|HOME||Myocardial Ischemia||Myocardial Injury|
The ECG is useful in the diagnosis of coronary artery disease in multiple stages. The ST-T waves may be altered with ST depression associated with ischemia. When acute injury is present as in acute myocardial infarction, there can be ST elevation. Following an MI, there can be distinct Q wave patterns indicative of damage. The ECG can provide information suggesting coronary disease before infarction (ischemia pattern), during infarction (injury pattern) and after infarction (Q waves). The various walls of the heart are represented on the surface ECG with different lead groups. There can be overlap in the walls on the ECG. The lead groups are listed below.
Frequently, the affected wall will be referred to and the specific leads will not be mentioned. The text will use the wall location and the ECGs that are provided will demonstrate the lead groupings.
There are many ST segment and T wave abnormalities that may be encountered on the ECG and it is important to discriminate between those suggesting ischemia and those that are non-specific. Ischemia is classically represented by J-point depression and ST segment depression of at least 1 mm in two contiguous leads (leads that are in the same group for a specific wall) and associated T wave inversions. As with many things in medicine, the more severe the abnormality, the more specific the abnormality is for a disease process. The more pronounced the degree of ST depression, the more it correlates to myocardial ischemia (ST depression of 4 mm is more suggestive of myocardial ischemia than ST depression of 1 mm). Representative ECGs of inferior, lateral, anterior, anterioseptal, anteriolateral, and high lateral wall ischemia are provided.
The other common causes of ST depression with T wave abnormalities include:
Digitalis, left ventricular hypertrophy with strain and hypokalemia are discussed elsewhere. Frequently, when there is ST elevation due to injury, there are what are known as reciprocal changes. For example, if there is ST elevation with an injury pattern in the inferior leads, there may be ST depression in the anterior or lateral leads on the ECG, which represents a mirror image of the ST changes in the inferior wall.
The identification of myocardial injury may be the most important clinical utility of the ECG. The ECG progresses through a series of changes involving the ST segment and T wave and potentially the QRS complex that is fairly standard with minor variations. The initial abnormality of the ECG is the peaked T wave. Following the peaked T wave, the ST segment will begin to rise. ST segment elevation of at least one millimeter in at least two contiguous leads is the minimal criteria for the ECG diagnosis of an injury pattern. The next phase of injury is inversion of the T waves. After T wave inversion, the ST segment may return to baseline and eventually, the T wave will return to normal. During this process, Q waves may develop. The development and timing of Q waves is the most variable element of the evolution of a myocardial infarction. It may take as little as an hour or as many as 6-7 hours for the Q waves to develop after the initial ST elevation.
When interpreting ECGs, the distinction between myocardial injury and myocardial infarction must be made. The presence of ST elevation cannot be diagnosed as myocardial infarction unless Q waves are present. If there are no Q waves present, myocardial injury must be diagnosed. The timing of a myocardial infarction may possibly be identified as well. If Q waves are present and there is still ST elevation or inverted T waves, the diagnosis of acute or recent myocardial infarction can be made. If Q waves are present but both the ST segment and T waves have returned to baseline then the interpretation of myocardial infarction, old are age indeterminate is made. Representative ECGs of injury pattern are provided for the anterioseptal, anteriolateral, and inferior walls.
As discussed above, reciprocal changes may occur with ST elevation. It is sometimes difficult to differentiate on the ECG if the ST depression is simply an electrical mirror or if there is additional ischemia in those areas. One important fact is that reciprocal changes are seen with ST elevation but not with ST depression. In other words, if there is ST elevation anteriorly and ST depression inferiorly, this represents an anterior wall injury pattern and reciprocal changes inferiorly; it does not represent inferior ischemia with reciprocal ST elevation anteriorly.
Finally, Q waves do not always represent prior infarction. Q waves can be normal in leads V1, aVL and III. Common causes of pathologic Q waves without myocardial infarction are listed.