Effects of graded reduction of brain blood flow on ventilation in unanesthetized goats

The ventilatory effects of graded reductions in brain blood flow (BBF) were studied in unanesthetized goats. At a BBF of 85% of control (PVO₂ = 29.2 Torr, PVCO₂ = 47.3 Torr) there were no clear ventilatory effects. At BBF of 70% of control (PVO₂ = 25.2, PVCO₂ = 50.5) and 50% of control (PVO₂ = 22.3, PVCO₂ = 53.0) there was hyperpnea, due primarily to an increase of tidal volume. Further reduction of BBF (avg of 42% of control) first produced intense tachypnea and then (30-40% of control) caused apnea that was reversible. At 50% BBF there was a reduction of brain O₂ consumption, (4.67-4.00 ml/min) and an increase in systemic O₂ consumption. β-Adrenergic blockade prevented the increase in systemic O₂ consumption and reduced the hyperpnea by two-thirds at 50% BBF; the residual hyperpnea was associated with hypocapnia in contrast to the hyperpnea prior to β-adrenergic blockade, which was virtually isocapnic. The data suggest that hyperpnea due to brain ischemia is a result of both brain acidosis and systemic hypermetabolism. The similarity of the pattern of responses to that previously reported for progressive carboxyhemoglobinemia suggests that brain hypoxia is a determinant of the ventilatory responses to brain ischemia.