The effect of heart rate variability on blood pressure is augmented in spinal cord injury and is unaltered by exercise training.

Abstract

To define differences in heart rate and blood pressure variability (HRV/BPV) after spinal cord injury (SCI) compared with uninjured controls, and to determine whether variabilities are impacted by whole-body exercise after SCI. Individuals with SCI (n = 40), aged 18-40, and uninjured age/sex-matched controls (n = 22) had HRV and BPV determined during supine paced (0.25Hz) breathing. Spectral and cross-spectral values were derived for fluctuations at low (LF 0.05-0.15Hz) and high (HF 0.20-0.30Hz) frequencies. Thirty-two individuals with SCI further underwent either 6months of whole-body exercise training (n = 17) or a control intervention (n = 15). Individuals with SCI had injuries graded A-C in severity, neurological levels of injury C1-T10. LF and HF HRV and LF BPV were significantly lower in individuals with SCI (p = 0.008-0.002), though HF BPV was similar. The LF cross-spectrum demonstrated similar phase and gain relationships between groups. The HF phase relationship between pressure and heart rate differed markedly: individuals with SCI demonstrated a -11.7 ± 3.4° phase lag (241 ± 70ms feedback mechanism of pressure into heart rate), whereas uninjured controls demonstrated a +21.5 ± 10.8° phase lead (443 ± 224ms feedforward mechanism of heart rate into pressure, p = 0.007). Whole-body exercise increased mean VO2peak by 2.09ml/kg, whereas HRV, BPV, and their cross-spectral relationships were not significantly altered relative to the control intervention after SCI. After SCI, marked frequency-specific differences exist in the relationship between heart rate and blood pressure variabilities. The high-frequency cross-spectral relationship indicates that a feedback mechanism of blood pressure into heart rate may predominate in this range.