[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.

[Age-related characteristics of the electrical activity of the cardiac fibers in the human embryo]. / Vozrastnye osobennosti élektricheskoi aktivnosti serdechnykh volokon émbriona cheloveka.

Transmembrane action potential and intercellular electric coupling have been studied in isolated heart during various stages of intrauterine growth of human embryos as well as in heart preparations of human adults. Formation of electrical properties of ventricular myocardium was slower than that of atrial fibres and was found to be fully completed in the first 14 weeks of intrauterine growth. Intercellular electrical coupling enhanced in the course of the embryonic development as indicated by a drop in the input resistance and an increase in electronic length.

[Automatism of the sino-atrial node and sinoatrial conduction of excitation in hypoxia]. / Avtomatiia sinusovogo uzla i sinoatrial'noe provedenie vozbuzhdeniia v usloviiakh gipoksii.

The effect of experimental hypoxia on the automatic activity of the sinus node and sinoauricular transmission of excitation was studied in 17 isolated preparations of rabbit hearts. A greater frequency of the shifts of sinoauricular delay was noted under hypoxia. Local electrophoretic applications of adrenalin demonstrated that the shifts of the delay occur because of the shifts of pacemakers. The mean frequency of the automatic activity recorded in the atria falls to zero between 6 and 20 min of hypoxia. Hypoxia causes the development of sinoauricular blocks and atrial arrhythmias in the form of individual extrasystoles, high-frequency discharges, or alternations of series of impulses with "mute" periods. The increased tendency towards arrhythmias in the sinoauricular zone is, according to the authors, due to the growing heterogeneity of the duration of action potentials that increases more than three-fold by the 10th minute of hypoxia. The possible reflection of the excitation wave ("echo") from the sinus node zone with an inhibited excitability was demonstrated.

[Intercellular coupling in the sinus node of the rabbit heart]. / Mezhkletochnaia sviaz' v sinusnom uzle serdtsa krolika

Passive electrical properties of SA node region of rabbit's heart were studied. Electrotonic anisotropism was characteristic of the entire SA node; anisotropism decreased moving from crista terminalis to interatrial septum. The input resistance, RBX, was the largest in the region of true pacemakers and decreased moving to the perinodal region. Analysis of the results yielded specific resistance values for contacts and electrogenic membrane of 3 Ohms-cm3 and 10(4) Ohms-cm2 respectively.