[2 or 3 level blocks in the Tawara node during atrial tachycardia]

In atrial flutter (or paroxysmal atrial tachycardia), the ventricular response is dependant on the passage through 3 superposed zones of conduction in the Tawara node, the zone of decremential conduction being the central zone N. When the ventricular response is between half and a quarter of the atrial rate there are two possible explanations: type B alternate Wenckebach period (mobitz I block in the central zone N, 2/1 block at the nodo-ventricular junction) or type A alternate Wenckebach period (Mobitz I block in the central zone N and 2/1 block at the atrio-nodal junction). These two responses may alternate in the same patient depending on the drug therapy or vagal activity due to a phenomenon similar to the "GAP" phenomenon. Inexactitudes in the working out of the arithmetic formulae may easily be explained by a certain degree of concealed conduction of blocked activation in one zone or more rarely by hisian extrasystoles. Type A alternate Wenckebach periods are always easier to construct than type B. Perfect 3/1 atrial flutter can only be explained by a type B alternate Wenckebach period with a 3/2 period with a 3/2 period in the N zone and a 2/1 block in the NH zone. When the ventricular rhythm is permanently very slow or when the RR intervals are greater than four times the atrial cycle, 3 zones of block are usually at issue (the third being located in the inferior part of the node or superior part of the bundle of His). Examples of 5/1, 6/1 flutter are thereby analysed. Rapid atrial pacing after termination of the atrial arrhythmia allows a better analysis of its mechanism and the successive reproduction of conduction defects in each zone of block.