Heart attacks (myocardial infarction, or MI) occur when the arteries supplying blood to the heart become clogged or narrowed, cutting off oxygen and causing the heart muscle to die. In 90 percent of patients, clots form in the injured artery within four to six hours of an MI. Opening the artery as quickly as possible is the best way to save lives. If available, angioplasty to open clogged blood vessels is the first line of therapy. In the procedure, a thin, balloon-tipped tube called a catheter is inserted into the artery under x-ray guidance. The balloon is inflated, compressing the clot or other substances blocking the artery and enlarging the inner diameter of the blood vessel. Angioplasty is frequently performed in conjunction with placing a stent, or metal support to hold the vessel open. Coronary artery bypass grafting (CABG) is a surgical procedure in which the blockage in the artery is bypassed with a blood vessel taken from elsewhere in the body. In rare cases, a manmade graft is used.
Angioplasty or bypass surgery have been shown to improve survival compared to clot-busting drugs after a heart attack, but only in hospitals where many of the procedures are done. Studies show the survival rates for drug treatment compared to procedures to open clogged vessels are the same at hospitals with less experience in angioplasty and/or bypass surgery.
Thrombolytic Drugs
Thrombolytic, or clot-busting, drugs are medications that are used in the early treatment of heart attacks, especially when angioplasty is not an option. These drugs dissolve the clot, or thrombus, responsible for causing the blocked artery. The earlier angioplasty is performed or thrombolytic drugs are administered (ideally within three hours of the heart attack), the better the chance of survival. Some studies have shown, however, that these drugs may not help - and may possibly harm -elderly patients over age 75, depending on their medical condition.
Currently, the standard thrombolytic drug is tissue-type plasminogen activator (tPA), also called alteplase (Activase®). According to one study, it was the most effective in restoring blood flow to the heart, followed by reteplase (Retavase®), urokinase (Abbokinase®), and finally streptokinase (Kabikinase®, Streptase®).
Other thrombolytics include:
- Lanoteplase®
- Anistreplase®
- Tenectaplase® is the newest drug of this class. It can be delivered more rapidly than tPA, but as yet it has not been shown to improve survival after a heart attack.
TPA and reteplase are thought to be best for patients who begin treatment more than three hours after symptoms start (but thrombolytic drugs and angioplasty are not effective after approximately 12 hours have passed).
Thrombolytic therapy improves survival after a heart attack. The medications must be started within a few hours after the attack.
Thrombolytic Regimens A thrombolytic agent is typically administered with intravenous heparin, an anticoagulant. (Heparin, like aspirin, does not break up existing blood clots, but it can prevent clots from reforming.)
The older form of heparin, called unfractionated heparin, is usually administered continuously through an intravenous (IV) tube into a vein, and frequent blood tests are needed to be certain the blood is not too "thin" (or too thick) - which increases the risk of hemorrhage (uncontrolled bleeding).
Newer heparin preparations, called low molecular weight heparins, can be injected under the skin - usually in the abdomen - and there is less risk of unwanted bleeding . Blood tests to monitor their effect are not necessary. Studies also suggest that some low molecular weight heparins may be superior to older medications in preventing recurrent heart attack or death. Low molecular weight heparin preparations include:
- Enoxaparin (Lovenox®)
- Dalteparin (Fragmin®)
Not all heart attacks can be treated with clot-busting drugs or anticoagulants, such as in patients who already are at high risk of bleeding. An electrocardiogram (ECG) of the heart's electrical activity can help determine which treatments are best.
Hemorrhage (uncontrolled bleeding) in the brain (a type of stroke) is the most serious complication of thrombolytic therapy, but it occurs rarely (in approximately 3 out of every 1,000 patients). Many more heart attack patients would die without clot-busting drugs (39 out of 1,000 patients). And evidence suggests that thrombolytic therapy, particularly in patients who take aspirin regularly, reduces the risk of a second heart attack or a stroke. Streptokinase given without heparin is least likely to cause hemorrhage, but it is not as effective as some other regimens in restoring blood flow to the heart.
Medications after a Heart Attack According to a recent study of America's hospitals, patients with the highest survival rates after heart attacks were more likely to receive aspirin and beta-blockers. Although their protective value and low cost has been proven in many studies, beta-blockers are not prescribed as often as they should be. ACE Inhibitors, and statins are also beneficial for the long-term after a heart attack. They frequently are prescribed in combination with aspirin and beta-blockers.
