[A method of determining the location of the pacemaker in the canine right atrium]. / Metod opredeleniia lokalizatsii voditelia ritma v pravom predserdii sobaki.

Theoretical calculations and experiments on hearts of open-chest dogs have demonstrated a possibility of localizing a spontaneous excitation focus by recording 2 intra-atrial delays representing differences in activation moments at 3 sites of the right atrium. Assuming myocardial homogeneity with respect to excitation spread velocity, it has been shown that the pacemaker can be localized by intersection of two hyperbola-like intra-atrial delay isochronisms with recording electrodes at their foci. In natural, i.e. nonhomogeneous myocardium, experiments have been conducted to simulate pacemakers with known co-ordinates by electrical stimulation of various sites within the right atrium. Comparison of experimental and theoretical data showed the present method to be able to localize the pacemaker with an error of +/- 3 mm, provided the pacemaker is between the recording electrodes where the intra-atrial delays do not exceed 60% of their maximum values.

[Transposition of the pacemaker in the right atrium during stimulation of the vagus nerve in the dog]. / Peremeshchenie voditelia ritma v pravom predserdii sobaki pri stimuliatsii bluzhdaiushchego nerva.

The heart rate and intraatrial latencies between epicardial electrograms from three sites of the right atrium have been studied during vagal stimulation in open-chest dogs. It has been shown that alterations of latencies started at a certain cardiac cycle length irrespective of pacing frequency. A transitional process of changes from a steady latency value in the control to another steady value during vagal stimulation has been observed. The transitional process has been simulated in experimental procedure in which two sites of the right atrium were paced at close and constant frequencies. To interpret the results obtained one-dimensional model of the sinus node has been constructed. According to the model, pacemaker shift within the sinus node results from a competition between two foci of automaticity with close intrinsic frequencies.

[Changes in the localization of the cardiac pacemaker in the rabbit sinus node during hyperkalemia]. / Izmeneniia lokalizatsii voditelia ritma serdtsa v sinusovom uzle krolika pri giperkaliemii.

Isolated preparations of the rabbit right atrium were studied by special techniques for changes in the localization of the cardiac pacemaker under conditions of elevated extracellular concentrations of potassium ions [K+]o. When [K+]o was 10.8 mM, the cardiac pacemaker moved from the superior to the inferior part of the sinus node (by 4-6 mm with 2-4 intermediate positions). Shifts in the position of the cardiac pacemaker were accompanied by changes in spontaneous contractions. They were also attended by changes in the pattern of the total electrogram of the preparation which was indicative of an altered electrical activation of the right atrium. It is noteworthy that these changes occurred only when the cardiac pacemaker shifted along the terminal crest. Perfusion by normal [K+]o-containing solution resulted in the migration of the pacemaker in the reversed direction, with the number of the intermediate positions rising to 3-5. The findings obtained point to a marked functional heterogeneity of the sinus node.