Lovering AT, Stickland MK, Kelso AJ, Eldridge MW. Direct demonstration of 25-and 50-m arteriovenous pathways in healthy human and baboon lungs. Am J Physiol Heart Circ Physiol 292: H1777-H1781, 2007. First published December 1, 2006; doi:10.1152/ajpheart.01024.2006.-Postmortem microsphere studies in adult human lungs have demonstrated the existence of intrapulmonary arteriovenous pathways using nonphysiological conditions. The aim of the current study was to determine whether large diameter (Ͼ25 and 50 m) intrapulmonary arteriovenous pathways are functional in human and baboon lungs under physiological perfusion and ventilation pressures. We used fresh healthy human donor lungs obtained for transplantion and fresh lungs from baboons (Papio c. anubis). Lungs were ventilated with room air by using a peak inflation pressure of 15 cmH 2O and a positive end-expiratory pressure of 5 cmH2O. Lungs were perfused between 10 and 20 cmH2O by using a phosphate-buffered saline solution with 5% albumin. We infused a mixture of 25-and 50-m microspheres (0.5 and 1 million total for baboons and human studies, respectively) into the pulmonary artery and collected the entire pulmonary venous outflow. Under these conditions, evidence of intrapulmonary arteriovenous anastomoses was found in baboon (n ϭ 3/4) and human (n ϭ 4/6) lungs. In those lungs showing evidence of arteriovenous pathways, 50-m microspheres were always able to traverse the pulmonary circulation, and the fraction of transpulmonary passage ranged from 0.0003 to 0.42%. These data show that intrapulmonary arteriovenous pathways Ͼ50 m in diameter are functional under physiological ventilation and perfusion pressures in the isolated lung. These pathways provide an alternative conduit for pulmonary blood flow that likely bypasses the areas of gas exchange at the capillary-alveolar interface that could compromise both gas exchange and the ability of the lung to filter out microemboli.intrapulmonary arteriovenous anastomoses; shunt; baboon; pulmonary circulation RECENT WORK has demonstrated the transpulmonary passage of saline contrast microbubbles during exercise, but not at rest, in healthy adult human subjects without cardiac anomalies, suggesting that intrapulmonary arteriovenous pathways are recruited during hyperdynamic conditions (5,14,22). Although the actual size of the saline contrast microbubbles is not known, theoretical and experimental data have estimated that the size distribution of microbubbles that survive to enter the pulmonary microcirculation to be 60 to 90 m in diameter (8,10,16,31,32). These estimates prompted Eldridge et al. (5) to suggest that these inducible intrapulmonary arteriovenous pathways must be at least 60 m in diameter. Postmortem studies in adult human lungs have documented the existence of intrapulmonary arteriovenous pathways for more than 100 years (20). More recent studies using microspheres have demonstrated that these vessels are functional under various conditions. For example, Tobin and Zariquiey (25) demonstrated that arteriove...

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