Reduced blood flow through intrapulmonary arteriovenous anastomoses during exercise in lowlanders acclimatizing to high altitude

Blood flow through intrapulmonary arteriovenous anastomoses ((Q) over dot(IPAVA)) is elevated during exercise at sea level (SL) and at rest in acute normobaric hypoxia. After high altitude (HA) acclimatization, resting QIPAVA is similar to that at SL, but it is unknown whether this is true during exercise at HA. We reasoned that exercise at HA (5050m) would exacerbate (Q) over dot(IPAVA) as a result of heightened pulmonary arterial pressure. Using a supine cycle ergometer, seven healthy adults free from intracardiac shunts underwent an incremental exercise test at SL [25, 50 and 75% of SL peak oxygen consumption ((V) over dot(O2 peak))] and at HA (25 and 50% of SL (V) over dot(O2 peak)). Echocardiography was used to determine cardiac output ( Q.) and pulmonary artery systolic pressure (PASP), and agitated saline contrast was used to determine (Q) over dot(IPAVA) (bubble score; 0-5). The principal findings were as follows: (i) (Q) over dot was similar at SL rest (3.9 +/- 0.47lmin(-1)) compared with HA rest (4.5 +/- 0.49lmin(-1); P=0.382), but increased from rest during both SL and HA exercise (P<0.001); (ii) PASP increased from SL rest (19.2 +/- 0.7mmHg) to HA rest (33.7 +/- 2.8mmHg; P=0.001) and, compared with SL, PASP was further elevated during HA exercise (P=0.003); (iii) (Q) over dot(IPAVA) was increased from SL rest (0) to HA rest (median=1; P=0.04) and increased from resting values during SL exercise (P<0.05), but was unchanged during HA exercise (P=0.91), despite significant increases in (Q) over dot and PASP. Theoretical modelling of microbubble dissolution suggests that the lack of (Q) over dot(IPAVA) in response to exercise at HA is unlikely to be caused by saline contrast instability.