Managing Canine Heart Failure by ABCD Stage

HeartQ&AMeongsiljang Veterinary Advisory Board

Canine heart failure is classified into four stages, known as ABCD. Here’s a summary of the symptoms, when to start treatment, and key home care tips for each stage.

What Are the ABCD Stages of Canine Heart Failure?

Illustration of a dog's heart failure stage examination

The ABCD classification system for canine heart failure, established by the American College of Veterinary Internal Medicine (ACVIM), is an international standard that categorizes the progression of chronic valvular disease into four stages. Treatment should begin at stage B2. Even if symptoms are not yet apparent, initiating medication once the heart has started to enlarge in stage B2 is essential to extend life expectancy. In stages C and D, heart failure medications such as diuretics, ACE inhibitors, and pimobendan are absolutely necessary.

Why Is It Divided into ABCD Stages?

Heart failure doesn’t just appear overnight; it develops gradually over several years. Even among dogs with the same heart condition, the type of medication, frequency of check-ups, and dietary management vary significantly depending on the stage, making it essential to classify the disease accurately. According to veterinary internal medicine textbooks, the ABCD classification system is an international standard that allows pet owners and veterinarians to share a common understanding of disease progression and develop a coordinated treatment plan.

At-a-Glance Comparison of the 4 ABCD Stages

Item	Stage A	Stage B1	Stage B2	Stage C	Stage D
Cardiac abnormality	None (high-risk group)	Murmur present, normal size	Murmur present, enlarged heart	Symptoms present	No response to medication
Symptoms	Asymptomatic	Asymptomatic	Asymptomatic	Coughing, dyspnea	Severe, refractory
Treatment	Observation	Observation, regular checkups	Start medication	Combination drug therapy	Hospitalization, intensive care
Checkup interval	Once a year	6-12 months	3-6 months	1-3 months	As needed

Based on the ACVIM 2019 guidelines.

Stages A and B1: Medication Is Not Yet Needed

- Stage A: Your dog belongs to a breed at high risk for heart disease but currently shows no signs of cardiac abnormalities. Breeds such as Cavalier King Charles Spaniels, Maltese, Pomeranians, and Dachshunds fall into this category. - Stage B1: A heart murmur is detected during auscultation, but chest X-rays and echocardiograms confirm that the heart size is normal. For both of these stages, we do not prescribe medication; instead, we monitor for any changes through regular check-ups.

A Cavalier King Charles Spaniel undergoing a cardiac examination

Stage B2: The Golden Window to Start Treatment

Stage B2 is characterized by the presence of a heart murmur and an enlarged heart visible on echocardiography or X-rays. Although dogs may appear healthy outwardly, large-scale clinical trials such as the EPIC study have proven that starting medication at this stage can extend lifespan. The primary medication used in Stage B2 is Pimobendan. According to veterinary internal medicine textbooks, there is no evidence that administering ACE inhibitors in the preclinical (Stage B) phase slows heart failure progression or improves survival. Furthermore, two large-scale randomized controlled clinical trials, SVEP and VetProof, found no significant difference between the placebo and treatment groups. Therefore, Pimobendan is the standard medication for Stage B2, and the decision to add an ACE inhibitor should be carefully made by your veterinarian based on the individual case. Accurate staging requires an echocardiogram.

Why Start Medication in Stage B2 Even Without Symptoms

Stage B2 is when there are no symptoms at all, so pet owners often hesitate, wondering, "My dog seems fine for now—should I really start medication?" However, studies show that dogs who begin medication at this stage experience the onset of heart failure symptoms an average of 15 months later than those who do not. If you’ve been told your dog has a heart murmur, be sure to confirm whether it’s Stage B1 or B2 with an echocardiogram.

Stage C: Congestive Heart Failure with Visible Symptoms

Starting in Stage C, symptoms become noticeable to pet owners. Fluid accumulation in the lungs (pulmonary edema) or abdomen (ascites) leads to the following signs: - Coughing: Especially severe, dry coughing at dawn or after exercise - Increased respiratory rate: More than 30 breaths per minute while resting - Exercise intolerance: Refusal to walk, easy fatigue - Syncope: Collapsing after excitement or coughing Treatment involves a combination of diuretics (furosemide), ACE inhibitors, pimobendan, and spironolactone.

Caring for an elderly dog with symptoms of heart failure

Stage D: Refractory End-Stage Heart Failure

Stage D is refractory heart failure where symptoms cannot be controlled with standard medication doses. It may be necessary to increase diuretic doses, use multiple medications together, and provide oxygen therapy or thoracentesis/abdominocentesis (procedures to drain fluid). It is important for owners to measure respiratory rate per minute and record weight changes at home. At this stage, restricting water intake and switching to a low-salt diet are also essential.

Signs That Require Immediate Emergency Care

If any of the following signs appear, do not delay—seek immediate care at an after-hours or emergency animal hospital. - Blue or purple gums (cyanosis) - Reluctance to lie down, breathing only while sitting upright - Respiratory rate exceeding 60 breaths per minute, or abdominal breathing - Failure to regain consciousness after fainting - Coughing with frothy discharge (suspected pulmonary edema) Pulmonary edema is a life-threatening emergency that can be fatal within hours.

Stage-Specific Home Care Points

- Measure resting respiratory rate: Count breaths for one minute while your pet is sleeping, at the same time each day. A rate of 30 or more breaths per minute is a warning sign. - Switch to a low-sodium diet: A veterinary cardiac prescription diet is recommended starting at Stage C. - Maintain a healthy weight: Obesity increases the workload on the heart. - Administer medication at consistent times: Diuretics should be given at the same time each day. - Minimize stress and excitement: Avoid strenuous exercise and exposure to heat. - Regular cardiac ultrasounds: From Stage B2 onward, echocardiograms are recommended every 3 to 6 months. Abruptly stopping heart medication can lead to acute deterioration.

Reviewed by a veterinarian

Dr. Tony — Punnawat Phongkittirak

Veterinarian

A veterinarian who majored in veterinary medicine at Khon Kaen University, Thailand, and completed the IVSA program at North Carolina State University in the United States. Drawing on clinical experience at animal hospitals, he works in the pet healthcare field and is dedicated to building a digital care environment that connects pet parents with veterinarians.

Frequently Asked Questions

What happens if you don’t give medication at stage B2?

While symptoms may not appear immediately, the EPIC study found that dogs not given the medication progress to Stage C an average of 15 months earlier than those that do. This has a significant impact on both lifespan and quality of life.

Does a heart murmur always mean heart failure?

Not at all. A heart murmur is merely a sign of heart disease, not an indicator of its stage. To determine the exact stage, we need to confirm whether the heart is normal in size (B1) or enlarged (B2) through an echocardiogram and X-rays.

How do you measure the respiratory rate during sleep?

Count one breath as the rise and fall of your dog’s chest while they are sleeping peacefully, and measure this for one minute. If their resting respiratory rate consistently exceeds 30 breaths per minute, it may signal the development or worsening of heart failure, so please consult your veterinarian.

Do heart failure medications need to be given for life?

Yes. Medications started at stage B2 should be continued for the rest of the pet’s life. Stopping these medications abruptly can cause suppressed symptoms to flare up rapidly, potentially leading to acute pulmonary edema. Always consult your veterinarian before adjusting the dosage.

When should I start feeding a low-sodium diet?

Generally, we transition to a cardiac prescription diet after Stage C (when symptoms appear). In Stage B2, an extremely low-sodium diet isn't necessary, but it's a good idea to start reducing the salt from treats and human foods at this point.