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 degrees to 7.8+/-1.8 degrees, P<0.05). Between high afterload and isovolumic loading there was no significant change in torsion (7.8+/-1.8 degrees to 6.2+/-1.5 degrees, 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.