AMINOPHYLLINE PARTIALLY BLOCKS VENTILATORY DEPRESSION WITH HYPOXIA INTHE AWAKE CAT

In humans and cats, the ventilatory response to 30 min of moderate hyp oxia is biphasic, an initial increase being followed by a decrease in ventilation to levels that are often less than halfway between the ini tial response and the air-breathing control level. The decrease, or hy poxic depression, is thought to be of central origin. In humans, intra venous aminophylline, an adenosine blocker, blunts hypoxic depression and may completely block it in anesthetized cats. We studied 11 adult cats while awake, measuring ventilation and end-tidal Po-2 (Peto(2)) a nd Pco(2) (Petco(2)) during 30 min of isocapnic hypoxia (Petco(2) 32 T orr(1 Torr = 133.3 Pa), Peto(2) 60 Torr) after intravenous aminophylli ne on 1 day and saline on another. On the saline day hypoxia initially produced a 75% increase in ventilation, which declined at 30 min to 1 10% of control, largely owing to a decrease in tidal volume. With amin ophylline, room-air ventilation was slightly increased, and hypoxia in itially produced a 50% increase in ventilation, which then declined to 130% of control at 30 min. Late in hypoxia, ventilation was significa ntly greater with aminophylline than with saline. The degree of hypoxi c depression was not related to blood theophylline levels and was simi lar after aminophylline doses of 10 and 13 mg/kg. We conclude that hyp oxic depression is at least partially due to adenosine accumulation, t he effect of aminophylline being likely due to central adenosine block ade.