Terminating atrial fibrillation by cooling the heart.

Disappointing results of pharmacological therapy in many patients with atrial fibrillation (AF) have motivated efforts to develop device therapy for this complex electrical disorder. Such devices have focused on mechanisms including ablation of tissue harboring triggers or substrates, mechanical devitalization of the left atrial appendage to provide antiarrhythmic as well as antithrombotic effects, and the application of low energy electrical fields to defibrillate AF. However, there are other mechanisms to modulate cardiac fibrillation that have rarely been translated into therapy, including hyperpolarizing myocytes, or stretching, or cooling the heart 6,7 to cause defibrillation. In this issue of HeartRhythm, Naksuk et al describe an innovative and creative prototype device to defibrillate AF by direct cooling. The authors developed a device that exploits the Peltier effect by using electrical energy (8.6– 14 V, 3.6 A) to create a cold surface placed on the epicardium and a hot surface that is cooled by a copper mesh and saline irrigation. The authors calibrated 4 × 4 or 2 × 2 cm devices in 2 ex vivo swine hearts to identify settings to achieve a o5°C target temperature. They then measured the physiological impact of this calibrated device in a proofof-concept in vivo canine study, applying the device repeatedly to distinct atrial regions through a sternotomy. From a technical perspective, target temperature was achieved in two-thirds of the trials in 3–4 minutes. Physiologically, the authors studied the impact of cooling on atrial pacing capture, conduction time, and AF. During atrial pacing, cooling produced a progressive increase in capture threshold cooling, ultimately leading to loss of capture. During atrial capture, conduction time prolonged substantially in two-thirds of the trials when lower epicardial and endocardial temperatures were achieved. AF was then induced and studied during rapid pacing. Epicardial cooling to −0.5°C caused AF to terminate into sinus rhythm, and AF