Effects of hypoxia, hyperoxia, hypercapnia and elevated carboxyhemoglobin concentration on VO(2)max and exercise capacity in goats

Humans and other animals sometimes have to function in extreme environmental conditions that result in changes in inspired gas pressures. Understanding how adverse conditions affect exercise performance and aerobic capacity enables us to better predict and prepare for exposures to these environments when aerobic exercise is required. The purpose of this study was to quantify if and how exercise capacity and maximal aerobic capacity ( V˙O2max) are affected in mammals breathing hypoxic, and/or hypercapnic gases with or without breathing small amounts of carbon monoxide (CO) to raise their carboxyhemoglobin fraction (FHbCO). The experiments tested if and how different inspired fractions of oxygen (O 2) (0.06, 0.09, 0.12, 0.15, 0.21 and 0.50), carbon dioxide (CO 2) (0 and 0.05) and inspired CO producing elevated FHbCO (0, 0.15, 0.30 and 0.45) alone and in combination cause dose-dependent responses to V˙O2max and treadmill speed eliciting V˙O2max . To answer these questions, a cross-flow indirect calorimetry system was constructed to measure oxygen consumption (V˙O2) and O2-transport variables in female goats exercising on a treadmill. Oxygen concentration was measured in arterial and mixed-venous blood and in combination with recorded heart rates (fH) enabled cardiac output (Q˙) and stroke volume (Vs) to be calculated. In order to quantify the effects of altered inspired gases and V˙O 2max on endurance capacity, goats breathed room air (FIO 2 0.21) or modest hypoxia (FIO2 0.12) with or without elevated FHbCO (0.30 FHbCO) while running until exhaustion at speeds above and below the speed required to elicit V˙O2max.;Both hypoxia and elevated FHbCO decreased V˙O 2max and running speed at V˙O 2max in a dosedependent manner while hyperoxia and hypercapnia showed no significant differences from room air (0.21 O2). A multiple stepwise regression showed that fractional reduction of V˙O 2max was best predicted from the O2 fraction in inspired air (FIO2) and FHbCO with the equation: V˙O2max gas/V˙O2max air = 0.306 + (3.00 * FIO2) -- (0.849 * F HbCO) (R2 = 0.884). Hypoxia and elevated FHbCO both decrease arterial O2 concentration (C aO2), albeit by different mechanisms -- decreased arterial O2 saturation (SaO2) of hemoglobin-binding sites vs. decreased capacity of the hemoglobin molecule (Hb) to bind O 2. Elevated FHbCO increased SaO2 at a given oxygen partial-pressure (PO2) and induced hyperventilation; both of which resulted in the combination of these gases affecting V˙O2max by less than the sum of each individually. The decrease in CaO2 was correlated with the decrease in V˙O2max which was correlated with the decrease in mechanical power output. Neither hypoxia nor elevated FHbCO significantly decreased Q˙, suggesting that Q˙ is unchanged at maximal exercise regardless of inspired gas concentrations despite a 60% decrease in V˙O2max. Finally, conditions that lower V˙O2max shift the speed vs. endurance time relationship as time-to-fatigue (TTF) was shortened at the same absolute V˙O2 whether the goats breathed hypoxic gas or had elevated FHbCO. Results from this study suggest exercise capacity can be predicted from FIO2 and FHbCO and can be used to predict the decrement in an individual's performance capabilities in these environments.