NSTE-ACS on ECG

What it looks like on ECG

The typical presentation is horizontal or downsloping ST-segment depression at the J point, measured relative to the isoelectric line. The preferred isoelectric reference is the TP segment; the PR segment is used if the TP segment cannot be reliably determined (e.g., during tachycardia with a short diastolic interval). Depression of 0.5 mm or more in two contiguous leads is more common in the lateral (I, aVL, V5–V6) and inferior (II, III, aVF) leads, and less common in the anteroseptal leads (V1–V4), depending on the territory of the affected artery. Concurrently, T-wave inversion may occur: diagnostically significant is a new symmetric inversion of ≥1 mm depth in two contiguous leads with a dominant R wave, which supports ischemia. Asymmetric or shallow T-wave inversion is often nonspecific and should be evaluated alongside clinical presentation and dynamics rather than as an equivalent ischemic criterion.

In multivessel disease or suspected left main coronary artery stenosis, widespread horizontal ST depression in 6–8 leads with possible ST elevation in aVR (and sometimes V1) may occur. This is not a formally recognized equivalent of acute coronary occlusion (OMI) and does not automatically imply left main disease — such a pattern is also seen in tachycardia, anemia, hypotension, and severe subendocardial ischemia without critical left main stenosis. It requires urgent evaluation of the cause and, if NSTE-ACS is confirmed, generally an early invasive strategy, but not automatic routing via the STEMI protocol.

Important: The ECG at the time of pain may be completely normal — ischemia does not always produce visible changes, especially if the episode has resolved by the time the tracing is recorded. Therefore, suspected ACS requires a series of ECGs (at presentation, upon recurrent pain, in 15–30 min, and during follow-up) correlated with troponin levels.

Mechanism

The underlying mechanism is an unstable atherosclerotic plaque with rupture or endothelial erosion and thrombus formation. In most cases, the thrombus is non-occlusive or limits blood flow partially and intermittently, but transient complete occlusion with spontaneous reperfusion, distal platelet microembolization, or vasospasm is also possible — meaning that the absence of ST elevation on ECG does not rule out an episode of acute occlusion. The subendocardium is most vulnerable to ischemia due to high intramural mechanical compression of vessels and the predominantly diastolic nature of perfusion in this layer. When coronary blood flow decreases, subendocardial ischemia typically manifests as horizontal or downsloping ST depression and altered T-wave morphology, though the specific distribution of changes across leads depends on the affected territory and the mechanism of ischemia rather than a single simple rule. If a platelet thrombus embolizes distal branches, micronecrosis develops with troponin elevation — this constitutes non-ST-elevation myocardial infarction (NSTEMI) rather than unstable angina alone, even though the ECG picture may be identical.

Causes

• Rupture or erosion of an unstable atherosclerotic plaque with non-occlusive or transiently occlusive thrombosis
• Vasospasm in the setting of endothelial dysfunction (including Prinzmetal's angina as a distinct entity)
• Progressive coronary artery stenosis without acute thrombosis
• Spontaneous coronary artery dissection (SCAD)
• Multivessel atherosclerotic disease with diffuse subendocardial ischemia

Separately, type 2 myocardial infarction involves myocardial injury due to an oxygen supply/demand mismatch (tachyarrhythmia, anemia, hypotension, respiratory failure) without plaque rupture. This is not synonymous with atherothrombotic NSTE-ACS: the clinical diagnosis of NSTE-ACS usually implies an atherothrombotic mechanism, whereas suspected type 2 MI requires actively ruling out non-ischemic myocardial injury and assessing whether true ischemic signs are present.

Differential diagnosis

• STEMI — persistent ST elevation meeting criteria in two contiguous leads, requires emergency reperfusion via primary PCI protocol rather than an NSTE-ACS strategy
• Patterns suspicious for acute occlusion (the so-called OMI concept — occlusion MI): de Winter waves, isolated posterior MI, Aslanger pattern. OMI is not a separate formal ESC diagnostic category, but denotes probable acute coronary occlusion based on the combination of clinical presentation and ECG; the absence of these patterns does not rule out occlusion, and the decision for urgent coronary angiography is made based on clinical findings, ECG dynamics, hemodynamics, and expert evaluation
• Isolated posterior MI — horizontal ST depression in V1–V3 with a tall R and upright T; when suspected, leads V7–V9 are recorded, and ST elevation ≥0.5 mm in two contiguous posterior leads (in some algorithms for men under 40, the threshold is ≥1 mm) in the appropriate clinical context is considered an equivalent of STEMI with an indication for immediate reperfusion strategy; ST depression in V1–V3 alone without confirmation in V7–V9 does not automatically indicate posterior MI and may have other causes
• Diffuse horizontal ST depression with ST elevation in aVR when left main or multivessel disease is suspected — a high-risk marker, but not a proven occlusion equivalent; the decision for emergency coronary angiography is made individually, taking into account clinical presentation and hemodynamics
• Wellens' syndrome — deep symmetric T-wave inversion or biphasic T wave in V2–V3, typically recorded in the asymptomatic period after a pain episode and indicating a high risk of critical LAD stenosis; this is a marker requiring early invasive evaluation and avoidance of stress testing, but not a recognized equivalent of acute occlusion or an independent indication for immediate reperfusion per the STEMI algorithm
• ST depression in left ventricular hypertrophy with systolic overload — asymmetric, combined with LVH voltage criteria, without dynamics or clinical signs
• ST depression associated with digoxin — sagging, diffuse, unrelated to pain
• Nonspecific ST-T changes in tachycardia, hyperventilation, electrolyte disturbances — no clear clinical correlation and dynamics

Clinical significance and management

NSTE-ACS combines unstable angina and NSTEMI — the distinction is based on biomarkers, not ECG. NSTEMI is diagnosed in the presence of a sharp rise and/or fall in cardiac troponin with at least one value above the 99th percentile upper reference limit, combined with clinical or ECG signs of ischemia. In unstable angina, such troponin dynamics are absent despite ischemic symptoms; a chronically elevated troponin without acute dynamics does not confirm an MI. The danger of NSTE-ACS is that plaque instability can lead to complete occlusion and transformation into STEMI, as well as life-threatening arrhythmias against the background of ischemia.

At the ECG stage, the physician must: rule out STEMI criteria; separately identify patterns suspicious for acute occlusion (de Winter waves, ST-elevation in V7–V9 in posterior MI, Aslanger pattern) and, if present, refer the patient for expert evaluation and, when indicated, urgent revascularization; assess diffuse ST depression with ST elevation in aVR as a high-risk marker requiring prompt determination of the cause and, as a rule, an early invasive strategy, but not automatically a STEMI protocol; identify Wellens' syndrome as an indication for early invasive evaluation without stress testing; document ST depression / T-wave inversion indicating leads and depth; record serial ECGs in case of ongoing pain or instability; correlate findings with dynamic troponin and clinical status (GRACE, HEART risk scores). According to ESC guidelines, the further strategy is early invasive (usually within 24 hours) for high risk, urgent (less than 2 hours) for signs of ongoing ischemia, hemodynamic instability, or life-threatening arrhythmias.

Common interpretation errors

• Mistaking ST depression in V1–V3 for "nonspecific changes" without considering posterior MI (leads V7–V9 must be recorded and ST elevation sought there)
• Automatically interpreting diffuse ST depression with ST elevation in aVR as proven left main coronary artery occlusion, instead of evaluating it as a high-risk marker requiring an individualized approach to management
• Classifying Wellens' syndrome as ordinary nonspecific T-wave changes and failing to refer the patient for early invasive evaluation
• Being reassured by a single normal ECG and failing to repeat the recording during ongoing pain
• Confusing the scoop-like ST depression associated with digoxin with ischemic depression, or failing to account for baseline LVH and LBBB (without evaluation using Sgarbossa criteria) as causes of ST depression unrelated to ACS

Frequently asked questions

Related terms

More in the section "Ischemia and Myocardial Infarction"

Entire section

Sources

• 2023 ESC Guidelines for the management of acute coronary syndromes without persistent ST-segment elevation
• AHA/ACCF/HRS Recommendations for the Standardization and Interpretation of the Electrocardiogram, 2009
• Surawicz, Knilans — Chou's Electrocardiography in Clinical Practice
• Clinical Guidelines of the Russian Ministry of Health on Acute Coronary Syndrome Without ST-Segment Elevation

Educational material for medical students and physicians. Does not replace clinical guidelines and physical examination; the ECG must always be interpreted in conjunction with the clinical picture.