EFFECT OF METABOLIC-RATE ON VENTILATORY ROLL-OFF DURING HYPOXIA

This study was done to determine 1) whether goats demonstrate the roll -off phenomenon, i.e., a secondary decrease in minute ventilation (VE) , after an initial hyperventilation during various levels of hypoxia a nd, if so, 2) whether roll-off could be due to changes in metabolic ra te. We hypothesized that roll-off occurs in the goat during hypoxia bu t is not due to hypometabolism. To answer question 1, eight unanesthet ized adult goats were exposed to 15-20 min of hypoxia at 0.15, 0.12, a nd 0.09 inspired O-2 fraction (FIO2), resulting in 60, 40, and 30 Torr arterial PO2, respectively. Goats were fitted with a face mask connec ted to a spirometer to measure VE, and arterial blood gas samples were obtained via carotid arterial catheters. Roll-off was seen with 0.15 and 0.12 FIO2), whereas VE steadily increased with 0.09 FIO2. During h ypoxia, arterial PCO2 fell 2, 3, and 7 Torr at 0.15, 0.12, and 0.09 FI O2, respectively. In the second series of experiments, nine different goats were exposed to 30 min of 0.12 FIO2. O-2 consumption and CO2 pro duction were measured five times during baseline and hypoxia. VE incre ased to 32% above baseline values after 2 min of hypoxia and then grad ually decreased by 18%. Changes in breathing frequency and tidal volum e contributed to the roll-off. O-2 consumption decreased (P = 0.0029, analysis of variance) and CO2 production increased (P = 0.0027) during hypoxia, although both changes were small (<7%) compared with the eve ntual 18% decrease in VE. We conclude that the adult goat demonstrates the roll-off phenomenon during moderate levels of hypoxia. Although m etabolic rate-does change slightly during hypoxia, this decrease canno t solely account for the roll-off that we observed.