Figure 5: Comparison of a self-limited PVC and two examples of selfsustaining VT/VF in terms of the percentage of myocardium occupied by active wave fronts that are 0 to 20 msec old. PVC: refractory period 50% of normal, conduction velocity 60% of normal. VT/VF: refractory period 60% of normal, conduction velocity 30% of normal.