Evaluation of autonomic function

Clinical laboratory evaluation of adrenergic function is lacking in sensitivity. Recent studies suggest that components of the Valsalva maneuver can be used to evaluate adrenergic function. Late phase II and phase IV are primary regulated by peripheral and cardiac sympathetic activity, respectively. With progressive adrenergic failure there is a loss of IIl, an increase in IIe and a loss of IV in that order. Baroreflex gain can be described by the heart period/blood pressure (BP) response. An alternative approach of expressing latency instead of gain is of interest but is of uncertain validity. Evaluation of heart rate recordings in the frequency domain using power spectral analysis can be used to study parasympathetic/sympathetic balance in humans. Its value as a clinical test remains to be established. Prolonged tilt is a sensitive and probably specific test of vasodepressor syncope. Isoproterenol infusion during tilt is of doubtful specificity.