Some diseases can narrow the blood vessels in the lungs, which leads to high blood pressure in the lungs (pulmonary hypertension) even when blood pressure in the rest of the body is normal. Pulmonary hypertension is a highly fatal disorder since the strain of pumping blood through narrowed vessels often leads to failure of the heart muscle. When the heart muscle fails, the lungs cannot supply enough oxygen to meet the body's needs. Treatments for this disorder widen the damaged blood vessels in the lung and make it easier for the heart to pump blood through the lungs. One such drug (epoprostenol) is effective only if given intravenously through a small plastic tube that must remain in the vein permanently. Another treatment involves continuous inhalation of a gas (nitric oxide) that also widens blood vessels in the lungs. A newer approach has been an inhaled mist form of epoprostenol called iloprost. Because each mist treatment lasts only about 60 minutes, up to 12 treatments each day are required. Each of these treatments has its drawbacks. Recently, researchers learned that a drug used for erectile dysfunction, sildenafil (Viagra), can enhance and prolong the actions of both iloprost and nitric oxide.