Postextrasystolic potentiation and echocardiography. The effect of varying basic heart rate, extrasystole coupling interval and postextrasystolic interval

Abstract

The purpose of this study was to determine whether echocardiography can be used to monitor postextrasystolic potentiation (PESP) and, if so, which of the echocardiographic variables are valid indicators of PESP and under what conditions. An implantable M-mode echocardiographic transducer was sewn to the right ventricular outflow tract of the dog heart to record continuously across the minor diameter of the left ventricle. The sinus node pacemaking region was excised and a pacing protocol was used to vary the basic heart rate, the extrasystole coupling interval and the postextrasystolic interval. The decreases in ejection time and diastolic dimension as well as the increases in the mean rate of circumferential fiber shortening (Vcf) and fractional shortening (% FS) that occur with increasing heart rate were detected by echocardiography. There were also increases in posterior wall maximal velocity and excursion and an increase in the rate of diastolic relaxation. Shortening the coupling interval of the extrasystole resulted in the expected potentiation of Vcf, % FS and the excursion, maximal velocity and rate of diastolic relaxation of the posterior wall. No correlation was observed for percent thickening of the posterior wall or interventricular septum, either because the technique was too insensitive or because wall thickening does not reflect the rapid-phase force-frequency perturbation of PESP. Allowing a full compensatory pause for the postextrasystolic interval resulted in a greater potentiation of all variables than if the postextrasystolic interval was timed to occur when postextrasystolic diastolic pressure and dimension were identical to preextrasystolic diastolic pressure and dimension (isolength). However, only at isolength was potentiation highly predictable, and then only if the heart rate and the coupling interval were known. We conclude that most of the variables measured echocardiographically can be used to determine whether the PESP response is equivalent to a predicted value, provided all intervals are known or carefully controlled.