Do nonlinear measures of heart rate variability track muscle sympathetic nerve activity during central hypovolemia in humans?

Abstract

Objective: Although linear measures of heart rate variability (HRV) (eg, RR interval high frequency, RR interval low frequency) have been associated with autonomic function, it is unclear whether nonlinear measures (ie, entropy and fractality) are similarly associated. We therefore hypothesized that nonlinear metrics of HRV would be correlated with the progressive increase in muscle sympathetic nerve activity (MSNA) induced by central hypovolemia. Methods: Twelve (7 male/5 female) healthy subjects (age, 32 ± 3 years; height, 172 ± 3 cm; weight, 80 ± 5 kg) underwent progressive lower body negative pressure (LBNP) to presyncope. Electrocardiogram and muscle sympathetic nerve activity from the peroneal nerve were measured continuously. Linear (RR interval high frequency, RR interval standard deviation, pNN50) and nonlinear (sample entropy, fractal dimensions by curve length and dispersion analysis, symbol dynamics) indices of HRV were calculated at each level of LBNP and recovery. Muscle sympathetic nerve activity was calculated as bursts per 100 heart beats (b/100 hb). Results: Muscle sympathetic nerve activity increased from 22.8 ± 2.9 b/100 hb at baseline to 46.6 ± 6.2 b/100 hb at –80 mmHg LBNP (P b .001) and returned to 21.6 ± 5.3 b/100 hb during recovery. All HRV indices decreased with LBNP and increased with recovery, resulting in inverse correlations withMSNA. The linear HRV indices were strongly correlated with MSNA (amalgamated R ≥ 0.79), whereas the only nonlinear index with an amalgamated R of 0.79 or higher was fractal dimension by dispersion analysis (R = 0.79). Conclusion: Although nonlinear indices of HRV decrease with reduced central blood volume, they are not as tightly associated with sympathetic responses as linear indices of HRV.