An Accelerometer Based Heart Monitor to Measure Changes of the Autonomic Nervous System

The electrocardiogram (ECG) and impedance cardiography (ICG) are typically combined to estimate electromechanical features such as the pre-ejection period (PEP) and left ventricular ejection time (LVET); indicators of changes in the cardiac specific drive of the autonomic nervous system (ANS). Current methods of ICG are time intensive in subject preparation and the measurements are vulnerable to non-reproducible subject-specific electrode configuration. Furthermore, analysis of impedance waveforms can be time consuming and labeling of key time points can suffer from experimenter bias. Here we present a wearable heart monitor that includes ECG, but replaces the commonly used 8 ICG electrodes with a single accelerometer (ACC) placed at the suprasternal notch. The ACC indirectly measures movement of the arterial pulse wave as blood is ejected into the aorta and great vessels. The resulting ACC waveform is processed into two smooth and readily identified waves, corresponding to the timing of the opening and closing of the aortic valve. We tested the ACC’s utility and reliability for tracking cardiac ANS tone by comparing PEP and LVET measurements obtained simultaneously with conventional ICG and the ACC. Participants were recorded in the sitting and supine position with ECG, ICG, and ACC. While seated, they engaged in a classic physical stress task known to modulate ANS activity. There were obvious and significant associations between ICG and ACC estimates of PEP and LVET derivatives with respect to time. These findings support ACC as a complementary method for tracking ANS that is robust, time efficient, and readily accessible to researchers.