SUMMARY Driving rabbit atrial trabeculae at a rapid rate for 15 minutes resulted in a decrease in the space constant for electrotonic decay from an average of 670 to 400 ;im. Input resistance, Ru,, as measured by use of a double-barrelled microelectrode, increased from a mean value of 380 kOhms to one of 600 kOhms. The time to return to control values after the end of rapid driving was 20-60 minutes. Similar effects of rapid driving were observed in the presence of atropine, propranolol, and atropine plus propranolol and phentolamine. According to the theory of current spread in a three-dimensional syncytium, a rise of input resistance should be interpreted mainly as an increase of cell-to-cell resistance. We advance the hypothesis that, when driven at their maximal possible rate (or when fibrillating), cardiac cells gain Na + and Ca 1 *, and that this results in partial but reversible uncoupling.

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Closed heart surgery without the use of cardiopulmonary bypass (CPB) is one of the trends of surgical treatment of tachyarrhythmias. Having rich experience of epicardial cryoablation, the authors introduced the original technique of intracardiac cryoablation. They have demonstrated the feasibility of creation of complete AV block in patients with supraventricular tachycardias by AV node-His-bundle cryoablation, elimination of AV junctional ("nodal") tachycardia by perinodal cryoablation, cryoablation of septal and paraseptal left posterior AV accessory pathways and ectopic foci in atrial septum, complete or partial cryoisolation or cryofragmentation of the atria in patients with atrial flutter and/or fibrillation, and cryoablation or arrhythmogenic zones in ventricles. Good results (arrhythmia-free patients) were obtained in 82%-100%. Cryoablation on the closed heart without the use of CPB has the following advantages: (a) the possibility of continuous monitoring of cardiac electrical activity during the operation; (b) ablation efficacy control; and (c) diminished trauma and little risk of surgical intervention.

Eighty-two patients with ectopic atrial tachycardia (EAT) were subjected to radical closed heart surgery (without cardiopulmonary bypass). The age of the patients ranged from 1 to 51 years. Permanent EAT was present in 19 patients, incessant EAT in 14, and paroxysmal EAT in 49 patients. Preoperative electrophysiological study included computed analysis of the P wave vector. Ectopic foci were established in the right atrium in 34 patients, in the left atrium in 11, in the interatrial septum in 32, and extracardially in 5 patients. For ablation or isolation of the foci, the cryogenic technique was used in 74 patients, cryo- and laser techniques in 4, and the laser technique alone in 1 patient. In 3 patients resection of the atrial auricles including the ectopic focus was undertaken. In 4 patients complete AV block was induced and a cardiac pacemaker implanted. After primary surgery, favourable results were obtained in 71 patients. In 11 patients recurrences were observed; 8 of these patients underwent successful repeat surgery. In 3 patients medical treatment was effective. Finally, the follow-up results have been promising-79 patients (96.4%) (including 4 patients in whom a complete AV block was created) have become arrhythmia-free. When EAT is resistant to medical treatment, closed heart ablation of the ectopic focus has proved to be safe (no mortality or morbidity) and effective. It can therefore be recommended not only for the termination of EAT but also for the prevention of dilated cardiomyopathy.