Intrapulmonary shunt during normoxic and hypoxic exercise in healthy humans

This review presents evidence for the recruitment of intrapulmonary arteriovenous shunts (IPAVS) during exercise in normal healthy humans. Support for pre-capillary connections between the arterial and venous circulation in lungs of humans and animals have existed for over one-hundred years. Right-to-left physiological shunt has not been detected during exercise with gas exchange-dependent techniques. However, fundamental assumptions of these techniques may not allow for measurement of a small (1-3%) anatomical shunt, the magnitude of which would explain the entire A-aDO2 typically observed during normoxic exercise. Data from contrast echocardiograph studies are presented demonstrating the development of IPAVS with exercise in 90% of subjects tested. Technetium-99m labeled macroaggregated albumin studies also found exercise IPAVS and calculated shunt to be approximately 2% at max exercise. These exercise IPAVS appear strongly related to the alveolar to arterial PO2 difference, pulmonary blood flow and mean pulmonary artery pressure. Hypoxic exercise was found to induce IPAVS at lower workloads than during normoxic exercise in 50% of subjects, while all subjects continued to shunt during recovery from hypoxic exercise, but only three subjects demonstrated intrapulmonary shunt during recovery from normoxic exercise. We suggest that these previously under-appreciated intrapulmonary arteriovenous shunts develop during exercise, contributing to the impairment in gas exchange typically observed with exercise. Future work will better define the conditions for shunt recruitment as well as their physiologic consequence.