Wednesday, November 10, 2021

With all those Q-waves, this can't be acute OMI, right?

Author's note: This post is guest-written by Brooks Walsh, an emergency physician with an interest in emergency electrocardiology and echocardiography, along with Steve Smith. I'm grateful for Steve's review and additional comments.

The case is not recent, and non-relevant details have been changed to make this case unidentifiable.

The case: A week of chest pain

An older gentleman was brought to the ED, complaining of burning abdominal pain. The pain had started about a week ago, but had worsened the day he came to the ED.

An ECG was obtained:

The ST elevation in the inferior leads does not clearly meet standard STEMI criteria. The ST elevation in the inferior leads is modest, and < 1 mm even in lead III.


Of course, there is reciprocal ST depression in aVL, which validates a diagnosis of an Occlusion MI, an “OMI.” But there are also fairly overt Q waves in the inferior leads - see aVF for example:

But a skeptical team member pointed out those inferior Q waves in the inferior leads, suggesting that angiography wouldn't be worth it:  To paraphrase them:

The ECG is Q’ed out, this is an old infarct.”



Should urgent angiography be canceled?

Smith Answer: No!  Presence of Q-waves is common in acute OMI.  When there are QS-waves (not QR-waves!!) in the precordial leads, then old MI is possible.  In such cases, use the LV aneurysm formula: See explanation here: Chest pain and ST Elevation.  Inferior OMI is much more difficult to differentiate from inferior aneurysm (Old inferior MI with persistent STE). There is no good rule to use. However, a small T-wave or inverted T-wave is strongly suggestive of old MI.  A QS-wave is suggestive.  But when there are OMI findings, NEVER attribute a QR-wave in ANY lead (inferior or precordial) to old MI.  Activate the cath lab!! 

A second ECG was obtained, about 15 minutes after the first:

Now there was increased ST elevation in the inferior leads.

A detail of V2 showed an evolving posterior component as well.

So, despite the Q waves, the dynamic changes further proved that this was an acute event. 

A troponin obtained in the ED was only trivially elevated, further supporting acuity.

The patient went to angiography and a TIMI 0 lesion of the RCA was stented.

So the inferior Q waves were at best a distraction, and at worst may have dissuaded some clinicians from immediate reperfusion. 

So did the Q waves lie? 

Or are they just misunderstood?

Five facts about Q waves to better understand them!

1. STEMIs with Q waves are indeed worse than those without.

STEMIs associated with Q waves indicate a larger infarct, and predict a poorer overall outcome. This is shown in both clinical studies, as well as cMR imaging studies.

2. But Q waves don’t reliably predict or localize necrotic myocardium.

When cardiac magnetic resonance is used to identify old infarcts, Q waves can be quite inaccurate.

3. Q waves are also misleading when used to predict the duration of the MI.

Up to 50% of patients with an LAD occlusion develop an anterior Q wave within the first hour of symptoms. Another study found that almost 40% of patients presenting with STEMI, with symptom onset < 6 hours prior, had Q waves

4. T wave inversion may be a better sign for predicting duration of ischemia.

In patients presenting with STEMI, the presence of Q waves seemed to predict the area of myocardium at risk, whereas T wave inversions were better at predicting the duration of ischemia.

5. They are often misleading when used to predict the success of reperfusion therapy. 

While it is overall worse to have Q waves present with a STEMI, they do not predict less benefit from immediate reperfusion therapy. So Q waves (by themselves) should not be a contradiction for fibrinolysis of PCI.

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