Table 1.

Cardiovascular features derived from ambulatory physiological data.

Feature	Definition	Interpretation
Interbeat interval (IBI)	Time (in ms) between heartbeats (inverse of heart rate)	IBI describes how fast the heart is beating; greater IBI values denote a slower heart rate
Respiratory sinus arrhythmia (RSA)	High frequency variability in IBI which occurs at the respiratory frequency	RSA is an estimate of parasympathetic (PNS) influence on the heart; greater RSA values typically indicate PNS activation
Pre-ejection period (PEP)	Time (in ms) between the beginning of electrical stimulation of the heart and the opening of the aortic valve	PEP is an inverse estimate of cardiac contractility and sympathetic (SNS) control of the heart; greater PEP values typically indicate reduced contractility and SNS withdrawal
Left ventricular ejection time (LVET)	Time (in ms) between the opening and closing of the aortic valve	LVET describes how long it takes the heart to pump blood out of the heart on a given heartbeat; greater LVET values are associated with greater time to eject blood per heartbeat
Stroke volume (SV)	Volume (in mL) of blood ejected by the heart with each beat	SV describes the volume of blood ejected from the heart during each heartbeat; greater SV values indicate greater blood volume per heartbeat
Cardiac output (CO)	Volume (in L) of blood circulated in the body per unit of time (min)	CO describes blood flow over time; greater CO values indicate greater blood flow rate (in L/min)