The heart is described as failing when the cardiac output is unable to maintain the circulation of sufficient blood to meet the needs of the body. In mild cases,cardiac output is adequate at rest and becomes inadequate only when increased cardiac output is required,eg.in exercise. Heart failure may affect either side of the heart,but since both sides of the heart are part of one circuit,when one half of the pump begins to fail if frequently leads to increased strain on,and eventual failure of,the other side. The main clinical manifestations depend on which side of the heart is most affected. Left ventricular failure is more common than right,because of the greater workload of the left ventricle.
COMPENSATORY MECHANISMS IN HEART FAILURE.
When heart failure happens acutely,the body has little time to make compensatory changes,but if the heart fails over a period of time the following changes are likely to occur in an attempt to maintain cardiac output and tissue perfusion,especially of vital organs:
- The cardiac muscle fibres enlarge and increases in number,which makes the walls of the chambers thicker.
- The heart chambers enlarge.
- Decreased renal blood flow activates the renin-angiotensin-aldoaterone system,which leads to salt and water retention. This increases blood volume and cardiac workload. The direct vasoconstrictor action of angiotensin 2 increases peripheral resistance and puts further strain on the failing heart.
