title: Unified rate-dependent atrioventricular nodal function: consistent recovery and fatigue properties revealed with S1S2S3 protocols and different recovery indexes.
abstract: BACKGROUND
  Rate-dependent nodal properties are commonly assessed with premature protocols performed at different basic rates. Because characteristics of responses differ with recovery time index, the true nature of nodal rate-dependent properties is elusive.
  
  
  OBJECTIVE
  The purpose of this study was to reveal consistent nodal rate-dependent properties regardless of selected recovery index.
  
  
  METHODS
  With S(1)S(2)S(3) protocols, we independently varied basic and pretest cycle lengths and thereby distinguished cumulative from noncumulative effects of rate on nodal conduction time in rabbit heart preparations. Nodal responses to 30 basic and pretest cycle length combinations (five with identical basic and pretest cycles as in standard protocols) were analyzed using both atrial (AA) and His-atrial (HA) intervals as recovery index.
  
  
  RESULTS
  AA and HA curves had an identical shape for any of 30 steady-state conditions. When assessed with constant pretest cycle lengths, cumulative effects (fatigue) of shortened basic cycle lengths were also independent of recovery index. Shortening of pretest cycle length at fixed basic rates led to apparent inhibitory and facilitatory effects when assessed with AA and HA curves, respectively. These effects vanished when a single long cycle was inserted after the pretest cycle. In all responses including those obtained with standard protocols, combined effects of basic and pretest cycle lengths set nodal conduction time.
  
  
  CONCLUSIONS
  S(1)S(2)S(3) protocols reveal consistent nodal recovery and fatigue properties regardless of recovery index used. Changes in nodal function curves arising from the use of different recovery indexes mainly depend on pretest effects. This study provides a new approach to a unified interpretation of nodal recovery and fatigue properties.
authors.affiliation: Département de Physiologie, Faculté de Médecine, Université de Montréal, Montréal, Quebec, Canada.
authors.forename: Rafik
authors.initials: R
authors.lastname: Tadros
authors.forename: Michel
authors.initials: M
authors.lastname: Lavallée
authors.forename: Jacques
authors.initials: J
authors.lastname: Billette
mesh_terms: D000704:Analysis of Variance; D000818:Animals; D016275:Atrial Function; D001283:Atrioventricular Node / Q000502:physiology*; D002036:Bundle of His / Q000502:physiology*; D005117:Cardiac Complexes, Premature / Q000503:physiopathology; D004195:Disease Models, Animal; D004562:Electrocardiography; D022062:Electrophysiologic Techniques, Cardiac; D006339:Heart Rate; D008297:Male; D018763:Muscle Fatigue / Q000502:physiology*; D011817:Rabbits; D020127:Recovery of Function