Bubble and macroaggregate method differ in detection of blood flow through intrapulmonary arteriovenous anastomoses in upright and supine hypoxia in humans

Blood flow through intrapulmonary arteriovenous anastomoses (Q_IPAVA) increases in healthy humans breathing hypoxic gas, and is potentially dependent on body position. Previous work in subjects breathing room air has shown an effect of body position when Q_IPAVA was detected using transthoracic saline contrast echocardiography (TTSCE). However, the potential effect of body position on Q_IPAVA has neither been investigated when breathing hypoxic gas, nor using a technique capable of quantifying Q_IPAVA. Thus, the purpose of this study was to quantify the effect of body position on Q_IPAVA when breathing normoxic and hypoxic gas at rest. We studied Q_IPAVA using TTSCE and quantified Q_IPAVA using filtered Technetium-99m-labeled macroaggregates of albumin (^99mTc-MAA) in 7 healthy men breathing normoxic and hypoxic gas (12% O₂) at rest, while supine and upright. Based on previous work using TTSCE, we hypothesized that the quantified Q_IPAVA would be greatest with hypoxia in the supine position. We found Q_IPAVA quantified with ^99mTc-MAA to significantly increase while breathing hypoxic gas in both supine and upright body positions (Q_IPAVA = 0.7 ± 0.4 vs. 2.5 ± 1.1% of cardiac output, respectively). Q_IPAVA detected with TTSCE increased from normoxia in supine hypoxia, but not in upright hypoxia (median hypoxia bubble score of 2 vs 0, respectively). Surprisingly, Q_IPAVA magnitude was greatest in upright hypoxia when Q_IPAVA was undetectable with TTSCE. These findings suggest the relationship between TTSCE and ^99mTc-MAA is more complex than previously appreciated perhaps because of the different physical properties of bubbles and MAA in solution.

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