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 inerior 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 sometimes grouped together with the anterior leads.
The “anterior” leads (V3 and V4) view the anterior wall of the left ventricle.
The “lateral” leads (I, aVL, V5 and V6) view the lateral wall of the left ventricle. Leads I and aVL are sometimes referred to as the “high lateral” leads, because their positive electrode is on the left shoulder. Leads V5 and V6 are sometimes 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.
This image from Rescue One EMS Prehospital Program © 1999 Centric Medical Communications, Inc. illustrates the point nicely. This was from a class sponsored by Centocor (makers of the drug Retavase) that was taught by a Miami-Dade Fire Captain. In case you weren’t aware, Miami-Dade was the largest enroller in ER-TIMI-19 which was a clinical trial involving prehospital administration of thrombolytic therapy.
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. Most acute STEMIs are documented in the distribution of the right coronary artery or the left anterior descending.
In other words, the standard 12 lead ECG does a relatively poor job examining the lateral and posterior walls of the left ventricle, so there’s a danger of missing STEMI in the distribution of the circumflex artery.
That’s the main reason it’s so important to carefully analyze 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.
Right ventricular infarction is another issue that will have to be addressed another time.
What do we mean when we say that a lead is reciprocal? 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 leads that face the “ischemic boundary”.
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 Bruce Hopenfeld. Jeroen Stinstra, and Rob MacLeod. J. Cardiovasc. Electrophys, 15(10), 1200–1206, 2004.
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. Some authors have suggested that the first sign of acute inferior STEMI is a downsloping ST segment in lead aVL, and I have seen this happen many times.
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).
Keep in mind that reciprocal changes can be subtle, and may present as nothing more than a flattening of the ST segment in the reciprocal leads.
*** Update 01/15/09 ***
Check out this case at Dr. Smith’s ECG blog to see just how subtle reciprocal changes can be! And how they can prevent you from discharging a patient home to experience cardiac arrest!
*** End update ***
You will sometimes notice reciprocal changes in the anterior leads (V1, V2, V3 and V4). These usually represent reciprocal changes associated with injury of the posterior wall of the left ventricle. Since we don’t usually view modified chest leads V7, V8 and V9, we most often see these changes associated with acute inferior STEMI, because the posterior descending artery branches off the right coronary artery (RCA), which also supplies the inferior wall of the left ventricle.
With anterior STEMI, the occlusion is often in the left anterior descending artery (LAD) which branches off the left main coronary artery. Depending on the patient’s coronary vasculature, the culprit artery, and the location of the occlusion, the blood supply may also effect the lateral wall of the left ventricle, which can create reciprocal changes in the inferior leads (sometimes very subtle depending on the stage of the infarct).
Reciprocal changes may not always be present, but when they are present, it is very strong supporting evidence that the patient is experiencing actue STEMI.