Abstract
Heartbeat interval in healthy subjects shows beat-to-beat fluctuation called heart rate variability (HRV). HRV is most prominent at rest and decreased with exercise and stress. Reduction of HRV in 24-hr ECG is an increased risk for death after acute myocardial infarction, in hemodialysis patients with end stage renal disease, and in general elderly populations. Respiratory sinus arrhythmia (RSA) is a major component of HRV and is also known as high-frequency component, an index of cardiac vagal function. RSA itself, however, is a phenomenon reflecting cardio-pulmonary interaction and seems to bare own physiological roles. Recent studies have shown that RSA improves pulmonary gas exchange efficiency through matching alveolar ventilation and perfusion within each breathing cycle. In resting animals and humans, RSA seems useful for saving circulatory and respiratory energy by suppressing unnecessary heartbeats during expiration and concentrating heartbeat effectively during inspiration. Indeed, the magnitude of RSA increases at rest, during slow-wave sleep, and with relaxation and it decreases with physical exercise and mental stress. Also, RSA decreases with advancing age and with severity of coronary artery stenosis and disappears in patients with severe heart failure. These suggest that RSA may be a resting function of cardiorespiratory system and its magnitude may reflect functional reserve of the cardiorespiratory system. Additionally, the association between RSA and cardiac vagal activity may be the indirect consequence of the fact that both of them change in the same direction at least against the physiological changes along the axis across resting and strain. RSA may be used as an index of physiological resting of cardiopulmonary system and may provide a useful marker for the effects of lifestyle modification and stress management in patients with cardiovascular diseases.
