This is a 66 year old male with severe substernal chest pain.  He was intermittently bradycardic down to the 30’s with a blood pressure in the 80’s systolic.

This is the first ECG:

There is sinus rhythm with first degree AV Block.  The QRS is slightly long (113 ms) but there is no bundle branch block.  The T-waves in inferior leads have high voltage proportional to the QRS, very suspicious for inferior MI.  aVL has T-wave inversion, which raises suspicion even higher

4 minutes later, a Right Sided ECG:

RIGHT SIDE ECG, t = 4 minutes — (the limb leads are standard, only the precordial leads are shifted to the right side) — there is still sinus rhythm with 1st deg AVB.  The T-waves in the limb leads have less voltage than in the previous ECG, suggesting evolution (artery opening!), which supports the diagnosis of ACS.  The R side leads do not have any significant ST elevation.

T = 18 minutes:

t = 18 minutes.   There is sinus bradycardia with a very prolonged PR interval and some nodal escape beats.  Sinus beats that happen AFTER the nodal beat can be seen in the upstroke of the T-wave in the 2nd, 4th, 5th, 7th, and 9th complexes at the bottom in lead II.  MORE IMPORTANTLY, there is now clear ST elevation in inferior leads (artery now closing!), with reciprocal ST depression in aVL and also in V2-V4, diagnostic of simultaneous posterior STEMI. Inferoposterior STEMI.

The patient was taken for immediate angiography and PCI of a 95% thrombotic occlusion of the RCA.

Here is another case in which aVL was critical to the diagnosis of inferior STEMI:

https://drsmithsecgblog.com/2010/07/inferolateral-st-elevation-might-be.html