Time-dependent bradycardia: a new effect?

Time-dependent bradycardia, a term introduced by the Laceys (Lacey and Lacey, 1980), refers to the fact that the duration of a cardiac cycle is related to when in that cycle, or the just preceding cycle, certain stimuli occur. Since stimulus effects on heart activity with very short latencies have been known for a long time, the question arises whether the effect is one in its own right, or whether it is simply a result of the new way of data reduction introduced by the Laceys. Theoretically, the effect can be explained without assuming anything beyond the well-known short latency bradycardia often observed upon stimulation. Derivation of the effect from an empirical curve of cardiac activity not related to cycle phase shows that we are not dealing with a new effect.