Effects of hypoglycaemia on ventilation and arousal responses to hypoxia, and newborn calves during active and quiet sleep.
Ventilatory and arousal responses to progressive, isocapnic hypoxia were assessed in five full-term calves, aged 1-8 d, during normo-glycaemia and insulin-induced hypoglycaemia; replicate tests were made during both quiet and active sleep. Hypoxia was produced by rebreathing 8-10% (W/V) oxygen; end-tidal PO2 and PCO2 confirmed the mask-seal, and the constancy of PCO2 achieved by a soda-lime CO2-absorber. Oxygen saturation was recorded continuously by aspirating aortic blood through a cuvette-oximeter. Airflow through a tightly fitting face-mask was digitized at 50 ms intervals to calculate breath-by-breath ventilation and rate. Sleep-state and arousal were characterized by EEG, electro-oculogram, nuchal electromyogram and behaviour. An intravenous dose of 2.5 units/kg of soluble insulin produced hypoglycaemia within 60 min (blood glucose less than 1 mmol/l). In the normoglycaemic control state, ventilation during quiet sleep increased linearly; typically the ventilation ratio to pre-hypoxic control was 1.5 at an arterial O2-saturation of 85-90%. In contrast, during active sleep, hypoxaemia progressed without any ventilatory response to a very low arterial O2-saturation of less than 70%. Severe hypoglycaemia did not alter these ventilatory responses during either quiet or active sleep. Thus, the relationship between ventilation ratio and arterial O2-saturation at a saturation of 75% did not differ significantly from control. In quiet sleep the mean ventilation ratio values at an arterial O2-saturation of 75% were 1.92 +/- 0.07 (SEM) and 1.62 +/- 0.07 (P greater than 0.05) for normoglycaemia and hypoglycaemia respectively; in active sleep these were 1.08 +/- 0.09 (SEM) and 1.06 +/- 0.04 (P greater than 0.20). The arterial O2-saturation at which reflex arousal occurred differed between sleep-states but was not altered by hypoglycaemia. In quiet sleep, values were 85.0 +/- 1.4 (SEM) and 84.4 +/- 2.5; in active sleep, 57.8 +/- 3.9 (SEM) and 60.4 +/- 1.4, for normoglycaemia and hypoglycaemia respectively.