IV. PATHOPHYSIOLOGY
A. Nature’s Defense Mechanisms
When heart failure is caused by any of the diseases or precipitating factors outlined above, the body’s defenses are called upon to assist. Nature always has a way to com¬pensate. Heart failure causes less blood to be ejected from the ventricles. Instead of about 5 liters per minute ejected at rest, the cardiac output can fall to less than 2 liters and not meet the needs of the body.
One compensatory response involves the nervous system and adrenal glands. They are stimulated to produce adrenaline and noradrenaline. Adrenaline constricts arteries and, therefore, increases the resistance in the arteries, which increases blood pressure to allow survival. This increase in resistance is a great load that can be likened to a steep hill against which the left ventricle must pump. The muscle is already very weak, and the increased workload increases heart failure. Imagine a poorly tuned 1934 car trying to climb a long, steep hill. Nature’s com¬pensatory responses are usually useful to the body, but in this case, they are counterproductive. The only way the heart muscle can do the work is to increase the contraction of the muscle and to reduce the resistance in the arteries. Nature unfortunately does not have a built-in answer and increases the resistance in arteries in order to increase blood pressure. The body is programmed to increase blood pressure when the cardiac output and blood pressure fall for any reason. Fortunately, medical scientists, by unraveling these mechanisms, were able to produce a series of drugs in the early 1980s that reduce this resistance. These drugs are called vasodilators (see Section VII).