Dynamics of the ventilatory response to central hypoxia in cats

The dynamics of the effect of central hypoxia on ventilation were investigated by the technique of artificial perfusion of the brain stem in alpha-chloralose-urethan-anesthetized cats. A two-channel roller pump and a four-way valve allowed switching the gas exchanger into and out of the extracorporeal circuit which controlled the brain stem perfusion. When isocapnic hypoxia (arterial PO2 range 18-59 Torr) was limited to the brain stem, a decline in ventilation was consistently found. In 12 cats 47 steps into and 48 steps out of central hypoxia were made. The ventilatory response was fitted using least squares with a model that consisted of a latency followed by a single-exponential function. The latencies for the steps into and out of hypoxia were not significantly different (P = 0.14) and were 32.3 +/- 4.0 and 25.1 +/- 3.6 (SE) s, respectively. The time constant for the steps into hypoxia (149.7 +/- 8.5 s) was significantly longer (P = 0.0002) than for the steps out of hypoxia (105.5 +/- 10.1 s). The time constants for the increase and decrease in ventilation after step changes in the central arterial PCO2 found in a previous study (J. Appl. Physiol. 66: 2168-2172, 1989) were not significantly different (P greater than 0.2) from the corresponding time constants in this study (for 7 cats common to both studies). Theories of the mechanisms behind hypoxic ventilatory decline need to account for the long latency, the similarity between the time constants for the ventilatory response to O2 and CO2, and the differences between the time constants for increasing and decreasing ventilation.