Respiratory Sinus Arrhythmia (RSA)
R es_riratory sinus arrhythmia (RSA) is a measure of vagal modulation of the sinus node1-5 Respi-ratory activity is able to modulate both parasympa-thetic and sympathetic cardiac outflow; however, when the breathing rate is more than nine breaths per minute (0.15 Hz), RSA is maximally dominate by the modulation of parasympathetic outflow.6·7 One explanation for respiratory RR interval fluctua-tions is that respiration first affects arterial pressure and then heart rate fluctuations mechanically via the baroreflex control.8 The role of baroreflex contribu-tion in the origin of RSA in healthy humans has been emphasized recently.9,l0
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