Electrophysiological and histological assessment of transmurality after epicardial ablation using unipolar radiofrequency energy.

BACKGROUND

The transmurality of the ablation lesions created on a beating heart has not been examined histologically or electrophysiologically. This study aimed to assess the feasibility of an atrial epicardial or endocardial ablation in an off-pump setting using unipolar radiofrequency (RF) energy.

METHODS

A linear ablation lesion of 5 cm was made in the lateral left atrium using unipolar RF energy with a temperature-controlled algorithm in 16 canines either epicardially (n=8) or endocardially (n=8) on the beating heart without any cardiopulmonary bypass. The ablation depth and transmurality were examined histologically two hours after the ablation. A conduction block across the linear ablation lesion was tested by epicardial mapping in two animals four weeks after each epicardial and endocardial ablation.

RESULTS

There was no significant difference in the ablation depth between the epicardial and endocardial ablations (2.5+/-0.7 mm vs. 3.0+/-1.4 mm, p=0.055) in the histological examination. However, the ablation lesion was transmural in only 14 of 30 (46.7%) evaluated points after the epicardial ablation, while in 28 of 30 (93.3%) after the endocardial ablation (p<0.0001). The thin atrial tissue adjacent to the endocardium survived after the epicardial ablation. The activation maps demonstrated a complete linear conduction block in all animals after the endocardial ablation, but in none after the epicardial ablation.

CONCLUSIONS

Epicardial unipolar radiofrequency ablation on the beating heart does not necessarily create a complete linear conduction block. An alternative ablation device that creates a transmural lesion is needed, and intraoperative electrophysiologic assessment of the lesion should be crucial in off-pump AF surgery.