Counterpoint: Exercise-induced intrapulmonary shunting is real.

intrapulmonary shunt is miniscule is further confirmed by a recent study reporting venous admixture in very fit athletes during exercise breathing pure O 2 (18). During 100% oxygen breathing, alveolar PO 2 is elevated to such an extent that ventilation-perfusion inequality and diffusion limitation no longer contribute to the AaDO 2 —it can be explained only by right to left shunting (18). In this study (16), venous admixture during 100% oxygen averaged 0.5%, a value also consistent with the previously reported microsphere and inert gas data. Fourth, it has never been shown that oxygen exchange across the vessels responsible for microbubble transmission is impaired. It is entirely possible that oxygen exchange is normal, and indeed, as stated above in exercising dogs (14), arterial oxygenation was not impaired, suggesting this to be the case. Finally, it has been argued by Drs. Stickland, Lovering, and Eldridge that proximal vessel (precapillary) gas inert gas exchange occurring by diffusion may result in an underestimation of intrapulmonary shunt (3, 17) by MIGET. This is because diffusion equilibration of inert gases is much faster than for O 2. However, were that the case, the problem for O 2 exchange becomes one of diffusion limitation and not shunt. But even here, there is spectrophotometric evidence (1) that O 2 can also take part in precapillary exchange, casting doubt on this explanation. In summary, flow through vessels responsible for micro-bubble transmission in exercising humans has never been shown to impair gas exchange and should not be equated to a shunt, which implies an absence of gas exchange. Furthermore, when intrapulmonary shunts have been quantified, irrespective of technique, they are tiny, like the Whos that Horton the Elephant heard, and can account for no more than 1.4 mmHg, or 7%, of the total AaDO 2 of 19 mmHg. We leave it to the reader to decide if microbubble transmission really implies a shunt, whether a " shunt is a shunt no matter how small, " and if the effect of intrapulmonary shunt on pulmonary gas exchange is significant.son JS. Exercise-induced intrapulmonary arteriovenous shunting in healthy humans. Effect of prolonged, heavy exercise on pulmonary gas exchange in athletes. Effect of acetazolamide on pulmonary and muscle gas exchange during normoxic and hypoxic exercise. Intrapulmonary shunting and pulmonary gas exchange during normoxic and hypoxic exercise in healthy humans. Direct demonstration of 25-and 50-␮m arteriovenous pathways in healthy human and baboon lungs. Intrapulmonary …