METHODS AND RESULTS: The first study population consisted of 90 patients with
syncope of unknown etiology and 12 control subjects. The second study population consisted of 43 patients with unexplained
syncope. In the first study, head-up tilt (80 degree angle) was conducted for 40 minutes, and
norepinephrine and
epinephrine levels were measured. In the second study, all patients were randomly allocated to either
isoproterenol test (20 patients) or
nitroglycerin test (23 patients) for 20-minute head-up tilt.
Isoproterenol infusion was given at a rate of 1 to 3 microg/min.
Intravenous infusion of
nitroglycerin was started at 250 microg/hour with increasing dosages up to 1,500 microg/hour.
Norepinephrine and
epinephrine were measured in peripheral venous blood. Left ventricular volumes were measured by echocardiography with patients in the supine position and during head-up tilt every 1 minute. End-diastolic volume and end-systolic volume were calculated. In the first study, 61 patients demonstrated a positive response and 29 patients demonstrated a negative response. Plasma
norepinephrine changes during head-up tilt were not significantly different, whereas
epinephrine levels were significantly higher in the positive patients than in the negative and control subjects (148 +/- 118 pg/mL vs 66 +/- 31 pg/mL and 55 +/- 27 pg/mL). Thirteen of the 20 patients given
isoproterenol and 15 of the 23 patients given
nitroglycerin showed a positive head-up tilt (65.0% vs 65.2%; P = NS). During
isoproterenol and
nitroglycerin infusion head-up tilt,
epinephrine in the positive group determined by the
nitroglycerin test was significantly higher than that in the other three groups (103 +/- 38 pg/mL vs 60 +/- 33 pg/mL, 31 +/- 21 pg/mL, and 50 +/- 52 pg/mL). In contrast, end-systolic volume was significantly smaller in the positive group than in the other three groups based on findings of the
isoproterenol test.
CONCLUSION: