Parasystolic Accelerated Idioventricular Rhythms Producing Bidirectional Tachycardia Patterns

Two patients with digitalis toxicity had markedly irregular accelerated idioventricular rhythms that were parasystolic in nature. The relatively abrupt appearance of extrasystoles with different morphologies resulted in patterns of the so-called rapid and 'slow' bidirectional ventricular tachycardias. The parasystolic rhythms seemed to have been classic rather than modulated since nonparasystolic beats occurring in different moments of the cycle failed to affect the latter in the expected, predictable manner. It is possible for the existence of marked, spontaneous variations of the degree of automatic activity to interfere with the diagnosis of parasystolic modulation.