Pericarditis vs STEMI

So what is a STEMI?
STEMI criteria:
The ST segment of an EKG will be elevated by more than 3 mm in the precordial leads and by more than 1 mm in the limb leads. Further, in the precordial leads, the shape should be convex up (like a tombstone). The shape matters more than the amount of elevation in some cases. You may have minimal elevation, but the shape is convex up, that could be considered a STEMI.
In the limb leads, the shape does not matter, just the elevation. Anything more than one small box, and you have a STEMI! Pretty straight forward. Unless, of course, it is an early repol!

So why do people get confused? And what is Early Repolarization?
There is an entity known as normal variant early repolarization that can give you ST segment elevation. But this is not a STEMI. It is a normal finding.
So how can you tell the difference? Look at the shape of the ST segment take off (the J point), it will usually be a notch and does not take off flat across. See the example below.

Above you see the first example on the right. The notch on the J point gives it away as early repol. In the far left example, you see the notch again in one of the precordial leads, usually V2 or V3. This is early repol, not a STEMI.
Here is an Early Repol EKG:


So what about pericarditis?
Pericarditis ST elevations look like the concave upwards ST elevations of early repol, but do not have the notch. Further, you may have PR segment depression in lead II. There will be no reciprocal changes or ST depressions anywhere.
 Good example of pericarditis EKG:
 
Notice the ST elevations all over the place with no reciprocal changes.
 Below is a good example of an Inferior STEMI.
 
Notice the reciprocal changes in the anterior leads. You see depressions. While it is not necessary to have reciprocal changes, they can happen, and it helps clinch your diagnosis.
Below is an anterior STEMI, without reciprocal changes.

 Below is another Anterior STEMI with the perfect tombstone pattern. No reciprocal changes.

 Here is a nice table comparing the various EKG findings in STEMI, early repol, and pericarditis.

Comparison of ECG Changes Associated with Acute Pericarditis, Myocardial Infarction and Early Repolarization
ECG finding	Acute pericarditis	Myocardial infarction	Early repolarization
ST-segment shape	Concave upward	Convex upward	Concave upward
Q waves	Absent	Sometimes Present	Absent
Reciprocal ST-segment changes	Absent	Sometimes Present	Absent
Location of ST-segment elevation	Limb and precordial leads	Area of involved artery	Limb and Precordial leads
ST/T ratio in lead V6*	>0.25	N/A	<0.25
Loss of R-wave voltage	Absent	Present	Absent
PR-segment depression	Sometimes in Lead II	Absent	Absent

If you are unsure if it is a STEMI or not, KEEP GETTING EKGs! PLASTER THE WALLS WITH EKG PAPER! STEMIs evolve! They don't look the same. If you have 10 EKGs in the last hour, and they all look the same.... it's not a STEMI!