Sino-Atrioventricular Symbiotic Rhythm (SA-AV)

 Quintiliano H. de Mesquita, MD

 In the beginning of 1970 decade, we described a new ECG rhythm denominated " SA-AV Symbiotic", characterized by addition phenomenon or fusion of sinusal and nodal stimulus, in the level of atrial mass (Mesquita, Q.H., Arq. Bras. Cardiol., 1970;23:179) .

The symbiotic is a transition rhythm with a different P wave in comparison with sinusal and nodal AV rhythm. It is resulted from atrial addition phenomenon, caused by SA and AV node synchronization, through the use of fast conduction way in one of the connection bundles of S A and AV nodes, which are responsible for the development and propagation of synchronic stimulus in anterograde and retrograde directions of atrial mass. That synchronization, involving SA and AV nodes, represents the basic mechanism for the development of the symbiotic rhythm.

The morphology of P wave, in the symbiotic rhythm, is dependent from the precedence or predominance of one of the SA or AV stimulus in the atrial activation. The resultant P wave records large variation in shape, since altered positivistic register with notches, flatten register with notches, diphase register (+- and -+ types) until reduced negativity register.

We have identified, in the diphase register of P wave type (+-) , the addition phenomenon with sinusal precedence and in the type (-+) , the addition phenomenon with nodal AV precedence .

The P wave were registered positive and notched or flatten with or without notches , showing the sinusal predominance and nodal AV predominance with aspects of reduced negativity.

After the desynchronization of SA and AV nodes , the symbiotic rhythm register stops and gives place to the sinusal or nodal A V rhythm.

We have selected L2 and L3 as the best leads for the analysis of the symbiotic rhythm with large variety of shapes, principally in continuous and prolonged recordings. The PR interval in the symbiotic rhythm may be with equal duration as the nodal AV rhythm type 1 or with equal or superior duration in comparison with the sinusal rhythm. The P wave presents the same duration as the nodal and sinusal rhythm.

# The symbiotic rhythm has been registered by us until now during the following phenomena: sinusal depression, transitory nodal A V rhythm, and pararrhythmia with isorhythmical AV dissociation.

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