ABSTRACT
BACKGROUND: Vasovagal syncope has been believed to account for the majority of syncope of unknown origin (SUO). Head-up tilt test (HUT) has been well recognized as a useful test in the evaluation of SUO. The purpose of this study was to determine the predictors of HUT to develop a less time-consuming test protocol and get an information on the pathophysiology of vasovagal syncope. METHODS: Eighty five patients (mean age, 43+/-18 years: 47 men, 38 women) underwent a 80 degree HUT without or with isoproterenol infusion (2 migcrogram/min and 5 migcrogram/min, each for 6 minutes) for unexplained syncope or pre-syncope. Positive HUT was defined as symptomatic hypotension (systolic blood pressure or = 10 seconds) or asystole > or = 3 seconds. The patients were divided into two groups according to the result of the HUT: Group I included 47 patients (M:F=25:22) with positive result, Group II, 38 patients (M:F=22:16) with negative result. RESULTS: Group I patients had more episodes of syncope than Group II (3.1+/-2.5/year vs. 2.0+/-2.1/year, p<0.05). There were no significant differences between the 2 groups in the heart rate (HR), systolic and diastolic blood pressure (BP) at the baseline supine position. BP after tilt was not significantly different between 2 groups except for systolic BP at 6 minutes after tilt, which was significantly lower in Group I than Group II (109.5+/-17.5 mm Hg vs. 118.1+/-18.2 mm Hg, p<0.05).However, HR after tilt was significantly faster in Group I than Group II (81.1+/-15.1/min vs. 74.2+/-14.9/min, p<0.05 at 2 minutes after tilt; 83.7+/-14.4/min vs. 74.6+/-14.7/min, p<0.01 at 4 minutes after tilt). The increase in HR was greater in Group I than Group II (19.9+/-12.6/min vs. 12.8+/-10.6/min, p<0.001). In the prediction of positive HUT with HR rise above 14/min during the early 6 minute of baseline head-up tilting, the specificity, sensitivity, and positive predictive value were 63.2%, 70.2%, and 70.2%, respectively. CONCLUSION: In patients with SUO, positive HUT can be predicted with the early HR response during head-up tilt. This result shows that vasovagal syncope is triggered by exaggerated HR response to the decreased venous return and allow us to develop a less time-consuming HUT protocol.