Ventricular Refractory Periods in Relation to Rate and Test-Site VT Intervals in Anesthetized and Conscious Dogs: A Canine Model for Conscious State Measurements

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Abstract
The functional relationship of ventricular effective refractory period (ERP) with basic cycle length (BCL) of stimulated ventricular depolarization and VT intervals of test-site unipolar ventricular electrograms were studied in five conscious and eight anesthetized (sodium pentobarbital, 30 mg/kg) open-chested dogs. The range of BCLs studied was 300 to 1000 ms, achieved through ventricular stimulation following chemically-induced complete AV block. In the conscious animal model developed for this study, two-to-four ventricular electrode leads were exteriorized and the ventricular rate was maintained by an implanted programmable VVI pacemaker. In all animals studied, the BCL-ERP relation was closely represented by the empirical equation ERP = A-B.Exp(-k.BCL), and the VT-ERP relation over the same range of BCLs was linear: ERP = C + D.VT. The correlation coefficients were in the range of 0.991 to 0.999. The mean values of the parameters in the above equations determined by the appropriate non-linear or linear regression analysis showed significant differences between the two groups of animals studied. In three conscious animals a strong linear correlation between the test-site VT intervals and simultaneously measured QT intervals measured from the lead II surface electrogram was demonstrated (r = 0.993 to 0.998). For a fixed site of stimulation, the morphology of ventricular depolarization complexes as well as the corresponding T-waves remained essentially unaltered with BCL for both myocardial and surface electrograms. The possible applications of developed canine model and the results of the present study include: (1) the study of the rate-dependent effects of cardioactive drugs on ventricular electrophysiology and, (2) the improved design of electronic refractory periods of rate programmable pacemakers.