Mr. M, 70, is a previously healthy, physically active nonsmoker who arrives in the ED complaining of severe midsternal chest pain. He rates the pain intensity as 10/0-10. The pain began while working on a construction project at his home. He described the pain as "squeezing" in character and radiating to both arms. His vital signs are: BP, 170/90 mm Hg; heart rate, 70 beats/minute; respiratory rate, 18/minute; SpO2 98% on room air. A stat 12-lead ECG was obtained (see Initial 12-lead ECG).
What is the medical diagnosis?
Based on his signs and symptoms and ECG, Mr. M was experiencing an inferolateral ST-elevation myocardial infarction. Recognizing what areas of the heart are affected relies on ST-segment and/or T wave changes in specific leads. Leads II, III, and aVF all view the inferior wall of the left ventricle. Leads V5 and V6 view the lateral wall of the left ventricle.1
The ST segment, the line between the QRS complex and the T wave, represents the time from the completion of ventricular depolarization (represented by the QRS complex) to ventricular repolarization (represented by the T wave). The ST segment is normally isoelectric (at the baseline or zero potential as identified by the T-P segment).1,2 New ST-segment elevation of 1 mm (0.1 millivolt [mV]) or more in two or more limb leads (or precordial leads V4-V6) or by 2 mm or more in two or more precordial leads V1-V3) can indicate myocardial injury.2,3 Locating the "J" point is helpful in determining if the ST segment is elevated. To find the J point, follow these steps:
* Identify the point where the QRS complex and the ST segment meet. This is the J point.
* Move one small box (0.04 second) to the right; this is the J point.
* Find the isoelectric line (baseline) of the cardiac cycle by looking at the T-P segment.4 (Some references suggest finding the isoelectric line by looking at the PR segment.2)
* Measure the height (amplitude) of the ST segment from the isoelectric line at the J point in mm (each small box is 1 mm) (see J point elevation).
Treatment options
Because Mr. M arrived at the hospital within 12 hours of symptom onset, he was a candidate for reperfusion therapy.5 His cardiac catheterization revealed that the culprit artery was a totally occluded obtuse marginal (OM) branch of the left circumflex coronary artery. The left circumflex coronary artery supplies the lateral wall of the left ventricle and in some individuals the inferior wall of the left ventricle. Mr. M did not undergo percutaneous coronary intervention because the culprit lesion was located in a small branch. An echocardiogram performed the next day showed preserved left ventricular function with a normal ejection fraction. Mr. M was discharged to home on a beta-blocker, an antiplatelet agent, and a statin. Discharge teaching included the need for follow-up appointments with his primary healthcare provider and cardiologist, dietary modifications, and a cardiac rehabilitation plan.
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