Sensitivity to orthostatic stress and beta-receptor activation in patientswith 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 cas e controlled study, all patients and controls were subjected to a standard protocol: baseline supine (10 min), baseline tilt (70 degrees, 45 min), iso proterenol supine (0.05 mu g/kg per min, 10 min), and isoproterenol tilt (7 0(degrees) 10 min). The participants were 11 consecutive patients referred for syncope evaluation (5 men, 6 women; mean age, 34.1 +/- 10.4 years; rang e, 18-56 years) and 11 age and sex matched controls (5 men, 6 women; mean a ge, 35.5 +/- 12.2 years; range, 19-63 years). On-line, beat-to-beat measure ments of cardiomotor functions (heart rate, stroke volume, and cardiac outp ut) and vasomotor functions (systolic, mean, and diastolic blood pressures and total peripheral resistance [TPR]) were detected noninvasively by volum e clamp photoplethysmography and impedance cardiography. Patients and contr ols had similar cardiomotor and vasomotor responses during passive tilt and during isoproterenol infusion in the supine position. Immediately after ti lt during isoproterenol infusion and before the onset of symptoms, decrease s in vasomotor functions were significant in study patients when compared w ith those in controls; whereas responses in cardiomotor functions were simi lar between the two groups. When compared with baseline supine findings, TP R decreased by 56.5% +/- 10.9% and 29.5% +/- 23.3% in the patient populatio n 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 pl ayed an important role in the initiation of a vasovagal response. These obs ervations hold the key to early detection of hemodynamic changes and potent ial therapeutic interventions before patients become symptomatic.