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Heart Disease and Congestive Heart Failure

Written By:

Nick A. Schroeder, DVM, DACVIM

Congestive heart failure is a clinical diagnosis – one that is made by the veterinarian taking into account a pet’s history and physical examination findings in addition to the results of testing, which may include chest x-rays, echocardiography (ultrasound of the heart), electrocardiography (EKG, ECG), blood pressure, blood work , etc. Heart failure happens because patients have heart disease, however just because a pet has heart disease, doesn’t automatically mean that it has heart failure. Many pets have mild heart disease and can live for years only to succumb to some other issue. Some pets with more serious heart disease however can develop heart failure. Heart failure occurs when the heart cannot do its job for whatever reason.

The heart anatomically is divided into two sides – the right and left. The right side of the heart takes deoxygenated blood coming back from the body and delivers it into the lungs. This is a relatively low-pressure system, so the right side of the heart is smaller and has a thinner muscle than the left side. The blood comes in from the great veins (the vena cavae) and enters the right atrium. From there it goes into the right ventricle (the pumping chamber) across the tricuspid valve. From the right ventricle, blood is pumped across the pulmonic valve and into the pulmonary arteries, which go into the lungs. The left side of the heart takes oxygenated blood coming in from the lungs and pumps it out to the rest of the body. This is a high-pressure system and the left side of the heart is normally larger with a thick muscle wall. The blood comes in from the pulmonary veins in the lungs and enters the left atrium. From there, the blood goes across the mitral valve into the left ventricle (the pumping chamber). Blood then travels across the aortic valve and goes out the arteries to the rest of the body. As long as the valves are all competent (i.e. they don’t leak), all the openings/outflow tracts are patent (wide open), the muscle pump function is adequate and the heart can fill properly, then the heart can do its job normally. If disease affects any or all of these functions, then problems can arise.

Heart disease in dogs and cats may lead to heart failure. Dogs most commonly develop age-related chronic mitral valvular disease (CMVDz, myxomatous valvular degeneration/MVD, endocardiosis – all equivalent terms), though they may occasionally be diagnosed with dilated cardiomyopathy (DCM). Cats are most often diagnosed with hypertrophic cardiomyopathy (HCM), and may occasionally be diagnosed with other conditions (restrictive or unclassified cardiomyopathy). Less often, congenital cardiac defects or pericardial disease may lead to heart failure. Fortunately heart attacks (myocardial infarctions) are essentially unheard of in dogs. Cats may have chronic myocardial infarctions from tiny clots, but rarely have associated chest pain. A heart attack in humans often occurs secondary to clot formation from a ruptured cholesterol plaque inside one of the small (“coronary”) arteries supplying blood to the heart muscle itself. The resulting obstruction to blood flow (ischemia) causes chest pain, heart muscle dysfunction potentially leading to heart failure, and occurs secondary to underlying coronary atherosclerosis – a disease which doesn’t occur in dogs and cats. The relative risk for heart failure is broadly determined by the degree of heart chamber enlargement as well as muscle pump function. On a very basic level, patients with no to mild heart chamber enlargement are considered to be at a low risk for heart failure while those with moderate to severe heart chamber enlargement are at a high risk, though there are exceptions.

Heart failure in dogs and cats most often is diagnosed as congestive heart failure(CHF). What this means is that congestion in the tissues occurs behind the side of the heart that is failing and fluid accumulation develops. Congestive heart failure is therefore also referred to as backward failure, resulting from elevated filling pressures. Hormonal activation occurs in patients with serious heart disease, and the body retains salt and water from the urine, leading to an overall increase in total blood volume. Initially, this improves heart function, but can eventually overload the heart. Most commonly, the left side of the heart is the one that fails. If the left side of the heart cannot do its job, the blood pressure backs up behind it in the lungs. The pulmonary veins become engorged and fluid starts to accumulate in the lungs, leading to coughing and difficulty breathing. This fluid is called pulmonary edema which can absolutely be life-threatening as the patient literally drowns. If the right side of the heart cannot do its job, the blood backs up behind it in the veins of the body. The liver and abdominal organs become congested and fluid can start to accumulate in the body cavities. Most of the time, free fluid builds up in the abdomen, which is referred to as ascites. If too much ascites develops, it puts pressure on the diaphragm, making it difficult for patients to breathe comfortably. Occasionally, free fluid termed pleural effusion builds up in the chest cavity around the lungs. If too much pleural effusion develops, the lungs cannot expand normally, leading to difficulty breathing. Rarely, patients may develop free fluid directly around the heart within the pericardial sac (pericardial effusion), or under the skin (subcutaneous edema) from right-sided congestive heart failure.

Rarely dogs or cats may be diagnosed with forward failure. What happens in these patients is that the heart muscle cannot generate a high enough blood pressure to maintain adequate perfusion of the body tissues. The systemic blood pressure is low (hypotension), and most patients as a result are weak and/or collapsed. These pets typically have a low body temperature (hypothermia), a low heart rate (bradycardia) and may or may not have concurrent fluid retention from backward failure/CHF. Patients with forward failure typically have a poor prognosis.

Heart disease in dogs and cats may be categorized into one of a number of different systems. The American College of Veterinary Internal Medicine (ACVIM) established a consensus statement in 2009 specifically regarding dogs with chronic mitral valvular disease. This system may be applied to dogs with other types of acquired heart disease as well as in cats.

Stage A: Asymptomatic patients with no evidence of structural heart disease on testing but is considered to be at a relatively high risk for developing heart disease over time based on breed, genetic factors, familial history, etc. These patients do not need medical therapy.

Stage B: Asymptomatic patients with evidence of structural heart disease on testing.

B1: No sign of heart enlargement (relatively low risk for heart failure). These patients typically do not need medical therapy.

B2: Heart enlargement present (increased risk for heart failure). These patients may benefit from medical therapy.

Stage C: Patients with evidence of structural heart disease with heart enlargement and symptoms of congestive heart failure. These patients need medical therapy.

Stage D: Patients with evidence of structural heart disease, generally quite severe heart enlargement and symptoms of congestive heart failure refractory to current medical management (end-stage disease).

Dogs and cats diagnosed with heart failure may be successfully managed over relatively long periods of time. The goals of medical management are to control fluid retention and blood pressure, as well as to improve muscle pump function and control the heart rate in certain circumstances. We want our pets with heart failure to have a good quality of life while on medication. Consultation with a veterinary cardiologist may be recommended for patients with heart disease and/or congestive heart failure.