PROSPECTIVE EVALUATION OF HIGH-DOSE OR LOW-DOSE ISOPROTERENOL UPRIGHTTILT PROTOCOL FOR UNEXPLAINED SYNCOPE IN YOUNG-ADULTS

The sensitivity of the passive head-up tilt test (HUT) in the evaluati on of unexplained short-lasting syncope in young adults remains insuff icient. The infusion of isoproterenol was proposed to improve the bene fit. To evaluate the sensitivity-specificity relationship during isopr oterenol dosing, we studied 76 young adults (aged 20.9 +/- 1.7 years) (group S) with recurrent (mean 3.8 +/- 1.6) losses of consciousness th at remained unexplained after clinical and noninvasive assessment and 35 young healthy volunteers (aged 22.6 +/- 2.7 years) (group V). Subje cts underwent either passive HUT (45 min, 60 degrees without drug dosi ng for 48 subjects in group S (S1) and 17 in group V (V1), or HUT with isoproterenol infusion at progressive doses (2 then 5 mu g/min) after 30 minutes of passive tilting for 28 patients in group S (S2) and 18 in group V (V2). During passive HUT, the test was positive (asystole, bradycardia, or fall in systolic blood pressure) in 2 of 17 (11.8%) pa tients in group V1 and in 7 of 48 (14.6%) in group S1 before 30 minute s, and in 3 of 17 (17.6%) in group V1 compared with 10 of 48 (20.8%) i n group S1 at the end of the 45-minute infusion, with no difference in delay before the appearance of a positive result. During HUT with iso proterenol dosing, the test was positive in 2 of 18 (11.1%) patients i n group V2 and in 18 of 28 (64.2%) in group S2 before 45 minutes (2 mu g/min; p < 0.01) in 7 of 18 (38.8%) in group V2 compared with 24 of 2 8 (85.7%) in group S2 before 60 min (5 mu g/min; p < 0.01). In both ca ses the mean delay in evoking a positive response was significantly sh orter. No asystolic response was observed in the volunteers regardless of the protocol used. The most characteristic response to isoproteren ol injection was the appearance of a junctional escape rate with a fal l in systolic blood pressure (61.5% of subjects in group S2). The infu sion of isoproterenol considerably improves the sensitivity of the HUT with satisfactory specificity if low doses are used (<3 mu g/min). Th ese results support the use of HUT with isoproterenol in the evaluatio n of unexplained syncope in young adults.