[Widening of the alveolar-arterial oxygen gradient during incremental exercise]

Excessive widening of the alveolar-arterial gradient for oxygen, with respect to the subject's age and oxygen uptake, is the most sensitive signal of a disturbance in pulmonary gas exchange, whether it is due to ventilation, circulation or diffusion. During incremental exercise, simultaneous measurements of metabolic power, ventilation and dead space may suggest a possible aetiological diagnosis without confirming the causal nature of the impairment. The magnitude and the kinetic of P(Ai-a)O(2) can suggest explanations of the mechanisms involved and lead to complementary functional or morphological investigations (CT, ultrasound, DLCO, etc.) in relation to the clinical setting. The purpose of this text is to review the physiology of the pathway of oxygen from the alveolar air to pulmonary capillary blood during incremental exercise, with emphasis on the predominant role of the dimensions of diffusion and pulmonary capillary capacity in relation to oxygen uptake and the functional responses (alveolar ventilation and cardiac output).