Comparison of early and late dimensions and arrhythmogenicity of cryolesions in the normothermic canine heart.

Little is known about myocardial cryoablation at normothermia and the effect of cryoprobe head size and duration of freeze on final lesion volume. In the present study, cryolesions were created with a carbon dioxide cryoprobe with two head sizes (cylindrical head 6 mm diameter and large circular head 18 mm diameter) in the normothermic canine heart during cardiopulmonary bypass. The duration of freeze (exposure time) varied from 2 to 3 or 4 minutes and the effects on immediate and chronic lesion size were evaluated. Lesions produced by epicardial exposures were compared with intramyocardial lesions created by placing the cylindrical head in a 6 mm stab incision. A minimum of four lesions were created in each dog. Lesion size was evaluated at 0 minutes (iceball) and 24 hours (two dogs), 7 days (one dog), or 4 weeks (five dogs). Iceball diameter was approximately 5 mm larger than chronic lesion diameter regardless of head size or exposure time. Prolongation of exposure time from 2 to 3 minutes resulted in significant increases in the volume of epicardial lesions (cylindrical head: 280 +/- 100 mm3 versus 740 +/- 200 mm3, p = 0.001; circular head: 1200 +/- 100 mm3 versus 2300 +/- 500 mm3, p = 0.007) because of increases in diameter and depth. No further increase in lesion size was observed when exposure time was prolonged from 3 to 4 minutes. A 3-minute intramyocardial exposure with the cylindrical head placed in a stab incision enabled production of transmural lesions (16 +/- 2 mm deep). Two and 4 weeks postoperatively, dogs underwent electrophysiologic study from the right and left ventricular apices. No animals had inducible ventricular tachycardia despite the heterogeneous configuration of the multiple cryolesions. In conclusion, it is possible to produce rapid and predictable ablation of clinically useful volumes of myocardium during normothermic bypass with the use of currently available equipment. Under these conditions, an exposure time of 3 minutes is optimal for a liquid carbon dioxide cryoprobe. Cryolesions should be overlapped by at least 2.5 mm to produce continuous areas of ablation. Multiple cryolesions do not form a chronic substrate for ventricular tachycardia.