Sensitivity to orthostatic stress and β-receptor activation in patients with isoproterenol-induced vasovagal syncope: A case controlled study

Cardiomotor and vasomotor responses were assessed during isoproterenol tilt-induced vasovagal reaction in patients with a history of syncope. In a case controlled study, all patients and controls were subjected to a standard protocol: baseline supine (10 min), baseline tilt (70°, 45 min), isoproterenol supine (0.05 μg/kg per min, 10 min), and isoproterenol tilt (70°, 10 min). The participants were 11 consecutive patients referred for syncope evaluation (5 men, 6 women; mean age, 34.1 ± 10.4 years; range, 18- 56 years) and 11 age and sex matched controls (5 men, 6 women; mean age, 35.5 ± 12.2 years; range, 19-63 years). On-line, beat-to-beat measurements of cardiomotor functions (heart rate, stroke volume, and cardiac output) and vasomotor functions (systolic, mean, and diastolic blood pressures and total peripheral resistance [TPR]) were detected noninvasively by volume clamp photoplethysmography and impedance cardiography. Patients and controls had similar cardiomotor and vasomotor responses during passive tilt and during isoproterenol infusion in the supine position. Immediately after tilt during isoproterenol infusion and before the onset of symptoms, decreases in vasomotor functions were significant in study patients when compared with those in controls; whereas responses in cardiomotor functions were similar between the two groups. When compared with baseline supine findings, TPR decreased by 56.5% ± 10.9% and 29.5% ± 23.3% in the patient population and controls, respectively (P = 0.005). When compared with isoproterenol supine findings, TPR decreased by 27.5% ± 22.8% in the study patients and increased by 22.6% ± 48.1% in the controls (P = 0.005). The inability to overcome isoproterenol-induced vasodilatation during orthostatic stress played an important role in the initiation of a vasovagal response. These observations hold the key to early detection of hemodynamic changes and potential therapeutic interventions before patients become symptomatic.