context: Thrombolysis, also known as thrombolytic therapy, is a treatment to dissolve dangerous clots in blood vessels, improve blood flow, and prevent damage to tissues and organs. Thrombolysis may involve the injection of clot-busting drugs through an intravenous ( IV) line or through a long catheter that delivers drugs directly to the site of the blockage. It also may involve the use of a long catheter with a mechanical device attached to the tip that either removes the clot or physically breaks it up. Thrombolysis is often used as an emergency treatment to dissolve blood clots that form in arteries feeding the heart and brain -- the main cause of heart attacks and ischemic strokes -- and in the arteries of the lungs (acute pulmonary embolism). Thrombolysis is also used to treat blood clots in: Veins that cause deep vein thrombosis ( DVT) or clots in the legs, pelvic area, and upper extremities; if left untreated, pieces of the clot can break off and travel to an artery in the lungs, resulting in an acute pulmonary embolism. Bypass grafts Dialysis catheters If a blood clot is determined to be life threatening, thrombolysis may be an option if initiated as soon as possible -- ideally within one to two hours -- after the onset of symptoms of a heart attack, stroke, or pulmonary embolism (once a diagnosis has been made). The most commonly used clot-busting drugs -- also known as thrombolytic agents -- include: Eminase (anistreplase) Retavase (reteplase) Streptase (streptokinase, kabikinase) t- PA (class of drugs that includes Activase) TNKase (tenecteplase) Abbokinase, Kinlytic (rokinase) Depending on the circumstances, a doctor may choose to inject clot-busting drugs into the access site through a catheter. More often, however, doctors insert a longer catheter into the blood vessel and guide it near the blood clot to deliver medications directly to the clot. During both types of thrombolysis, doctors use radiologic imaging to see if the blood clot is dissolving. If the clot is relatively small, the process may take several hours. But treatment for a severe blockage may be necessary for several days. Doctors also may opt for another type of thrombolysis called mechanical thrombectomy. During this procedure, a long catheter tipped with a tiny suction cup, rotating device, high-speed fluid jet, or ultrasound device is used to physically break up the clot. Although thrombolysis can safely and effectively improve blood flow and relieve or eliminate symptoms in many patients without the need for more invasive surgery, it's not recommended for everyone. Thrombolysis may not be recommended for patients who use blood-thinning medication, herbs, or dietary supplements, or for people with certain conditions associated with an increased risk of bleeding. These conditions include: Severe high blood pressure Active bleeding or severe blood loss Hemorrhagic stroke from bleeding in the brain Severe kidney disease Recent surgery Thrombolysis also may be associated with an increased risk of complications in patients who are pregnant or at an advanced age, and in people with other conditions. Patients who undergo thrombolysis have a small risk of infection (less than one in 1,000) as well as a slight risk of an allergic reaction to the contrast dye that may be needed for imaging. Besides risk of serious internal bleeding, other possible risks include: Bruising or bleeding at the access site Damage to the blood vessel Migration of the blood clot to another part of vascular system Kidney damage in patients with diabetes or other pre-existing kidney disease The most serious possible complication is intracranial bleeding, which is potentially fatal. But this complication is rare. Bleeding in the brain that causes stroke occurs in less than 1% of patients. Although thrombolysis is usually successful, the treatment is not able to dissolve the blood clot in up to 25% of patients. Another 12% of patients subsequently redevelop the clot or blockage in the blood vessel. In addition, thrombolysis alone -- even when successful -- cannot treat tissue that has already been damaged by compromised blood circulation. So, further treatment may be needed to address the underlying causes of the blood clot and repair damaged tissues and organs.