The circulation in sleep in newborn lambs.

Interaction of the cardiovascular, homeothermic and behavioural systems has particular significance in early life as this period of development sees high cardiac output, limited circulatory reserves, a particular vulnerability to thermal stress, and long periods of rapid eye movement (REM) sleep during which thermal regulation may be suspended. In unstressed lambs, significant cardiovascular differences exist between behavioural states of sleep and wakefulness, with the REM sleep-state being outstanding. In REM sleep there is wide cardiac output variation, a low heart rate, and a high stroke volume. High levels of cardiac output occurring in combination with poorer oxygenation and respiration in REM may represent a risk to the newborn, as cardiac performance is already high and reserves are low in comparison with the adult. Lambs respond to cool stress with increases of cardiac output and oxygen consumption in wakefulness and quiet sleep, but not in REM sleep. Suspension of thermoregulatory processes in REM sleep also points to this as the more vulnerable state for the newborn. Arousal responses from quiet sleep or REM sleep to wakefulness occur if lambs are subjected to acute reductions of blood pressure. There is a delay before arousal which is longer in REM sleep than in quiet sleep. Animals fail to arouse in approximately 40% of hypotensive tests in both states. This shared feature of the sleep-states (failure to arouse) may be more important than the state-related difference (longer delay before arousal in REM) in understanding a cardiovascular mechanism of Sudden Infant Death Syndrome.