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

Purpose 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. Methods Individuals with SCI (n = 40), aged 18-40, and uninjured age/sex-matched controls (n = 22) had HRV and BPV determined during supine paced (0.25 Hz) breathing. Spectral and cross-spectral values were derived for fluctuations at low (LF 0.05-0.15 Hz) and high (HF 0.20-0.30 Hz) frequencies. Thirty-two individuals with SCI further underwent either 6 months of whole-body exercise training (n = 17) or a control intervention (n = 15). Results 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 degrees phase lag (241 +/- 70 ms feedback mechanism of pressure into heart rate), whereas uninjured controls demonstrated a +21.5 +/- 10.8 degrees phase lead (443 +/- 224 ms feedforward mechanism of heart rate into pressure,p = 0.007). Whole-body exercise increased mean VO(2peak)by 2.09 ml/kg, whereas HRV, BPV, and their cross-spectral relationships were not significantly altered relative to the control intervention after SCI. Conclusion 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.

Automated summary

Individuals with SCI exhibit significantly lower LF and HF HRV and LF BPV compared to uninjured controls, though HF BPV is similar. The relationship between heart rate and blood pressure variabilities differs in SCI individuals, with a predominance of blood pressure feedback mechanism in the high-frequency range. Whole-body exercise does not significantly alter HRV, BPV, or their cross-spectral relationships after SCI.