Divergence between refractoriness of His-Purkinje system and ventricular muscle with abrupt changes in cycle length.

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Abstract
The concept that refractoriness of the His-Purkinje system (HPS) and ventricular muscle both vary directly with cycle length is based on observations during the use of constant cycle length. During abrupt changes in ventricular cycle length, refractoriness of the ventricular muscle is known to reflect the cumulative durations of preceding cycle lengths. The effect of such changes on retrograde refractoriness of the HPS is not known. In this study refractoriness of ventricular muscle and of the HPS was evaluated in 30 patients with normal intraventricular conduction by the ventricular extrastimulus (V2) technique during constant cycle length (method I) and during abrupt cycle length changes (method II). During method II the cycle length immediately before V2 was identical to the constant cycle length of method I and therefore was designated as the reference cycle length (CLR); however, the cycle length preceding (CLP) CLR was either longer than CLR (method IIA) by 100 to 300 msec in 11 patients or shorter than CLR (method IIB) by 100 to 300 msec in 30 patients. Results showed that compared with method I, method IIA shortened the relative refractory period (RRP) of the HPS from 350 +/- 29 to 344 +/- 29 msec (p less than .04), whereas the effective refractory period (ERP) of the ventricular muscle increased from 225 +/- 21 to 233 +/- 20 msec (p less than .0001). In contrast, compared with method I, method IIB lengthened the RRP of the HPS from 335 +/- 30 to 351 +/- 35 msec (p less than .0001), whereas ERP of the ventricular muscle decreased from 223 +/- 23 to 213 +/- 22 msec (p less than .0001). Similar to the inverse relationship between CLP and RRP of the HPS, ERP of the HPS was prolonged with short CLP (method IIB) compared with long CLP (method IIA). The results indicate a marked divergence between refractoriness of the HPS and of ventricular muscle during abrupt cycle length changes; these results were not previously anticipated. Whereas ventricular muscle responded to cumulative effects of preceding cycle lengths and varied directly with CLP, the HPS appeared to respond to directional and/or dynamic changes in cycle length and varied inversely with CLP. Moreover, in contrast to ventricular muscle, the HPS appeared to be responsive to rate of change in cycle length whereby short-to-long change in cycle length had a greater effect than long-to-short change in cycle length.