Long-term observation of cardiac rhythm and automaticity in the dog after excision of the sinoatrial node.

Twenty-nine dogs having undergone aseptic excision of the sinoatrial node were kept under observation, in most cases, for more than a year. Observation confirmed the instability of the new rhythm or rhythms taking over the automaticity after excision of the physiological pacemaker. This instability was particularly marked during the first ten days following surgery. The new pacemaking sites were mainly junctional. Spontaneous pauses occurred. Later on, in more than 50% of the cases, pacemaker sites became localized in the high zone of the atrium, and the rhythm was of sinus node origin. Through the evolution, rhythm disorders were observed as sequences of bradycardia, tachycardia, atrial fibrillation, and flutter. Unexcised nodal cells were found in three animals, through their presence did not make any significant difference to their ECGs as compared with animals in which removal of nodal cells was complete. Surgical implantation of electrodes enabled a study to be made of recovery time, which was always well within physiological limits, but was particularly long during the ten days following surgery. The values varied greatly from one examination to another. Propranolol (0.5 mg/kg i.v.) aggravated the disorders and lengthened corrected recovery time; atropine had varying effects. An analogy is apparent between the disorders described here and those observed in man in the sick sinus syndrome.