Derivation of chronotropic indices of autonomic nervous system activity using impedance cardiography

The impedance cardiograph has been widely used to examine cardiac sympathetic inotropic activity. Recently evidence was presented indicating that left ventricular ejection time (LVET) may be a useful index of sympathetic chronotropic influences derivable from the impedance cardiograph. The goal of the present research was to derive an index of parasympathetic chronotropic influences. In addition, further support for LVET as a sympathetic chronotropic index was sought. Eight healthy college-age subjects participated in a within-subjects design experiment. Tasks were chosen to elicit a wide range of cardiovascular responses resulting from differing combinations of sympathetic and parasympathetic activity. Results indicated that heart rate corrected for LVET (HR-LVET) correlated highly with respiratory sinus arrhythmia (RSA), an index of vagal cardiac chronotropic activity. In addition, principal components analysis indicated that heart rate, RSA, LVET, HR-LVET, and HR corrected for RSA (HR-RSA) all loaded on a single factor distinct from inotropic or vascular indices. These results provide support for the use of impedance cardiography to derive indices of sympathetic (LVET) and parasympathetic (HR-LVET) chronotropic influences.