Congenital and Inherited Disorders of the Cardiovascular System of Cats

The ductus arteriosus is a short, broad vessel in the unborn kitten that connects the pulmonary artery with the aorta and allows most of the blood to flow directly from the right ventricle to the aorta. Before birth, oxygenated blood within the main pulmonary artery passes into the descending aorta through the ductus arteriosus, bypassing the nonfunctional lungs. At birth, inflation of the lungs with the kitten’s first breath causes the ductus to close and allows the blood to flow to the lungs.

If the ductus does not close, the blood flow is forced from chambers of the left side of the heart to those of the right side (left-to-right shunting), resulting in many problems with circulation of blood and development of abnormal heart rhythms (arrhythmias) and enlarged chambers of the heart. Over time, heart failure develops in untreated animals. Surgery is successful when left-to-right shunting is present, but the outcome is poor when right-to-left shunting occurs.

The aorta is the large artery that carries blood away from the heart. Obstruction (stenosis) of the aorta is an abnormality that can affect the flow of blood. Emptying of the left ventricle may be obstructed at 3 locations: subvalvular, also called subaortic, consisting of a fibrous ridge of tissue within the outflow tract of the left ventricle; valvular (the valve itself); and supravalvular (obstruction past the aortic valve). Treatment options include medication to reduce signs of heart failure; surgery may also help but is costly and may be risky for some pets.

Pulmonic stenosis causes an obstruction to the blood flow from the right ventricle. In most cases, the obstruction is due to abnormal development of the flaps on the valves between the pulmonary artery and the right ventricle. The stenosis can also occur in the opening, the subvalvular region (within the outflow tract of the right ventricle), or in the area beyond the pulmonary valve. Signs include heart failure (including fluid accumulation in the abdomen or limbs), weakness, and a failure to thrive. Surgery may help in some cases; medications to improve signs of heart failure can help lessen signs.

The septa (plural of septum) are the membranes that divide the chambers of the heart. In an unborn kitten, there is an opening in this membrane between the atria that allows shunting of blood from the right to the left side in order to bypass the nonfunctional lungs. At birth, the opening closes and shunting stops. However, increased right atrial pressure may reopen the flap where the membranes have not sealed and allow shunting to resume. This is not a true atrial septal defect because the membranes have formed normally. A true atrial septal defect is a consistent opening of the membranes, which allows blood to shunt from the atrium with the greater pressure. Some of these defects cause few problems for the affected animal, while others lead to heart failure and other associated problems.

Ventricular septal defects (openings in the membrane separating the ventricles) vary in size and effects on blood circulation. These defects may occur with other abnormalities of the heart present at birth. Treatment depends on the severity of the signs and the direction of the shunting of blood.

Tetralogy of Fallot is a defect that produces a bluish tinge to skin and membranes because there is not enough oxygen in the blood. It is caused by a combination of defects: pulmonic stenosis (see Congenital and Inherited Disorders of the Cardiovascular System of Cats : Pulmonic Stenosis), a typically high and large ventricular septal defect (see Congenital and Inherited Disorders of the Cardiovascular System of Cats : Atrial and Ventricular Septal Defects), thickening of the muscle fibers of the right ventricle, and varying degrees of the aorta rotating to the right. The effect of these abnormalities depends on their severity and size. In addition to the bluish tinge of the skin and mucous membranes, signs may include fatigue, shortness of breath, stunted growth, and seizures. Certain medications may be prescribed to help alleviate signs.

Defects of the mitral and tricuspid valves occur more commonly in cats than in many other species. Mitral valve dysplasia refers to abnormal development or malformation of the mitral valve of the heart, allowing blood to flow back into the left atrium (regurgitation). Any component of the mitral valve (valve flaps, the structures that anchor the flaps, or the small muscular columns that attach the flaps to the ventricle) may be malformed, and often more than one component is defective. If the defect is severe, signs of heart failure (such as coughing and difficulty breathing) and arrhythmias may develop. Although these signs can be treated, the outlook is poor in most cases.

Tricuspid valve dysplasia (abnormal development) is seen occasionally in newborn kittens. This prevents the tricuspid valve from performing adequately and leads to blood being regurgitated back into the right atrium. Other defects of the heart may also be noted in affected cats.

Signs of right-sided congestive heart failure, such as accumulation of fluid in the abdomen and lungs, may be seen. A loud heart murmur is very noticeable. Arrhythmias, especially the sudden onset of a very high heart rate, are common and may cause death. Electrocardiography and x-rays may show enlargement of the right ventricle and atrium, while the malformed tricuspid valve can sometimes be seen using echocardiography (ultrasonography).

The outlook for cats with these signs is guarded. Periodic draining of fluid from the abdomen may be needed. Medications such as diuretics, vasodilators, and digoxin may also be prescribed.

Endocardial fibroelastosis is a congenital defect that is seen most commonly in Siamese and Burmese cats. The wall of the left ventricle is enlarged, and the membrane that surrounds and lines the heart becomes thickened and fibrous. Signs, such as difficulty breathing, usually appear when affected kittens are from 1 to 4 months old. The aortic valve may also be affected. Treatment is rarely successful and the outlook is poor.

Cor triatriatum sinister results from a fibrous membrane dividing the left atrium and has been reported in cats. (“Sinister” means left.) The affected atrium is divided into 2 chambers. There are commonly one or more perforations in the separating membrane, allowing communication between the 2 portions of the atrium. Surgery can be performed to correct this disease.