This study was done to determine 1) whether goats demonstrate the roll- off phenomenon, i.e., a secondary decrease in minute ventilation (V̇E), after an initial hyperventilation during various levels of hypoxia and, if so, 2) whether roll-off could be due to changes in metabolic rate. We hypothesized that roll-off occurs in the goat during hypoxia but is not due to hypometabolism. To answer question 1, eight unanesthetized adult goats were exposed to 15-20 min of hypoxia at 0.15, 0.12, and 0.09 inspired O2 fraction (FI(O2)), resulting in 60, 40, and 30 Torr arterial PO2, respectively. Goats were fitted with a face mask connected to a spirometer to measure V̇E, and arterial blood gas samples were obtained via carotid arterial catheters. Roll-off was seen with 0.15 and 0.12 FI(O2), whereas V̇E steadily increased with 0.09 FI(O2). During hypoxia, 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 FI(O2). O2 consumption and CO2 production were measured five times during baseline and hypoxia. V̇E increased to 32% above baseline values after 2 min of hypoxia and then gradually decreased by 18%. Changes in breathing frequency and tidal volume contributed to the roll-off. O2 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 eventual 18% decrease in V̇E. We conclude that the adult goat demonstrates the roll-off phenomenon during moderate levels of hypoxia. Although metabolic rate does change slightly during hypoxia, this decrease cannot solely account for the roll-off that we observed.
- biphasic ventilatory response
- control of breathing
- oxygen consumption
- roll-off phenomenon