Background: Pimobendan is reported to reduce left heart size and extend the preclinical period in dogs with ACVIM stage B2 degenerative mitral valve disease (DMVD). The precise mechanism underlying this benefit is unclear. Hypothesis: Pimobendan enhances systolic contraction of the mitral annulus thereby reducing regurgitant orifice and severity of mitral regurgitation. Animals: Twenty client-owned dogs with newly diagnosed ACVIM stage B2 DMVD.
Methods: Prospective clinical trial. All dogs underwent cardiac computed tomography (CCT) before and 14 days after starting oral pimobendan. Geometric indices of the mitral annulus and cardiac volumes were measured at multiple cardiac phases by CCT. Regurgitant volume and fraction were estimated by calculating left ventricular total stroke volume compared to right ventricular stroke volume. Paired t-test or Wilcoxon signed-rank test was applied for parametric or non-parametric comparisons between time points.
Results: Minimal adverse events were observed (n=1, mild diarrhea). End-systolic mitral annular area (p<0.001), septo-parietal distance (p<0.001), inter-commissural distance (p<0.001), and trigone-to-trigone distance (p=0.041) were decreased after 14 days of pimobendan, whereas end-diastolic geometric indices were not different (all p values>0.05). Left ventricular end-diastolic volume (p<0.001), end-systolic volume (p<0.01), and stroke volume (p=0.023) decreased after pimobendan. Left ventricular ejection fraction was unchanged (p=0.35). Regurgitant volume (p<0.01), regurgitant fraction (p<0.001), and left atrial volumes (p<0.001) reduced after pimobendan.
Conclusion: Short-term administration of pimobendan augmented systolic contraction of the mitral annulus thereby decreasing the severity of mitral regurgitation. These dynamic changes may underlie observed clinical benefits in dogs with subclinical DMVD.
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