Different electrocardiographic manifestations of the cardioinhibitory vasovagal reflex.

Syncope is a transient symptom. Typically patients affected by vasovagal syncope (VVS) are asymptomatic at the time of evaluation, and the opportunity to capture a spontaneous event is rare. The increasing use, during the last two decades, of tilt testing and prolonged electrocardiographic (ECG) monitoring in the clinical practice, especially implantable loop recorder (ILR), has made easier the ECG documentation of an attack, thus increasing our knowledge on the mechanism of the cardioinhibitory vasovagal reflex. Very little has been reported in the literature on the ECG characteristics of vagally induced atrioventricular (AV) block. Atrioventricular block during tilt testing is a rare finding. The majority of previous reports described only few cases. 4 Zyśko et al. report a systematic evaluation in a consecutive series of 31 patients who had an AV block induced during tilt testing. Since a positive hypotension/bradycardia response during tilt testing is considered highly specific of VVS, the authors correctly argue that a comprehensive description of the different patterns of reflex AV block could help to distinguish reflex from intrinsic cardiac AV block due to a degenerative disease of the conduction system. Nevertheless, arrhythmias induced in laboratory remain somewhat artificial and how these correlate with the spontaneous event remains uncertain. Some recent evidences show a weak correlation with the ECG characteristic of the spontaneous VVS documented by ILR. However, also this latter method has pitfalls. Indeed, while with ILR the true ECG pattern responsible for syncope is obtained, its nature cannot be proven with certainty because the present technology does not allow the simultaneous recording of blood pressure and other signals, and the aetiology of VVS still remains uncertain. The results of the study of Zyśko et al. are helpful for a better understanding of the mechanism of spontaneous syncope. Table 1 shows comparative results of the findings observed during tilt testing in the study of Zyśko et al. and during spontaneous syncope diagnosed as likely neurally mediated and documented by ILR in the ISSUE 2 study. The spontaneous cardioinhibitory reflex was twice as frequent as that of the induced one. Lone sinus arrest was about four times more frequent than AV block þ sinus bradycardia/arrest in both situations. Lone AV block was present only during spontaneous events. How to interpret these findings?