Adenosine-Induced Atrioventricular Block in Patients With Unexplained Syncope

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Abstract
Background ATP and its related nucleoside, adenosine, are ubiquitous biological compounds with potent depressant activity on the atrioventricular node. We hypothesized that an increased susceptibility of the atrioventricular node to adenosine may, in some cases, play a role in the genesis of syncope. Methods and Results The study was performed in two parts. In part 1, we evaluated the effects of a bolus injection of 20 mg ATP in a group of 60 patients (57±19 years, 31 men) with syncope of unexplained origin and in 90 control subjects without syncope (55±17 years, 46 men). In control subjects, the upper 95th percentile of the maximum RR interval distribution, during ATP-induced atrioventricular block (AVB), was 6000 ms. In the syncope group, 28% of patients had a maximum RR interval above this limit ( P =.000). The distribution of the maximum RR interval below the 95th percentile was similar in the two groups. In part 2, we validated the ATP test in 24 patients who had the fortuitous ECG recording of a spontaneous syncope caused by a transient asystolic pause (AVB in 15 and sinus arrest in 9). The ATP test caused AVB with an asystolic pause of ≥6000 ms in 53% of the patients with documented AVB but in none (0%) of the patients with documented sinus arrest ( P =.01). Among the patients with spontaneous AVB, the ATP test was abnormal in 6 of the 7 patients (86%) in whom all conventional investigations for syncope had been negative and in 2 of the 8 patients (25%) who had shown positivity ( P =.03). Conclusions An increased susceptibility to ATP testing is present in patients with SUO and patients with syncope due to paroxysmal AVB. Thus, a logical inference is that ATP testing can be used to identify patients with syncope due to paroxysmal AVB. The results of this study form the necessary background for future prospective studies with an aim to validate this assumption.

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