The heart attempts to compensate for the congestion, or backup, of blood in a number of ways. It beats faster and expands somewhat more than usual as it fills with blood, so that when it contracts, more blood is forced out to the body. In addition, the decreased volume of blood reaching the kidneys causes them to start a hormonal cascade, which causes them to retain sodium and water, resulting in swelling or edema. These efforts help meet the body's demands in the short term, but they ultimately have very harmful long-term effects. Faster beating allows less time for the heart to refill after contraction, so less blood ends up being circulated. The increased effort means the heart muscle needs more oxygen, and if this need isn't met, it can be fatal.
Congestive heart failure is progressive and should not be confused with a heart attack, which involves sudden tissue death of the heart muscle. Although heart failure may occur suddenly in some cases, gradual loss of function is more common. Fatigue, shortness of breath on exertion and increased frequency of nighttime urination develop and worsen over time.
Failure of the left side of the heart (left-sided failure) is most common. It leads to increased pressure in the pulmonary veins in the lungs, which forces fluid into the surrounding microscopic air sacs (alveoli) that transfer oxygen to the bloodstream. As the alveoli fill with fluid, they no longer work properly, which limits the amount of oxygen available to the body and produces the most characteristic symptoms of congestive heart failure: fatigue and shortness of breath. In right-sided failure, the increased pressure in the veins returning blood from the rest of the body combined with the compensatory retention of sodium and water leads to fluid accumulation and swelling in the abdomen, liver and legs. Often, both left-and right-sided heart failure occur together.