Uses and limitations of the electrocardiogram in acute coronary syndromes

Primary use — immediate triage. The 12-lead ECG must be obtained and interpreted within 10 minutes of first medical contact. Its principal function is to separate ST-elevation myocardial infarction, which requires immediate reperfusion, from non-ST-elevation ACS, which is managed with antithrombotic therapy and risk-stratified angiography. Because the greatest benefit from reperfusion occurs within the first two to three hours after symptom onset, the ECG is the single time-critical investigation in the whole pathway.

STEMI criteria and localisation. New ST elevation at the J point in two contiguous leads: ≥ 1 mm in all leads except V2–V3, where the threshold is ≥ 2 mm in men ≥ 40 years, ≥ 2.5 mm in men < 40, and ≥ 1.5 mm in women. New left bundle branch block with ischaemic symptoms is treated as a STEMI equivalent. Territory localisation: II, III, aVF = inferior (right coronary); V1–V4 = anteroseptal (LAD); I, aVL, V5–V6 = lateral (circumflex). Always record right-sided (V4R) leads in inferior infarction to detect right ventricular involvement, and posterior leads (V7–V9) when there is ST depression in V1–V3.

NSTE-ACS findings and serial recording. Look for ST depression (horizontal or downsloping ≥ 0.5 mm), transient ST elevation and T-wave inversion. Dynamic changes — appearing with pain and resolving with relief — are the most specific and confer high risk. A single ECG is therefore inadequate: repeat at 15–30 minute intervals, with every episode of pain, and after any change in symptoms. Continuous ST-segment monitoring is preferable where available.

Additional uses beyond diagnosis. The ECG gives prognostic information — the extent of ST deviation and the number of leads involved are components of the GRACE risk score. It detects arrhythmic and conduction complications — atrial fibrillation, ventricular tachycardia, and complete heart block, which most often complicates inferior infarction. It suggests mechanical and other complications such as pericarditis (widespread saddle-shaped ST elevation with PR depression) and left ventricular aneurysm (persistent ST elevation weeks later). It also provides a baseline against which subsequent tracings are compared, which is why obtaining old ECGs is so valuable.

Limitations in the older patient. The resting ECG is less valuable in older people because of common age-related changes. Confounders that mask or mimic ischaemia: left bundle branch block, paced rhythm, left ventricular hypertrophy with strain, atrial fibrillation, digoxin effect, electrolyte disturbance and pre-existing Q waves or T-wave abnormalities. A normal ECG does NOT exclude acute coronary syndrome — up to a third of NSTEMIs have a normal or non-diagnostic tracing, and the left circumflex territory and the posterior wall are electrocardiographically "silent". Conversely the ECG cannot distinguish type 1 (plaque rupture) from type 2 (supply–demand mismatch) infarction, which is a very common and important distinction in the old.

Practical consequences. Because of these limitations the ECG must always be interpreted together with the clinical context, serial high-sensitivity troponin measurement and echocardiography. Compare with a previous tracing wherever one exists. Where left bundle branch block or a paced rhythm is present, apply the Sgarbossa (or modified Smith-Sgarbossa) criteria and use urgent echocardiography to look for a regional wall motion abnormality. Never withhold reperfusion from a patient with a convincing history and evolving symptoms merely because the ECG is non-diagnostic — repeat it, and escalate to imaging or angiography.

The 2023 ESC ACS guideline formally recognises "STEMI-equivalent" patterns that indicate acute coronary occlusion despite the absence of classical ST elevation, and recommends an immediate invasive strategy for these patients. They include: de Winter's sign (upsloping ST depression at the J point in the precordial leads with tall symmetrical T waves — proximal LAD occlusion); posterior infarction (ST depression in V1–V3 with tall R waves, confirmed on posterior leads V7–V9); ST elevation in aVR with widespread ST depression (left main or severe three-vessel disease); and isolated right ventricular infarction on V4R. This reflects the wider shift in emphasis from "STEMI versus NSTEMI" toward the occlusion myocardial infarction (OMI) concept, and it is precisely the kind of high-yield detail examiners reward.