Validation of Sympathetic Activity Index from Heartbeat series: a Preliminary Study on Pre-ejection periods

Reliable estimates of cardiac sympathetic activity may not be retrieved from spectral analysis of heart rate variability (HRV) series due to overlap with vagal activity in the low-frequency (0.04-0.14Hz) band. Recently, we proposed a HRV-based index, namely the Sympathetic Activity Index (SAI), which may provide instantaneous estimates of sympa-thetic activity through an orthonormal Laguerre expansion of heartbeat interval series. In this preliminary study, we further validate SAI by performing a correlation analysis with sympathetic- related pre-ejection period (PEP) measurements. Using data gathered from 17 healthy volunteers during a virtual-reality emotional elicitation, results indicate a group-wise SAI-PEP correlation of 0.216 (p < 0.001), with an intra-subject correlation as high as 0.852 (p < 10
⁻⁶
). These findings further support the use of SAI as a proxy measure for the non-invasive, instantaneous estimation of cardiac sympathetic activity.