COMPARISON OF VENTILATORY RESPONSES TO SUSTAINED REDUCTION IN ARTERIAL OXYGEN-TENSION VS CONTENT IN AWAKE PONIES

To gain insight into central and peripheral contributions to changes i n breathing during hypoxia, we compared effects on breathing of reduci ng inspired PO2 (hypoxic hypoxia) with reducing arterial O-2 content ( Ca-O2) through elevation of carboxy hemoglobin (COHb) (CO hypoxia). Tw elve awake ponies were studied during 1 h of breathing room air follow ed by 6 h when COHb was increased to 25% and Ca-O2 was decreased by 17 %. When COHb was increased, arterial P-CO2 (Pa-CO2) increased graduall y to 1.3 Torr above (P < 0.05) control level between 30 and 45 min of CO exposure. Pulmonary ventilation (VE) decreased (P = 0.09) similar t o 1 liter the first 30 min of CO exposure. After similar to 45 min, Pa -CO2 began to decrease, steadily reaching 1.5 Torr below (P < 0.05) co ntrol level by 4.5 h of CO hypoxia. VE did not change significantly af ter 30 min of elevated COHb. Eight ponies were also studied during 5 h of hypoxic hypoxia (arterial PO2 similar to 40 Torr). Pa-CO2 decrease d 5 Torr (P < 0.05) within 5 min of hypoxia and decreased another 4 To rr (P < 0.05) between 30 min and 5 h of hypoxia consistent with hypoxi c ventilatory acclimatization. VE increased (P < 0.05) within 3 min of hypoxic hypoxia but then decreased (P < 0.05; VE roll off) toward con trol and did not increase significantly with ac climatization. Because CO and hypoxic hypoxia both decrease brain oxygenation but only hypox ic hypoxia increases carotid chemoreceptor activity, we conclude that initial hypoventilation with CO hypoxia and VE roll off with hypoxic h ypoxia are consistent with hypoxic ventilatory depression within the b rain. In addition, hyperventilation with prolonged CO hypoxia is consi stent with a central nervous system mechanism contributing to this pha se of hypoxic ventilatory acclimatization in ponies.