes only. DO NOT use it to set or select actual incubator settings! When working with incubators, the user is to exercise independent medical judgmement.

Neither Draeger Medical nor its distributors can be held responsible for any losses, damage to property or bodily injuries including death resulting directly or indirectly from the use of this calculation program.

For further information please contact:

Draeger Medical Systems, Inc.
NC&T Product Management
3135 Quarry Road
Telford, PA 18969
USA

Tel.: +1-215-721-5400
Fax: +1-215-660-2645
www.draeger.com

Changing parameters
Click on the up or down arrow next to the parameter in the settings field in order to modify it. The parameters are grouped as "Patient Information", "Environment Settings" and "Incubator Settings".

Calculating optimal incubator settings
In order to simulate an optimal heatbalance under defined environmental conditions and with a set relative humidity simply click the "Optimize Temp." Button. The program will then change the indicated incubator temperature to a value that simulates a thermally neutral condition.

If you wish to optimize the thermal environment, including setting the optimal humidity level, click the "Optimize Temp./Humidity" Button. The program will then change the indicated incubator settings including temperature and humidity to an optimal combination.

Remember that all settings provided for in this application are approximations intended solely for educational purposes, and must not be used to establish actual incubator settings.

Patient:
Body weight at birth [g]
Age [Days]
Gestational Age at birth [Weeks]
Ventilated [y/n]
Diaper/Sweater/Cap [y/n]

Incubator:
Temperature [°C]
rel. Humidity [%]
Phototherapy [y/n]
Incubator Type

Environment:
Room Temperature [°C]

Results

Displayed as horizontal bars:
Heat Production

• Q Phototherapy
• Q Convection
• Q Radiation
• Q Evaporation
• Heat Balance

Displayed as vertical bar:
Water loss

In incubators using a mattress made from insulating material, conductive heat loss is negligible and is therefore not taken into account in this program.

Four different types of incubator can be selected from the pull down menu. Once the desired type of incubator has been selected, all settings will immediately adjust to simulate the capabilities of the selected incubator. Incubator parameters are dependent on the type of incubator selected. By changing the type of incubator, the data from the previous incubator will be stored. If selected again, the data from the previous incubator will be re-displayed. This gives the possibility to compare the effect of different types of incubator on the same patient settings.

Four different incubator types are available in this calculation program:

Caleo
Incubator Caleo with standard hood.

Caleo with double wall
Incubator Caleo with optional double wall on top of the hood.

Isolette 8000
Isolette 8000 Incubator with double walls.

Isolette C2000
Isolette C2000 Incubator with double walls

For more information about these incubators visit www.draeger.com

Heat Loss

Heat is primarily lost through the skin. The lower the body weight, the greater the body surface to weight ratio. The risk of cooling in Newborns is not only due to an immature thermoregulatory system, but can also be attributed to the larger surface area per gram of body weight which is four times greater than it is in an adult.[10]

In premature infants the central nervous system control of temperature regulation is not fully developed. The brain is unable to regulate metabolism in order to adjust heat production in response to environmental temperature changes. Therefore when environmental temperature changes occur the premature infant is unable to maintain thermal stability.

Types of Heat Gain or Heat Losses

Any heat exchange with the environment ocurs via these different types of a heat gain or loss:

Radiation

Heat transfer due to radiation depends on the surface temperature and the surface structure of two bodies facing each other. It is independent of properties relating to the gaseous medium inbetween the two bodies, such as air temperature. Energy uptake is effected by absorption of heat radiation directed towards the body. Two opposite surfaces exchange radiation energy with each other. If the inner wall of the incubator is cooler than the infants skin, then the infant will lose heat by radiation to the inner wall of the incubator.

LeBlanc reported that radiant heat loss accounted for 64% of the total dry (non-evaporative) heat loss in premature infants [11]. Figure 1 shows radiant heat loss for newborns weighing 1400g, at different incubator inner wall temperatures. The skin temperature of the newborns was 36°C.

The temperature of the incubator's inner walls depends on both incubator and ambient temperatures. In order to reduce radiant heat loss, it is important to minimise the temperature difference between the incubator's inner walls and the skin of the infant.

Mark et. al. revealed that the temperature gradient between the skin temperature of the child and the incubator's inner walls could be reduced considerably by using incubators with warm air flowing between double walls [11]. In addition, infants nursed in double walled incubators were found to have lower metabolic heat production.

If incubator air flow is directed between the double walls, the effect of temperature differences between the child and the ambient atmosphere can be reduced to such an extent that radiant heat loss becomes negligible.

The calculations for radiant heat transfer are derived from the laws of physics [1].

The temperatures of the top and sidewalls are effected by the incubator design, the incubator set temperature and the room temperature [3][4].

See also: References

Convection

A resting body is surrounded by a 4 - 8 mm layer of static air; it is mainly the air outside this layer that moves and thus withdraws heat from the body. The extent of this heat loss is a function of the difference between body surface and ambient temperature, air velocity and body surface area [10].

The major variable is the air movement: as the air velocity is doubled, the cooling effect is 'squared'. The majority of all incubators in use today are convection incubators. Heat exchange by convection is effected between the premature infant and the air. The effects of climatic fluctuations on the body temperature are greater in the premature infant than they are in the full term newborn. Compared to a full term newborn, the ratio between surface area and weight is less favourable in the premature infant (Fig. 2). Heat insulation values of the tissue are lower in premature infants because of their thinner body shell and thinner fat deposits.

Less dense layers of air expand and rise upwards leaving behind a static layer of air surrounding the body. This process is called ' natural convection'. In contrast, 'forced convection' means that the air currents are generated by external forces such as a fan. In accordance with international standards, the air velocity above the incubator bed should not exceed 35 cm/s at any time. (CEI/IEC 601-2-19, 12/90) [14]. If the air velocity is reduced from 35 cm/s to 11 cm/s, heat loss by convection drops by about 40% at a given skin temperature of 36°C, (Figure 3).

Decreasing the air velocity in an incubator is therefore an effective way to reduce convective heat loss.

Respiratory convective heat loss takes place as the body warms inspired air. As a rule, inspired air has a temperature lower than the body core temperature. Respiratory convective heat loss is a function of the temperature difference between inhaled and exhaled air and the minute ventilation of the premature infant. Measurements on a premature baby of 1400 grams, at a minute volume of 280 ml/min and a temperature difference of 7°C between the inhaled and exhaled air temperature, resulted in a heat flow of 0.04 Watts. This is only about 3% of the heat loss due to natural convection via the skin surface. It is therefore negligible [1].

The calculations for convective heat transfer are derived from the laws of physics [1].

See also: References

Evaporation

Transepidermal Water Loss

Loss of heat by evaporation in newborns is mainly caused by transepidermal water loss. Such insensible (non-visible) water loss can be explained as water leakage through the skin, caused by differences in water concentration between the interior of the body and ambient atmosphere. The quantity of water transported through the skin per unit is proportional to the difference in water concentration ( the concentration gradient ).

As the relative humidity increases in the air surrounding the infant and as the infants skin becomes thicker and less permeable to water, transepidermal water loss decreases.

Transepiderminal water loss is also influenced by air velocity in the incubator or ambient atmosphere surrounding the premature infant. A lower air velocity results in less water loss. [9]

A mature newborn on the day of birth, nursed in an ambient atmosphere of 50% relative humidity, will lose 5 mL / (m2 * h) by evaporation. A premature infant of 26 weeks gestation, nursed in an ambient atmosphere of 50% relative humidity, will lose 60 mL / (m2 * h). Evaporative water loss from the premature infant is 12 times greater than from the mature newborn [8] (Figure 3)

A premature infant of 30 weeks gestation, in the first one or two days after birth, nursed in an incubator without humidity, may suffer heat loss caused by evaporation which exceeds its capacity for metabolic heat production.

In this instance, even in a relatively high incubator temperature of 37°C, the infant will not maintain the necessary body temperature.

Evaporative water loss means heat loss of 560 cal/ml, which in turn places a great burden on the metabolism of the premature infant. Evaporative water loss of up to 6 g/kg/h [8], must be replaced parenterally to prevent the blood from thickening. The immature kidneys in a premature infant are unable to cope with rapid compensation of fluid imbalance. Parenteral fluid administration is a challenge to both doctors and nursing staff and the difficulties should not be underestimated.

Administration of too much fluid stresses the infants circulation. There is also an increased risk of a persistent ductus arteriosus, cerebral hemorrhage and necrotising enterocolitis. [15], [16], [17], [18]

Attempts should therefore be made to prevent evaporative water loss right from the beginning. Figure 5 depicts how water loss from the skin can be reduced by humidifying the atmosphere surrounding the baby.

Fig.4

Given a relative humidity of 50% in an incubator, a baby ( < 30 weeks gestation, weighing 0,9 kg ) will lose 60ml water from the skin each day. The energy required for evaporating this amount of water is 33,6 Kcal/day. Assuming that a premature infant (1st day of life) received 40,5 Kcal (45 Kcal/kg [19]), energy loss by evaporation amounts to 80% of the energy supplied.

The calculation of the evaporative heatloss depends on [7][8][9].

Evaporation by sweating

Sweating plays a minor role during the first few days of life. Mature newborns begin to sweat when their body temperature rises to more than approximately 37.2°C [20]; however, the maximum heat loss per unit of body surface area amounts to only about 1/10 of the heat loss experienced by an adult [21].

Sweating in premature babies, particularly on their first day of life, begins at much higher body temperatures than in mature newborns. Neonates of 30 weeks gestation failed to produce any sweat response in spite of high body temperatures. This may be due to immaturity of their sweat glands [21].

Respiratory evaporation

Respiratory water loss is of less significance than evaporative water loss from the skin. However, respiratory water loss does account for approximately 1/4 of the total insensible water loss. Adequate humidification must therefore be provided when ventilating newborns. The ISO standard for breathing-gas humidifiers demands that the water vapour content of breathing gas should be greater than 33 mg per litre at the patient connection. Related to the body temperature, this corresponds to a relative humidity of more than 75%, (Figure 5). As a rule, a value of >90% is considered to be optimum.

Conduction

This type of heat loss takes place whenever the body makes direct contact with the mattress. The degree of heat loss is a function of the material from which the mattress is made. If the temperature of the mattress is lower than the skin temperature of the premature infant at the area of contact, the infant loses heat by conduction. If the mattress temperature is higher, the infant gains heat by conduction. If a premature infant is placed onto a well insulated mattress, the skin temperature at the area of contact corresponds to the core temperature of the baby. Given that a well insulated mattress is used, heat loss due to convection will be negligible.

All Draeger incubators use a well insulated mattress and therefore the conductive heat loss is not calculated in this program.

1. Set the weight of the baby to 1500g, first day of life and 28th week of gestation. The result is a heat production of 2,54 Watts. The Room temperature is 26°C, the Incubator type is a Caleo with double wall and is set to an incubator temperature of 36,5°C and rel. Humidity of 80%. The baby is now in a thermally neutral condition.

2. Set the weight of the baby to 1000g. Do not change the age. The result is a heat production of 1,64 Watts. The Incubator temperature in this tool now needs to be increased to 36,8°C in order to keep a thermally neutral condition.

The results are approximated using a calculation to simulate the baby's surface area [2] and heat production [1][5][6]. The baby is in thermal balance if heat production equals total heat loss.

The different types of heat exchange are:

radiation
convection
evaporation
conduction

The calculation of radiant and convective heat exchange is obtained using the formulas of physics. [1] The temperatures of the incubator top and sidewalls are related to the incubator design, the incubator set-temperature and the room temperature [3][4]. The evaporative heat loss is related to [7][8][9] The conductive heat loss is neglected in this tool due to the insulating material of the mattress.

Phototherapy is an additional heat source when in use.

The program shows the results of heat loss from the infants body: Heat balance is the sum of the different types of heat loss compared with heat production by the baby.

If the result is near zero, the baby is in thermal balance
If the result is negative, temperature and/or humidity is set too low. The baby gets too cold!
If the result is positive, temperature and/or humidity is set too high. The baby overheats!

See also: References

[1] Frankenberger, H., Güthe, A., Inkubatoren Verlag TUV Rheinland (1991)

[2] Boyd, E.: The growth in surface area of the human body. University of Minnesota Press (1935)

[3] Drägerwerk Luebeck: Laboratory-Measurements on Incubator 8000 IC/SC (1992)

[4] Drägerwerk Luebeck: Laboratory-Measurements on Incubator 8000 IC/SC (1992)

[5] Bruek, K.: Waermehaushalt und Temperaturregelung, in: Pysiologie des Menschen, Schmidt, R.F., Thews, G., (Hrsg.), 21. Auflage, S.583 ff., Berlin, Heidelberg, New York (1983)

[6] Bruek, K.: Temperature regulation in new-born infant, in: Biol. Neonate. 3, 65, (1961)

[7] Hammarlund, K., Sedin, G.: Transepidermal water loss in new-born infants, III. Relation to gestational age, in: Acta Paeatr. Scand. 68, 795, (1979)

[8] Hammarlund, K., Sedin, G.: Transepidermal water loss in new-born infants, VIII. Relation to gestational age and post-natal age in appropriate and small for gestational age, in: Acta Paeatr. Scand. 72, 721, (1983)

[9] Okken, A.; Blijham, C.; Franz, W.; Bohn, E.: Effects of forced convection of heated air on insensible water loss and heat loss in preterm infants in incubators, in: The journal of Paediatrics 101,108, (1982)

[10] Obladen, M.; Thermoregulation und Thermoregulationsstörungen bei Neugeborenen. Beitr. Intensiv-Notfallmed. vol. 6, pp. 82-99 (Karger, Basel 1987)

[11] Marks, K.H.; Incubators. Medical Instrumentation 1987 vol. 21, No.

[12] Yeh TF et. al.: Oxygen consumption and insebsible water loss in premature infants in single-versus double walled incubators. J. Pediatr. 97: 967,1980

[13] Boyd. E.: The growth in surface area of the human body. University of Minnesota Press (1935)

[14] Internatinal Standard. CEI/IEC 601-2-19 First edition 1990-12

[15] Stevenson, Fluid administration of patient ductus arteriosus complicating respiratory distress syndrome, J. Pediatr. 1977; 90:257-261

[16] Bell, Effects of fluid administration on the developement of symptomatic patient ductus arteriosus and congestive heart failure in premature infants, N Engl J Med 1980; 302; 598-604

[17] Bell, High-volume Fluid intake prediposes premature infants to necrotising enterocolitis lancet 1979; ll : 90

[18] Thomas, Hyperosmolality and intraventricular haemorrhage in premature babies, Acta Paediatr Scand 1976; 65: 429-432

[19] Halliday, H.L., McClure, G.,Reid, M.: Handbook of Neonatal Intensive Care; 3. Edition, London Philadelphia Toronto Sydney Tokyo (1989)

[20] Hey, E.N., Katz, G.: The optimum thermal environment for naked babies Arch. Dis. Child, 45: 328-334 (1979)

[21] Hey, E.N., Katz, G.: Evaporative water loss in the newborn baby. J. Physiol. 200: 605-619 (1969)