General causes of heart failure in dogs and cats

The causes of heart failure in dogs and cats are quite diverse, and it is useful to think of them in terms of general pathophysiologic groups. These groups are myocardial failure, pressure overload, volume overload, and reduced ventricular compliance (impaired filling). The major underlying abnormality for most cases of heart failure falls into one of these pathophysiologic groups, although other pathophysiologic abnormalities often are also present. In advanced failure, abnormalities of both systolic and diastolic function are common.

Myocardial failure is characterized by poor ventricular contractile function; valvular insufficiency may or may not be present. Diseases that cause a volume or flow overload to the heart usually involve a primary ”plumbing” problem (e.g. a leaky valve or abnormal systemic-to-pulmonary connection). Cardiac pump function is often maintained near normal for a prolonged time, but myocardial contractility does eventually deteriorate. Pressure overload results when the ventricle must generate higher than normal systolic pressure to eject blood. Concentric hypertrophy increases ventricular wall thickness and stiffness and predisposes to ischemia. Excessive pressure loads eventually cause myocardial contractility to decline.

Diseases that restrict ventricular filling cause abnormal diastolic function. Contractile ability is usually normal initially, but inadequate filling causes congestion behind one or both ventricles and may diminish cardiac output. Therapy for these cases centers on enhancing ventricular filling.

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