How pimobendan can change echocardiographic findings

December 15, 2025

Echocardiographic Changes in Dogs with Stage B2 Myxomatous Mitral Valve Disease Treated with Pimobendan Monotherapy. Crosland A, Cortes-Sanchez PM, Sudunagunta S, et al. Vet Sci. 2024;11(12):594. Published 2024 Nov 25. doi:10.3390/vetsci11120594

What harm is there in starting pimobendan without echocardiography when you’re pretty sure that heart murmur is mitral valve disease?

We all know that pimobendan is indicated only in ACVIM stages B2 and C, but it can still be tempting to start it without investigation – especially when progression feels inevitable. We frequently see changes in echocardiographic measurements once patients have been on pimobendan for a while. This study highlights an important point to consider before taking that shortcut.

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What was the study objective?

To assess how pimobendan monotherapy affected heart size over time (specifically left atrial and left ventricular dimensions) in dogs with myxomatous mitral valve disease.

How was this done?

Echocardiographic data from 31 dogs with myxomatous mitral valve disease presenting to two referral centres was retrospectively analysed. Dogs were divided into a control group (7 dogs not receiving pimobendan) and an intervention group (24 dogs receiving pimobendan in accordance with the 2019 ACVIM consensus criteria). All dogs had no evidence of congestive heart failure, had no concurrent congenital or acquired structural cardiac disease, had no systemic hypertension or significant systemic illness, and were not receiving any other cardiac medications (including diuretics, ACE inhibitors, spironolactone or amlodipine).

Patients were assessed longitudinally at two time points: the initial assessment and the first follow-up examination.

What were the outcomes?

Dogs receiving pimobendan showed a reduction in normalised diastolic left ventricular internal dimension (LVIDdN) and a decrease in LA:Ao at follow-up, showing evidence of reverse remodelling in patients with Stage B2 MMVD treated with chronic pimobendan monotherapy.

What are the pros and cons of the study?

Pros

The study addresses a common clinical scenario in which dogs may be misclassified as Stage B1 after starting pimobendan, leading to inappropriate cessation of treatment. Notably, 42% of dogs no longer met Stage B2 criteria at recheck.
The study demonstrates a measurable treatment effect using standardised echocardiographic techniques.

Cons

The overall sample size was small, and the control group was particularly small.
The patient population consisted almost exclusively of small-breed dogs (8–9 kg), limiting applicability to medium and large breeds.
As a retrospective study, data collection time points were not standardised, reducing comparability between dogs and potentially influencing results.
As a referral-based population, it may not reflect the broader spectrum of cases seen in general practice; a wider cohort may have yielded a larger control group.
Although LVIDdN is a recognised metric, comparing values over time may be affected by changes in body weight between assessments.
The unexpected reduction in LA:Ao within the control group may reflect regression to the mean rather than a true physiological change.
Inter- and intra-observer variability was likely, as multiple clinicians from two centres performed echocardiography; measurements were not limited to specific operators.

Our thoughts (for now)

Although far from a perfect study design, the findings mirror what we regularly see in clinical practice, and this is the first published evidence explaining how misclassification of MMVD can occur. It reinforces the importance of performing echocardiography before starting pimobendan and of re-evaluating patients over time. Crucially, dogs demonstrating reverse remodelling should not have pimobendan withdrawn simply because they no longer meet the numerical criteria for Stage B2 at follow-up.

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