Accumulation of fluid in the lung tissues causing disturbance of the gas exchange that may lead to respiratory failure. It is caused by direct injury to the lung parenchyma or congestive heart failure. The symptoms may appear suddenly or gradually. Suddenly appearing symptoms include difficulty breathing, feeling of suffocation, and coughing associated with frothy sputum. Gradually appearing symptoms include difficulty breathing while lying in bed, shortness of breath during activity, and weight gain (in patients with congestive heart failure). [ NCIT:P378 ]