Validation of Sympathetic Activity Index from Heart Rate Variability series: A Preliminary Muscle Sympathetic Nerve Activity Study

We recently proposed a Sympathetic Activity Index (SAI) through an orthonormal Laguerre expansion of linear RR-interval autoregressive kernels. The resulting heart rate variability (HRV) instantaneous indices may be used for the effective estimation of cardiac sympathetic outflow because the model is independent from the overlapping dynamics of the sympathetic and vagus nerves in the low frequency (LF) band (0.04-0.15Hz). In this study, we perform a preliminary validation of the SAI performance through concurrent estimates from efferent muscle sympathetic nerve activity (MSNA) recordings. ECG and MSNA were simultaneously recorded in 12 hypertensive patients during a 10min resting state in the supine position and up to 30min sodium nitroprusside (SNP, 0.4 µg/kg per minute) administration. Results show a characteristic increase of the MSNA during SNP intake with respect to the resting state (p< 0.001). While SAI was associated with a significant increase during SNP (p<0.001), LF power did not show significant changes between sessions (p>0.05). Spearman analysis highlighted a significant correlation between SAI and MSNA (r=0.47; p<0.02) and a nonsignificant correlation between LF power and MSNA (r=-0.09; p>0.05). This study provides a further validation step of the SAI and supports the use of HRV parameters as a reliable proxy of cardiac sympathetic outflow dynamics.