Abstract
PURPOSE: We have demonstrated that acetylcholinesterase inhibition (AcHi: pyridostigmine bromide: PB) improves orthostatic blood pressure (BP) responses to head-up tilt (HUT) compared to no drug (ND) in some individuals with tetraplegia and theorize that difference between subjects with an orthostatic BP response to PB (responders [R]) and those without a BP response (non-responders [nR]) may relate to the autonomic completeness of injury. Although we cannot directly asses the degree of autonomic cardiovascular impairment, we determined differences in the autonomic response to HUT using heart rate (HR) variability (HFV) and BP variability (BPV) after administration of PB (60 mg) compared to ND in subjects with tetraplegia. METHODS: Mixed factor ANOVA was used to determine main and interaction effects for high frequency HRV (HF: vagal) and low frequency BPV (LFsbp: sympathetic) responses to HUT (0°, 15°, 25°, 35°) following PB and ND administration in the R and nR. Beat-to-beat HR and BP were assessed using a 3-lead ECG and finger photoplethysmography, respectively; data were collected at 500 Hz for 5-minutes at each angle of HUT. RESULTS: Eleven individuals with SCI participated; however, one subject was unable to tolerate the HUT maneuver during ND therefore, results are presented in 10 subjects (age: 38±10 years; duration of injury: 13±10 years; level: C4-6; AIS: A, B, C). Five individuals were R and 5 were nR (+6.9±5.5 vs. -7.1±8.1 mmHg, respectively; p=0.012); demographics did not differ between the groups. Supine HR was lower in R compared to nR (48±3 vs. 57±8 bpm, respectively; p=0.049); supine BP did not differ. Although supine HF did not differ, supine LFsbp was increased in R compared to the nR (4.11±0.12 vs.3.82±0.24 ln, respectively; p=0.041). The group (R, nR) by condition (PB, ND) by HUT interaction effect was significant for HF (3.639; p=0.044), but not for LFsbp. CONCLUSION: These data suggest that tetraplegic individuals with increased resting sympathetic nervous system activity (LFsbp) respond to AcHi with an increased BP and vagal response to HUT, indicative of improved orthostatic baroreceptor function.
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