METHODOLOGY OF ISOPROTERENOL-TILT TABLE TESTING IN PATIENTS WITH SYNCOPE

To assess the impact of isoproterenol, duration of tilt, symptom development and hemodynamic changes on the outcome of tilt table tests, 100 patients with syncope underwent successive 80-degrees head-up tilt for 10 min during infusions of 0, 2 and 5-mu-g/min of isoproterenol. All 15 patients with another cause of syncope had a normal test result and 66 (78%) of the 85 patients with syncope of unknown origin had a test that resulted in syncope or presyncope. Isoproterenol was required to produce syncope or presyncope in > 90% of positive tests and 66% to 80% of positive tests required a dose of 5-mu-g/min of isoproterenol. Without isoproterenol, symptoms did not develop until after greater-than-or-equal-to 4 min of head-up tilt. With either 2 or 5-mu-g/min of isoproterenol, the half-time of symptom onset was 0.7 to 1.9 min and the rate of symptom development did not depend on the dose of isoproterenol. During syncope, the mean heart rate, systolic blood pressure and rate-pressure product each decreased significantly from 132 +/- 21 to 67 +/- 25 beats/min, 117 +/- 19 to 60 +/- 16 mm Hg and 15.3 +/- 2.9 to 4.2 +/- 2.2 x 10(3) mm Hg/min, respectively. During presyncope, mean trough rate-pressure product (5.5 +/- 2 x 10(3) mm Hg/min) was significantly higher (p = 0.027) than during syncope. Receiver-operating characteristic analysis showed that a trough rate-pressure product less-than-or-equal-to 9,000 mm Hg/min best separated tests ending in syncope or presyncope from asymptomatic tests (positive predictive value 0.96 to 1.0, negative predictive value 0.91 to 0.94) regardless of isoproterenol dose. These results 1) demonstrate the need for isoproterenol to induce syncope or presyncope within 5 min of head-up tilt, 2) indicate that presyncope can be used as an outcome, and 3) define an objective hemodynamic outcome variable.