Cardiovascular autonomic regulation in fighter pilots: Lessons from active standing tests

Fighter pilots (FP) are exposed to flight accelerations and stressful situations that defy cardiovascular control during and after flight. FP presents a smaller adjustment in sympatho-vagal balance during tilt test after flight compared to baseline, suggesting a huge impact of flight on autonomic modulation to the heart. We undertake to test the hypothesis that FP will have a smaller vagal reentrance and lower sympathetic withdrawal during the recovery at the supine position after a prolonged active standing test that mimics flight hemodynamic demands. Twenty-one military personnel (20-34 years old), composed of 9 FP and 12 non-pilots (NP) matched by age, V̉O_2max and body mass index were enroled in the experimental protocol. R-R intervals were continuously recorded in the supine position for 15 min (SUP_baseline ), during the prolonged active standing test (45 min) windowed in six 5 min time frames (from ORT1 to ORT6), and a recovery period in the supine position for 15 min (SUP_recovery ). Heart rate variability was performed by spectral analysis to obtain the normalized low (LFn) and high (HFn) frequency components. The variation (Δ) from baseline (Δ = ORT6 - SUP_baseline ) and from recovery (Δ = SUP_recovery -ORT6) periods were calculated. FP had a smaller ΔLFn (sympathetic mediated) and ΔHFn (vagal meditated) during recovery after active standing as compared to NP. Both groups showed similar changes in ΔLFn and ΔHFn during orthostatic stress compared to baseline, with no differences over time. Therefore, FP show a smaller vagal reentrance and a lower sympathetic reduction during recovery at supine following active standing compared to NP.