The correct designation of the type of MI doctors treat is not just a disease reporting requirement. It is also necessary for billing and payment for services. In addition, the type of MI often determines the course of the event and the eventual outcome for the patient in time. It is thus a major determinant of the most appropriate form of treatment.
The two clues on the ECG test that make it possible to identify a heart attack by type are changes in the ST segment and the appearance of Q waves. The ST segment may become elevated, depressed, or may not change at all during a heart attack. If a change does occur it usually does not persist unless an aneurysm of the heart muscle forms, in which case it adjoins a Q wave. Q waves usually, but not always, form later in the process of the acute event or after it is complete.
Q waves might appear in leads where they were not prior to the heart attack or in place of R waves. When Q waves replace R waves the initial deflection representing contraction of the ventricles is downward rather than upward. Either type of change can appear in leads whose sensors face the damaged area(s) of the heart.
Most heart attacks involve the left ventricle because of its oxygen requirements and the anatomy of the coronary arteries. The injury can be to the front, left side, bottom, or septum of the chamber. It can also involve a combination of these sites. Less frequently the event can affect the back portion of the left ventricle. On rare occasions it might involve the right ventricle. Sole involvement of the left or right atrium is very rare and most difficult to detect on ECG.
The medical terms for the most common types of heart attacks based on the area of the left ventricle involved as revealed by ECG are as follows:
- Anterior MI – front wall
- Apical MI (inferolateral myocardial infarction) – left lateral and bottom wall (apex)
- Lateral MI – left lateral wall
- Anteroseptal MI – front wall and interventricular septum
- Septal MI – interventricular septum
- Anteroapical MI – front wall and apex
- Inferolateral MI – inferior and left lateral walls
- Posterior MI – back wall
Types of heart attacks in terms of extent of injury are based on how much of the total thickness of the wall of the heart muscle is involved. Elevation of the ST segment on ECG correlates with
full-thickness injury. When the ECG does not show elevation of the ST segment only a part of the full thickness of the muscle was involved. ST segment depression (a downward shift) during a heart attack might signal damage to a thin layer of heart muscle just beneath the endocardium.
Full-thickness injury means a blood clot has totally blocked the flow of blood to the area of involved muscle. The clot forms in a major artery of the heart which has a preexisting cholesterol plaque.
Names for types of heart attacks depend on what happens with the ST segment during the acute event. A heart attack during which there is elevation of the ST segment is termed an ST segment elevation myocardial infarction (STEMI). When ST segment elevation does not occur it is termed a non-ST segment elevation myocardial infarction (NSTEMI). When ST segment depression occurs it is termed a subendocardial myocardial infarction (SEMI).
Use of the term SEMI has become less popular in recent years though. The reason is autopsy studies have not always shown a good correlation between ST segment depression and the true amount of heart muscle damage. Thus, in many circles ST segment depression is a marker for a NSTEMI.
Q waves also appear with total blockage of an artery resulting in full-thickness damage to the part of the heart muscle it supplies. The reason they appear is the lack of passage of electrical activity into areas of dead tissue. Thus, the sensors of the leads facing those areas sense only electrical activity moving away from them. The result is negative deflections coupled with the heart beats.
Names also exist for types of heart attacks depending on whether or not Q waves form during or after the event. Those names are Q-wave MI and non-Q-wave MI. The names imply their meanings. Classifying MIs based on Q waves has also become less popular though in recent years because of the occasional mismatch between them and autopsy findings.
The prognosis for types of heart attacks based on ECG findings depends on the type. Research has compared the forecast for types of heart attacks based on the Q-wave and the ST segment factors. The most up-to-date data relate to the ST segment aspects though. Although data is available for various disease-course and outcome variables, the one of most concern is the death rate.
The death rate is worse for STEMIs short-term (in the hospital up to 30 days) than for NSTEMIs, but is better than for NSTEMIs long term. The difference in the death rate for STEMIs compared to NSTEMIs continues to widen over time as far out as 4 years from the time of the heart attack based on research. Also, a future heart attack is less likely to occur if there is ST segment elevation.