EKG

Find your baseline

Sometimes the ground you’re walking on is smooth.  Sometimes it isn’t (insert photograph of the Andes Mountains here).

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What does this have to do with EKG’s?  Read on…

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50’s y.o. man with chest pain

The RR intervals look to be just a bit over 3 big boxes wide meaning the rate is just under 100, probably low 90’s.  We see P waves throughout, it’s a sinus rhythm.  Axis is normal.  QRS is narrow.  The ST segments look elevated in II, III, and aVF, so inferior STEMI right?  There actually looks to be some elevation to the lateral chest leads so a big inferolateral MI? Let’s take a closer look.

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The ST segments are elevated, but they’re most elevated relative to the PR segment.  If you compare the ST segment to the TP segment all of a sudden most to all of the elevation goes away.

Let’s zoom in laterally.

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With magnification the lateral elevation disappears completely when you use the TP segment as the baseline rather than the PR.  This patient doesn’t have ST elevation, they have PR depression.

Do you know of any conditions that cause diffuse upcoving ST elevation (or apparent ST elevation) with PR depression?  This patient had an unclear enough story that they were taken to heart cath to make sure there was no heart attack.  There wasn’t.  The patient had Pericarditis.

This is a fairly common EKG to see on boards where STEMI mimic EKG’s are more common than STEMI’s.  In real life there’s a few things to emphasize.  Make sure you’re defining your baseline.  The best segment to compare the ST segment with is the TP segment (even though the PR can be easier to see if they’re tachy).  Because the PR segments can be depressed in pericarditis, double check that you’re dealing with ST elevation at all.  Next, the elevation in pericarditis should always cove upwards, it should never take on the shape of tombstones.  It also generally involves multiple coronary artery distributions.  And while there can be some changes in aVR and V1 that can look like reciprocal changes, there cannot be reciprocal changes elsewhere.  If there’s ST depression anywhere but aVR and V1 then you’re probably dealing with a STEMI.  A soft point when inferior STEMI is being questioned is to look at where the most elevation is.  Most of the time a STEMI will have the most elevation in III, most of the time in pericarditis it will be in II.  This last point doesn’t hold up every time but it’s frequently true.

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