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It’s perhaps the most commonly missed STEMI in medicine. Some call it a STEMI equivalent because it does not present with ST-elevation in two anatomically contiguous leads on the conventional 12-lead ECG.
The guidelines go on to state that fibrinolytic therapy is a Class III intervention (evidence of harm) for patients with ST depression “except when a true posterior (inferobasal) MI is suspected or when associated with ST elevation in lead aVR.”
Overall this seems a lukewarm endorsement for urgent reperfusion therapy for patients suffering suspected acute isolated posterior STEMI, but these patients deserve reperfusion!
Let’s look at a case study. This was graciously contributed by Oli Smith, a paramedic in London.
The patient is a 58-year-old male who awoke from sleep with left arm pain. Within 15 minutes he also experienced chest discomfort, nausea and vomiting. He contacted 9-9-9 (the equivalent of 9-1-1 in the U.K.).
On EMS arrival the patient appeared acutely ill. His skin was pale and diaphoretic. He had a grimace on his face. The crew performed an OPQRST assessment:
Onset: At rest;
Provoke: Nothing makes the pain better or worse;
Time: 25 minutes.
The patient’s medical history included hypertension, dyslipidemia and acute myocardial infarction. He said he had a stent placed three years ago. Medications were aspirin, lisinopril and rosuvastatin, with no known drug allergies.
Temp: 37ºC (98.6ºF);
SpO2: 97% on room air;
BGL: 7.2 mmol/L (130 mg/dL).
A 12-lead ECG (Figure 1) showed sinus rhythm with ST depression in leads V2–V4.
There are two important teaching points here.
The ST depression of cardiac ischemia (as opposed to acute injury) does not typically localize to a particular myocardial territory. We know this from stress testing. With ischemia we would expect to see ST elevation in lead aVR with widespread ST-segment depression. When ST depression is maximal in the right precordial leads (V1–V3) over the left precordial leads (V4–V6), then posterior STEMI is more likely than subendocardial ischemia.
In this case the treating paramedic chose to capture an additional 12-lead ECG with leads V5 and V6 in the position of V8 and V9.
Correct positions for modified posterior chest leads (Figure 2) are:
Lead V7: posterior axillary line (when used);
Lead V8: midscapular;
Lead V9: paraspinal.
Due to the relatively small size of the QRS complexes in the posterior leads, 0.5 mm of ST-segment elevation is considered to be a positive finding.
It appears (to me) that the modified posterior chest leads V8 and V9 are positive in Figure 3. But this is a potential problem! Modified posterior chest leads can either confirm or confuse the diagnosis.
I am not against them (especially because some physicians will not take a patient to the cardiac cath lab without an ECG showing ST-segment elevation), but it’s important to know you cannot rule out acute posterior STEMI with modified posterior chest leads!
You can find several cases at Dr. Smith’s ECG Blog (http://hqmeded-ecg.blogspot.com/; @SmithECGBlog) where the modified posterior chest leads were negative and yet the patient was found to have a totally occluded circumflex artery.
So what was the outcome of this case?
The patient was transported straight to the cardiac cath lab (as is the policy in London). He experienced ventricular fibrillation twice during the procedure and was successfully defibrillated. The circumflex was found to be totally occluded and was stented. He had a full recovery.