Hypoxia offsets the decline in brachial artery flow-mediated dilation after acute inactivity

Intermittent (IH), as opposed to continuous hypoxia (CH), is thought to have beneficial effects on cardiovascular function and health. In the present study, we examined the acute effects of IH and CH (∼80% pulse oxygen saturation via 10% oxygen tank) on peripheral vascular function. Brachial artery flow-mediated dilation (FMD) was used to assess vascular function in 12 young adults (23 ± 5 yr; 8 M/4 F) before and after 50 min of IH (5 cycles; 4-min normoxia/6-min hypoxia per cycle), CH (20-min normoxia followed by 30-min hypoxia), or time control (50-min normoxia) interventions. Brachial artery diameter and velocity were measured using Doppler ultrasound to assess blood flow and shear rate. The total change in shear rate was greater during IH (634 ± 1,073·s^-1, P < 0.05) and CH (321 ± 833·s^-1, P = 0.05) than during time control (-412 ± 789·s^-1). %FMD was reduced following time control (7.4 ± 1.2 to 5.9 ± 1.1%, P < 0.05) but was maintained following both hypoxia trials (IH: 7.2 ± 1.5 to 7.5 ± 1.5%, P = 0.52; CH: 6.9 ± 1.6 to 6.8 ± 1.4%, P = 0.73). Normalized %FMD for shear rate area under the curve (%FMDSR_AUC) was reduced following the time control trial (4.2 ± 1.4 to 3.7 ± 0.9%, P < 0.05) with no change observed with CH (4.0 ± 1.5 to 3.9 ± 1.4%, P = 0.71). However, %FMDSR_AUC increased with IH (3.8 ± 1.1 to 4.5 ± 1.5%, P < 0.05). Our data suggest that acute exposure to hypoxia (both intermittently and continuously) offsets the decline in vascular function after brief inactivity. The potential beneficial effect of hypoxia on peripheral vascular function observed in the current study may be associated with enhanced brachial artery shear in response to the hypoxic challenge.