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 (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.