Failure of parasystolic impulses to appear on schedule. Exit block due to concealed conduction of sinus impulses.

A 45-year-old patient free of any heart disease was admitted to the hospital with an electrocardiographic pattern of ventricular parasystole. The parasystolic rhythm was relatively fast, such that several consecutive ectopic complexes manifested. A later tracing reflected only isolated parasystolic complexes with long and fixed coupling intervals. The interectopic intervals, however, were once more in multiple of the parasystolic cycle as directly measured during the phases of undisturbed parasystolic rhythm. In the latter tracing, several scheduled parasystolic impulses did not yield a response, despite calculation suggesting that these impulses occurred outside the refractory period. In other words, an exit block was present. Analysis of the tracing suggests that the exit block was caused by concealed penetration of the sinus impulses into the ectopic-ventricular junction. That is, any sinus impulse penetrates into the junction and renders it refractory, in such a way that only parasystolic impulses that are relatively late within the sinus cycle may be conducted to the surrounding myocardium and result in a parasystolic complex.