Heart failure may be classified as cardiomyopathy (disease of the heart muscle) or valvular disease (disease of the flow regulating valves in the heart). Sub-categories of cardiomyopathy are Dilatative (larger dogs are over-represented) and Hypertrophic (affecting mostly cats). Mitral valve insufficiency is a problem of mainly small breeds of dogs. The term Congestive Heart Failure (CHF) may be used to describe any disease process of the heart involving reduced blood flow.
The heart is simply a pump. Its function is to keep oxygenated and nutrient-rich blood moving in one direction to reach the organs and tissues. When the heart becomes less capable of doing its job, all systems in the body suffer consequences.
Dilatative Cardiomyopathy (DCM) is the most common type of heart disease affecting dogs. It can occur in any breed, with larger dogs being genetically predisposed. It occurs when the heart muscle loses contractility, or the ability to squeeze blood out of the heart chambers. It can involve either side of the heart, although left-sided DCM is most common. In this case, the left ventricle cannot pump blood out of the heart as quickly as it fills with blood returning from the lungs. The left side of the heart muscle will stretch out and congestion will occur in the lungs as a result of increased pressure. Coughing is the first symptom of left-sided DCM. In right-sided DCM, the blood is backing up in the abdomen instead. The right side of the heart receives spent blood from the organs and returns it to the lungs for re-oxygenation. The increased blood pressure in the abdominal vessels causes water to leak from the vessel walls into the abdominal cavity. Fluid accumulation in the abdomen is called ascites. As more fluid builds up, the diaphragm cannot expand into the abdomen, and respiratory distress will occur with no cough necessarily present. Sometimes DCM affects both sides of the heart simultaneously. On x-ray, the cardiac silhouette will show enlargement. Treatment for DCM of any origin involves removal of excess fluid by using diuretic drugs like furosemide and spironolactone. Improvement of contractility and reduction of blood pressure is gained with the use of ACE inhibitors such as enalapril and benazepril and vasodilators like amlodipine and diltiazem. A newer drug called pimobendan is used in combination with diuretics and blood pressure medications to enhance blood delivery to the organs. Taurine deficiency causes dietary related DCM especially in cats who cannot produce this amino acid on their own. Taurine is added to all commercially prepared dog and cat foods.
Hypertrophic Cardiomyopathy (HCM) is a heart disease affecting mainly cats. It is very rare in dogs. In this form of heart failure, the heart muscle becomes hypertrophied or thickened. It becomes less efficient at pumping blood, and the heart chamber decreases in size. Blood slows as it passes through the ventricles leading to the possibility of the formation of blood clots. Eventually, a heart murmur (turbulence and regurgitation of blood) can be heard with a stethoscope, but not until the disease has progressed to a significant degree. The murmur may also disappear, making this an unreliable method for diagnosing HCM. On x-ray, the overall cardiac silhouette may appear to be a normal size as well. Symptoms of HCM are respiratory distress caused by fluid build up in the lungs, though cats rarely cough. A blood clot may dislodge from the heart and wind up at a narrowing part of the arteries. The condition is called thrombus (clot). It can occur on the left side of the heart and cause pulmonary embolism in the lungs; or, on the right side of the heart the clot moves to the hind-limbs causing saddle-thrombus. Complete or partial paralysis and anoxia occur in the rear legs, leaving them cold and lifeless. The prognosis of a thrombus is poor or guarded at best. Unfortunately, sudden death is the first “symptom” of many cats with HCM. If caught early, HCM is treated with blood thinners and blood pressure medications. Maine Coon cats have a genetic predisposition for HCM.
Mitral valve insufficiency occurs in small breed dogs primarily. For some reason in these animals, the mitral valve that controls the direction of blood flow becomes thickened and less flexible. A significant murmur can be heard with a stethoscope on examination. The murmur is produced by regurgitation of blood back into the heart chamber. The valve should act as a one-way gate, but in this type of heart disease it is not effective. As a result of the back up in blood flow, left side heart enlargement occurs as in DCM, and coughing results from pulmonary congestion. Less often, the valve disease involves the tricuspid valve. In this case, the heart failure involves the right side of the heart, and symptoms reflect those related to right sided DCM including abdominal ascites. Treatment includes the medications described to control the effects of DCM. Surgical repair or replacement of the affected valve is very risky and cost-prohibitive at this point in time.
The overall prognosis for CHF depends on the mechanism and progression of the disease. Treatment for symptoms and consequences of CHF can greatly improve quality of life as well as extend life-expectancy in most cases.