Exercise-Induced Intrapulmonary Arteriovenous Shunting and Pulmonary Gas Exchange
Authors: Michael K. Stickland, Andrew T. Lovering
DOI / Source: https://doi.org/10.1249/00003677-200607000-00003
Date: 01 July 2006
Reading level: Intermediate
Why This Matters for Freedivers
This paper helps explain a surprising idea: under hard effort, the lungs may open “shortcut” vessels that let some blood bypass the smallest capillaries. For freedivers, that matters because (1) it can affect oxygen transfer when you’re working hard, and (2) it may weaken the lung’s normal role as a filter—potentially relevant to discussions about bubbles/emboli reaching the brain in certain diving situations.
Synopsis
We usually picture the lungs like a fine sieve: all blood from the right side of the heart is forced through tiny capillaries wrapped around alveoli, where gas exchange happens, before returning to the left side of the heart. That model is tidy—and it mostly works. But this review explores evidence for something messier: the lungs also appear to contain larger “bypass” pathways (intrapulmonary arteriovenous anastomoses). These are bigger-than-capillary vessels that connect arteries to veins more directly.
The puzzle is that classic gas-exchange tools (especially MIGET and 100% oxygen tests) generally don’t show a meaningful right-to-left shunt during exercise in healthy people, so for years the assumption was: “no real shunt exists.” Yet anatomical studies (including casting studies and bead experiments in lungs) strongly suggest these larger connections do exist, sometimes up to surprisingly large diameters.
So who’s wrong. The authors argue: maybe nobody. Different methods are “looking” at different things. The review walks through newer evidence using contrast echocardiography (agitated saline microbubbles). In these studies, many healthy people show bubbles appearing on the left side of the heart during exercise—suggesting some transpulmonary passage consistent with opened bypass pathways. The likelihood of seeing this increases with higher cardiac output and higher pulmonary pressures during harder exercise.
Then comes the practical consequence: if some blood takes a shortcut, it can reduce oxygen loading efficiency—especially when combined with very fast blood transit times during intense effort. The review also points out something that’s very relevant to divers: one of the lungs’ “side jobs” is filtering particles and small emboli coming from the venous circulation. If larger pathways open, that filter can be partially bypassed, which could theoretically increase the chance of particles/bubbles reaching the arterial side.
A really important nuance: the authors emphasize that these pathways might not behave like a “pure shunt” in the simplistic sense. There may be some gas exchange happening upstream of the capillaries (precapillary diffusion), and large vessels have thicker walls and different diffusion distances. That means a pathway could allow the passage of bubbles (anatomical shunt) while still doing some gas exchange (so it doesn’t show up the same way on certain “functional shunt” tests).
Bottom line: during hard exercise, the lung circulation may be more dynamic than the textbook diagram—opening pathways that can influence oxygenation and the filtering function of the lungs. For freedivers, this is a useful piece of the bigger puzzle when thinking about effort, oxygen drops, and why certain “bubble/emboli” discussions keep coming up in breath-hold diving science.
Abstract
This review discusses evidence that exercise can recruit intrapulmonary arteriovenous pathways that allow some blood to bypass the smallest pulmonary capillaries. While traditional gas-exchange methods often do not detect a large physiological shunt during exercise, anatomical data and contrast echocardiography studies suggest these larger-diameter connections can open with increasing cardiac output and pulmonary pressures. The authors explain how this could contribute to exercise-related impairment in pulmonary gas exchange and how it may also reduce the lung’s filtering function for larger particles/emboli. The paper highlights the methodological reasons different techniques may disagree and outlines key unanswered questions about vessel size, regulation, and physiological impact.