Direct demonstration of sinus node reentry in the rabbit heart.

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Abstract
In spontaneously beating isolated right atria of the rabbit, ectopic beats of varying prematurity were elicited. In some preparations, very early premature beats were followed by abnormal extra responses 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, the pathway of the impulse in the sinus node during this phenomenon was followed. In 1 case a complete and detailed picture of the electrical behavior of the sinus node was obtained, revealing some basic mechanisms underlying occurrence of sinus echoes. Analogous to longitudinal dissociation in the atrioventricular node, dissociation in conduction may occur in the sinoatrial node. Circus movement within the sinus node is possible. The dimensions of an intranodal circuit are extremely small, with a diameter of 1-2 mm; the average conduction velocity in the circuitous pathway is 2.5 cm/s. No anatomical or pathological obstacle was involved in 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. 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 to the site of block, not being discharged by neighboring fibers, reached threshold, temporarily becoming the pacemaker of the heart.