Rate dependence and adrenergic dependence of arrhythmias

The presence or absence of ventricular premature beats is often closely dependent on the heart rate, and the analysis of Holter recordings permits definition of the thresholds of frequency that are compatible with the extrasystolic phenomenon. The upper threshold above which the extrasystoles disappear, and the lower threshold below which they are not observed can be defined for longer or shorter periods of recording, and they can vary according to changes in the autonomic nervous system balance. The coupling interval of extrasystoles tends to shorten when the adrenergic drive increases, but is also strongly dependent on the duration of the immediately preceding RR/cycle lengths: the rate dependence and the adrenergic dependence explain why the coupling interval is often slightly variable rather than classically fixed. The complex interactions of these phenomena are more compatible with an underlying mechanism of parasystole, and determine the existence and the importance of repetitive activity following the initial premature beat in many forms of ventricular arrhythmias.