In parasystole, be it atrial or ventricular, an ectopic focus is protected, or shielded, from the depolarizing effects of the normally conducted beats. Thus, it is theoretically capable of producing a premature response whenever the impulse from the ectopic focus is able to propagate to the rest of the myocardium. The characteristics of ectopic premature beats from a parasystolic focus are shown here.
Note that when an impulse from the parasystoalic focus and a normally conducted beat reach the myocardium at the same time, the resultant complex will be the fusion of the two waveforms. This fusion of normally conducted and parasystolic beats will result in the variable morphology of the associated P waves or QRS complexes.