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Some aspects of body temperature in children

The internal body temperature or the temperature at which all normal body functions take place is very strictly controlled by several regulatory mechanisms. The normal body temperature is carefully and rigidly maintained around 98.6 degrees Fahrenheit (F) or 37 degrees Celsius (C). The reason for this strict control is that all types of body processes, whether simple or complex, function best at this temperature.

Although most of us do not consciously think of the body temperature except when it rises and gives rise to fever, there is a highly developed mechanism by which the body temperature is carefully regulated. There is a temperature controlling centre in the brain which works like a thermostat. It is exquisitely sensitive to the internal temperature as well as the environmental temperature. The normal body processes and activity or exercise levels constantly generate heat and unless a specialised controlling system is in existence, the body temperature could get completely out of hand and the tissues could get overheated. As this is such a dynamic process, the measured body temperature is generally considered to be normal around a narrow range. It is also important to note that the surface temperature of the human body varies even in the same individual depending on the sites at which it is measured. This is true even when the readings are taken from different sites at the same time.

The normal body temperature is kept at a very even keel by several automatic adjustments. On the one hand, if the body gets too hot, messages from the controlling centre from the brain leads to an increased blood flow through the skin by opening up of more and more of the tiny capillary blood vessels. The end result of this process is loss of heat from the body by radiation. This mechanism is complemented by sweating and the evaporation of sweat helps to cool the body. On the other hand if the body is too cold, there is another mechanism to shut down skin blood vessels and conserve heat within the internal organs. If necessary more heat could be generated by shivering which is due to contraction of muscles. The temperature control mechanism is not fully developed in the newborn baby, especially in preterm babies and they are prone to showing up wide fluctuations in the body temperature. This is a particularly tricky problem when they are ill with some sickness. A drop in the body temperature leads to several deleterious effects and is associated with significant morbidity and mortality.

Measurement of the body temperature is an important step in determining the state of health. Fever is a common symptom in children and is an important feature of a number of diseases. There are several different types of measuring devices that could be used to check the body temperature in children. The time honoured instrument is the standard mercury thermometer in which the column of mercury may be encased in glass or plastic. The more modern types of instruments include digital thermometers, electronic ear thermometers and measuring strips that could be placed on the surface of the skin. In the Western world, the traditional mercury thermometers are being gradually phased out because of the potential dangers posed by the mercury in them in case of accidental biting on it or breaking of the bulb.. In many of those countries there is a marked shift of the preferences to the more modern digital thermometers

The newer probe-type digital thermometers are quicker to use and are most convenient. They are also probably a little bit more reliable and are much safer. These can be used to check the temperature on the surface of the skin, inside the mouth or in the rectum. For accurate readings, the instruction provided with the device should be scrupulously followed.

Another refinement is the availability of electronic ear thermometers. They are very easy to use and provide a reading in seconds. These devices work by measuring the heat radiating from the ear drum. In these special thermometers, to get a reliable temperature measurement, the thermometer must be used exactly as directed and one needs to read the instruction manual or leaflet carefully before using the instrument. The procedure of measurement needs a steady hand and the thermometer has to be directed in the line of the ear drum. This could become quite tricky, especially with very young children. If the child has been lying with their head on a warm pillow, or has just come inside out of the cold, one needs to wait 10 to 15 minutes before the ear thermometer can provide an accurate measurement of body temperature. These instruments are quite expensive.

Strip thermometers that measure temperature on the skin are very convenient to use but are not very accurate. They are not reliable enough for taking an exact measurement but are useful to indicate the presence of fever in infants and very young children. Some of these do not give the exact temperature but indicate only the presence of an elevated temperature.

A body temperature reading can be taken from several sites. The common and traditional ones are the mouth, armpit, ear, skin surface or the rectum. The site at which the instrument is used is important in recording the temperature. The traditional mercury thermometers could be used under the armpit, inside the mouth and inside the rectum. The selection of the site depends on the age and type of child. In small children the mouth should not be used as there is a risk of the child biting on the instrument. It may also not be possible to keep it in the armpit in some children who are restless and not able to lie still. Checking it rectally is of course unpleasant for the child.

If a standard mercury column thermometer is used, it is important to shake down the column to below the normal range before the procedure. Whatever the site that is selected, it is important to keep the thermometer in place for the necessary prescribed period of time for it to equilibrate with the temperature level. Although there is no universal agreement on the duration for which the thermometer has to be kept in place, generally with children who are old enough to co-operate and keep still, one needs to keep the thermometer under their armpit for at least 5 minutes. In older children and adolescents oral temperature measurement could be used. The thermometer is placed in the mouth, under the tongue and it will take two to three minutes for the mercury column to stabilise.

It is important to note that if the child has just eaten or drunk anything hot or cold, one needs to wait for at least 10 minutes before an accurate temperature reading can be taken. Temperature readings from the armpit are not absolutely and totally reliable and are about 0.5ºC lower than mouth temperature. Although a rectal temperature reading is the most accurate and is quite often used in hospital it is not necessary to be so precise when taking temperature readings at home. Rectal temperature reading is therefore not recommended for home use. Rectal temperatures are the closest to central ‘core’ temperature and are about 0.5ºC (2ºF) higher than readings taken from the mouth or ear.

One of these several different methods may be used in a given child to record the temperature. In very young children, especially those under 3 months of age, a digitally read rectal temperature is probably the most accurate. Electronic ear thermometers should not be used in these very young children as their ear canals are too small. In those between three months to about 4 or 5 years, a digital thermometer under the armpit would give a reliable reading. In a child over 4 years of age, a digital thermometer could be used to take the oral temperature if the child is cooperative. However, kids who have frequent coughs or are breathing through their mouths because of stuffy noses might not be able to keep their mouths closed long enough for an accurate oral reading. In such cases an electronic ear thermometer or a digital thermometer under the armpit could be used.

It is a common practice in our country to just assess fever in children by the feeling of warmth detected by a hand placed on the surface of the body. This is a practice that is debatable and quite unreliable. Besides, it does not give any idea of the height of the fever. It would be a useful exercise to invest in a reliable thermometer in the home and to familiarise oneself in the correct way of using the instrument. It is such a useful thing for a parent to be able to tell the doctor the exact level to which the temperature of a child has risen. This information definitely helps in arriving at a possible cause for the fever.

The writer appreciates feed-back from readers. Please email him at bjcp@sltnet.lk or write to him at the following address:

Dr. B. J. C. Perera, Consultant Paediatrician, Asiri Medical Hospital, 181, Kirula Road, Colombo 5.

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