Comparing a cardiac sympathetic activity index with pre-ejection period in time series

Over the last decade, cardiology research has yielded a Sympathetic Activity Index (SAI) that captures the non-linear response patterns of the sympathetic nervous system. We investigated this chronotropic index alongside pre-ejection period (PEP), an inotropic index. While SAI has been validated in physiology, cardiology, and biomedical engineering research, this study introduces SAI to biopsychology. SAI is calculated exclusively from ECG, while PEP requires both ECG and impedance cardiography (ICG) as inputs. An average of 1468 time series observations were analysed per participant per sympathetic index (SAI, PEP) across 17 participants (13 female). The mean SAI-PEP correlation increased significantly from baseline to stimulus (r_B->S(16) = .22, p = 0.042), and then dropped from stimulus to recovery, back to near baseline levels (r_S->R(16) = -.21, p = 0.047). Ideographic patterns emerged, although overall average PEP-SAI correlations were lower than expected, as the procedure did not include a physical stressor. Participants with the strongest positive SAI-PEP correlations (mean r(1565) = .579, p < 0.001) had a matching pattern of psychological distress, as measured by Subjective Units of Distress Scale time series. When psychological distress patterns diverged from both SNS indices, SAI and PEP also diverged from each other. Results suggest that cardiac rate (SAI) and contractility (PEP) may reflect similar temporal dynamics when psychological and physiological stress patterns are aligned. PEP's lability in time series was over 10 times higher than that for SAI. While theoretical and methodological advantages are associated with SAI, further research is needed to comprehensively assess it as a cardiac sympathetic index.