Microbubbles during radiofrequency catheter ablation: Composition and formation

Abstract

The purpose of this study was to measure tissue temperatures associated with microbubble formation during radiofrequency (RF) ablation. Microbubble formation visualized by echocardiography has been used to indicate excessive tissue heating during RF pulmonary vein isolation. However, little is known about the tissue temperatures associated with microbubble formation. Optical fluorometric thermometry probes were used to record tissue temperatures in isolated porcine atrium overlying either lung or esophageal tissue in a saline bath. RF energy was delivered through an irrigated ablation electrode during echocardiographic monitoring for microbubble formation. The maximal recorded tissue temperatures were 81.0 +/- 5.0 degrees C and 88.3 +/- 8.1 degrees C at the time of intermittent (type 1) microbubble formation for lung and esophageal preparations, respectively. During continuous (type 2) microbubble formation, the temperatures were 91.4 +/- 8.2 degrees C and 99.2 +/- 7.8 degrees C, respectively (both P < .001 vs type 1). Tissue temperatures averaged >100 degrees C at the time of "pops." The maximal recorded temperature occurred up to 4 mm deep in the tissues and frequently occurred external to the atrial tissue. The total RF lesion volumes for lung and esophageal preparations were related to the pattern of microbubble formation but not to total power delivered. After generation of type 1 bubbles, up to 60% reductions in RF energy were needed to restore target tissue temperatures of 65 degrees C. Gas chromatographic analysis of the microbubbles was consistent with steam formation. Microbubble formation during RF ablation represents excessive tissue heating to the point of steam formation. Maximal tissue heating may occur in the adjacent lung and esophagus during cooled ablation.