An Overview of Cardiac Physiology
In order to understand heart irregularities, it is important to understand how the heart functions normally. The heart is made up of four chambers - a right and left ventricle and a right and left atrium – that act in series to pump blood (and thereby carry oxygen, hormones and other nutrients) throughout the body. These chambers are separated, connected and surrounded by muscle walls and by valves that open and close in a particular manner based on electrical stimulation, pressure changes and other complex factors.
Arteries are the vessels that carry oxygenated blood from the heart to body tissues. Veins are the vessels that return unoxygenated blood to the heart from those tissues. Unoxygenated blood enters the heart on the right side through large veins called the cranial and caudal vena cava. Blood goes first into the right atrium and then into the right ventricle, which are two separate chambers. From there, blood is pumped out through the large pulmonary artery and dispersed throughout the lungs, where the red blood cells pick up a new supply of oxygen and then the blood reenters the left side of the heart (left atrium, then left ventricle) through the pulmonary veins. "Pulmonary" simply means pertaining to the lungs. Finally, blood is forced out of the left ventricle into a large artery called the "aorta." From there, it travels through a complex maze of arteries that get progressively smaller until they become "capillaries" - the tiny blood vessels that are the site of oxygen, gas and nutrient exchange. After delivering its oxygen supply and picking up carbon dioxide and other waste products, the blood returns to the right side of the heart through increasingly larger veins, and ultimately back through the vena cava. When functioning normally, this circuit repeats itself in a predictable and finely-regulated fashion. The heart sounds are caused by the opening and closing of the various valves, and by the relaxation, filling and contraction of the heart chambers.
Definition of Heart Disease
In its most basic sense, heart disease is any condition which causes the heart to function abnormally. In most cases, the effects of heart disease interfere with the heart's normal rhythm and ability to effectively pump blood. Fortunately, treatment for heart disease in cats has come a long way, and many cats live for years after they have been diagnosed with heart disease. Heart disease does not equal heart failure. Once heart failure is confirmed, the prognosis becomes more guarded.
How Heart Disease Affects Cats
Cats with heart disease can be asymptomatic for years. When clinical signs do appear, they most often involve some sort of respiratory distress, the degree of which can vary widely. Cats may have shallow, rapid and labored breathing, panting, exercise intolerance, pain in their legs, a bloated appearance, accumulation of fluid in the chest cavity, and less commonly coughing, vomiting and/or weight loss. Sometimes, lethargy and inappetance may be the only things owners notice. Unfortunately, in other cases, affected cats may die suddenly, with no prior noticeable signs. Stress (travel, new house members, trips to the veterinarian) can exacerbate the effects of heart disease in otherwise compensated cats. The signs of heart disease often do not show up until the condition is in an advanced state. Many cats require lifelong medication to help forestall further deterioration of their heart, which is not always possible.
Causes of Heart Disease in Cats
The causes of heart disease in cats include genetic predisposition, injury or damage to the heart caused by trauma, poisoning or heartworm infection, underlying medical conditions such as hyperthyroidism, and anatomic genetic or congenital abnormalities. Owners who feed home-made diets should be aware that a particular type of feline heart disease, dilated cardiomyopathy, can be caused or greatly exacerbated by a dietary deficiency of taurine, which is a dietary essential amino acid in cats. Breeds that are especially susceptible to heart disease include Maine Coons, Persians, Ragdolls and American Shorthairs.
In rare cases, some cats die from heart disease without ever showing clinical signs of disease. Middle-aged to older cats, typically over 6 years of age, are at an increased risk of heart disease connected with hyperthyroidism.
Cats can develop a number of diverse heart diseases. The most common of these are diseases that involve hypertrophy, or enlargement, of the heart muscle, which is the myocardium. The general term for this in veterinary medicine is cardiomyopathy. Cats can develop hypertrophic cardiomyopathy (HCM), restrictive cardiomyopathy (RCM) and much less frequently dilated cardiomyopathy (DCM). Cats also can suffer from something called arrhythmogenic right ventricular cardiomyopathy, myocarditis and arterial thromboembolism. Some feline heart disease cannot easily be classified. These conditions may be genetic, caused by an anatomic or physical abnormality or be caused by an underlying primary medical condition. Sometimes, the cause is never determined.
Types of Heart Disease in Cats
Hypertrophic Cardiomyopathy (HCM)
Hypertrophic cardiomyopathy occurs when areas of the heart muscle become thickened and noncompliant, which eventually adversely affects the heart's ability to pump blood properly. HCM is the most common heart disease of domestic cats. There is thought to be a genetic basis to HCM in cats. Certain breeds are predisposed, including the Maine Coon, Persian, Ragdoll and American Shorthair; littermates and close relatives of affected cats are also at an increased risk of developing the disease. Many cats develop HCM as a secondary response to some other stress or disease. For example, cats over 6 years of age are predisposed to hyperthyroidism, which puts abnormal demands on heart function and commonly contributes to cardiac complications. Clinical HCM tends to be more common in middle-aged to older male cats but can occur in either gender and at any age.
Restrictive Cardiomyopathy (RCM)
Restrictive cardiomyopathy occurs when the elasticity of the heart muscle is reduced through a process called "fibrosis", which eventually thickens, scars and deforms the heart muscle and chamber walls. The cause of RCM is not well understood but probably is due to a number of things acting together. RCM is usually a disease of middle-aged to older cats.
Dilated Cardiomyopathy (DCM)
Dilated cardiomyopathy involves poor heart contractility with an accompanying thinning of the walls around all heart chambers. In time, the chambers become enlarged/dilated, and the heart is unable circulate the body's required volume of oxygenated blood. While DCM used to be common in companion cats of all ages, with no breed or gender predisposition, it is rarely seen today. In the late 1980s, it was discovered that most commercial cat foods were deficient in taurine, a nutrient essential to the health of cats. This dietary taurine deficiency was closely related with feline DCM. Once the dietary corrections were made, DCM decreased dramatically in domestic cats.
Arrhythmogenic Right Ventricular Cardiomyopathy
Cats can also develop cardiomyopathy that primarily affects the right ventricle, with chamber enlargement and wall thinning similar to that seen more diffusely with DCM.
Mycarditis is inflammation of the heart muscle and adjacent structures. A viral cause has been suggested but not conclusively established. Bacterial causes are reported as well. Acute sudden death is the most common clinical presentation of myocarditis in cats.
Cats are especially prone to arterial thrombosis and embolization that can accompany any of the forms of feline cardiomyopathy. The association is due to diminished or interrupted blood flow through the arteries and veins, possible clotting abnormalities and accompanying clot formation. The signs normally come on very suddenly and are dramatic, involving slight or incomplete paralysis of one or more limbs (paresis), decreased ability to ambulate, pain, vocalization and visible distress, low body temperature, loss of sensation, organ failure and sometimes death, among others. These thromboembolisms tend to lodge in particular parts of the cat's circulatory system.
Veterinarians have a number of tools available to help them diagnose feline heart disease. Many of these are routinely available in a general companion animal veterinary clinic. The more advanced diagnostic procedures generally will require referral to a veterinary cardiologist or possibly to a veterinary teaching hospital.
Diagnosing Heart Disease in Cats
Heart disease in cats is usually diagnosed based upon some combination of a through history, a complete physical examination, a complete blood count and chemistry panel, assessment of systolic blood pressure and possibly thyroid hormone levels, thoracic radiographs, an electrocardiogram and an echocardiogram. These tests are largely non-invasive and not painful to the animal.
Once heart disease has been diagnosed, further tests may be performed to assess whether an underlying condition is causing the disease or if the cat is having concurrent problems with infection or damage to other organs. Skilled veterinary cardiologists can diagnose cardiac disease without a tremendous amount of difficulty.
The proper treatment for feline heart disease depends on the type of condition involved. Generally speaking, the therapeutic options include supportive care, activity restriction, dietary management and selection from a number of medications, each of which has its own pros and cons in managing heart disease in cats. New treatments and treatment protocols are constantly being developed, and a veterinarian is the only one who can assess a given cat and come up with the best therapeutic plan for that patient.
Treating Heart Disease in Cats
Most affected cats will benefit from minimization of stress, oxygen supplementation if they are having respiratory difficulty, and a nice warm, safe, quiet living environment. The treating veterinarian may recommend a sodium-restricted diet and activity restriction as well. Hospitalization is usually necessary at least initially to stabilize cats that have reached the point of having clinical signs of heart disease.
One of the more dangerous consequences of heart disease is accumulation of fluid in and around the lungs, referred to as pulmonary edema, pleural effusion or ascites. Once these conditions develop, cats typically are approaching congestive heart failure. In acute cases where a cat is in severe respiratory distress, a procedure called thoracocentesis can be used to pull fluid out of the thoracic cavity through a needle inserted directly into the chest. A medication known as furosemide is frequently prescribed to cats having extreme difficulty breathing due to this fluid build up, although it should not be used in very high doses long-term as cats seem especially sensitive to developing dehydration and renal complications from this drug. In cases where the symptoms are severe, the medication typically is given intramuscularly rather than intravenously to avoid excess stress to the cat, and then is transitioned to oral administration once the clinical signs are controlled. Application of nitroglycerin ointment topically is also often used to help stabilize critical cats on the theory that it might reduce pleural effusion and pulmonary edema due to its vasodilation properties. Some reports suggest that nitroglycerin may have benefits for long-term management of these conditions in cats.
Vasodilators such as enalapril (which is called an ACE-inhibitor) are available to help conserve the strength of the heart and reduce "wear and tear" by lowering blood pressure through dilation of the blood vessels. Enalapril has also been shown to reduce enlargement of the left side of the heart in cats that are suffering from hypertrophic cardiomyopathy. Beta blockers, particularly atenolol but there are others, are quite effective in lowering heart rate and controlling arrhythmias. Calcium channel blockers such as diltiazem can be prescribed to increase elasticity of the heart, promote vasodilation and enhance ventricular relaxation. Diltiazam is generally less effective than the beta blocker drugs in reducing heart rate, although it has some effect. Diltiazem or atenolol are presently the hallmarks of long-term treatment for cats with HCM. Long-term therapy also usually includes use of an anticoagulant such as aspirin or warfarin to help reduce the serious risk of thromboembolism that accompanies
Feline Heart Disease.
It is critically important for owners to recognize that there are no cookie-cutter approaches to heart disease in cats. Each animal is an individual requiring individualized assessment, treatment, monitoring and follow-up care. Each of the available drugs carries risks and benefits, and there can be severe complications from adverse drug interactions that your veterinarian will want to watch for.