Exercise-induced Arteriovenous Intrapulmonary Shunting in Dogs

Abstract

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 m) intrapulmonary arteriovenous pathways are present in the dog, and whether exercise recruits these conduits. Methods: Through the right forelimb, 10.8 million 25-m stable isotope-labeled microspheres (BioPAL, Inc., Worcester, MA) were injected either at rest (n ϭ 8) or during high-intensity exercise (6-8 mph, 10-15% grade, n ϭ 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-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 ϭ 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.Keywords: intrapulmonary arteriovenous anastomoses; shunt; pulmonary gas exchange; pulmonary circulationThe classic understanding of the normal pulmonary circulation is that all the blood leaving the right ventricle passes through the pulmonary microcirculation via capillaries before returning to the left side of the heart through the pulmonary venous circulation. Studies using saline contrast echocardiography in humans have demonstrated that exercise results in the transpulmonary passage of contrast bubbles, suggesting that large-diameter intrapulmonary (I-P) arteriovenous pathways are recruited with exercise (1-3). These findings are controversial in that they are in contrast with prior research, using inert gas and/or inspired 100% Previous studies, using contrast echocardiography, suggest that intrapulmonary shunts are recruited during exercise in health. However, the size of the contrast is unknown, and thus arteriovenous diameter is undefined. What This Study Adds to the FieldLarge-diameter intrapulmonary arteriovenous shunts are functional in healthy animals, and exercise recruits these vessels.oxygen (4)(5)(6)(7)(8), that has failed to detect substantial right-to-left physiological I-P shunt during exercise.Despite an absence of right-to-left I-P...

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