Effect of temperature on regulation of breathing and sleep/wake state in fetal lambs

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Abstract
Lamb fetuses (n = 16) were employed to study the effect of ambient liquid temperature on behavioral (n = 16) and recorded (n = 8) sleep/wake states and on absolute breathing threshold and responsivity to CO2 (n = 8). Sleep/wake states were estimated by electroencephalogram, nuchal electromyogram, electroculogram and by behavioral criteria ex utero. CO2 threshold and responsivity were assessed with fetal CO2 tests in which fetal breathing was related to increasing fetal arterial CO2 tension (PaCO2) during CO2 rebreathing by the ewe. The high normal CO2 threshold of the fetus (55.7 .+-. 2.4 [SE] torr) at warm core temperature (39.6 .+-. 0.3.degree. C) was lowered significantly (to 41.6 .+-. 1.7 torr) by cooling and decrease of core temperature (to 37.1 .+-. 0.5.degree. C). Similarly cooling increased responsivity to CO2 (i.e., decreased X-axis intercept and increased slope) of maximal intratracheal pressure (ITP), ITP 100 ms after onset of a breath, and fetal ventilatory equivalent. The increased breathing frequency with cooling was less linear. Response to cooling characteristically included rapid onset of vigorous breathing movements, which is attributed to a cutaneous receptor reflex response, and development of less vigorous but more regular breathing movements concomitantly with the combined reduction of skin and core temperature. The relationship of breathing to sleep/wake state was clear. Apnea was associated with sleep; induced breathing movemnets were associated with change in sleep state, i.e., entry into active or transitional sleep and then wakefulness.

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