Mechanism of reduction in alveolar-arterial PO2 difference by helium breathing in the exercising horse

Abstract

Previous work has shown that replacing N2 in air with He at the same inspired O2 fraction reduces the exercise-induced alveolar-arterial PO2 difference (AaPO2) in horses but has provided no mechanism explaining this effect. We sought to distinguish among possible causes by using the multiple inert gas elimination technique. Six horses were studied on a high-speed treadmill while they breathed either ambient air or normoxic He-O2. O2 uptake reached 138.0 ml.min-1.kg-1 and was not affected by He-O2. Temperature-corrected arterial PO2 was 76.7 Torr (air) and 86.9 Torr (He-O2) (P < 0.01). Corresponding AaPO2 was 22.3 and 15.9 Torr, respectively (P < 0.01). Mean AaPO2 predicted from ventilation-perfusion inequality did not change with He-O2 (12.7 Torr with air and 11.9 Torr with He-O2). Mean arterial PCO2 was 50.1 Torr with air and 44.1 Torr with He-O2 (P < 0.01); minute ventilation and tidal volume were correspondingly higher by 140 l/min and 1.0 liter, respectively, with He-O2. Pulmonary O2 diffusing capacity, cardiac output, and all ventilation-perfusion dispersion indexes did not change with He-O2. Intrapulmonary shunt was insignificant. Higher ventilation with He-O2 explained only approximately 4 Torr of the 10-Torr rise observed in arterial PO2. The remainder (and the corresponding fall in AaPO2) was due to more complete diffusion equilibration as a consequence of the higher minute ventilation and thus alveolar PO2, which reduced the average slope of the O2 dissociation curve, thereby increasing the ratio of diffusive to perfusive conductance.

The Effects of Breathing He–O2 Mixtures on Maximal Oxygen Consumption in Normoxic and Hypoxic Men

“…This was not the case in normoxia. The present results agree with those obtained by others in the horse (Erickson et al 1994(Erickson et al , 1995, and appear compatible with the tested hypothesis that the role played by pulmonary ventilation and the gas exchange system in limiting ýOµ,max is important in hypoxia, but negligible in normoxia.…”

“…Such a reduction can be achieved, for instance, by breathing low-density gas mixtures, such as He-Oµ. In fact in the horse, He-Oµ breathing had no effect on ýOµ,max in normoxia (Erickson et al 1994), but increased (Erickson et al 1995) ýOµ,max in hypoxia . As far as humans are concerned, He-Oµ breathing has been reported to increase (Robertson & McRae, 1966;Brice & Welch, 1983), decrease (Murphy, Clark, Buckingham & Young, 1969;Spitler, Horvath, Kobayashi & Wagner, 1980) or have no effects (Bowers & Fox, 1967) on ýOµ,max in normoxia.…”

Plasma Catecholamine, Adrenocorticotropin and Cortisol Responses to Exhaustive Incremental Treadmill Exercise of the Thoroughbred Horse.

“…During . the incremental exercise test, VO2 was continuously measured by means of an open flow respirometry system [7] and determined during the final 30 sec of each step.…”

Effects of Caffeine on Performance, Cardiorespiratory Function and Plasma Hormonal Responses during Exhaustive Treadmill Exercise in the Thoroughbred Horse.

“…VO2 was continuously measured during the exercise test using a bias flow ventilatory system for horses [8]. HR was continuously recorded during the exercise test with a heart rate monitor (Bandage-XL, Pollar, Finland).…”

“…Arterial and mixed venous blood gases, O2 saturation (O2 sat) and hemoglobin (Hb) were directly measured at 37.0°C with ABL 510 (Radiometer, Copenhagen, Denmark) and were corrected to body temperature with use of the corresponding mixed venous blood temperature [8]. SV was calculated based on the following expressions: SV = CO/HR [CO (cardiac .…”