Blood volume measurement as a tool in diagnosing syncope.

BACKGROUND The cause of syncope remains unknown in 24% to 37% of cases even after standard diagnostic tests. Measuring blood volume may elucidate the mechanisms of syncope in the individual patient and prove helpful in determining optimal therapy. This report includes the largest set of blood volume measurements performed in syncope patients to date. METHODS We performed radioisotopic blood volume measurement (Daxor BVA-100) on 539 patients who presented to our center with syncope/presyncope of unclear etiology. There were 202 men and 337 women, ages 16 to 88 years; many were receiving treatment at the time they were referred to our center. We also measured blood pressure, heart rate, and cardiac index before and during tilt, with complete data available for 411 patients. RESULTS Blood volume derangements ranged from -32% to +116% deviation from normal. Hematocrit could not be used to predict volume status. Volume depletion was found in 241 (44.7%) patients and volume expansion was found in 63 (11.7%). Blood pressure, heart rate, and cardiac index before and during tilt did not correlate with any component of blood volume and could not be used to predict volume status. CONCLUSIONS Syncope patients are heterogeneous with respect to blood volume, and blood volume derangements are common and are not identified through tilt table testing. Empirically prescribed pharmacological treatment for syncope is frequently inappropriate. Blood volume measurement should be included in syncope diagnosis.