Mechanisms of Cardiomyoplasty: Comparative Effects of Adynamic Versus Dynamic Cardiomyoplasty

Abstract

Background. The apparent paradox seen in patients who have undergone dynamic cardiomyoplasty and shown substantial clinical and functional improvements with only modest hemodynamic changes may be due to inappropriate end points chosen for study, a result of incomplete understanding of mechanisms involved. The purpose of this study was to compare the relative role of the passive “girdling effect” and the dynamic “systolic squeezing effect” of the wrapped muscle in cardiomyoplasty. Methods. The control group of 6 dogs underwent 4 weeks of rapid pacing (250 beats/min) to induce severe heart failure followed by 8 weeks of observation without rapid pacing. The trajectory of recovery in hemodynamics and cardiac dimensions was followed with echocardiography and Swan-Ganz catheters. In the “adynamic” cardiomyoplasty group (n = 4), the left latissimus dorsi muscle was wrapped around the ventricles and allowed to stabilize and mature for 4 weeks. This was followed by rapid pacing and recovery as in the control group. In the “dynamic” cardiomyoplasty group (n = 3), the same protocol for the adynamic group was followed except that a synchronizable cardiomyostimulator was attached to the thoracodorsal nerve of the muscle wrap. This allowed the latter to be transformed during the rapid-pacing phase and permitted dynamic squeezing of the muscle wrap to be generated by burst stimulation synchronized with cardiac contraction in a 1:2 ratio. Results. Baseline data were comparable in all groups prior to rapid pacing. After 4 weeks of rapid pacing, the left ventricular ejection fraction was higher in the adynamic (27.0% ± 3.9%; p < 0.05) and dynamic (33.3% ± 2.3%; p < 0.02) cardiomyoplasty groups compared with controls (18.8% ± 8.3%). Similarly, ventricular dilatation in both systole and diastole was less in the adynamic (51.8 ± 8.7 mL, [ p < 0.002] and 38.2 ± 7.2 mL [ p < 0.001], respectively) and dynamic (62.0 ± 7.2 [ p < 0.02] and 41.3 ± 3.5 mL [ p < 0.005], respectively) cardiomyoplasty groups compared with controls. In the dynamic group, on and off studies were carried out after cessation of rapid pacing while the heart was still in severe failure, and they demonstrated a systolic squeezing effect in stimulated beats. Only this group recovered fully to baseline after 8 weeks. Conclusions. By reducing myocardial stress, both the passive girdling effect and the dynamic systolic squeezing effect have complementary roles in the mechanisms of dynamic cardiomyoplasty.