Baroreflex regulation of sympathetic nerve activity in patients with vasovagal syncope.

Activity in Patients With Vasovagal Syncope To the Editor: In their study, “Dysfunctional Baroreflex Regulation of Sympathetic Nerve Activity in Patients With Vasovagal Syncope,” Béchir et al1 implied a unilateral causal relationship between reduction of arterial baroreflex control of heart rate and increase of muscle sympathetic nerve activity (MSNA) in patients with syncope. The authors concluded that dysfunctional baroreflex regulation of sympathetic activity provides “new insights into the mechanisms of vasovagal syncope...” We have two comments. Baroreflex function can be depressed by suprabulbar central influences and also by vagal, somatic, or sympathetic2 afferents. This experimental evidence has provided strong support for the concept that baroreceptive input should be considered as one of the neural mechanisms underlying cardiovascular regulation3 and not the only mechanism, as suggested by the Béchir et al1 study. The observation of enhanced MSNA values in patients with syncope both at rest and during lower body negative pressure1 is in contrast to the reduced MSNA already reported4 and raises the possibility of an alternative interpretation of the results. Indeed, the depressed baroreflex function observed in patients with syncope1 may be the consequence of an increase in central sympathetic drive. In the Béchir et al1 study, baroreflex inhibitory modulation of MSNA was inferred by using the alpha index that only assesses arterial baroreflex control of heart rate. A comprehensive approach to arterial baroreflex modulation of heart rate and MSNA in patients with syncope has been previously published in a study4 misquoted in the Béchir et al1 paper. An exhaustive assessment of baroreflex function should consider several aspects besides alpha index, including the definition of the entire sigmoidal baroreflex curve during drug infusion and the evaluation of spontaneous reciprocal changes of heart rate, arterial pressure, central venous pressure, and MSNA during progressive tilt (or lower body negative pressure).5