Effects of afterload on regional left ventricular torsion

Objective: To determine if left ventricular torsion, as measured by magnetic resonance tissue tagging, is afterload dependent in a canine isolated heart model in which neurohumoral responses are absent, and preload is constant. Methods: In ten isolated, blood perfused, ejecting, canine hearts, three afterloads were studied, while keeping preload constant: low afterload, high afterload (stroke volume reduced by approx. 50% of low afterload), and isovolumic loading (infinite afterload). Results: There were significant effects of afterload on both torsion (P < 0.05) and circumferential shortening (P < 0.0005). Between low and high afterloads, at the anterior region of the endocardium only, where torsion was maximal, there was a significant reduction in torsion (15.1 ± 2.2° to 7.8 ± 1.8°, P < 0.05). Between high afterload and isovolumic loading there was no significant change in torsion (7.8 ± 1.80 to 6.2 ± 1.5°, P = NS). Circumferential shortening at the anterior endocardium was significantly reduced both between low and high afterload (-0.19±0.02 to -0.11 ± 0.02, P < 0.0005), and also between high afterload and isovolumic loading (-0.11 ± 0.02 to 0.00 ± 0.02, P < 0.05). Plots of strains with respect to end-systolic volume demonstrated a reduction in both torsion and shortening with afterload-induced increases in end-systolic volume. Torsion, but not circumferential shortening, persisted at isovolumic loading. Conclusions: Maximal regional torsion of the left ventricle is afterload dependent. The afterload response of torsion appears related to the effects of afterload on end-systolic volume.