Syncope and Prehospital Cath Lab Activation for Posterior STEMI

A middle-aged man had syncope.

This ECG was recorded prehospital; here are the limb leads:

What do you think?

Notice that there are inverted P-waves and a very short PR interval.  This is a junctional rhythm with retrograde P-waves that come slightly before the QRS.  Otherwise, it is unremarkable. 

Here are the Precordial leads:

What do you think?

Here is what the computer read:

Why did it read this?

The computer sees ST depression in V3 and V4, which normally is indeed nearly diagnostic of posterior STEMI.

Is it posterior STEMI?

The computer measures ST elevation or depression at the J-point, relative to the PQ junction.  There is indeed quite a bit of ST depression relative to the PQ junction, but the PQ junction is artificially elevated as an artifact of the P-wave, which is fused with the QRS:

The black arrow shows the PQ junction.  Note that the P-wave occurs just prior to the QRS and is fused with the QRS.  This artificially elevates the PQ junction.  The red arrow shows where the PQ junction really should be.  The blue arrow shows the J-point.
So there really is zero ST depression.

We saw this and de-activated the cath lab immediately.

Learning points:

1. When the patient does not have chest pain, scrutinize the ECG even more closely.  There should always be some suspicion for a false positive when syncope only is the presenting complaint.

2. Read the entire 12-lead ECG.  Our eyes always want to look for ischemia by looking at ST segments and T-waves.  But abnormalities, or apparent abnormalities, of repolarization may be entirely a result of abnormal rhythm or abnormal QRS.

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