VENTILATION AND CARBON-DIOXIDE EXCHANGE IN EXERCISING HORSES - EFFECTOF INSPIRED OXYGEN FRACTION

Thoroughbred horses (TB) have no ventilatory response to added CO2 dur ing near-maximal exercise. To see whether that reflects mechanical lim its to ventilation or the control of breathing, we examined the effect s of varying inspired O-2 fraction (0.16, 0.21, or 0.30) in five norma l TB standing quietly and galloping at 10 and 14 m/s on a level treadm ill. We measured gas exchange (O-2 consumption and CO2 production) and ventilation with a flow-through mask system. We also measured PO2, PC O2, and O-2 contents in arterial and mixed venous blood and calculated cardiac output by using the Fick equation. Low inspired O-2 fraction (0.16 vs. 0.21) had significant effects in TB galloping at 14 m/s. Art erial Pot then was 38 Torr compared with 56 Torr for horses on air. Ti dal volume and minute ventilation were 20% greater than their correspo nding values on air, which were 12 liters and 1,475 l/min, respectivel y, whereas respiratory frequency did not change. O-2 consumption and C O2 produc tion were unchanged, but alveolar ventilation was 6% greater , despite increased alveolar and physiological dead spaces, so arteria l Pco(2) was lower (45 vs. 50 Torr on air). Thus, hypoxia was an effec tive stimulus to breathing, and minute ventilation was not mechanicall y limited in TB breathing air at the speeds studied.