Provoking vasodepressor syncope with head-up tilt-table testing.

Head-up tilt testing has proven effective in identifying individuals prone to vasodepressor syncope (VDS). VDS refers to the transient loss of consciousness/cerebral anoxia seen with hypotension produced by autonomic imbalance. In this case, the hypotension is the result of parasympathetic domination. Most episodes appear to be triggered by reduced venous return which stimulates the cardiac mechanoreceptors in the inferior-posterior left ventricle. Once activated, these receptors send out afferent signals along the unmyelinated C of the vagus nerve and cause vasodilation. Once venous return is restored, the usual sympathetic compensations (increased heart rate/force of contraction and vasoconstriction) overcome the parasympathetic domination. A tilt-study allows one to passively tilt the patient up to 40-80 degrees and abruptly reduce venous return in a controlled environment. One can then determine which mechanism will dominate--the usual sympathetic vasoconstriction or the parasympathetic reflex (Bezold-Jarisch)--by frequent observations of blood pressure and ECG. Bradycardia/ventricular standstill may also occur during parasympathetic domination. Once susceptibility to vasodepressor syncope is identified by a tilt study, medications to expand the blood volume and/or minimize venous pooling are often needed. Other drugs to block the parasympathetic pathway and/or the effects of excessive catecholamine levels may also be ordered. Dual chamber pacing may be required for malignant episodes of bradycardia or ventricular standstill.