To help determine severity, physicians use a calculation called an ejection fraction, which is the percent of the blood pumped out during each heart beat. In most cases of heart failure the left side fails, causing systolic dysfunction , in which fluid backs up and accumulates in the lungs. Right-sided heart failure, which is much less common, causes diastolic dysfunction , in which fluid entering the heart backs up, causing the veins in the body and tissues surrounding them to swell. In such cases, the ejection fraction is paradoxically normal or high. Given the interconnected nature between the chambers of the heart, left-sided failure may ultimately precipitate failure in the right side of the heart.
Heart failure can occur in several ways.
The ventricle muscles of the heart may become thin and weakened and dilate to the extent that they cannot pump the blood with enough force to reach all the body's tissues. Mechanisms that the body uses to compensate for inefficient pumping action can, over time, change the architecture of the heart and finally lead to irreversible problems. As a result of these changes, the body's vital organs do not receive enough oxygen and nutrients, and the body's wastes are removed more slowly. Eventually vital systems break down.