ECG Changes in Acute Myocardial Infarction: Bedside Reference

It is three in the morning in a district hospital. The nurse hands you a tracing on a 45-year-old man with an hour of chest pain, and there is 1.5 mm of ST elevation in the inferior leads that could be ischaemia or could be his baseline. There is no cardiologist rostered, the fibrinolytic is in the fridge, and the nearest interventional centre is two hours away in good weather. The clinical question is not what the ECG shows in a textbook sense; it is whether this tracing commits you to reperfusion in the next ten minutes. This reference is organised around that decision and the ones that follow it.

What the tracing is actually telling you

Occlusion produces a predictable sequence, but the sequence is fast and you may catch it at any point. Tall, broad, symmetrical T waves with a widened base appear within minutes and are frequently dismissed as normal variant. Convex ST elevation follows. Loss of R wave voltage and pathological Q formation begin within hours and continue over days, T waves invert as the injury current resolves, and after weeks a tracing may show nothing but Q waves and poor progression. The clinically useful consequence is that the absence of ST elevation on a tracing taken twenty minutes into symptoms tells you very little, and a single ECG is never a negative test in a convincing history.

The decision points

1. Verify the recording before you interpret it

Check calibration, paper speed, filter settings and lead placement. Reversed limb leads and high V1 and V2 placement generate spurious patterns that have sent many patients to unnecessary angiography and, more dangerously, have masked real disease. If the appearance is unexpected, repeat the tracing with the electrodes checked before acting on it.

2. Ask whether the pattern meets reperfusion criteria, and whether that is the right question

Conventional thresholds require new ST elevation at the J point in two contiguous leads, with sex and age specific cutoffs in V2 and V3 and a lower threshold elsewhere. But a meaningful minority of acute occlusions never meet those thresholds. The de Winter pattern of upsloping ST depression with tall symmetrical T waves in the precordial leads, hyperacute T waves without elevation, and isolated posterior infarction all represent occluded arteries and all are commonly delayed. Treat the clinical picture combined with a convincing pattern as an indication to act rather than waiting for millimetres to accumulate.

3. Extend the tracing when the standard twelve leads are equivocal

Two additional acquisitions cost ninety seconds and change management more often than most investigations you will order all night. Right sided leads, particularly V4R, identify right ventricular involvement in inferior infarction and change fluid and nitrate decisions immediately. Posterior leads V7 to V9 confirm the infarction suggested by tall R waves and ST depression in V1 to V3. Make both routine in inferior and in isolated anterior ST depression presentations.

4. Localise, then predict what is going to go wrong

Localisation is not an academic exercise; it tells you which complication to prepare for. Inferior infarction predicts bradyarrhythmia, atrioventricular block and, when the right ventricle is involved, profound hypotension with nitrates. Proximal anterior infarction predicts pump failure, bundle branch and high-grade conduction block, and mechanical complications. Elevation in aVR with widespread depression in a shocked patient suggests left main or severe multivessel disease and is a transfer decision rather than a fibrinolysis decision.

5. Use reciprocal change as your confirmation tool

Reciprocal ST depression in a territory opposite the elevation is the most reliable bedside discriminator between infarction and its mimics. Pericarditis produces widespread saddle-shaped elevation with PR depression and no reciprocal pattern except in aVR. Early repolarisation gives concave elevation with notched J points in a young patient with stable serial tracings. Left ventricular hypertrophy and left bundle branch block produce discordant elevation proportional to the QRS, and in the latter the concordance rules attributed to Sgarbossa, with the proportionality modification, remain the practical tool. Takotsubo and myocarditis will fool you and can only be excluded downstream.

6. Repeat the tracing rather than waiting for the biomarker

In an ongoing presentation, serial ECGs at ten to fifteen minute intervals with continuous ST monitoring will resolve the question faster and more safely than a second troponin. Evolution across serial tracings is diagnostic in itself; a static equivocal pattern in a resolving patient is reassuring in a way that a single tracing can never be. Systematic technique for this is set out in our clinical ECG interpretation reference.

7. Decide reperfusion strategy on geography and clock

If timely primary angioplasty is achievable, activate transfer and start antithrombotic therapy. If it is not, fibrinolysis followed by transfer for angiography is the pathway, and the decision should be made in minutes rather than after a departmental discussion. Assess reperfusion at sixty to ninety minutes on ST resolution rather than on symptom relief, since pain is an unreliable marker. Failure of resolution mandates rescue angiography, not a second dose. First-hour sequencing for hospitals without on-site intervention is laid out in our cardiac emergency first hour protocols.

When to escalate, transfer or call for help

• Activate transfer immediately, before further investigation: ST elevation meeting criteria, new left bundle branch block with a convincing history, de Winter or hyperacute patterns, or elevation in aVR with shock.
• Prepare pacing: inferior infarction with second degree block or a ventricular rate under 40 per minute. Transcutaneous pads on before transport, not during it.
• Escalate for haemodynamic support: right ventricular infarction with hypotension unresponsive to volume, or anterior infarction with pulmonary oedema. Stabilisation approaches are covered in the acute cardiac care worldwide reference and across the interventional and acute care hub.
• Seek a second opinion on the tracing rather than waiting: a photograph sent to a colleague resolves most equivocal patterns in under five minutes, and that habit is worth building deliberately before the night you need it.

What to document

• Time of symptom onset, time of first medical contact, and time of every ECG, written on the tracing itself.
• Lead placement confirmation and whether right sided or posterior leads were recorded.
• The specific pattern, with the leads named, and the reciprocal changes present or absent.
• Comparison with any previous tracing, and where that tracing came from.
• The reperfusion decision, the reasoning, and the time therapy was given or transfer activated.
• ST resolution at sixty to ninety minutes, expressed as a proportion, and the action taken if resolution failed.

Bench card

Patterns that should never be filed as non-diagnostic: hyperacute T waves in an ongoing presentation, de Winter upsloping depression, biphasic or deeply inverted precordial T waves in a pain-free patient with recent symptoms, and any new bundle branch block with a convincing history. Structured pattern libraries for each sit in the ECG reference for physicians and the wider cardiology reference library. A more didactic walk through the same electrocardiographic sequence is published as this guide to ECG changes in acute myocardial infarction.

Questions from the floor

Can an infarction occur with a genuinely normal ECG?

Yes, and it happens often enough to matter. Circumflex territory infarction is electrocardiographically silent in a substantial proportion of cases, and very early presentations may show nothing. Serial tracings, posterior leads and biomarkers together are what close the gap; a single normal tracing does not.

How do I approach ST elevation in a patient with a paced rhythm?

Apply the same concordance principles used in left bundle branch block, recognising that they are less well validated in ventricular pacing. In practice, a convincing history with concordant elevation or excessively discordant elevation should be treated as an occlusion and transferred.

Is ST depression ever an indication for immediate reperfusion?

Isolated posterior infarction is, once confirmed on posterior leads. Widespread depression with aVR elevation is an indication for urgent angiography rather than fibrinolysis. Ordinary anterior ST depression from demand ischaemia is neither.

What if the elevation persists weeks later?

Persistent elevation with Q waves in a pain-free patient several weeks after a large anterior infarction suggests ventricular aneurysm and warrants echocardiography, assessment for mural thrombus and a review of anticoagulation, rather than repeat reperfusion.

Reviewed by Dr A M Thirugnanam, MD, MSICP, FSCAI, Ph.D., Senior Interventional Cardiologist — written for physicians who must commit to a reperfusion decision before any cardiologist sees the tracing.