Introduction: T he classic ECG criteria for preexcitation syndrome includes: a short PR interval, the presence of a delta wave and an increased QRS duration. When there is minimal preexcitation on a 12 lead ECG, the diagnosis can be challenging. There are 3 criteria to confirm an accessory pathway: absence of a Q wave in V6, presence of an R wave in V1and absence of an R wave in avR.
Case presentation: We present the case of a 14-year-old female patient which was hospitalized for an elec-trophysiological study (EPS), after recurrent episodes of paroxysmal supraventricular tachycardia, despite antiarrhythmic drugs. We studied the 3 ECG criteria which failed to confirm the presence of an accessory pathway, although the EPS found a left lateral accessory pathway with orthodromic reentrant tachycardia. We thus analyzed another criterion: the variation of the PR interval which is the difference of the longest PR interval and the shortest PR interval of the 12 lead ECG. Methods: During EPS, 4 catheters were introduced through the right femoral vein: 2 quadripolar cathe-ters in the high right atrium and at the level of the His bundle, a decapolar catheter at the level of the co-ronary sinus and the ablation catheter at the level of the right ventricle. During ventricular stimulation the retrograde conduction was through a left lateral accessory pathway, which was further confirmed by the activation through the coronary sinus catheter. Thus, a transseptal approach was performed using the Brockenborg needle for the transseptal punctu-re. After the puncture a sheath was introduced inside the left atrium and a catheter ablation at the level of the left lateral ring. Catheter ablation of the accesso-ry pathway was performed using 35 W and 55 C. Results: The 12 lead ECG preablation showed minimal preexcitation, with a small delta wave in V2-V3 but with a normal QRS complex of 100 ms and a normal PR interval in II of 160 ms. The 3 classical criteria for confirmation/exclusion of an accessory pathway were analyzed: there was a Q wave in lead V6, the precordial transition was after lead V1 and a R wave was present in lead avR. Therefore the 3 criteria failed to confirm the presence of an accessory pathway. We measured the longest and the shortest PR interval and found a difference of 40 ms between leads II and V2. After ablation there was no anterograde or retrograde con-duction through the accessory pathway. The 12 lead ECG performed after catheter ablation was compared with the ECG before ablation and the 3 classical cri-teria which were consistent in excluding an accessory pathway. Our criteria of PR variation of >30 ms con-firmed the presence of the accessory pathway befo-re ablation and the absence of the accessory pathway after ablation, with a PR interval variation after abla-tion of 5 ms. Furthermore an adenosine test was performed to exclude the presence of the accessory pathway after ablation which produced a 2:1 AV block. Conclusions: In unclear cases, another tool for ventri-cular preexcitation is the difference between the lon-gest and the shortest PR interval on the same ECG re-ading. A difference of more than 30ms in our case was successful in confirming the presence of the accessory pathway before ablation, and excluding it after success-ful treatment. The use of a selective AV nodal blocking agent (such as adenosine) can non-invasively confirm or exclude an accessory pathway.