The effect of cooling on breathing and shivering in unanaesthetized fetal lambs in utero.

The effects of cooling have been studied in chronically instrumented unanaesthetized fetal lambs of 106-132 days gestation. Cold stimulation of cutaneous thermoreceptors by means of a coil placed around the fetus in the amniotic cavity induced continuous breathing which was associated with high-voltage electrocortical activity throughout the cooling period. Direct internal cooling by means of an intragastric cooling coil did not alter fetal breathing or sleep-state patterns. Shivering occurred in response to both external and internal cooling, and this response was present as early as 106 days gestation, well before the appearance of high-voltage sleep. In older fetuses there was evidence of a reduction in thermoregulatory responses during low-voltage electrocortical activity, but not of the complete suspension of thermoregulation characteristic of low-voltage (r.e.m.) sleep seen in mature animals. It is concluded that deep regular breathing associated with high-voltage electrocortical activity can be induced by cold stimulation of cutaneous thermoreceptors, but not by a reduction in core temperature alone. Furthermore, fetal responses to cooling are present as early as 106 days gestation.