Heart failure is the result of poor cardiac function and is reflected by a decreased volume of blood pumped out by the heart, called cardiac output. Heart failure can be caused by weakness of the heart muscle, which pumps blood out through the arteries to the entire body, or by dysfunction of the heart valves, which regulate the flow of blood between the chambers of the heart. The diminished volume of blood pumped out by the heart (decreased cardiac output) is responsible for a decreased flow of blood to the kidneys. As a result, the kidneys sense that there is a reduction of the blood volume in the body. To counter the seeming loss of fluid, the kidneys retain salt and water. In this instance, the kidneys are fooled into thinking that the body needs to retain more fluid volume when, in fact, the body already is holding too much fluid.
This fluid increase ultimately results in the buildup of fluid within the lungs, which causes shortness of breath. Because of the decreased volume of blood pumped out by the heart (decreased cardiac output), the volume of blood in the arteries is also decreased, despite the actual increase in the body's total fluid volume. An associated increase in the amount of fluid in the blood vessels of the lungs causes shortness of breath because the excess fluid from the lungs' blood vessels leaks into the airspaces (alveoli) and interstitium in the lungs. This accumulation of fluid in the lung is called pulmonary edema. At the same time, accumulation of fluid in the legs causes pitting edema. This edema occurs because the build-up of blood in the veins of the legs causes leakage of fluid from the legs' capillaries (tiny blood vessels) into the interstitial spaces.
An understanding of how the heart and lungs interact will help you to better comprehend how fluid retention works in heart failure. The heart has four chambers; an auricle and a ventricle on the left side of the heart and an auricle and ventricle on the right. The left auricle receives oxygenated blood from the lungs and transfers it to the left ventricle, which then pumps it through the arteries to the entire body. The blood then is transported back to the heart by veins into the right auricle and transferred to the right ventricle, which then pumps it to the lungs for re-oxygenation.
Left-sided heart failure, which is due primarily to a weak left ventricle, usually is caused by coronary artery disease, hypertension, or disease of the heart valves. Typically, when these patients initially come to the doctor they are troubled by shortness of breath with exertion and when lying down at night (orthopnea). These symptoms are due to pulmonary edema that is caused by pooling of the blood in the vessels of the lungs.
In contrast, right-sided heart failure, which often is due to chronic lung diseases such as emphysema, initially causes salt retention and edema. Persistent salt retention in these patients, however, may lead to an expanded blood volume in the blood vessels, thereby causing fluid accumulation in the lungs (pulmonary congestion) and shortness of breath.
In patients with heart failure due to weak heart muscle (cardiomyopathy), both the right and left ventricles of the heart are usually affected. These patients, therefore, can initially suffer from swelling both in the lungs (pulmonary edema) and in the legs and feet (peripheral edema). The physician examining a patient who has congestive heart failure with fluid retention looks for certain signs. These include:
- pitting edema of the legs and feet,
- rales in the lungs (moist crackle sounds from the excess fluid that can be heard with a stethoscope),
- a gallop rhythm (three heart sounds instead of the normal two due to muscle weakness), and
- distended neck veins. The distended neck veins reflect the accumulation of blood in the veins that are returning blood to the heart.
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