Why and How does "Prone-Sleeping" concern with Sudden Infant Death Syndrome(SIDS)?

Shiro Kubota, MD
Masatoshi Sano, MD

Kubota Maternity Clinic
Hirao-2-12-18, Chuoku, Fukuoka, Japan 810-0014
Phone: 092-531-7530, Fax: 092-531-9840
    After the "Back-to Sleep" campaign, the incidence of SIDS has remarkably decreased in the world1.2.3. However, it has been still unknown why an infant sleep at supine-position is safer against SIDS than at prone-position, and how an infant dies with SIDS. By the observations of central(C-) and peripheral(P-) deep-body temperature(DBT) of the infants who slept in supine and in prone-position, we clarified that the infants in prone-sleeping get heat-stroke by the disturbance of thermolysis from abdomen, when they are put on clothes excessively. When the cold stimulation is lost continuously, the warming makes an infant sleep more deeply and reduces arousal response and respiration, as a result, hypoxemia of the infant advanced. It is strongly assumed that SIDS is not a desease, but is the irreversible stage of hypoxemia caused by the infant's thermoregulation during sleep in warm condition. To prevent from SIDS and to liberate mothers from nursing apprehensions, it is very important to know the reasons why infants are sensitive to heat and to understand the intrinsic and extrinsic problems surrounding them.
 As there are many descriptions about the epidemiological surveys and the autopsy findings of SIDS which relate to body and environmental temperetures of infants, it is suggest that the characteristics of this syndrome may have relevance to the thermoregulation of infants. The relationship between SIDS and over-heating had been mentioned in1970's4.5.6. However, negative relevances between SIDS and hyperthermia had been recognized in 1980's. In those days it was difficult to differ scientifically between fever and heat-stroke in the infants with hyperthermia. Fever is usually accompanied with infections. In the case of an infection, thermoregulation of the patient corresponds to the person who regulates the body temperature at low-temperature environment, because thermoregulation-center is set erroneously at a high level by pyrogens.  Therefore, thermogenesis is promoted by muscular strain like shiverings, but thermolysis is inhibited by peripheral vasoconstriction. As a result, hands and feet become cooler and do not sweat. Sweating is generally noted when the patient begin to recover and peripheral vessels are dilated. Different from fever, heat-stroke is generally caused by the the disturbance of thermolysis and/or the increase thermogenesis. In the infants, however, the thermolysis is declined under the environmental conditions of high temperature and humidity. It will be easy to make infant over-heating, because he cannot say a word, cannot turn over by himself, or cannot remove (bed)clothes by himself. Under such conditions, his peripheral blood vesseles are dilated, the hands and feet are warm, his skin is moist and sweat, that is, the P- DBT is elevated.
   The deep-body thermometers using in this experiments is reported to measure the DBT at the depth of 9 mm from surface7. One was attached on the sternum and measured the temperature of the blood in the heart and the other on the foot sole measured the DBT at the same part which was correlated with the peripheral blood flows8.  Fig. 1 shows one of the representative records of them. In the supine-position, the C-DBT and back-environmental temperature(B-ET) were higher than the P-DBT and abdominal-ET(A-ET). The B-ET was equal to C-DBT. After changing the position from "supine" to "prone", three temperatures altered as follows. The A-ET and B-ET rapidly altered within a few minutes. The A-ET elevated higher than the C-DBT while the B-ET was declined. B-ET in the prone position was lower than A-ET in supine position. On the other hand, the P-DBT gradually elevated interestingly to reach in equilibrium about 1 hour later. The C-DBT showed little variations throughout the observation.
Figure 1
Fig.1 The alternations of body temperatures by changing positions.
    The body temperatures of an infant weighed 2,620 grams at the second day after full-term deliverty was recorded by two deep-body thermometers(Terumo Co.:PD-3) continuously and simultaneously . The environmental temperatures (exactly, between the inner and the outer clothes) at both the abdomen(A-ET) and the back(B-ET) were also recorded. The abbreviations in this figure are as follows, C-DBT: central deep-body temperature, P-DBP: peripheral deep-body temperature, A-ET; abdominal environmantal temperature, B-ET: back environmental temperature. The obserbations were performed under the superintendence of one or more doctors and nurses. Mothers of all infants on this studies were informed and consented previously.
   From the alternations of A-ET and B-ET by their position change, it was recognized that the heat of infants is released much more from their abdomnen than from their back. The more important phenomena was observed. The P-DBT gradually rose with the prone-position as compared with that of the supine-position.  The elevation of P-DBT means as follows. The vesseles of extremities are dilated to carry inner heat to outside. Accordingly, it means that the thermolysis is suppressed at the prone-position compared with supine-position. If an infant in prone-position is covered excessively and insides of clothes become hot and humid,  the heat of the infant will not be released and be accumulated. Presumably, it is possible that the P-DBT of the infant in prone-position and excessive-clothes reach equilibrium to C-DBT within a few ten minutes, which will cause "heat-stroke". The high temperature and humidity of environment do not mean only the high temperature and humidity in their room, but also mean those of  the inside of clothes. Caps and socks also disturb their thermolysis at prone-position. The homeothermal animals produce heat as long as they live. If one of them breathe it last, it did not produce heat any more. Then, its body temperature become to be variable by ET. When the ET is high, its body temperature is still high. This might be the reason why the body temperature of an infant who died with SIDS was high.
Figure 2
Fig.2 The thermoregulation of an infant at normal environmental temperature.
The deep-doby temperatures were monitored continuously and simultaneously on a full term delivery infant in the supine-position, and the alternation of the heart rate and his behaviers(awakening or sleeping) were observed in room temperature(25℃). The abbreviations in this figure are as follows, C; crying, A; awakening, S; sleeping, HR; heart rate, RT; room temperature. Other abbreviations are same as Fig.1.
  In the cool environment8, to keep their C-DBT normal, infants reduce the thermolysis by the peripheral vasoconstriction and produce heat by muscular movements, like crying. In the normal environment(Fig.2), C-DBT shows little fluctuations, but P-DBT altered by the changes of peripheral vasoconstriction and vasodilatation. The decline of P-DBT is associated with vasoconstriction, the increase heart rate and awakening, while the elevation of P-DBT is associated with vasodilatation, unfructuated heart rate and sleeping, respectively. Therefore, it is assumed that the thermoregulation of the infants is controlled by the ON/OFF switch of stimulation via neurotransmitter secretion. In high temperature environment, however, infants must continue to release heat by peripheral vasodilatation, sweat and stop to produce heat by relaxing muscles. In cool environment, the cold stimulation makes infants arousal response and respiratory acceleration. But in high temperature environment, infants sleep well and do not receive any cold stimulation. If over-heating continues for a long time and the switch of the stimulations continues to be OFF, an infant will sleep more deeply and relax the muscles more than usual, as a result, his/her respiration is more inhibited and hypoxemia is advanced. It was also observed9 that heart rate of sleeping infants in warm condition(34℃) showed little fluctuations and TcPO2 in warm condition was lower than those of the infants at room-temperature(25℃). Furthermore, in high environmental temperature, his/her oxygen consumption also increases. If the neurotransmitter-secretions is continuously reduced, it is easy to understand that his/her hypoxemia will be more advanced and will not be recovered any more. Therefore, it might be considered that SIDS will happen when an infant is sleeping deeply without arousal response and his/her hypoxemia become irreversible.
 Although there have been at least hundreds of thousands of victims during the last century, no one sees the moment of SIDS. The infants areusually asleep after feeding at night or in the busy day-care center. If someone watches the sleeping sick infant by heat-stroke and the person touches him/her, this touch stimulation makes him/her arousal and respiration reactions which let him/her escape from SIDS. This might be the reason why SIDS has no eyewitness. Nevertheless, the cold stimulation exists at night and in the day-care center as long as the sleeping infants are not covered excessively, even though there are little stimulation or no eyewitness. Therefore, the cold stimulation is the matter of great importance for the infants to prevent from SIDS. From our experiments, it was highly suggested that the cold stimulation disappear not only by the heating but also by the warming with excessive clothes. Because, wrapping an infant keeps away the cold stimulation from him/her by the high temperature and humid environment of the inside of clothes. Consequently, the  P-DBT elevates continuously without rhythmical up-and-down fructuations. This is the most dangerous for sleeping infants.  If you watch carefully his/her behavior, it is possible to know whether the environmental temperature is suitable for him/her or not. Because the behavior of infants represents their present conditions. If he/she cries drawing his/her extremites, he/she feels cold or uncomfortable. If he/she is asleep without sweat or warm hands
and feet, he/she might be comfortable. However, if he/she sleeps deeply with sweat and warm hands and feet, he/she might be too warm. It is much better to think that infants are sensitive to "warmth" rather than "coolness". Never over-wrap him/her with the clothes, especially made of unventilated or bad hygroscopic materials.
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   We are grateful to Prof. Keiko Shimizu helped English translation.
(June 7,2001)