Individual Variability in the Leg Blood Flow Response to Expiratory and Inspiratory Resistive Loading

Abstract

BACKGROUND: Fatiguing inspiratory muscle work initiates the inspiratory muscle metaboreflex causing a time-dependent increase in muscle sympathetic nerve activity (MSNA) and a decrease in leg blood flow (Q[Combining Dot Above]L). Fatiguing contractions of the expiratory muscles also facilitates an increase in MSNA. However, the effect of fatiguing expiratory muscle work on Q[Combining Dot Above]L is somewhat unknown. PURPOSE: To determine the effect of expiratory resistive loading (ERL) compared to inspiratory resistive loading (IRL) on Q[Combining Dot Above]L in healthy humans. METHODS: Five healthy men (n = 2, 30 ± 6 y) and women (n = 3, 29 ± 5 y) performed ERL and IRL at 65% of maximal expiratory and maximal inspiratory mouth pressure (MEP; MIP) to task failure. Respiratory frequency was maintained at 15 breaths/min with an inspiratory duty cycle of 0.5. Q[Combining Dot Above]L (via Doppler ultrasound) and mean arterial pressure (MAP) (via finger photoplethysmography) were measured before, during and up to 3 min after ERL and IRL. Expiratory and inspiratory muscle fatigue was assessed by measuring the reduction in MEP and MIP from pre- to post-ERL and IRL. EMG was measured in each leg to ensure no muscular contraction occurred. RESULTS: Task failure occurred at 10.6 ± 2.4 min for ERL and at 20.6 ± 8.8 min for IRL; each subject performed ≥ 7 min of ERL and IRL. There was a 21 ± 4% reduction in MEP and a 7 ± 5% reduction in MIP from before to after ERL and IRL, respectively (P < 0.05). Despite an increase in group mean MAP from rest to during ERL (14 ± 14 mmHg, P = 0.035) and IRL (12 ± 6 mmHg, P = 0.021), there was no change in group mean Q[Combining Dot Above]L across time during either ERL or IRL (P > 0.05). There was, however, substantial individual subject variability in the Q[Combining Dot Above]L response to loaded breathing. During ERL, Q[Combining Dot Above]L decreased relative to baseline values at min 3 (−29 ± 20%) and min 7 (−17 ± 6%) in 2 of the 5 subjects; Q[Combining Dot Above]L increased by 15 ± 7% from rest to min 7 in the remaining 3 subjects. Similarly, there was a reduction in Q[Combining Dot Above]L from rest to min 3 (−23 ± 9%) and min 7 (−16 ± 3%) in 2 of the 5 subjects during IRL; Q[Combining Dot Above]L increased by 60 ± 26% from baseline to min 7 in the remaining 3 subjects. There was no significant change in MAP or Q[Combining Dot Above]L during ERL and IRL trials at 2% of MEP and MIP. CONCLUSION: Leg blood flow appears to decrease in some but not all healthy humans in response to ERL. Indeed, we report substantial individual variability in the leg blood flow response to both ERL and IRL.