Comparison of Heart Rate Variability Parameters to the Autonomic Reflex Screen in Postural Orthostatic Tachycardia Syndrome and Neurogenic Orthostatic Hypotension

Purpose: The clinical significance of heart rate variability (HRV) in the context of autonomic dysfunction continues to be a matter of debate. Therefore, the purpose of the current study was to investigate the clinical relevance of HRV in the context of autonomic dysfunction. Methods: Heart rate variability data from 225 volunteers consisting of controls (n = 166) and patients with mild (n = 25) and severe (n = 34) autonomic dysfunction were retrospectively analyzed. Time and frequency parameters were correlated against baseline and standardized tests of autonomic function. Results: During baseline, resting HR was negatively correlated with time (SD of all normal RR interval, r = -0.511; RMSSD, r = -0.585; pNN50, r = -0.545) and frequency (low-frequency, r = -0.362; high-frequency, r = -0.421) parameters (P < 0.01). Resting systolic blood pressure demonstrated similar significant correlations (P, 0.01). During head-up tilt, SD of all normal RR intervals was positively correlated with Delta HR and change in systolic blood pressure (r = 0.340; r = 0.538, respectively; P < 0.01). Similarly, low-frequency, high-frequency, and low-frequency/high-frequency ratios were correlated with Delta HR (r = 0.422, r = 0.176, r = 0.470) and change in systolic blood pressure (r = 0.451, r = 0.407, and r = 0.185) (P < 0.01). Time parameters (SD of all normal RR intervals, RMSSD, and pNN50) were all significantly correlated with deep breathing (r = 0.600; r = 0.556; r = 0.516; P < 0.01, respectively). Low-frequency and high-frequency power were also correlated (r = 0.596; r = 0.580, respectively) (P < 0.01). Conclusions: Time and frequency parameters showed significant negative correlations with baseline hemodynamics. During a test of sympathetic activation and parasympathetic withdrawal, this relationship shifted to reveal significant positive correlations between HRV parameters and hemodynamics. Last, during a test of parasympathetic activation, there were significant positive correlations with cardiovagally mediated HRV parameters. Overall, these results suggest broader clinical relevance for HRV parameters within the spectrum of autonomic functioning.