Hyperoxia-induced stepwise reduction in blood flow through intrapulmonary, but not intracardiac, shunt during exercise

Blood flow through intrapulmonary arteriovenous anastomoses (IPAVA) (Q_IPAVA) increases during exercise breathing air, but it has been proposed that Q_IPAVA is reduced during exercise while breathing a fraction of inspired oxygen (FI_O2) of 1.00. It has been argued that the reduction in saline contrast bubbles through IPAVA is due to altered in vivo microbubble dynamics with hyperoxia reducing bubble stability, rather than closure of IPAVA. To definitively determine whether breathing hyperoxia decreases saline contrast bubble stability in vivo, the present study included individuals with and without patent foramen ovale (PFO) to determine if hyperoxia also eliminates left heart contrast in people with an intracardiac right-to-left shunt. Thirty-two participants consisted of 16 without a PFO; 8 females, 8 with a PFO; 4 females, and 8 with late-appearing left-sided contrast (4 females) completed five, 4-min bouts of constant-load cycle ergometer exercise (males: 250 W, females: 175 W), breathing an FI_O2 = 0.21, 0.40, 0.60, 0.80, and 1.00 in a balanced Latin Squares design. Q_IPAVA was assessed at rest and 3 min into each exercise bout via transthoracic saline contrast echocardiography and our previously used bubble scoring system. Bubble scores at FI_O2= 0.21, 0.40, and 0.60 were unchanged and significantly greater than at FI_O2= 0.80 and 1.00 in those without a PFO. Participants with a PFO had greater bubble scores at FI_O2= 1.00 than those without a PFO. These data suggest that hyperoxia-induced decreases in Q_IPAVA during exercise occur when FI_O2 ≥ 0.80