Cowl, Andrielle

Activation of group III/IV skeletal muscle afferents (ergoreflex) and hypoxia (chemoreflex) are two potent stimuli of the sympathetic nervous system during exercise. Venous distention by graded sub‐systolic occlusion contributes to the ergoreflex during exercise. Although the ergoreflex causes greater increases in blood pressure (BP) compared with the chemoreflex, the interaction of the ergoreflex and hypoxia during lower extremity dynamic exercise is unknown. Objective To determine the effect of hypoxia and graded cuff occlusion on hemodynamic responses during low‐intensity cycling exercise. Methods 19 healthy adults (9 men, 25±5 yr) completed two sessions. Session 1 included a maximal cycle ergometry exercise test. Session 2 included two randomized bouts (hypoxia: 12.5% FiO 2 and normoxia: 21% FiO 2 ) of constant‐load cycling at 30% peak workload for 3 min (control, CTL) followed by randomized inflation of bilateral thigh pressure tourniquets to 20, 40, 60, 80, 100 mmHg for 2 min with 2 min of deflation between occlusions. The bouts were separated by 30 minutes of rest. Blood pressure, heart rate and cardiac output (CO) were measured continuously. Results During hypoxia, the average SPO 2 was 80%. Venous occlusion and hypoxia had differential effects on mean arterial pressure (MAP, occlusion × hypoxia, p=0.02). Significant increases of MAP from CTL occurred at 60 (Δ9 ± 6mmHg), 80 (Δ15 ± 8mmHg), and 100mmHg (Δ17 ± 9mmHg) under normoxia (p<0.05). MAP only increased at 100mmHg (Δ14 ± 9mmHg) under hypoxia (p<0.05). Under hypoxia, MAP was much greater at CTL and 20mmHg as compared to normoxia (p<0.05). SBP exhibited responses similar to MAP under hypoxia (100mmHg increase, p<0.001, occlusion × hypoxia interaction, p=0.02). CO was greater with venous occlusion as mediated by increases in HR (p<0.001). In addition, CO increased under hypoxia (p=0.007) but had no interaction with venous occlusion. Conclusion During low pressure venous occlusion, hypoxia had a greater effect on BP, but as pressure increased (60–100mmHg), there were no significant differences in BP between normoxia and hypoxia. Therefore, sympathetically mediated increases of BP during exercise attributed to hypoxia may negate any responses mediated by the ergoreflex when elicited by venous occlusion at low pressures.

Background Locomotor muscle group III/IV afferent feedback contributes to sympathoexcitation. Sub‐systolic lower limb occlusion may stimulate this reflex. Purpose To examine the cardiovascular response to lower limb venous occlusion in healthy humans during rest and exercise. Methods Nineteen adults (9 men, 25±5 yr; mean±SD) participated in two visits. V1: 3 min seated rest followed by randomized bilateral thigh tourniquet inflation to 20, 40, 60, 80, 100 mmHg for 2 min with 2 min deflation between occlusions. V2: Recumbent cycle ergometer exercise at 30% peak work with occlusion described in V1. We measured mean, systolic and diastolic pressure, cardiac output, heart rate, and stroke volume (MAP, SBP, DBP, CO, HR, and SV, respectively). Results At rest, MAP (85±18 mmHg) increased at 100 mmHg (Δ4±5, p<0.05) mediated by increased DBP (71±15 mmHg) at 100 mmHg (Δ5±4, p<0.05) with no change in SBP. During exercise, MAP (89±11 mmHg) increased at 60 (Δ7±7), 80 (Δ14±8), and 100 (Δ16±8) mmHg occlusions via increased SBP (133±22 mmHg) at 60 (Δ12±14), 80 (Δ20±13), and 100 (Δ26±13) mmHg and DBP (68±9 mmHg) at 60 (Δ5±5), 80 (Δ10±7), and 100 (Δ12±7) (p<0.05, all). Although, CO and SV did not change, HR (108±11 bpm) increased at 40 (Δ5±4), 60 (Δ8±5), 80 (Δ13±7), and 100 mmHg (Δ15±7) (p<0.05, all). Conclusion Stimulation of locomotor muscle group III/IV afferents via sub‐systolic occlusion contributes to cardiovascular control.