Direct Demonstration of Sinus Node Reentry in the Rabbit Heart

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In spontaneously beating isolated right atria of the rabbit, ectopic premature beats of varying prematurity were elicited. In some preparations, very early premature beats were followed by abnormal extra responses which were suspected to be sinus echo beats. By simultaneous recording of multiple atrial electrograms and consecutive impalement of as many sinus node fibers as possible during repeated induction of sinus echoes, we tried to follow the pathway of the impulse in the sinus node during this phenomenon. In one case we succeeded in obtaining a complete and detailed picture of the electricial behavior of the sinus node, revealing some basic mechanisms underlying the occurrence of sinus echoes. Thus the following could be established. (1) Analogous to longitudinal dissociation in the atrioventricular node, in the sinoatrial node, also, dissociation in conduction may occur. (2) Circus movement within the sinus node is possible. However, the dimensions of such an intranodal circuit are extremely small, the diameter being between 1 and 2 mm; the average conduction velocity in the circuitous pathway being 2.5 cm/sec. (3) No anatomical or pathological obstacle was involved in the present observation of sinus node reentry. The fibers in the center of the circuit, showing completely normal characteristics during sinus rhythm, were kept depolarized by electrotonic depolarizing current during circulation of the impulse around them. (4) After termination of sinus node reentry, a temporary shift of the dominant pacemaker occurred. This pacemaker shift was due to the fact that, at the moment the circulating impulse was blocked, the group of fibers lying distal from the site of block, while now not being discharged by their neighboring fibers, got the opportunity to reach threshold themselves, thus temporarily taking over the role of pacemaker of the heart Circ Res 44: 557-568, 1979

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