Background-Right ventricular pacing (RVP) during supraventricular tachycardia produces progressive QRS fusion before the QRS morphology becomes stable. This transition zone (TZ) may provide useful information for differentiating orthodromic reciprocating tachycardia (ORT) from atrioventricular nodal reentrant tachycardia and atrial tachycardia independent of entrainment success. Methods and Results-We studied the effect of properly timed RVP on atrial timing during the TZ in 92 patients with supraventricular tachycardia who had RVP within 40 ms of the tachycardia cycle length. The TZ during RVP includes progressively fused QRS complexes and the first paced complex with a stable QRS morphology based on analysis of the 12-lead ECG. We also measured the stimulus-atrial interval from the end of the TZ and with each QRS complex thereafter until pacing was terminated or ventriculo-atrial block occurred. A fixed stimulus-atrial interval was defined as variation Ͻ10 ms during RVP. Atrial preexcitation, postexcitation, or supraventricular tachycardia termination with abrupt ventriculo-atrial block was observed within the TZ in 32 of 34 patients with ORT. A fixed stimulus-atrial interval was established within the TZ in 33 of 34 patients with ORT. At least 1 of these 2 responses was observed in all patients with ORT. None of the patients with atrioventricular nodal reentrant tachycardia or atrial tachycardia had atrial timing perturbed or a fixed stimulus-atrial interval established within the TZ. Conclusions-During RVP within 40 ms of the tachycardia cycle length, ORT is the likely mechanism when atrial timing is perturbed or a fixed stimulus-atrial interval is established within the TZ. (Circ Arrhythm Electrophysiol. 2010;3:578-584.)

Background-Postoperative (postop) atrial fibrillation (AF) occurs in up to 60% of patients after cardiac surgery, leading to longer hospital stays and increased healthcare costs. Recently, B-type natriuretic peptide (BNP) has been reported to predict occurrence of nonpostoperative AF. This study evaluates whether elevated preoperative (preop) plasma BNP levels predict the occurrence of postop AF. Methods and Results-One hundred eighty-seven patients with no history of atrial arrhythmia who had a preoperative BNP level and had undergone cardiac surgery were identified. Their records were reviewed, and postoperative ECG and telemetry strips were analyzed for AF until the time of discharge. Postop AF was documented in 80 patients (42.8%). AF patients were older (68Ϯ11 versus 64Ϯ14 years, Pϭ0.04), but there was no difference in sex distribution, hypertension, left ventricular (LV) function, LV hypertrophy (LVH), left atrial size, history of coronary artery disease (CAD), or ␤-blocker use. Preop plasma BNP levels were higher in the postop AF patients (615 versus 444 pg/mL, Pϭ0.005). After adjustment for age, sex, type of surgery, hypertension, LV function, LVH, left atrial size, CAD, and ␤-blocker use, the odds ratios of postop AF according to increasing quartiles, compared with patients with lowest quartile, were 1.8, 2.5, and 3.7 (P trend ϭ0.03). Conclusions-An elevated preop plasma BNP level is a strong and independent predictor of postop AF. This finding has important implications for identifying patients at higher risk of postop AF who could be considered for prophylactic antiarrhythmic or ␤-blocker therapy.