Heart disease is defined as any functional, structural, or electrical abnormality of the heart. It encompasses a wide range of abnormalities, including congenital abnormalities (see Congenital and Inherited Anomalies of the Cardiovascular System) as well as anatomic and physiologic disorders of varying causes. Heart disease can be classified by various characteristics, including whether the disease was present at birth or not (eg, congenital or acquired), cause (eg, infectious, degenerative, genetic or heritable), duration (eg, chronic or acute), clinical status (eg, left heart failure, right heart failure, or biventricular failure), by anatomic malformation (eg, ventricular septal defect), or by electrical disturbance (eg, atrial fibrillation, ventricular premature complexes).
Heart failure is usually the end result of severe, overwhelming systolic and/or diastolic cardiac dysfunction that most commonly results in the formation of edema or effusion (eg, pulmonary edema, pleural effusion, ascites) severe enough to cause clinical signs (eg, tachypnea, dyspnea, abdominal distension). It may also produce signs referable to poor perfusion (low cardiac output). This most commonly happens in late-stage chronic heart failure; rarely, it may be due to acute heart failure (eg, ruptured chorda tendineae). The most common abnormalities in systole that lead to heart failure are decreased contractility (eg, dilated cardiomyopathy), severe valvular regurgitation (eg, mitral regurgitation), and left-to-right shunting cardiac defects (eg, ventricular septal defect, patent ductus arteriosus). Myocardial fibrosis is the most common abnormality that causes severe diastolic dysfunction, as seen in cats with hypertrophic cardiomyopathy. Heart disease can be present without ever leading to heart failure. Heart failure, however, can occur only if severe heart disease is present.