What is the Optimal Increase in Resting Heart Rate with Low Dose Isoproterenol Infusion for Tilt-Induced Vasovagal Response?

Isoproterenol is widely used as a provocative medium for vasovagal responses during tilt testing. Dose of isoproterenol infusion is generally titrated empirically by increase in resting heart rate before tilt up. To determine the optimal increase in resting heart rate with isoproterenol for tilt-induced vasovagal responses, we studied 97 consecutive patients with unexplained syncope. After the end of a negative baseline tilt (80 degrees for 30 min), the isoproterenol tilt was performed using one of two protocols: two-stage isoproterenol-tilt protocol, with doses of 0.01 and 0.02 microg/kg per min for 10 min each, or one-stage isoproterenol-tilt protocol, with a dose of 1 or 2 microg/min for 10 min. The resting heart rate increase was defined as a percentage increase in the resting heart rate after isoproterenol infusion, compared to the baseline heart rate before the tilt test. In 117 tilt procedures, 28 (93%) of the 30 positive responses occurred with a resting heart rate increase of > or = 21%. With the resting heart rate increase of 60 and 100%, 18 (60%) and 27 (90%) positive responses were observed, respectively. In conclusion, the minimum resting heart rate increase of > or = 21% was required to provoke a vasovagal response during subsequent isoproterenol-tilt (80 degrees for 10 min). Preferably, heart rate should be increased to 60-100% by isoproterenol titration before tilting.