A step forward in neonatal intensive care

Thermo Monitoring

The importance of maintaining the temperature of the premature baby has been well documented over the years, however the temperature can vary through out the body. The so called core temperature is not constant through out the entire body.

The body will attempt to maintain its temperature within narrow limits and, if exposed to thermal stresses, will utilise energy to achieve temperature stability.  If a baby is exposed to cold stress, the extremities of the body will cool before more central temperatures drop. By measuring both areas of the body the difference will indicate whether the baby is suffering thermal stress long before any difference would be seen in the central temperature of the body.

Thermal stress has been associated with an increase in mortality, making early detection of this problem an important part of the monitoring of premature and sick babies.

Temperature Monitoring involves a continuous display of the central and external temperatures, as well as the air temperature and humidity inside the babies’ incubator.

Although a baby may be exposed to excessive heat, the more common clinical problem, particularly in premature babies, is cold stress due to heat loss.

Babies lose heat during birth, resuscitation and transportation to the neonatal unit as well as during handling. Cold stress has been associated with serious illness and an increase in mortality

Isolette® C2000 Incubator

What happens if a baby is too cold?

Cold Stress:

• Exacerbates lung problems and increases the need for ventilation

• Results in poor growth and increases length of time in hospital

• Increases the risk of serious illness and possible fatality

Cold Stress – early effects

Cold stress occurs when heat loss is such that the baby has to call up thermoregulatory mechanisms to increase heat production or to prevent heat loss.

These mechanisms will allow the baby to maintain central temperatures, but only at a large metabolic cost. If we do not act until we see the central temperature falling we will have exposed the baby to a critical period of metabolic stress, which may have detrimental effects.

It is standard practice to measure the temperature of a baby undergoing intensive care. Usually only a single temperature is measured, often at infrequent intervals. This practice will not detect the early changes of cold stress, as the baby will initially use energy to maintain its central temperature.

An early response to cold stress is vasoconstriction (narrowing of the blood vessels) to reduce heat loss from the peripheries. As adults we know that our hands and feet are first to feel cold as the body reduces blood flow to the peripheries in response to cold exposure.  By measuring both central and extremities temperatures it is possible to obtain more detailed information on the thermal state of the baby.

The response of a baby to a procedure during which the incubator was opened for a brief period may result soon after the start of the procedure to the baby’s extremities temperature dropping but the central temperature will remain stable.

Measurement of a central and an extremities temperature and continuous display of the difference between them will give an early indication of developing cold stress.

An increase in the central extremities temperature differential occurs before the fall in core temperature. This is the principle of Thermo Monitoring.

Cold Stress – later effects

During the resulting effects of a prolonged handling, the central extremities temperature difference increases as the baby is exposed to a cold stress and the peripheries cool.   Eventually the baby can no longer compensate by activating thermoregulatory mechanisms and the central temperature drops.

If only a single temperature had been measured at infrequent intervals, this cold stress would not have been detected until the central temperature had decreased. The baby would have been exposed to a period of increased metabolic stress.

premature baby

Image by maria mono via Flickr

Conclusion

If there is no single core temperature in the body, what can we measure to give us some idea of the central temperature. What we need to monitor is a temperature which is close to an “average” core temperature and which will show changes in the central temperature.

Rectal temperature:

Traditionally the rectum has been used as a measure of the core temperature.

However taking a rectal temperature is an invasive procedure, also the measurement is not always reliable as rectal temperature is dependent on the depth to which the probe is inserted.

Where can temperature be measured?

• Rectal temperature is also affected by the temperature of the blood returning from the lower limbs. If there is extremities vasoconstriction (narrowing of the blood vessels) and the baby is centralising its circulation, the cold blood returning from the legs will significantly lower the measured rectal temperature.

• From a practical point of view, it is impossible to keep a rectal probe in the same position for any length of time and therefore this is not a suitable site for continuous temperature monitoring.

Axilla and abdominal skin temperatures

The axilla and the abdomen (over the area of the liver) are alternative sites commonly used to represent central temperature. In the newborn baby these sites, although on the skin, do not appear to react to lower temperatures with vasoconstriction (narrowing of the blood vessels). This means that, although the temperature measurements in the axilla or on the abdomen are slightly lower than the true central temperature, they will change in the same way as the central temperature.

Monitoring the trends in the axilla or abdominal skin measurements will therefore give information on the way the central temperature is changing. The axilla is a good site for a probe as it is not easily affected by changes in environmental temperature and the position of the probe does not interfere with X ray fields.

Zero heat flux temperature

If the baby is lying on a non conducting mattress, the skin adjacent to the mattress will be unable to lose heat and will therefore warm up to the temperature of the body’s core.  This is called the Zero Heat Flux Temperature.  With the baby on its back on a non conducting mattress, this temperature can be measured by a probe placed in the interscapular region.  It is important to use small flat probes that will not cause pressure damage to the baby’s delicate skin.

Extremities temperature

In the newborn baby the foot has been shown to respond to lower temperatures with vasoconstriction.

With Thermo Monitoring it is recommended that:

• There is continuous monitoring of a central temperature and a extremities temperature.

• Abdominal skin, axilla or interscapular probes are used to monitor central temperature.

• The sole of the foot is used to monitor extremities temperature.

• Both temperatures and the central extremities temperature difference can  be displayed as a continuous trend graph.  The continuous measurement of both a central and a extremities temperature is important in thermal monitoring of the newborn baby.  As there is no single central temperature it is difficult to give recommendations for this value.  The figure will also depend on the site from which the temperature readings are taken.  Most staff caring for babies feel that the central temperature measured from rectum, abdominal skin or axilla should be around 37°C.

In a study of 150 babies under 1500 grams birthweight, the median abdominal skin temperature was found to be 36.9°C with an interquartile range of 36.8 to 37.2°C.  Some babies however had an abdominal skin temperature significantly above 37.5°C .  In these babies any attempt to lower their temperature resulted in a widening of the central extremities temperature difference, suggesting increasing cold stress.  It would seem that some babies, in the first few days after birth, set their own central temperature to above 37°C.

The mean central extremities temperature difference increases over the first 5 days of life.  In babies less than 1000g birth weight the temperature difference increased from 0.5°C on day 1 to 1.0°C by day 3.  In heavier babies the temperature difference was between 1.0 and 1.3°C

Thermoregulation and the Baby

Within the “normo thermal” range the body is able to cope with the environmental temperature by increasing energy expenditure.  Below a certain environmental temperature the central temperature drops; hypothermia then results. Likewise an adult starts sweating to cool the body when the environmental temperature is too warm.  At high environmental temperatures the body can initially compensate by using energy for heat loss but at a certain point these mechanisms fail resulting in hyperthermia.

Within this “normo-thermal” range there is a point at which the body is using a minimum of energy for thermoregulation. This is the “thermo-neutral” range and corresponds to the most comfortable environmental temperature for the body.  The mature baby at birth has a smaller “normo-thermal” range than the adult but this rapidly matures.  In premature babies and low birthweight babies both the “normo-thermal” and “thermo-neutral” ranges are very narrow.

Premature Baby

In fact, the premature baby is not provided with mechanisms like sweating and shivering and its metabolism is limited as well. The only response to cold stress is vasoconstriction, which is seen with Thermo Monitoring.

Therefore, it should be the target of nursing care to keep the baby in its “thermo neutral” range to provide the best conditions for growth and maturity.

Thermo Monitoring gives us the information, whether babies are in their “thermo neutral” range. Maintaining the optimum environmental conditions for the pre-term baby is therefore a challenge.

The feotus does not have a system for temperature regulation.  Before birth the baby is poikilothermic which means that it adapts to the temperature of the mother.  This means that the foetal central and extremities temperatures are the same and have a value determined by the central temperature of the mother.  What happens in the premature baby after delivery?  Just after birth the central and extremities temperatures depend directly on the environmental temperature but with time the baby activates its own system for the regulation of body temperature, changing from a poikilothermic to a homeothermic state.

As this occurs the “normo-thermal” and “thermoneutral” ranges both increase.  The more mature the baby, the quicker this change occurs but in some premature babies it can take several days.  This period of increased thermal instability with all its potential complications adds to the baby’s problems at a time when it is often already critically ill.

Once the baby has activated its own body temperature regulation systems it is more stable and able to cope better with changes in environmental conditions without any effect on central body temperature.

Resuscitation

All babies cool rapidly after birth because of heat loss from evaporation of amniotic fluid from the skin.  Drying and wrapping the premature baby prevents such heat loss.  The premature baby is at higher risk of significant cold stress, particularly if resuscitation is needed.  In premature babies low temperature on admission to the neonatal unit has been associated with increased mortality and morbidity.  Evaporation, the major source of heat loss  can be significantly reduced using occlusive dressings (material, usually gauze or absorbent cotton).

Clean plastic bags are easier to use and prevent hypothermia immediately after delivery.  The baby can be slid into the bag up to the neck whilst still wet.  The head is covered with a hat and no blankets are used, thus allowing radiant heat to warm the infant through the bag.  Clinical inspection and auscultation (The act of listening for sounds within the body, chiefly for ascertaining the condition of the lungs, heart, pleura, abdomen) during resuscitation can be done through the bag and if vascular access is needed a small hole can be cut in the plastic.

It is important to prevent heat loss during transport to the neonatal unit. The infant can be transported while still in the bag, which is only removed once the baby is in a humidified environment.

Types of Heat Loss

Radiation

Radiation is the transfer of heat from a warm to a cooler surface.  If you sit in front of an open fire the side of your body facing the fire becomes hot while your back becomes cold.  If the baby is surrounded by cooler surfaces there will be heat loss through thermal radiation.

To reduce heat loss through thermal radiation the  incubator has double walls with a stream of warm air between the surfaces.  This raises the temperature of the walls, thus reducing thermal radiation.

Conduction

Heat will be transferred from the baby to the mattress with which it is in contact.  If the mattress is a good heat conductor then heat will be lost from the baby.  In everyday life you become aware of the difference between heat gain from a warm foot bath compared with the heat loss from sitting on a cold stone.

Evaporation

Whenever liquid evaporates there will be heat loss because of the heat of evaporation.  This is the reason why we feel cold when we open the door after taking a hot shower.  The wet and thin skin of a premature baby can result in significant fluid and heat loss if evaporation is not prevented.  By using high ambient humidity in the incubator the loss of heat by evaporation is greatly reduced.

By considering all the methods that premature babies can lose substantial heat from within different regions of their bodies, also by implementing substantial and well monitored methods to reduce or eliminate those losses the chances of the infant succumbing to thermal stress are lowered significantly.  Thus lowering the risks of serious illness or mortality of the baby at their most vulnerable time, when they are unable to self regulate their body temperature in the manner that adults are able. Full term babies have more resilience to heat loss than their premature counterparts.

For premature baby clothes suitable for babies from 1 lb please visit  prem2pram

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1 Comment (+add yours?)

  1. JoLynn
    Nov 28, 2009 @ 18:28:25

    Thanks for stopping by my blog. You have a lot of great info here. I am always grateful for those who can relate

    Reply

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