Guide to Heart Disease

The person with heart failure and edema who has been standing or sitting much of the day may experience a strange development known as nocturnal dyspnea (night-shortness of breath) when he lies down in bed at the end of the day. The fluid which has accumulated in the legs and feet is no longer held there by gravity, and the excess fluid gradually returns to the blood stream and overloads the circulation. Pressure rises in the blood vessels and starts to force excess amounts of plasma out of the small blood vessels (capillaries) into the tissue. The lungs, which were previously protected by gravity and their high location in the body when the person was erect, are now affected. The plasma oozes from the capillaries into the tiny air spaces in the lungs and blocks off the gas exchange that normally occurs there. If the person is still awake as this is happening, he may notice a gradually increasing shortness of breath and find that if he raises his head and shoulders higher by propping himself up on pillows, he is more comfortable. If asleep at this time, he may instead suddenly awaken and find himself gasping for breath. The normal reaction in this instance is to sit up or get out of bed and walk around, which again throws gravity into the situation and helps to correct the situation temporarily by drawing the excess fluid back down to the legs. Another medical term for this congestion of the lungs is pulmonary edema (pulmonary = lungs).
Pulmonary edema may occur in another circumstance in heart failure. The heart is basically a double pump. The right side of the heart receives blood from the body via the veins and pumps the blood through the lungs to receive oxygen. The left side of the heart now receives this blood from the lungs and pumps it back to the body. The left side of the heart has more work to do and is therefore equipped with larger muscles which contain the majority of the blood vessels that feed the heart (coronary arteries). In the event of a myocardial infarction, therefore, the left side is usually more severely, or exclusively, injured. The result is that one pump is now weaker than it should be. The right side of the heart will still be able to pump a normal amount of blood into the lungs, but the left side may not be able to accept the total volume and return it to the body. As a result, the blood vessels of the lungs become abnormally engorged, and the increased pressure inside the vessels can force plasma through the walls of the capillaries into the air spaces of the lungs.

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