Role of sympathetic failure in the development of reflex syncope in children
Objective: to investigate the time course of changes in autonomic cardiac regulation parameters in children with reflex syncope (RS) and to determine the causes of negative tilt test (TT) responses. Subjects. Seventy-nine patients (mean age 14,2±2,5 years) with RS, including 39 children with positive TT responses (a study group) and 40 children with negative TT responses (a comparison group) were examined. TT was performed according to the Westminster protocol, by estimating Irequency domain heart rate variability (HRV) parameters at baseline (point 1), after tilt (point 2), and before tilt back (point 3). Results. At baseline and after tilt, there were no differences in the parameters of HRV, heart rate (HR), and blood pressure (BP) in children of two subgroups. Before fainting, 72% of cases showed a dramatic (63±31%) reduction in the power of high frequencies (HF) and a moderate decrease in that of low and very low frequencies (LF and VLF). Before tilt back, 28% of the study group displayed, on the contrary, a 20±6% increase in LF with higher HR; the comparison group exhibited a fall in LF in 48% of cases, which was followed by an isolated rise in HR by 15±3% with stable BP values. In the subgroup with higher sympathetic influences, there were increases in HR by 11±3% and in diastolic BP (DBP) by 8±3%. Conclusion. In 72% of the cases, the development of RS was preceded by a reduction in sympathetic and parasympathetic tones, which was followed by a decrease in systolic BP. The important mechanism of resistance to the development of syncope during TT in children is a moderate increase in sympathetic influences with a gradual rise in DBP, which can be regarded as a successful adaptive response. Sympathetic failure in patients with RS is associated with the significantly higher sensitivity of TT.