Acute intermittent hypoxia augments the cardiovascular response to an orthostatic challenge preceded by locomotor exercise.
McMillan D, Maher J, Nash M
University of Miami, Coral Gables, FL, United states
Objective: Exercise acutely affects cardiovascular function, and acute intermittent hypoxia (AIH) is thought to potentiate autonomic cardiovascular control. A combination of these therapies may benefit the cardiovascular response of individuals with spinal cord injury (SCI) to orthostatic challenge. This study tested the effect of body weight supported treadmill training (BWSTT) with and without AIH on the cardiovascular response to a head up tilt (HUT) in persons with chronic SCI.
Design/Method: Five adult males with chronic spinal cord injury (SCI) completed testing. On two separate days participants conducted HUT testing before (seated baseline; CON) and after either BWSTT or combined BWSTT+AIH. A HUT maneuver was performed, where arterial blood pressure (BP) was assessed via automated brachial cuff during the first 10 and last 30 seconds of each 5 min stage (successive tilt angles 0°, 30°, 60°, and 90°). Heart rate variability (HRV) was assessed via telemetric heart rate monitor (CS600, Polar) for the entire duration of testing. Robotic-driven BWSTT (ReoAmbulator, Motorika) was conducted for 45 min with indirect calorimetry (Oxycon Mobile, CareFusion) and impedance cardiography (PF07 Enduro, PhysioFlow) measurement. On the second trial BWSTT was preceded by AIH (HYP123, Hypoxico) conducted for 15 bouts of 90 sec hypoxic intervals (FIO2 = 0.09) separated by 60 sec normoxic intervals (FIO2 = 0.21).
Results: No participants experienced syncopal or pre-syncopal symptoms. Systolic BP (SBP) at the onset of 60° tilt was higher in BWSTT+AIH (125±32 mmHg) compared to CON and BWSTT (119±31 and 117±27 mmHg, respectively). At 60°, BWSTT+AIH also showed a greater recovery (last 30 sec – first 10 sec) of SBP (7.8±2.7 mmHg) compared to CON and BWSTT (4.6±2.9 and 4.6±1.5 mmHg, respectively). A progressive increase was observed in HRV LF:HF from BWSTT+AIH to BWSTT to CON (2.08±0.57 to 3.48±0.45 to 4.01±0.68 ms2, respectively).
At a fixed exercise intensity (39.9±8.1 %VO2peak), BWSTT+AIH resulted in an increased stroke volume (131.0±3.2 ml/bt) and decreased heart rate (81.6±5.9 bt/min) compared to BWSTT alone (95.9±3.2 ml/bt and 103.5±5.9 bt/min, respectively).
Conclusion: BWSTT+AIH, but not BWSTT alone, stimulated a greater recovery in SBP at 60° HUT. HRV data suggest that differences in the cardiovascular response between conditions could be due to changes in autonomic cardiovascular control. This analysis is supported by increases in cardiac contractility observed during exercise when BWSTT was preceded by AIH. However, the time-course of loss of these acute benefits requires additional investigation.
Support: The Miami Project to Cure Paralysis.
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