Abstract: ISMICS 2006 Annual Meeting Abstracts
Microwave energy is currently being used to create lesions for the surgical treatment of atrial fibrillation. However, lesion transmurality has not been consistently achieved on the beating heart, likely due to the heat-sink effect of circulating blood. This study was designed to determine the relationship between cardiac output(CO) and atrial lesion depth and width with the Flex 10 (Guidant, Inc.) microwave device.
Six pigs underwent median sternotomy and were placed on cardiopulmonary bypass (CPB). On each animal, four microwave lesions on each atrium were performed for 60 seconds at 65 Watts. By controlling CPB flow rates, CO was maintained at either 0.0–0.5 L/min, 0.6–1.9 L/min, 2.0–3.9 L/min or 4.0 L/min for each lesion. CO was measured with a pulmonary artery flow probe. Lesion placement was randomized to control for varying thickness of the atria. In the last two animals, two additional lesions on each atrium were done for 120 seconds at 65 Watts with 0.0–0.5L/min cardiac output. The animals were sacrificed and tissue was stained and sectioned at 5mm intervals. Lesion depth and width were determined from digital photomicrographs.
Lesions created for 60 seconds were transmural 90% of the time when the CO was 0.0–0.5 L/min (52/58 sections). Only 26/40 (65%), 31/57 (54%), and 11/24 (46%) of atrial sections were transmural when CO was 0.6–1.9 L/min, 2.0–3.9 L/min, and 4.0L/min, respectively (p<0.001, Figure). When lesions were done for 120 seconds with a CO of 0.0–0.5 L/min, 27/27 (100%) sections were transmural. Lesion widths were likewise found to be inversely correlated with CO. When ablating for 60 seconds, the widths were 4.4±2.0 mm, 5.0±1.5 mm, and 4.9±1.8 mm for CO of 4.0L/min, 2.0–3.9 L/min, and 0.6–1.9 L/min, respectively. When the CO was maintained at 0.0–0.5 L/min, the 60-second lesion widths were 6.0±2.8 mm (p<0.05). When the ablation times were increased to 120 seconds, the lesion width was 9.7±3.9 mm for CO 0.0–0.5 L/min (p<0.001).
Microwave ablation lesion width and depth are strongly dependent on cardiac output. Transmural lesions can be reliably produced on the porcine heart only while on full cardiopulmonary bypass.