Heart Disease in Dogs
Slightly more than 10% of the animals examined by a veterinarian have some form of cardiovascular disease. Unlike diseases of many other organ systems, cardiovascular diseases generally do not go away but almost always become more serious and may lead to death. In addition, cardiovascular diseases may be more difficult to detect and quantify because the heart cannot be seen and is protected so well by the rib cage.
Heart disease can be defined as any abnormality of the heart. It encompasses a wide range of conditions, including congenital abnormalities (see Congenital and Inherited Disorders of the Cardiovascular System in Dogs) and disorders of physical structure and function. It can be classified by various methods, including whether the disease was present at birth or not (that is, congenital or acquired), causes (for example, infectious or degenerative), duration (for example, long- or short-term), clinical status (for example, left heart failure, right heart failure, or biventricular failure), or by physical structure malformation (for example, ventricular septal defect).
Heart failure is any heart abnormality that results in failure of the heart to pump enough blood to meet the body’s needs. It is not a specific disease; rather, it is a condition in which congestion or an abnormal accumulation of fluid, decreased blood flow to the body, and/or abnormally low blood pressure arise as the final consequence of severe heart disease. Heart disease can be present without ever leading to heart failure. Heart failure, however, can only occur if heart disease is present because it is a consequence of heart disease (Veterinary.heading on page Heart Failure in Dogs).
The following abnormalities of the cardiovascular system can lead to heart disease: 1) the heart valves fail to close or open properly (valvular disease); 2) the heart muscle pumps too weakly or relaxes inadequately (myocardial disease); 3) the heart beats too slowly, too rapidly, or too irregularly (arrhythmia); 4) the blood vessels offer too great an interference to blood flow (vascular disease); 5) there may be openings between chambers of the left side and right side of the heart (cardiac shunts); 6) there is too little or too much blood compared with the ability of the blood vessels to store that blood; and 7) there is parasitism of the cardiovascular system, such as heartworm disease.
Signs associated with any of these diseases are due either to inadequate blood flow through the organs (signs include exercise intolerance, weakness, and fainting) or to blood damming up in organs, which causes fluid to leak from blood vessels into tissues (signs include abnormal accumulation of fluid in the lungs or abdomen). A dog showing signs of having too little blood in the tissues to sustain normal function is said to be in heart failure. A dog showing signs caused by blood damming up in poorly drained organs is said to be in congestive heart failure. When there is not enough oxygen in the blood, the mucous membranes develop a blue tinge, and often there is an increased concentration of red blood cells.
The diseases of greatest importance in dogs, due to the number of cases that exist, are mitral regurgitation due to mitral valve dysplasia (see Congenital and Inherited Disorders of the Cardiovascular System in Dogs : Mitral Valve Dysplasia), dilated cardiomyopathy (see Acquired Heart and Blood Vessel Disorders in Dogs : Dilated Cardiomyopathy), arrhythmic cardiomyopathy in Boxers (see Other Causes of Heart Muscle Failure), and heartworm disease.