Pimobendan Reverse Remodeling in Stage B2 MMVD: Misclassification Risk and Monitoring Implications (Crosland 2024)

Bottom line

• A 2024 prospective cohort study (Crosland et al., Vet Sci) found that pimobendan monotherapy in Stage B2 MMVD dogs produced a significant reduction in normalized left ventricular internal diameter in diastole (LVIDdN) over time (P = .038) — a reverse remodeling effect not observed in untreated controls (P = .216).¹
• The study found a significant group × time interaction for LVIDdN (P = .011) between diagnosis and initial follow-up (median 3–6 months), while LA/Ao decreased in both groups without a significant treatment-group difference.¹
• Importantly, some Stage B2 dogs on pimobendan no longer met the echocardiographic classification criteria for Stage B2 after treatment — these dogs risk inappropriate withdrawal of pimobendan if misclassified as Stage B1; the authors recommend calling them "reverse remodelled Stage B2."¹
• The authors caution that echocardiographic staging should be performed before initiating pimobendan based on a murmur detection to avoid this misclassification risk if the first echocardiogram is performed after treatment has already started.¹
• This is a clinician-facing evidence summary. It is not a dosing protocol; confirm regimen, monitoring and contraindications against current product labeling and a veterinary formulary.

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What the evidence shows

Study design and population

This was a retrospective-prospective cohort study from cardiology services at the University of Liverpool and the University of Edinburgh.¹ Data from 31 dogs diagnosed with MMVD and cardiomegaly (LA/Ao ≥1.6 and LVIDdN ≥1.7 at time of diagnosis, consistent with Stage B2 and EPIC criteria) were included. The intervention group comprised 24 dogs treated with pimobendan; 7 dogs not receiving any cardiac medication at the time of staging served as controls. Echocardiographic changes in left atrial and left ventricular dimensions were compared over time, with particular attention to the interval between initial diagnosis and first follow-up (median 3–6 months).¹

Reverse remodeling findings

There was a significant group × time interaction for LVIDdN (P = .011) between diagnosis and initial follow-up: LVIDdN decreased significantly over time in the pimobendan group (P = .038) but not in the control group (P = .216).¹ For LA/Ao, there was no significant group × time interaction and no effect of treatment group (P = .561), but LA/Ao in both groups decreased over time (P = .01), suggesting that LA/Ao may decrease as part of natural variation or measurement variability independent of treatment.¹

The clinically significant finding is that following pimobendan treatment, some dogs fell below both the LA/Ao ≥1.6 and LVIDdN ≥1.7 thresholds used to define Stage B2. The study authors state that these dogs "could have been misclassified as stage B1 and had their medication inappropriately withdrawn" and suggest calling them "reverse remodelled Stage B2."¹

Clinical misclassification risk

The authors further advise: "Restraint is advised when prescribing pimobendan based on the detection of a heart murmur where echocardiographic staging is an option." This highlights that pre-treatment echocardiography is critical — if the first echocardiogram is performed after pimobendan has been started and the dog has already experienced reverse remodeling, the clinician may be unaware the dog ever met Stage B2 criteria, and could withhold a drug from which the dog is benefiting.

This study does not argue against pimobendan in Stage B2 MMVD — the reverse remodeling it describes is a favorable response — but it raises an important clinical workflow consideration: echo before drug, not drug before echo, to correctly stage the patient and ensure continuity of appropriate therapy.

How this fits clinical practice

The reverse remodeling finding from Crosland et al. 2024 adds mechanistic granularity to the EPIC trial data and informs monitoring strategy in Stage B2 dogs. A dog whose LVIDdN normalizes below 1.7 after starting pimobendan has likely experienced a beneficial cardiac response and should continue therapy, not have it withdrawn because of a lower number on a repeat echo. Echocardiographic benchmarks used to justify initiation (EPIC eligibility criteria, ACVIM 2019 Stage B2 thresholds²) should be documented at diagnosis before treatment begins, so that subsequent reverse remodeling is recognized for what it is. Monitoring plans should be discussed with the owner in light of this possibility. Do not infer specific doses from this summary; confirm regimen and contraindications against current product labeling and a veterinary formulary.

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References

1. Crosland A, Cortes-Sanchez PM, Sudunagunta S, Bouvard J, Bode E, Culshaw G, Dukes-McEwan J. 2024. Echocardiographic changes in dogs with Stage B2 myxomatous mitral valve disease treated with pimobendan monotherapy. Vet Sci 11(12):594. https://europepmc.org/article/MED/39728934
2. Keene BW, Atkins CE, Bonagura JD, Fox PR, Häggström J, Luis Fuentes V, Oyama MA, Rush JE, Stepien R, Uechi M. 2019. ACVIM consensus guidelines for the diagnosis and treatment of myxomatous mitral valve disease in dogs. J Vet Intern Med 33(3):1127-1140. https://europepmc.org/article/MED/30974015 [via]

Changelog

• 2026-06-22: First published.