916-89 Isoproterenol-induced Vasovagal Syncope: A Detailed Hemodynamic Analysis

Abstract

Usefulness of head-up tilt (HUTI ± Isoproterenol (Iso) testing in the provocation of a vasovagal response has been well recognized. Due to the invasive and intermittent nature of cardiovascular monitoring, understanding of the mechanisms underlying the HUT ± Iso-mediated reactions has been limited. By using impedance cardiography, volume clamp photoplethysmography, and internally developed computer technology, we have been able to examine noninvasively, on-line, beat-to-beat, hemodynamic parameters including: heart rate (HR). blood pressure, cardiac output, stroke volume, end diastolic volume (EDV), peak flow (PF), and peripheral resistance (PR) during HUT ± Iso testing. In this study, hemodynamic changes were examined in 10 consecutive patients (mean age 53 ± 21, range 22–84 yrs; male/female = 6/41 during an induction of a vasovagal response by HUT ± Iso testing. These changes are shown below. Intervention HR EDV PF PR Tilt (10 min) 8.9 ± 10.3 -14.7 ± 4.3 * -9.3 ± 79 53.8 ± 12.1 * Iso/Supine ** 407 ± 15.3 * 7.9 ± 6.2 * 23.8 ± 8.6 * -272 ± 8.5 * Iso/Tilt ** 67.8 ± 10.9 * -4.8 ± 5.2 23.3 ± 142 * -22.6 ± 8.2 * Syncope 4.6 ± 22.8 71.6 ± 20.4 * 63.3 ± 22.4 * -54.7 ± 11.5 * * p < 0.05 ** Iso = 3 mcg/min We interpret these findings as HUT induces a decrease in preload (EDV), an increase in sympathetic-mediated vascular tone (PR), and an insignificant change in cardiomotor tone as indirectly reflected by HR and PF. Iso blunts HUT-induced changes in EDV & PR but significantly increases PF. During HUT + Iso-provoked vasovagal response, there is a further decrease in PR but a significant increase in EDV & PF. These observations suggest: (1) Iso-mediated increase in cardiomotor tone and decrease in vasomotor tone contribute to the induction of vasovagal syncope; (2) Contrary to the conventional belief, a significant decrease in preload may not occur during HUT + Iso-induced vasovagal response.