Common Canine Cardiac Diseases
Chronic valve disease (CVD)
The most common acquired cardiac disease in dogs is CVD. The initial problem with this disease is a thickening of the heart valves, most commonly the mitral valve, although other valves can be affected. When the valves thicken, they become irregularly shaped and do not close normally. This allows blood to leak backwards across the valve, often detected as a murmur by your dog’s veterinarian. When blood leaks across the mitral valve, blood is being pushed from the left ventricle (high pressure chamber) to the left atrium (low pressure chamber) and to the aorta (high pressure chamber) rather than all of the blood being pushed to the aorta. Because the left atrium is a lower pressure chamber, blood follows the path of least resistance and preferentially is shunted backwards rather than forwards. This sets up a volume overload to the left heart, and eventual heart enlargement, especially the left atrium and left ventricle. Over time, the body enacts certain neurohormonal systems to also retain fluid, which is a maladaptive response and causes more problems for the heart.
Ultimately, this volume overload and heart enlargement can lead to the development of congestive heart failure, where fluid backs up into the lungs (pulmonary edema) or the body cavities (effusion).
In addition, dogs with enlarged left hearts can develop coughing from the mainstem bronchi (airways) being compressed. They can develop an arrhythmia called atrial fibrillation, and in rare cases, can have a split left atrium. Some dogs with severely enlarged left atria can develop pulmonary hypertension, or high blood pressure in the lung vessels.
In humans, the treatment of choice for this disease is surgical valve repair or replacement. Unfortunately in dogs, these treatments have been largely unsuccessful and the surgery alone for the main patient population of < 10 kg sized dogs has about a 50% reported mortality rate. Thus, we are left to medically manage the clinical signs associated with CVD once the disease becomes apparent enough to cause noticeable problems such as coughing, labored breathing, fainting.
If your dog has a diagnosis of CVD, it will not necessarily go on to develop any clinical signs. Some dogs have a very slowly progressing form of the disease and ultimately never have a problem. However some dogs have a rapidly progressing form, and it is often difficult to know which dogs are which at the initial visit. Close monitoring of attitude, activity level, appetite and breathing are important in dogs diagnosed with CVD so that early evaluation and treatment can be initiated should clinical signs develop. As long as your dog remains symptom free, then often semiannual to annual checkups are appropriate.
Dilated cardiomyopathy (DCM)
Dilated cardiomyopathy is a disease of the heart muscle, where the heart is not able to pump normally, and over time the heart chambers become markedly enlarged and the ventricles are poorly contracting. The result is altered geometry and mechanics of the heart. This leads to a decreased amount of blood being ejected with each beat, and this leads to low blood pressure. The body detects this and begins to enact several neurohormonal pathways that ultimately accelerate the progression of the disease. These include increased epinephrine and norepinephrine (adrenaline) to increase the heart rate, activation of the renin angiotensin aldosterone system that acts to retain fluid, constrict blood vessels, and cause heart muscle fibrosis. These pathways help lead to the syndrome of congestive heart failure.
Both dogs and cats can develop DCM. In cats it has been associated with dietary taurine deficiency, however since commercial diets have been supplemented with taurine, this disease is rare in the feline population. In dogs, it is typically a disease of larger and giant breed dogs, including Boxers, Newfoundlands, Great Danes, Dobermans, Wolfhounds and Deerhounds, and some Cocker Spaniels. Dobermans & Boxers have a form of the disease that is often rapidly progressing and has a much higher risk of sudden death from arrhythmias.
Sequelae of dilated cardiomyopathy include arrhythmias, especially atrial fibrillation and ventricular tachycardia, congestive heart failure, and forward heart failure.
Treatment of DCM is based on clinical signs, but will often include several medications.
If your pet has been diagnosed with DCM, it may need to be on medication for the remainder of its life. Typically this disease is progressive, which requires frequent rechecks and vigilant monitoring of respiration and attitude at home. One good way to help stay on top of things is to count the sleeping respiratory rate at home. The rise and fall of the chest wall is one breath. Normal in dogs and cats is < 30 breaths/minute. Establish a baseline for your pet, and if you notice the respiratory rate increases persistently over several days this may be an indication that fluid is building up in the lungs.