SUMMARY1. Serial respiratory recordings using impedance pneumography and barometric plethysmography were made from shortly after birth to 7 months in fifteen normal full-term infants. Each recording was made with the infant asleep and sleep state was estimated from records of electroencephalogram and electro-oculogram made in parallel.2. The respiratory records obtained during non-rapid eye movement (r.e.m.) sleep were analysed with computer assistance and stretches of the record, approximately 1 min before and up to 2 min after a spontaneous sigh and ensuing apnoeic pause, were processed and presented as sequential values of the fractional deviation of '. the breath by breath minute volume, from the mean. That part of the sequence which represented the respiratory response to the sigh was then fitted with second order equations representing the critically or underdamped response. The results were presented for each curve in terms of 6, the damping ratio and Wn, the frequency of the undamped respiratory oscillation.3. Three-quarters of the responses could be so fitted with an error of 20 % or less.The residual responses were mainly from infants within a few days of birth. In the youngest infants (4 days or less), the respiratory response to a sigh was highly stable but sluggish: during the period 4-8 days to 3-4 months, the oscillatory period diminished from ca. 25-12 s and respiration was potentially unstable since a small reduction in the damping factor would cause prolonged oscillation while, from 3-4 months, the more mature type of response which was stable with a rapid recovery supervened.4. The possible mechanisms responsible for this trend are discussed in terms of the factors thought to determine respiratory stability in the adult together with the possible relevance of the results to the normal process of respiratory adaptation at birth and to the respiratory difficulties encountered by some infants in the new-born period and early infancy.

ABSTRACX. We have investigated the effect of changing environmental temperature on metabolic rate, sleep state, and water loss in a longitudinal study of 22 lightly clothed babies from 2 d to 3 mo of age. Studies were performed in a modified barometric plethysmograph while recording sleep state, oxygen consumption, and skin and axillary temperatures. Oxygen consumption was higher in rapid eye movement sleep than in quiet sleep at all ages and varied widely between infants at each temperature. Within the first week, there was a 19% rise in oxygen consumption on cooling to 19-22OC during rapid eye movement sleep and a 6% rise during quiet sleep. The medim duration of quiet sleep periods was reduced from 17 to 12 min on cooling within the first week. No such change was seen at 1, 2, and 3 mo. Axillary temperature was reduced at 3 mo during cooling. This may be a part of normal patterns of change in temperature during sleep, unrelated to cooling. At each age, total evaporative water loss fell linearly with falling environmental temperature both within and below the temperature range at which metabolic rate was minimal. The evaporative water losses were greater than expected and suggested that sweating was occurring, both at temperatures at which metabolic rate was minimal and at those at which it was increased. The metabolic response to cooling and the process of sweating appear to be in dynamic equilibrium across this temperature range. Thus, it was not possible to define a temperature range over which both metabolic rate and evaporative water loss were at minimum values. REM, rapid eye movement sleepThe relationship between environmental temperature and V02 has been previously studied in term infants (1-3). The lower end of the thermoneutral range has been defined and is lower at 3 wk of age than at 1 d, in both naked and cot-nursed babies, and might be assumed to fall further with increasing age (4). The effect of sleep state on metabolic rate has been studied both in adults (5) and in babies within the first week at thermoneutral and cooler temperatures (6-8). In all these studies, metabolic rate was higher in REM. However, little information is available on the relationship between sleep state and environmental temperature, or on thermoneutrality, at older ages in infancy.We have investigated the effect of mild cold stress on metabolism and water loss (both respiratory and transcutaneous) during both REM and QS in infants from the first week to 3 mo of age. MATERIALS AND METHODSTwenty-two normal, healthy infants, free from known adverse risk factors were selected. All the infants were born at term (gestational age from 37.5 to 42 wk) to healthy mothers and had a median birth weight of 3.44 kg (range 3.1-4.74 kg). None experienced significant perinatal problems or had any major illnesses during the first 3 mo. Each infant was studied once during the first week (median age 49 h, range 16-143 h) and on one to three further occasions, at 1, 2, and 3 mo. Polygraphic recordings of sleep state and respiration were pe...

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In summary, there is evidence that developmental changes in respiratory control and in thermoregulation have effects upon each other. Theoretically, such effects could give rise to failure of the respiratory system and there is some circumstantial evidence to support the concept that on occasions such interactions may be of importance in sudden unexpected death in infancy. Future research in this area should focus on the nature of the interactions between thermoregulation and respiration, and on the effects of infection and infection-related mediators on those interactions.

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