Figure 1.

Diagrammatic representation of the mechanisms of re-entry. (1) A wavefront (arrows), initiated in a normal fashion in the sinus node, passes around an obstacle (disc) to electrical activation in a uniform fashion. This obstacle may be formed by an anatomical feature (fixed conduction block) like the tricuspid annulus, or by a physiological abnormality (functional conduction block) like an area of ischaemic myocardium, which may or may not result in block depending on a variety of conditions like vagal tone or coupling interval. (2) A premature impulse results in block of conduction on one side of the obstacle while conduction continues on the other. This is functional block because it is the result of a short electrical coupling interval which means that the myocardium in this region has not recovered its excitability in time to conduct the premature beat. (3) This wavefront takes sufficient time to circulate around the obstacle that repolarisation occurs and the area previously resulting in block recovers its excitability, so that this wavefront continually encounters excitable tissue and perpetuates as a re-entry circuit.