In view of the broad spectrum of arrhythmias and their considerable spontaneous variability, there is a need for a classification of arrhythmias as a basis for scientific and clinical decision making. From the clinical point of view, a classification should consider (a) hemodynamic consequences, (b) prognostic significance of arrhythmias, and (c) should allow assessment of efficacy of antiarrhythmic treatment. Hemodynamic consequences of tachycardias are related to the degree of heart rate: The shorter the RR interval the shorter the diastolic filling period, resulting in a decrease of the stroke volume and--above a critical heart rate--in a decrease of the cardiac output. The critical heart rate, on the other hand, is essentially related to the functional status of the heart: The more pronounced the cardiac impairment, the lower the critical heart rate. A second factor favoring hemodynamic impairment due to arrhythmias is the loss of the sequence of atrioventricular contractions. Despite the clinical relevance of hemodynamic consequences of arrhythmias, there is no accepted classification taking these aspects into account. In the past, more interest was directed toward the prognostic significance of arrhythmias. In 1971, Lown and Wolf published a classification of ventricular arrhythmias, assigning risk to advanced grades. This proposal, as those from others, took into account arrhythmias of ventricular origin only. The major concern about the Lown classification, however, relates to the consequences of maximal grading: a patient is assigned to a grade depending on the highest ranking. Thus, a person can only be in one grade, leaving all other arrhythmias grouped below as well as the true frequency of ventricular arrhythmia obscure.(ABSTRACT TRUNCATED AT 250 WORDS)