Here are some charts to help you identify and localize acute STEMI on the 12 lead ECG.
Contiguous leads are next to one another anatomically speaking. They view the same general area of the heart (specifically the left ventricle).
For example, these states in the upper-midwest are contiguous, because they are all touching and in the same region of the country.
The inferior leads (II, III and aVF) view the inferior wall of the left ventricle. Remember that the inferior leads make up the lower-left corner of the 12 lead ECG.
The septal leads (V1 and V2) view the septal wall of the left ventricle. They are often grouped together with the anterior leads.
The anterior leads (V3 and V4) view the anterior wall of the left ventricle. When there is ST-segment elevation in leads V1-V4 we often simply say “LAD occlusion.”
The lateral leads (I, aVL, V5 and V6) view the lateral wall of the left ventricle. Leads I and aVL are often referred to as the “high lateral” leads, because their positive electrode is on the left shoulder. Leads V5 and V6 are often referred to as the “low lateral” leads because their positive electrodes are on the lateral left chest.
In addition, any two precordial leads that a next to one another are contiguous. In other words, V4 and V5 are contiguous, even though V4 is an anterior lead and V5 is a lateral lead. This makes sense when you consider that leads V4 and V5 are next to each other on the patient’s chest.
It’s worth mentioning that the standard 12 lead ECG does a relatively poor job examining the lateral wall of the left ventricle, and does not directly examine the posterior wall of the left ventricle. That’s the reason we sometimes miss acute STEMI in the distribution of the circumflex artery.
Think of it this way. There are 3 main epicardial coronary arteries, the right coronary artery (RCA), left anterior descending (LAD) and the circumflex (LCX).
It stands to reason that approximately 33% of documented acute STEMIs should occur in the distribution of each of the 3 main arteries. But that’s not what we find! About 40% of STEMIs are documented in the RCA, 40% in the LAD, and only 20% in the LCX!
That’s who it’s important to look for so-called “electrocardiographically silent” high lateral STEMI and scrutinize the right precordial leads (V1-V3) for reciprocal changes that may indicate posterior STEMI! You can also consider using modified leads V7, V8 and V9 to increase the sensitivity.
You can read about right ventricular infarction here.
It means that during an acute STEMI, when ST-segment elevation is present in leads that face the acute injury, ST-segment depression will often be present in other leads.
Many theories have been advanced to help explain reciprocal changes. I can’t go into all of them here, but consider this diagram modified from A Mechanism for ST Depression Associated with Contiguous Subendocardial Ischemia by Hopenfeld et al. J Cardiovasc Electrophys, 2004: 15(10), 1200–1206.
Computer modeling has shown that as the ischemic zone extends from the endocardium to the epicardium, it creates a relatively positive area above the ischemic zone, and a relatively negative area at the ischemic boundaries.
This computer model helps explain why reciprocal changes may appear prior to ST-segment elevation! A downsloping ST-segment in lead aVL is often the first ECG sign of acute inferior STEMI!
Regardless of why reciprocal changes occur, clinical experience shows that the most important reciprocal changes can be viewed between the high lateral leads (I and aVL) and the inferior leads (II, III and aVF).
You will sometimes notice reciprocal changes in the anterior leads (V1, V2, V3 and V4). These changes are associated with injury of the posterior wall of the left ventricle.
Reciprocal changes may not always be present (in fact they are frequently absent with LAD occlusion) but when they are present, it is very strong supporting evidence that the patient is experiencing acute STEMI.
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