Echocardiographic Changes in Dogs with Stage B2 Myxomatous Mitral Valve Disease Treated with Pimobendan Monotherapy

The present study aimed to evaluate the effects of chronic pimobendan monotherapy on cardiac size in dogs with stage B2 myxomatous mitral valve disease (MMVD). Data from 31 dogs diagnosed with MMVD and cardiomegaly (LA/Ao ≥ 1.6 and LVIDdn ≥ 1.7) were included. The intervention group were dogs treated with pimobendan (n = 24). Dogs not receiving any cardiac medication were controls (n = 7). Echocardiographic changes in left atrial and left ventricular dimensions were compared over time. There was significant group × time interaction for LVIDdN (<i>p</i> = 0.011) between diagnosis and initial follow-up (median 3-6 months). There was a significant reduction in LVIDdN over time in the pimobendan group (<i>p</i> = 0.038) but not in the control group (<i>p</i> = 0.216). There was no significant group × time interaction for LA/Ao, and there was no effect of group (<i>p</i> = 0.561), but LA/Ao in both groups decreased over time (<i>p</i> = 0.01). Restraint is advised when prescribing pimobendan based on the detection of a heart murmur where echocardiographic staging is an option. Some stage B2 dogs that received pimobendan no longer met the echocardiographic classification criteria for stage B2 MMVD and could have been misclassified as stage B1 and had their medication inappropriately withdrawn. We suggest these dogs are referred to as reverse remodelled stage B2.