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Defining heart disease and heart failure
Heart disease is said to exist if any finding related to the cardiovascular system is outside the accepted limits of normality (eg, the finding of a murmur or dysrhythmia on physical examination or the identification of an electrocardiogram abnormality), which may or may not progress to heart failure.
There are many proposed definitions of heart failure, but none is universally accepted. In general, it is the pathophysiological state that results from the inability of the heart to deliver enough blood to the peripheral tissues to meet metabolic demands. Most forms of heart failure are associated with a low resting cardiac output, or at least an inability to elevate cardiac output during exercise, often despite elevated filling pressures.
Heart failure usually develops slowly. The most common causes of heart disease and heart failure in dogs are acquired during life and result in a slowly progressive deterioration of cardiac function during which the body has time to compensate for the disease. In the majority of patients that develop heart failure, reduced forward flow leads to a perceived fall in blood pressure and activation of the compensatory responses. The response is similar whatever the primary cause.
Compensatory mechanisms activated in heart failure
The heart itself is capable of responding directly to changes in blood flow and venous return. The myocardium has an intrinsic ability to adjust the force of ventricular contraction to accommodate changes in venous return, ensuring that the amount of blood pumped out of the ventricle in systole is the same as that entering it during diastole. Up to a certain limit, the greater the volume of blood in the ventricles when the heart has finished filling, the more vigorous the ventricular contraction and the greater the volume of blood ejected into the circulation during systole. This balance of input and output is known as the Frank–Starling …
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