Speaking at the World Conference on Interventional Oncology (WCIO; 9–12 June, Boston, USA), George Carberry (University of Wisconsin School of Medicine and Public Health, Madison, WI, USA) reported that positioning an antenna at least 5mm away from the heart when performing microwave ablation of the lung is associated with a significant reduction in arrhythmia and cardiac tissue injury compared with positioning the antenna nearer to the heart.
Carberry noted that “little data” exist for the safety of using any ablation modality—radiofrequency ablation, cryoablation, or irreversible electroporation—near the heart, adding: “however, the use of stereotactic body radiation therapy near the heart has been shown to result in severe toxicities due to collateral tissue injury”. Therefore, he and his colleagues sought “to determine the shortest distance between the antenna and heart at which percutaneous pulmonary microwave ablation can be safely performed”.
In the study, they performed microwave ablation—using fluoroscopic computed tomography (CT) guidance—in 11 anesthetised swine. Ablations were performed with a single 17 gauge, gas-cooled, antenna in, following randomisation, either a perpendicular (90±20 degrees) or parallel (180±20 degrees) orientation with respect to the heart. “Distance between the antenna and the heart was randomised to 0.75mm increments between 0mm and 10mm, with the final antenna distance measured during cardiac diastole with the use of continuous CT fluoroscopy,” Carberry commented. He added that the ablations were performed for five minutes at 65W unless a significant arrhythmia—asystole, heart block, bradycardia, supraventricular or ventricular tachycardia—developed. Continuous ECG was monitored and recorded during the ablations.
After the ablations, the swine were euthanised and tissue changes assessed with vital stain and histology. ECG rhythms were retrospectively interpreted by a cardiologist. Mixed effects logistic regression was used to assess the risk of cardiac tissue injury and significant arrhythmia that was related to the orientation and distance from the heart to the antenna.
Thirty-four ablations were performed overall and in all of these, the antenna was located <10mm from the heart (range 0—9.9mm). Fifty per cent (17) were performed in the perpendicular orientation and 50% were performed in the parallel orientation. According to Carberry, significant arrhythmias developed during 18% of ablations (at an average of 69±22 seconds into the procedure) and cardiac tissue injury occurred in 50% of ablations. Furthermore, ablations performed with the antenna in a parallel orientation were associated with a higher incidence of cardiac tissue injury for a given antenna-heart distance when compared to ablations performed with a perpendicular orientation.
Carberry commented: “As you might expect, the risk of arrhythmia and cardiac tissue injury decreased with increasing antenna distance from heart in both antenna orientations. However, no significant arrhythmia or tissue injury occurred with an antenna distance ≥4.4mm from the heart when either the parallel and perpendicular antenna orientation was used.”
“The risk of cardiac tissue injury or significant arrhythmia during percutaneous microwave ablation in the lung can be minimised by positioning the antenna at least 5mm from the heart,” he concluded.