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Dual AV node physiology often persists after successful slow pathway (SP) ablation, and the mechanism of tachycardia elimination is unresolved. Therefore, AV node conduction curves were analyzed following successful ablation (4 ± 1 energy applications) in 85 consecutive patients (58 women, age 50 ± 2 years) with typical AVNRT. Twenty-seven patients (32%) had complete elimination (group 1) whereas 58 (68%) patients had persistence (group 2) of dual AV node physiology. A significant increase in the AV node Wenckebach cycle length (WB-CL) was observed in both groups (310 ± 9 to 351 ±15 ms in group 1, and 325 ± 8 to 369 ± 9 ms in group 2.P < 0.05). A decrease in the fast pathway (FP) ERP (339 ± 15 to 279 ± 12 ms) and an increase in the maximum FP AH interval (141 ± 5 to 171 ±7) were observed only in group 1 (P < 0.05). In group 2, no change in the SP FRP (267 ± 7 to 280 ± 10 ms) was observed, and the change in the maximum SP-AH following ablation showed a significant inverse relation to the maximum SP-AH at baseline in group 2. In conclusion, (1) an increase in the WB-CL is observed independent of the persistence or elimination of dual physiology after successful ablation; (2) when dual physiology is eliminated, significant changes in the FP ERP and the maximum FP-AH occur; (3) when dual physiology persists, FP physiology and the SP FRP remain unchanged, and a significant inverse relation between the change in the maximum SP-AH following ablation and the maximum baseline SP-AH is observed. [ABSTRACT FROM AUTHOR]

An increase in sinus rate has been previously described in patients with AV node reentry (AVNRT) following successful AV node modification. This increase could either be a specific sign of elimination of slow pathway conduction or it could be a consequence of energy application in the posteroseptal area. Thus, we compared the changes in sinus cycle length following successful slow pathway ablation (defined as complete elimination of dual AV node physiology) in patients having AVNRT with those in patients undergoing successful ablation of a posteroseptal atrioventricular accessory connection. Twenty five patients (16 women and 9 men, mean age 41 ± 4 years) with typical AVNRT (cycle length 378 ± 12 ms) and 29 patients (16 women and 13 men, age 34 ± 5 years) with an accessory connection (17 manifest and 12 concealed) were studied. The electrophysiology study was performed during sedation with Fentanyl and Midazolam. The mean number of energy applications was 3 ± 1 for successful slow pathway ablation and 4 ± 1 for successful ablation of the accessory connection (p:NS). Following the successful energy application, the sinus cycle length decreased significantly 776 ms at baseline to 691 ms in patients with AVNRT. Following successful ablation of the posteroseptal AC, sinus cycle length decreased from 755 ms at baseline to 664 ms (p < 0.05 in both groups [difference between groups not significant]). The decrease in sinus cycle length did not correlate with the number of RF energy applications required for successful ablation or the total energy delivered. In conclusion, ablation of the AV node slow pathway and a posteroseptal accessory connection results in similar increases in the sinus rate. Thus, the increase in sinus rate is probably due to energy application in the posteroseptal space, possibly due to concomitant destruction of vagal inputs, and it is not specific for elimination of slow pathway conduction. [ABSTRACT FROM AUTHOR]

The purpose of the current study was to characterize the effects of transvenous ICD shocks on myocardial impedance. Rather than recording impedance during shocks, it was measured during continuous pacing in order to minimize confounding effects such as electrode polarization. Pacing impedance (reflecting the combined impedances of the electrode-tissue interface, myocardium, and blood pool) was measured every 5 seconds before and after 58 single shocks in 22 patients undergoing ICD implantation with a Transvene (n =14) or Endotak (n = 8) lead. There was a progressive and longlasting decrease in impedance after shocks. The magnitude of this change was similar for 0.6-J test shocks and shocks ≥ 5 J (28 ± 32 Ω vs 23 T 16 Ω; P = 0.8). However, the drop in impedance was more abrupt after high energy shocks. Because impedance continued to decline throughout the 5-minute interval between shocks, successive shocks had a cumulative effect, with a decrease of 46 ± 42 Ω after four discharges. In conclusion, a progressive decline in pacing impedance is a characteristic response to transvenous ICD discharges. [ABSTRACT FROM AUTHOR]