Causes of Dilated Cardiomyopathy

Dilated cardiomyopathy can be thought of as a compensatory reaction of the heart to a weakened heart muscle. The cause of the weakening may be unknown or it may be associated with many different risk factors including viral infections, exposure to toxins and drugs, or coronary artery disease, as in ischemic cardiomyopathy.

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In people with dilated cardiomyopathy, the heart enlarges to compensate for the inability of a weakened muscle to eject the amount of blood needed by the body. Stretching helps compensate for the weakened muscle in two ways. First, the overall volume of the heart is increased, thereby increasing the volume of blood ejected into the circulatory system. This action is analogous to a person squeezing a flexible plastic bottle. A strong person might be able to squirt out 50 percent of the contents of the bottle with each squeeze. If the person's grip is weakened and he is only able to eject 25 percent of the contents, only half as much liquid will exit the bottle. However, if the person had a bottle that holds twice as much, 25 percent of that bottle would be twice as much volume. In dilated cardiomyopathy, the heart is acting like the plastic squeeze bottle, dilating its chambers to try to maintain the amount of blood pumped to the tissues of the body by a heart whose ejection fraction is diminishing.

Stretching the heart muscle can also increase the strength of the contraction. Starling's Law, a basic principle involving the function of the heart, describes this phenomenon: the more the heart muscle stretches, the greater the force of its squeeze. The heart functions much like a spring whose coils are overstretched and then released to snap back into shape. If this action continues repeatedly, the heart will eventually lose the ability to regain its original shape, like an overstretched spring.

The heart can also compensate by beating faster. Although beating faster can improve circulation temporarily--normal hearts do this during exercise--a chronic increase in heart rate (tachycardia) can lead to dilated cardiomyopathy or serious arrhythmias.

The kidneys may also try to compensate for the decreased circulation by increasing the volume of blood and fluid in the circulatory system. Ironically, this compensatory mechanism exacerbates the situation by causing swelling in the legs and abdomen, and fluid accumulation in the lungs.

All of these compensatory mechanisms--dilating, stretching, beating faster, retaining fluid--evolved to keep the blood pumping to the body on a short-term basis following an injury or dehydration. In the long term, though, the heart responds like a spring that's been overextended; eventually, it can no longer return to its shape and function effectively. Then the dilated heart becomes more of a dam than a pump, with blood pooling inside an enlarged heart that is no longer capable of circulating blood. The most effective treatments for dilated cardiomyopathy focus on blocking these compensatory responses.