A modulated parasystole (MP) can be recognized only by the demonstration that there is a nonrandom relationship between the timing of an intervening sinus beat (R) (or beats) and the interectopic cycle length (IECL). This relation is expressed by the ‘phase-response curve’ (PRC). The aim of this paper is to describe an alternative method to draw the PRC from Holter recordings in order to recognize the presence of MP in case of ventricular premature beats(X) with varying coupling intervals, fusions and irregular IECL. To draw a PRC it is mandatory to know the value of the nonmodulated IECL and the value of the reversal point (RP); the last is usually found by a method of trial and error. We start from the observation that in presence of MP the distribution of R-X intervals is not random because every X, owing to the modulating effect of R, is shifted away from the scheduled point, toward the right if X-R is shorter and toward the left if X-R is longer than RP. As a consequence, the R-X distribution curve has a bimodal shape with a nadir located in a point equal to ‘nonmodulated IECL minus RP’. The RP can be easily calculated because the nonmodulated IECL (when absent on the actual record) is usually supposed to be equal to the mean of all the observed single IECLs. Hence the drawing of a PRC can be quickly tried by sampling a series of R in close proximity of the supposed RP and by measuring the length of the corresponding X-X intervals which embrace a single R.

Keywords:
Arrhythmias, Ventricular parasystole
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1994
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