An Experimental Study on Ectopic Atrial Rhythm

Since Mirowski et al. have published a series of papers on the left atrial rhythm, a number of studies have been carried out clinically or experimentally without clear differentiation of the left atrial rhythm from the other supraventricular ectopic rhythms, especially the A-V junctional rhythm and the coronary sinus rhythm. In order to clarify the differences among these ectopic rhythms and to establish criteria in the diagnosis of these ectopic rhythms including the left atrial rhythm, electrical stimulation was given at various of the left and right atria of isolated perfused hearts of adult mongrel dogs and P wave polarity, morphology and P-R interval in leads VF, V1 and V6 were examined during electrical stimulation. Effects of heart position and conduction disturbance on the P waves were also examined. Methods: Forty adult mongrel dogs weighing 9 to 14 kg were used. An isolate heart was perfused with Tyrode's solution kept at about 35°C by means of Langendorff's methofs in a cylindrical torso model simulated human thorax. The isolated heart was placed anatomically and electrocardiographically most similar to the normal heart in the human body, as described in a previous report. This position was used for the control experiment. Experiment 1. Thirty isolated perfused hearts placed in the control position were stimulated electrically at several sites of the left and right atria and P wave polarity and morphology in leads VF, V1 and V6 were examined. A region, in which electrical stimulation produced negative P wave in lead VF, "dome and dart" in lead V1 and negative in lead V6, was determined in each heart and conduction times from electrical stimulus to Q wave, i. e. St-Q time, were measured from the records to study intra-atrial and atrioventricular conduction velocity. P wave changes during atrial stimulation were examined with the heart turned clockwise around the ventrical axis by 45° and then counterclockwise from control position, and also lying horizontally. Experiment 2. P waves produced by the atrial stimulation at various sites were examined in the isolated perfused hearts with conduction disturbances of the atria and the atrial septum. The conduction disturbances were produced by an extensive incision to separate the lower part of the right atrium including the internodal tract (incision (I)) and a incision to separate the atrial septum including the anterior and middle internodal tract (incision (II)). Conduction times to various sites of both were measured using stimulating electrodes fixed at the right caudal atrium (the lower part of the incision (I)). Bipolar platinum electrodes with an inter-electrode distance of 1 mm were used for atrial stimulation in connection with the battery-powered pacemaker, Medtronic 5880A, both in Experiments 1 and 2. The intensity of the stimulation was twice as much as the threshold value. ECG's amplified with a main amplifier ( Yokokawa Electric Type 3122) via preamplifier were recorded on a photorecorder at a paper speed of 100mm/sec. The sensitivity was 1.5 cm/20 μv.