Papillary muscles contribute significantly more to left ventricular work in dilated hearts

Aims: Left ventricular (LV) dilatation results in increased sphericity and affects position and orientation of papillary muscles (PMs), which may influence their performed work. The aim of this study was to assess the contribution of PM to LV function and its changes with dilatation. Methods and results: Fifteen sheep were investigated. Ten animals were subjected to 8 weeks of rapid (180 bpm) pacing, inducing LV dilatation. Five animals served as controls. High-resolution gated computed tomography was performed to assess LV volumes, left ventricular ejection fraction (LVEF), global longitudinal strain (GLS), sphericity index, and PM angle, width and fractional shortening. 18F-fluorodeoxyglucose positron emission tomography (PET) was used to measure glucose metabolism as surrogate of regional myocardial work. Spatial resolution of PET images was maximized by electrocardiogram- and respiratory-gating. 18F-fluorodeoxyglucose uptake was measured in PM and compared with remaining left ventricular myocardium (MYO) to obtain a PM/MYO ratio. Animals with dilated heart had a more spherical left ventricle, with reduced LVEF (P < 0.0001) and GLS (P < 0.0001). In dilated hearts, PET analysis revealed a higher contribution of both PM to LV myocardial work (P < 0.0001); and PM angle towards LV wall correlated with PM work, together with PM width and the LV sphericity index. Sphericity index and posterior PM angle were strongest determinants of posterior PM/MYO ratio (R2 = 0.754; P < 0.0001), while anterior PM/MYO was mostly determined by sphericity index and the PM width (R2 = 0.805; P < 0.0001). Conclusion: In dilated hearts, PM contribute relatively more to LV myocardial work. We hypothesize that this is caused by the more cross-sectional orientation of the subvalvular apparatus, which leads to a higher stress on the PM compared with the spherical LV walls. The reduced cross-sectional area of the PM may further explain their increased stress.