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

 Blood flow through intrapulmonary arteriovenous anastomoses (https://physoc.onlinelibrary.wiley.com/cms/asset/2f1e2691-c2b9-42ea-ab59-3a220b20216e/eph12083-math-0004.png) is elevated during exercise at sea level (SL) and at rest in acute normobaric hypoxia. After high altitude (HA) acclimatization, resting https://physoc.onlinelibrary.wiley.com/cms/asset/fe8a0385-2b0a-4b82-9a88-868767df56fa/eph12083-math-0005.png is similar to that at SL, but it is unknown whether this is true during exercise at HA. We reasoned that exercise at HA (5050 m) would exacerbate https://physoc.onlinelibrary.wiley.com/cms/asset/08f69e24-3362-4255-aa5d-2ce3e5732f5a/eph12083-math-0006.png 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 (https://physoc.onlinelibrary.wiley.com/cms/asset/d12be8d0-cd10-4d11-b956-307988e87954/eph12083-math-0007.png)] and at HA (25 and 50% of SL https://physoc.onlinelibrary.wiley.com/cms/asset/9fd947cf-c4e8-4a21-947b-f571330854e0/eph12083-math-0008.png). Echocardiography was used to determine cardiac output (https://physoc.onlinelibrary.wiley.com/cms/asset/2eccf5d1-4ecb-4bcb-912e-0e23b48b7f7f/eph12083-math-0009.png) and pulmonary artery systolic pressure (PASP), and agitated saline contrast was used to determine https://physoc.onlinelibrary.wiley.com/cms/asset/f4e5ef28-b95d-4a65-be2d-860d23eb0f59/eph12083-math-0010.png (bubble score; 0–5). The principal findings were as follows: (i) https://physoc.onlinelibrary.wiley.com/cms/asset/272f17f1-d153-48ed-88fe-2b5622ea8bc4/eph12083-math-0011.png was similar at SL rest (3.9 ± 0.47 l min−1) compared with HA rest (4.5 ± 0.49 l min−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 ± 0.7 mmHg) to HA rest (33.7 ± 2.8 mmHg; P = 0.001) and, compared with SL, PASP was further elevated during HA exercise (P = 0.003); (iii) https://physoc.onlinelibrary.wiley.com/cms/asset/696fa7ce-002b-4434-9459-3e003ad4a74a/eph12083-math-0012.png 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 https://physoc.onlinelibrary.wiley.com/cms/asset/dad1aaa3-6180-4b53-b2bd-86ee4968fd09/eph12083-math-0013.png and PASP. Theoretical modelling of microbubble dissolution suggests that the lack of https://physoc.onlinelibrary.wiley.com/cms/asset/e48fcce9-50d3-4db5-ba9b-39789e23b0b7/eph12083-math-0014.png in response to exercise at HA is unlikely to be caused by saline contrast instability.