Rate-dependent AV nodal refractoriness: a new functional framework based on concurrent effects of basic and pretest cycle length.

The atrioventricular (AV) node filters atrial impulses. Underlying rate-dependent refractory properties are assessed with the effective (ERPN; longest nonconducted atrial cycle length) and functional (FRPN; shortest His bundle cycle) refractory period determined with premature protocols at different basic rates. Fast rates prolong ERPN and shorten FRPN, but these effects vary with subjects, age, and species. We propose that these opposite and variable effects reflect the net sum of concurrent cumulative and noncumulative effects associated with basic (BCL) and pretest cycle length (PTCL), respectively. To test this hypothesis, we assessed selective and combined effects of five BCL (S1S1) and six PTCL (S1S2) on ERPN, FRPN, and their subintervals (ERPN=A2H2+H2A3 and FRPN=H2A3+A3H3, where A is atrium and H is His bundle) with S1S2S3 protocols in six rabbit heart preparations. At control BCL, PTCL shortenings prolonged ERPN (113+/-12 vs. 101+/-14 ms, P<0.01) as a net result of prolonged A2H2 and curtailed H2A3. At control PTCL, BCL shortenings increased ERPN (127+/-20 vs. 101+/-14 ms, P<0.01) by prolonging A2H2. FRPN did not vary with BCL but decreased (163+/-6 vs. 175+/-10 ms, P<0.01) with PTCL that curtailed H2A3. Equal BCL and PTCL shortenings as in standard protocols prolonged ERPN but left FRPN unchanged. Notably, ERPN and FRPN significantly correlated through their H2A3 subinterval. In conclusion, BCL and PTCL are both important determinants of AV nodal refractoriness and together account for rate-induced changes in ERPN and FRPN observed during standard premature protocols. ERPN and FRPN are related variables. Similar functional rules may govern nodal refractory behavior during supraventricular tachyarrhythmias.