Components of alveolar-arterial O2 gradient during rest and exercise at sea level and high altitude

Abstract

To determine the effects of exercise and high altitude on the contributions of shunt, ventilation-perfusion (V/Q) nonhomogeneity, and diffusion limitation to the alveolar-arterial O2 gradient (AaDo2), we measured pulmonary exchange of O2, CO2, and six inert gases (SF6, ethane, cyclopropane, halothane, diethyl ether, and acetone) during rest and exercise in unanesthetized dogs at sea level and after acute exposure to an altitude of 6,096 m in a hypobaric chamber. Shunt and dead-space fractions, calculated from inert gas measurements, did not change. High altitude decreased the inert gas partial pressure gradients between mixed alveolar gas and mixed end-capillary blood, indicating that V/Q relationships became more homogeneous. Exercise had no effect on these gradients. At sea level, AaDo2 was mainly due to V/Q nonhomogeneity, with a small portion due to shunt. At high altitude, the contribution of shunt became negligible and that of V/Q nonhomogeneity diminished. These improvements were partially offset, however, by a gradient due to diffusion limitation. Exercise had no effect on AaDo2 or any of its components. At high altitude, estimated pulmonary O2 diffusing capacity averaged 20.8 ml.min-1 at rest and 35.3 ml-min-1.Torr-1 during exercise.

Pulmonary gas exchange at maximal exercise in Danish lowlanders during 8 wk of acclimatization to 4,100 m and in high-altitude Aymara natives

“…A previous study in conscious dogs concluded that ventilation-perfusion mismatch actually decreased, whereas diffusion limitation increased, during exercise at acute exposure to simulated high altitude (ϳ6,000 m) compared with sea level (22). However, in humans at a more moderate altitude (3,800 m), augmented pulmonary arterial pressure and ventilation-perfusion mismatch (shunting) have been demonstrated in lowlanders during exercise within the first days of acclimatization (15).…”

End tidal-to-arterial CO2and O2gas gradients at low- and high-altitude during dynamic end-tidal forcing

“…Although experiments investigating the alveolar-to-arterial oxygen difference (AaDO 2 ) between LA and HA exist, it is difficult to assess the AaDO 2 during DEF necessitating the study of the P ET -Pa O 2 gradient in its place. However, literature regarding the AaDO 2 is inconsistent suggesting that the AaDO 2 can widen or narrow following ascent to HA at baseline (22,33,45). During DEF, inspired gas concentrations are constantly changing making it difficult to assess alveolar PO 2 .…”

Pro: Hypoxic Pulmonary Vasoconstriction Is a Limiting Factor of Exercise at High Altitude

“…Low O 2 breathing increases mean pulmonary artery pressure (Ppa m ), which further counteracts the effects of gravity and makes the distribution of perfusion more homogeneous with no significant change in the distribution of ventilation (Dawson, 1968, Dawson,1972. This improves lung diffusing capacity (Dehnert et al, 2010) and ventilation/perfusion (V A /Q) matching (Mélot et al, 1987;Sylvester et al, 1981). The impact of it on arterial blood gases is difficult to detect as the normal alveolar-arterial PO2 gradient (A-aPO2) is already very small and decreases with inspiratory PO 2 because of the shape of the hemoglobin disscociation curve ((Farhi and Rahn, 1955).…”

“…The impact of it on arterial blood gases is difficult to detect as the normal alveolar-arterial PO2 gradient (A-aPO2) is already very small and decreases with inspiratory PO 2 because of the shape of the hemoglobin disscociation curve ((Farhi and Rahn, 1955). The improvement in V A /Q matching induced by global hypoxic vasoconstriction can be demonstrated by the multiple inert gas elimination method (MIGET), but only if either cardiac output remains naturally unchanged (Sylvester et al, 1981) or is normalized by mathematical manipulation (Mélot et al, 1987).…”