The American Heart Association 2006 statistical data indicates 8.5 million Americans suffered a heart attack in that year. Annually, heart disease causes one-third of all deaths in the United States, according to the 2006 data from the AHA. Physicians use an electrocardiogram to diagnose a heart attack, or myocardial infarction (MI). An EKG will be abnormal if the heart muscle has been damaged.
Causes
An MI occurs from a thrombus, or clot, blocking one or more arteries in the heart. A clot forms in the artery due to the damaging effects of atherosclerosis. In atherosclerosis, plaques of fat and cholesterol form on the inside of arteries. The plaques become unstable and rupture. When a plaque ruptures, the site on the artery where it occurs becomes inflamed and bleeds. The body's normal response is to create a thrombus to protect the area. The clot obstructs blood flow to the area of the heart, which that particular artery fed.
Location
Kathleen Ouimet Perrin, Ph.D., author of "Understanding the Essentials of Critical Care Nursing," explains that the left ventricle of the heart experiences the most MIs because it has the greatest oxygen demand. The location of a heart attack is based on which wall of the left ventricle the blocked artery is located. The anterior, lateral, septal, inferior and posterior walls make up the left ventricle. The left anterior descending artery (LAD) and the circumflex artery feed the left ventricle. The LAD supplies oxygen to the greatest part of the left ventricle and causes significant muscle damage.
Types
The abnormalities present on an electrocardiogram (EKG) after an MI show an elevated ST-segment or a depressed ST-segment. Sometimes referred to as Q-wave or non-Q wave changes, these terms indicate whether the MI affected the inner lining of the left ventricle wall, called a subendocardial infarction, or the entire thickness of the wall, called a transmural infarction. A transmural infarction causes the most damage.
Effects
A thrombus that partially blocks an artery and decreases blood flow to a section of the heart will appear as a downward deflected or inverted T-wave--T-wave depression indicates ischemia, which is tissue injury from inadequate oxygen supply. A completely blocked artery supplies no oxygen and the heart muscle begins to die. The ST-segment on the EKG will be elevated a couple of millimeters or more above the baseline. Reciprocal EKG changes will affect the areas of the heart opposite the MI. Dead heart muscle is reflected by a Q-wave or downward deflection on the EKG that occurs before the QRS complex. The QRS complex represents the contraction of the left ventricle and normally is a positive deflection from the baseline followed by a negative deflection, which returns to the baseline.
Considerations
An abnormal EKG after a heart attack may resolve. Heart muscle that was ischemic as indicated by a T-wave inversion or ST-segment elevation can heal, and the EKG will return to normal after approximately six months. Other changes become permanent with a chronic Q-wave displayed on the EKG, or the development of rhythm disturbances such as a left bundle branch block.
References
- American Heart Association: Cardiovascular Disease Statistics
- "Understanding the Essentials of Critical Care Nursing"; Kathleen Ouimet Perrin, PhD; 2009
- "Advanced Cardiovascular Life Support"; American Heart Association; 2006
