Exercise-induced arteriovenous intrapulmonary shunting in dogs

Rationale: We have previously shown, using contrast echocardiography, that intrapulmonary arteriovenous pathways are inducible in healthy humans during exercise; however, this technique does not allow for determination of arteriovenous vessel size or shunt magnitude. Objectives: The purpose of this study was to determine whether large-diameter (more than 25 mu m) intrapulmonary arteriovenous pathways are present in the dog, and whether exercise recruits these conduits. Methods: Through the right forelimb, 10.8 million 25-mu m stable isotope-labeled microspheres (BioPAL, Inc., Worcester, MA) were injected either at rest (n = 8) or during high-intensity exercise (68 mph, 10-15% grade, n = 6). Systemic arterial blood was continuously sampled during and for 3 minutes after injection. After euthanasia, tissue samples were obtained from the heart, liver, kidney, and skeletal muscle. In addition, 25- and 50-mu m microspheres were infused into four isolated dog lungs that were ventilated and perfused at constant pressures similar to exercise. Measurements and Main Results: Blood and tissue samples were commercially analyzed for the presence of microspheres. No microspheres were detected in the arterial blood or tissue samples from resting dogs. In contrast, five of six exercising dogs showed evidence of exercise-induced intrapulmonary arteriovenous shunting, as microspheres were detected in arterial blood and/or tissue. Furthermore, shunt magnitude was calculated to be 1.4 +/- 0.8% of cardiac output (n = 3). Evidence of intrapulmonary arteriovenous anastomoses was also found in three of four isolated lungs. Conclusions: Consistent with previous human findings, these data demonstrate that intrapulmonary arteriovenous pathways are functional in the dog and are recruited with exercise.