Comparison of diagnostic accuracy of sublingual nitroglycerin test and low-dose isoproterenol test in patients with unexplained syncope

Despite the widespread use of head-up tilt testing as a means for diagnosin g vasovagal syncope, standardization of test methodology remains a controve rsial issue. The aim of this study was to compare the diagnostic value of h ead-up tilt testing potentiated with sublingual nitroglycerin with that of head-vp tilt testing potentiated with law-dose isoproterenol. For this purp ose, 71 patients with unexplained syncope (mean age 43 years) and 30 asympt omatic controls were studied. All underwent the sublingual nitroglycerin an d low-dose isoproterenol tests on separate days in a randomized fashion. Th e protocol of the 2 tests was similar and consisted of 2 phases. Initially, subjects were tilted at 60 degrees for 20 minutes without medication; then , if syncope did not occur, patients and controls received sublingual nitro glycerin (300 mu g) or low-dose intravenous isoproterenol (mean infusion ra te 1.3 +/- 0.5 mu g/min) and continued to be tilted at 60 degrees for a fur ther 20 minutes. During the sublingual nitroglycerin test, a positive respo nse (syncope associated with sudden hypotension and bradycardia) occurred i n 35 patients (49%), a negative response in 36 (51%), and drug intolerance in none (0%). During the low-dose isoproterenol test, these percentages wer e 41%, 59%, and 6%, respectively. A concordant response was observed in 53 cases (75%). Among controls, 3 subjects (10%) had a positive response to th e sublingual nitroglycerin test and 4 (13%) to the low-dose isoproterenol t est. It is concluded that sublingual nitroglycerin and low-dose isoproteren ol are equivalent tests for evaluating patients with unexplained syncope. T he sublingual nitroglycerin test, however, is simpler, better tolerated, an d safer than the low-dose isoproterenol test and, thus, more suitable for r outine clinical use. (C) 2000 by Excerpta Medica, Inc.