ST-elevation myocardial infarction
An ST-elevation myocardial infarction (STEMI) is a clinical form of acute myocardial infarction and one form of acute coronary syndrome. It is defined as the presence of ischaemic symptoms, ST-elevations in a pattern consistent with the blood supply of one coronary artery and a rise and/or fall in troponin levels. Ischaemic symptoms are classically angina, but may also be dyspnoea, nausea, sweating, anxiety, or non-anginal chest pain. For initial evaluation, see acute coronary syndrome.
STEMI must be immediately transferred to a PCI centre for treatment.
Diagnosis and evaluation
If a patient has typical chest pain and the ECG shows ST-elevations in a pattern consistent with the blood supply of one coronary artery, we do not waste time with a troponin test, as the probability that it is a STEMI is high. However, in many cases a blood test can be performed in the ambulance, thereby not delaying the time to PCI.
These are the ECG leads corresponding to various regions of the heart, and the coronary artery they’re supplied by[1]:
| Leads with ST elevations | Affected myocardium | Supplying coronary artery |
|---|---|---|
| V1 – V2 | Septal | Proximal LAD |
| V3 – V4 | Anterior | LAD |
| V5 – V6 | Apical | Distal LAD, LCx, or RCA |
| I, aVL | Lateral | LCx |
| II, III, aVF | Inferior | RCA. LCx in 10% of cases |
| V7 – V9 (with reciprocal ST depression in V1 – V3) | Posterior | RCA or LCx |
A new-onset LBBB is considered equivalent to ST-elevation in the case of STEMI. Patients with typical chest pain and new-onset LBBB are treated as STEMI patients.
ST-depression in leads V1 – V4 may be reciprocal changes of a posterior wall infarct due to a circumflex (CX) artery occlusion. These cases are considered as equivalent to ST-elevation and is treated like STEMI. Posterior ECG leads (V7 – V9) would show ST-elevation.
ST-elevation or depression in the inferior leads (II, III, aVF) suggests inferior wall infarct. In these cases, the right-sided leads V4R, V5R, and V6R should be obtained to assess for a possible right ventricular infarct. In this case, ST-elevation would be present in leads V4R – V6R.
ST-elevations can be present in other conditions which may present similarly, like pericarditis, myocarditis, left ventricular hypertrophy, etc. Care must be used in ambiguous cases, but it’s often better to perform a PCI to be safe.
Pathomechanism
Initial management
If the expected time to a PCI lab is < 120 minutes, the patient is rushed straight there, with as low time from symptom onset to catheter insertion as possible. If the expected time is > 120 minutes, the patient should receive thrombolysis and then be rushed to a PCI lab.
The “classic” acute management for STEMI used to be “MONA”: Morphine, oxygen, nitroglycerine, aspirin. Nowadays we know that morphine may worsen the outcome of patients with AMI, and it should therefore only be given if the patient has strong pain which does not subside after administration of nitro.
Oxygen should only be given if the oxygen saturation is < 90%, as giving it at normal levels of oxygen saturation has not shown any benefit and may even cause harm.
Nitroglycerine, either sublingual tablets or sprays, should only be given if the patient has pain. It may be administered to the patient up to three times but does not improve outcome and is therefore not given routinely to all. Nitroglycerine is contraindicated in right ventricular infarction and should therefore not be administered if right ventricular infarction is suspected.
Aspirin (300 mg) should be administered to all patients to be chewed and swallowed in all cases of STEMI. In addition to this, a loading dose of a P2Y12 inhibitor like prasugrel or ticagrelor may be administered.
Beta blocker, most often metoprolol, should be administered in all cases where the patient does not have signs of heart failure and is haemodynamically stable.
Atorvastatin (80 mg) should be administered to all patients as soon as possible.
Unfractionated heparin (UFH) is administered to all patients unless they’re already anticoagulated.
If the anatomy is unsuitable for PCI and the infarcted myocardial area is large, or there is cardiogenic shock, emergency coronary artery bypass surgery (CABG) should be performed.
Summary:
- Aspirin (300 mg loading dose) – in all cases
- P2Y12 inhibitor loading dose (prasugrel or ticagrelor) – in all cases
- Atorvastatin – in all cases
- Unfractionated heparin – in all cases except with pre-existing anticoagulation
- Metoprolol – in all cases except with heart failure
- Nitroglycerine – in case of chest pain
- Morphine – in case of chest pain not responsive to nitro
- Oxygen – in case of O2 sat < 90%