Myocardial infarctions vary greatly in severity. Many cases of myocardial infarction are identified by ambulance staff, emergency room doctors and cardiac specialist nurse practitioners quickly. Other, often smaller myocardial infarctions sometimes are not recognized by victims, never receive medical attention, and can result in heart weakness and other complications. Adequate diagnosis requires a medical history, an electrocardiogram, and blood tests for heart muscle cell damage. Other information, including results of myocardial perfusion tests (see stress tests) and echocardiograms can also help establish the diagnosis of MI.
12-lead electrocardiogram (ECG) with ST-segment elevation in leads II, III and aVF, suggestive of an inferior acute myocardial infarction (AMI).
Electrocardiogram
Electrocardiogram (ECG/EKG) findings suggestive of MI are elevations of the ST segment and changes in the T wave. After a myocardial infarction, changes can often be seen on the ECG called Q waves, representing scarred heart tissue. However, a normal ECG/EKG does not rule out a myocardial infarction.
The ST segment elevation distinguishes between:
STEMI ("ST-Elevation Myocardial Infarction")
NSTEMI ("Non-ST-Elevation Myocardial Infarction") -- diagnosed when cardiac enzymes are elevated.
The leads with abnormalities on the ECG may help identify the location:
| Wall affected |
Leads |
Artery involved |
Reciprocal changes |
| Anterior |
V2-V4 |
Left coronary artery, Left Anterior descending (LAD) |
II, III, aVF |
| Anterolateral |
I, aVL, V3-V6 |
LAD and diagonal branches, circumflex and marginal branches |
II, III, aVF |
| Anteroseptal |
V1-V4 |
LAD |
- |
| Inferior |
II, III, aVF |
right coronary artery (RCA) |
I, aVL |
| Lateral |
I, aVL, V5, V6 |
circumflex branch or left coronary artery |
II, III, aVF |
Posterior |
V8, V9 |
RCA or circumflex artery |
V1-V4 (R greater than S in V1 & V2, ST-segment depression, elevated T wave) |
| Right ventricular |
V4R-V6R |
RCA |
- |
Cardiac markers
Cardiac markers or cardiac enzymes are proteins from cardiac tissue found in the blood. Until the 1980s, the enzymes SGOT and LDH were used to assess cardiac injury. Then it was found that disproportional elevation of the MB subtype of the enzyme creatine phosphokinase (CPK) was very specific for myocardial injury. Current guidelines are generally in favor of troponin sub-units I or T, which are very specific for the myocardium , are thought to rise before permanent injury develops. A positive troponin in the setting of chest pain may accurately predict a high likelihood of a myocardial infarction in the near future.
The diagnosis of myocardial infarction requires two out of three components (history, ECG, and enzymes) to be positive for MI. Currently the cardiac markers, namely the troponins have become so reliable that enzyme elevations alone are considered reliable measures of cardiac injury, with ECG serving to determine where in the heart the damage has occurred, and history serving to screen patients for further enzyme and ECG testing.
In difficult cases or in situations where intervention to restore blood flow is appropriate, an angiogram can be done (see below for an image). Using a catheter inserted into an artery (usually the femoral artery), obstructed or narrowed vessels can be identified, and angioplasty applied as a therapeutic measure (see below). Angiography requires extensive skill, especially in emergency settings, and may not always be available out of hours. It is commonly performed by cardiologists. There is a very small risk of plaque and vessel rupture on balloon inflation; should this occur, then emergency open-chest cardiac surgery may be required. Patients commonly experience bruising at the catheter insertion point in the groin and occasionally a hematoma. Dissection (tearing) of the blood vessel is rare but usually managed with a local thrombotic injection.
Diagnostic criteria
WHO criteria have classically been used to diagnose MI; a patient is diagnosed with myocardial infarction if two (probable) or three (definite) of the following criteria are satisfied:
Clinical history of ischaemic type chest pain lasting for more than 20 minutes
Changes in serial ECG tracings
Rise and fall of serum cardiac enzymes (biomarkers) such as creatine kinase, troponin I, and lactate dehydrogenase isozymes specific for the heart.
The WHO criteria were refined in 2000 to give more prominence to cardiac biomarkers. According to the new guidelines, a cardiac troponin rise accompanied by either typical symptoms, pathological Q waves, ST elevation or depression or coronary intervention are diagnostic of MI.
Source http://www.wikipedia.org\
