Noninvasive cardiovascular tests are reliable and reproducible and are widely used to evaluate autonomic function in human subjects. The heart rate response to deep breathing is probably the most reliable test for assessing the integrity of the vagal afferent and efferent pathways to the heart. This is because respiratory sinus arrhythmia is a relatively pure test of cardiovagal function, whereas many other conditions, such as plasma volume, antecedent rest, and cardiac and peripheral sympathetic functions, factor into the Valsalva response. Heart rate variability to deep breathing is usually tested at a breathing frequency of 5 or 6 respirations per minute and decreases linearly with age. The Valsalva maneuver consists of a forced expiratory effort against resistance and produces mechanical (phases I and III) and reflex (phases II and IV) changes in arterial pressure and heart rate. When performed under continuous arterial pressure monitoring with a noninvasive technique, the Valsalva maneuver provides valuable information about the integrity of the cardiac parasympathetic, cardiac sympathetic, and sympathetic vasomotor outputs. The responses to the Valsalva maneuver are affected by the position of the subject and the magnitude and duration of the expiratory effort. In general, it is performed at an expiratory pressure of 40 mm Hg sustained for 15 seconds. The Valsalva ratio, the relationship between the maximal heart rate response during phase II (straining) and phase IV (after release of straining), has been considered a test of cardiac parasympathetic function. However, without simultaneous recording of arterial pressure, this may be misleading. An exaggerated decrease in arterial pressure during phase II suggests sympathetic vasomotor failure, whereas an absence of overshoot during phase IV indicates the inability to increase cardiac output and cardiac adrenergic failure.
