Atrial flow dynamics as a determinant of tissue temperature during balloon cryoablation

Balloon cryoablation is an accepted method of achieving pulmonary vein isolation for the treatment of atrial fibrillation. The relationship between blood flow in the atrium and cryo energy delivery to the tissue remains poorly understood. Controlled cryoablations were performed in vitro using a pulmonary vein phantom constructed from bovine muscle, providing a 20 mm vein ostium. A temperature sensor was mounted within the 'vein wall' at a 1 mm tissue depth. Apparatus was constructed to assess the effect of incomplete pulmonary venous occlusion causing a leak, simulated atrial stasis, atrial circulation, and mitral regurgitation. Controlled ablations using the 2nd generation 28 mm cryoballoon catheter were repeated three times and mean values compared. Leak volume significantly affected both balloon temperatures and tissue temperatures. Simulated mitral regurgitation (MR) significantly impaired the effectiveness of cryo energy delivery resulting in significantly warmer balloon and tissue temperatures. With high leak volumes and moderate to severe MR there was a marked disparity between the cryoballoon temperature and the tissue temperature of approximately 60 degrees. Balloon warming times varied inversely with both leak volume and simulated MR flow volume. Incomplete venous occlusion and MR result in warmer balloon and tissue temperatures, and shorter balloon warming times, and are likely to significantly impair the effectiveness of cryoablation. Balloon temperature is poor indicator of tissue temperature under higher flow conditions.