Blood Flow Through Intrapulmonary Arteriovenous Anastomoses: Reconciliation of Inconsistent Data Obtained in Hypobaria and Body Position Studies
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Intrapulmonary arteriovenous anastomoses (IPAVA) are vascular conduits through which blood flow bypasses the pulmonary circulation, and does not participate in pulmonary gas exchange. Blood flow through IPAVA (QIPAVA) is known to increase with increasing cardiac output, such as exercise, and while breathing normobaric, hypoxic gas at rest or during exercise. Previous studies demonstrate that QIPAVA is decreased at rest and during exercise in hypobaria compared to equivalent normobaric conditions. Studies involving postural changes have shown that QIPAVA may change with body position. In human studies, QIPAVA is measured either by transthoracic saline contrast echocardiography (TTSCE) or by injection of 99mTc-labeled macroaggregates of albumin (99mTc-MAA). It is unknown if discrepancies in measuring QIPAVA in normobaria and hypobaria, and in different body positions, represent real physiological changes or if they are methodological artifacts. In Chapter IV, the effect of hypobaria on QIPAVA was investigated. QIPAVA was reduced during exercise in hypobaria in normoxia and hypoxia compared to normobaric conditions, however gas exchange efficiency was unimpaired. This suggests that pulmonary blood flow may change in hypobaria such that blood flow is directed away from IPAVA. Alternatively, it may suggest that saline contrast is less stable at high altitude and not detected by TTSCE. In Chapter V, the effect of changing body position on QIPAVA as detected by TTSCE was investigated in human subjects at rest. No significant changes were observed in QIPAVA with postural changes. In Chapter VI, a perfusion model was used to investigate behavior of saline contrast microbubbles, MAA, and microspheres (20 µm and 50 µm diameter) encountering a vertical bifurcation. The results indicated that microbubbles and 20 µm microspheres tend to enter the upper branch of the bifurcation, whereas MAA and 50 µm microspheres tend to enter the lower branch. In Chapter VII, the effect of atmospheric pressure on the initial microbubble radius (Ro) of agitated saline contrast microbubbles was investigated. The results of this study demonstrated that the Ro of microbubbles created at sea level pressure was significantly smaller than Ro of microbubbles created at higher altitudes (1,668 m and 5,260 m).