Several pieces of gold in the FOAM world from doctors Stephen Smith and Amal Mattu have highlighted the importance of ST changes in ECG lead aVL for diagnosing and excluding inferior and lateral STEMIs.
aVL as a high lateral lead is the area of the heart electrically opposite to the the inferior leads, in particular lead III. Consequently ST depression/T wave inversion in aVL may be a reciprocal change for an inferior STEMI … and vice versa.
Stephen Smith was supervising author on this key paper by Bischoff et al, which showed that aVL was the critically important and diagnostic lead in distinguishing inferior STEMI from pericarditis. In this retrospective evaluation, they found that in all cases of pericarditis with some inferior ST elevation, none had any ST depression in aVL. By contrast in almost all reviewed cases of inferior STEMI there was at least some ST depression in aVL (as a reciprocal change). The conclude that: “when there is inferior ST-segment elevation, the presence of any ST depression in lead aVL is highly sensitive for coronary occlusion in inferior myocardial infarction and very specific for differentiating inferior myocardial infarction from pericarditis.”
The paper is discussed in further detail on Stephen Smith’s ECG blog with examples. Additionally he provides examples of situations where even when the inferior ST elevation was combined with lateral elevation in V5/V6, this still was unlikely to be STEMI in the absence of the reciprocal changes in aVL.
This Amal Mattu video of his talk, “Catching STEMIs Before They Happen” at Essentials of Emergency Medicine, demonstrates with examples how ST depression/T inversion in aVL can precede the evolution of inferior STEMI on the ECG. The normal ECG has an isoelectric ST segment with upright T waves in aVL so any flipping of the T wave +/- ST depression in a patient with possible ischaemic symptoms should give you pause to consider an evolving STEMI requiring careful observation and serial ECG’s to detect.
Smith also provides a great example of an evolving inferior STEMI predicted in advance by a flipped T with ST depression in aVL with hyper acute T waves inferiorly
One resounding and reiterated Stephen Smith lesson in ECG interpretation has been that the ECG is proportional so ST and T wave changes should be interpreted by considering their relative size compared to their associated QRS. Subtle ST elevation in the high lateral leads of I and aVL can be easy to miss or confidently call a STEMI. However reciprocal changes inferiorly, particularly in lead III, can help confirm the diagnosis of high lateral STEMI despite only subtle/minimal ST elevation in I and aVL as demonstrated in this Smith example. Smith further discusses how to differentiate true high lateral STEMI’s with inferior reciprocal changes from false positives, so called “pseudo-high lateral STEMI”. Here’s another false positive example with explanation of the key discrminatory features.
Take home messages:
- The ECG is proportional. Small ST/T wave changes in the limb leads such as aVL and III can be significant,
- When there is any ST elevation in the inferior leads, look closely at aVL. Reciprocal ST depression/T inversion can confirm inferior STEMI and differentiate from other causes of inferior ST elevation such as pericarditis and early repolarisation
- ST depression/T inversion on its own in aVL may precede and predict an evolving inferior STEMI – get serial ECG’s!
- ST elevation in aVL and I by themselves can indicate a high lateral STEMI especially with inferior reciprocal changes. Look closely for subtle ST elevation in aVL and I and actively seek out any inferior reciprocal changes that may provide early confirmation of high lateral STEMI.
If ever in doubt or suspicious about an ECG, particularly in a patient with potentially concerning symptoms, keep the patient for a period of observation, do serial ECG’s, consider troponins and consult someone with more experience/expertise.