The available epidemiological and clinical data implicate increased sympathetic nervous system activity in increased cardiovascular morbidity and mortality and show that it has strong predictive power for mortality and cardiovascular events. Analysis of heart rate variability (HRV) has been widely used as a noninvasive assessment tool for autonomic nervous system function, and results show that reduced and/or abnormal HRV is associated with an increased risk of mortality in cardiac patients such as patients after acute myocardial infarction and patients with congestive heart failure. However, most indices derived from HRV primarily reflect vagal function. In contrast, few indices have been suggested as markers of sympathetic nervous system activity. This chapter reviews characteristics of HRV that have been proposed as potential markers of cardiac sympathetic activity, such as (in the frequency domain) low-frequency (LF) power, short-term scaling exponent, and non-Gaussianity index. While there is no widely accepted and well-tested HRV-based index of cardiac sympathetic activity, we discuss the key issues for the assessment of cardiac sympathetic activity based on HRV analysis.
All Science Journal Classification (ASJC) codes