Autonomic modulation and cardiac contractility in vasovagal syncope

BackgroundPrevious studies proposed as one of the main mechanisms involved in neurally mediated syncope, the stimulation of ventricular mechanoreceptors as the final trigger for vagal discharge.ObjectivesThis study aimed to verify the presence of a sympathetic driven increase of cardiac contractility before vasovagal syncope.MethodsWe studied 23 patients with recurrent syncope. All underwent a 60° tilt with pharmacologic challenge (sublingual spray nitrate). Two conditions were used to assess autonomic activity by heart rate variability analysis: in a supine position after 5 min of rest and after 15 min of tilt. Simultaneously, cardiac contractility was quantified by tissue-Doppler echocardiography at the base of the free walls of left ventricle. The peak myocardial velocities during systole (Sw) and late diastole (Aw) were considered.ResultsPassive tilt induced a significant increase of the low frequency component (LF) as well as a decrease of the high frequency component (HF) in positive patients (LF: from 49 ± 18 to 65 ± 18 nu, p < 0.05; HF: from 41 ± 21 to 26 ± 16 nu, p < 0.05). Tissue-Doppler showed a similar increase in Sw in both positive and negative patients but showed a significant decrease of Aw in syncopal subjects (p < 0.005).ConclusionsOur results do not show an increase in ventricular contractility before tilt-induced syncope, or in presence of a valuable increase of sympathetic activity. Instead, we observe a reduction of atrial contractility, which may be a contributory component in the pathogenesis of vasovagal syncope.