Thread by @sargsyanz

ECG quiz/minithread:

In this ECG, PR segment is depressed in II, III, aVF, and elevated in aVR.
The STs are depressed in II, aVF, and I, and elevated in aVR.
There are (old) inferior Q-waves.

The patient came in with chest pain, CT shows PE. Baseline ECG had similar, but less prominent PR/ST changes. Cath in 2017 showed normal coronaries.

What's causing the PR depressions and ST depressions?

These changes are from atrial repolarization (AR). AR is usually not noted because it's diffuse, low-amplitude, and largely obscured by ventricular electricity. But when the atrial voltage is prominent, AR can mimic pathology. Here's a great diagram from Dr. Smith's ECG Blog.

Other choices: LM ischemia can cause diffuse STD and ST elevation in aVR, but doesn't fit with PR changes or normal cath. Pericarditis can cause PR depressions (and elevation in R), but STs should be up, not down. Both less likely once PE diagnosed (Occam).

If you picked D thinking RV pressure overload -> acute RA dilation -> more prominent atrial voltage, you were right too. But RV ischemia should not directly cause these ST changes, certainly not the PRs.

Conclusion:

Don't forget that the atria repolarize too. When their voltage is prominent and when there is tachycardia / short PR, this can cause ST depression in leads with a positive P-wave.