Response of neurocardiac syncope to β-blocker therapy: Interaction between age and parasympathetic tone
Parasympathetic Nervous System
Beta-blockers are a first line therapy for neurocardiac syncope, but are not always effective. The purpose of this study was to determine whether differential autonomic responses to orthostasis predict the response of patients with neurocardiac syncope to beta-adrenergic blockade. We computed the RMS successive difference of the RR intervals (RMSSD: a measure of cardiac parasympathetic tone) during supine and upright phases of the initial tilt test in 28 patients with syncope and positive tilt tests who were treated with atenolol. Follow-up tilt testing was performed to assess the efficacy of the drug in preventing tilt induced neurocardiac syncope. RMSSD did not differ at baseline (supine) between those who did (n = 20) and did not (n = 8) respond to beta-blockade. However, withdrawal of parasympathetic tone in response to tilt varied inversely with age (r = -0.69; P < 0.01). Reduced age adjusted parasympathetic withdrawal during orthostasis was associated with a 47% versus 8% risk of beta-blockade failure (odds ratio = 11; P = 0.01). Patients with diminished age adjusted parasympathetic withdrawal during orthostatic stress are less likely to respond to beta-blocker therapy of neurocardiac syncope than their counterparts. This may reflect a correspondingly greater sympathetic response to orthostasis in these patients, but the mechanism for this interaction is undetermined.