Figure 3.

Conditions underlying generation of re-entrant arrhythmia. A: basic features of arrhythmic substrate, consisting of slow conducting myocardial pathway (path 1; dark gray), nonconducting myocardium, and second normally conducting pathway (path 2; white) (i). Normal action potential (blue arrow) propagates with velocity θ and effective refractory period (ERP) resulting in propagation wavelength (λ = θ × ERP) (yellow region) along path 2. It initiates a slow conducting impulse traveling along path 1 (i). In normal activity, the latter impulse cannot re-enter the circuit as it collides with refractory tissue in path 2 (ii). B: an abnormal triggered impulse immediately following the normal action potential cannot enter path 1 as this remains refractory. C: self-perpetuating re-entrant excitation occurs when a retrogradely conducting AP along path 1 (i) enters the beginning of path 2 with reduced conduction velocity and effective refractory period and therefore reduced excitation wavelength smaller than the dimensions of the propagation pathways. [From King et al. (567).]