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.