Effects of Changes in the L‐Type Calcium Current on Hysteresis in Restitution of Action Potential Duration

Background : Electrical restitution and memory of action potential duration (APD) are considered key determinants of stability of activation. Calcium handling also impacts stability of activation via its modification of the above and independently. We determined the effects of reduction in an important calcium current, I_CaL, on hysteresis in restitution of APD, which provides assessment of restitution and memory. Methods : Transmembrane potentials were recorded from isolated ventricular tissues from pigs. The pacing protocols consisted of sequential and oscillatory changes in diastolic intervals (DI). Two protocols were used, one where DIs ranged between 100 and 700 ms and the other where DIs ranged between 10 and 290 ms. Verapamil (2 μM/L) was used to reduce I_CaL. From the hysteresis observed in restitution, loop area, thickness, overall tilt, and the maximum and minimum delays between DIs and APDs were calculated to obtain measures of memory and restitution. Results : Restitution during sequential and oscillatory changes in DI displayed hysteresis. All measures of hysteresis decreased after reduction in I_CaL. The decreases in area, thickness, and overall tilt were most pronounced (50%, 50%, and 33%). Conclusion : Sequential and oscillatory changes in DIs reveal that while the overall slope of restitution of APD decreases with decrease in I_CaL, so do measures of memory. Because slopes and memory are considered to have contrasting effects on stability, our results suggest that predicting effects of alteration in I_CaL on stability of activation using slope alone may not be adequate. (PACE 2010; 451–459)     

Use of the Saline Infusion Electrode Catheter for Improved Energy Delivery and Increased Lesion Size in Radiofrequency Catheter Ablation

Although radiofrequency catheter ablation has undergone explosive growth as the treatment for a variety of arrhythmias, a limiting factor with the existing catheter delivery system has been the relatively small size of the lesions, which appears to be in part due to coagulum formation around the catheter tip, producing a rise in impedance and limiting energy delivery. In order to test the hypothesis that infusion of saline during radiofrequency current application can increase the lesion size and decrease the incidence of impedance rise, ten dogs were each given two radiofrequency ablation lesions to the left ventricular endocardium. One of these lesions was delivered with a standard 7 French quadripolar catheter with a 2-mm tip, and the second was done with a 7 French Iuminal electrode catheter (also with a 2-mm tip) for the infusion of normal saline during the delivery of radiofrequency energy. Energy was delivered for 60 seconds at either 10 or 20 watts at two distinct sites in the left ventricle for each animal. Four to 7 days following ablation, the animals were sacrificed for pathological examination. The lesions created with the saline infusion catheter were significantly bigger than those produced with a standard catheter (7.3 × 7.0 × 5.1 vs 5.2 × 4.9 × 3.5 mm, respectively, P < 0.001). At the lower energy level (10 W), none of the animals with the saline infusion catheter experienced an impedance rise versus 3 of 5 of the animals in whom the standard catheter was used. At the higher level (20 W), only 1 of 5 dogs had an impedance rise with the saline infusion catheter versus 5 of 5 with the standard catheter. We conclude that the use of a saline infusion catheter for radiofrequency energy delivery during catheter ablation produces a significantly larger lesion than that produced with a standard catheter and is effective in preventing impedance rise.