ECG criteria
- Hyperacute stage: tall, pointed, symmetrical T waves ("hyperacute T waves") may appear in leads corresponding to the suspected infarction zone — an optional and non-specific sign, which can be transient or absent
- Hyperacute stage: ST-segment elevation, often upwardly convex or straight, frequently merging with the T wave — a key sign of acute coronary occlusion, which may appear simultaneously with or earlier than the hyperacute T wave
- Acute stage (hours to days): maximal ST elevation, appearance and progression of the Q wave, decrease in R-wave amplitude
- Pathological Q/QS wave: in two contiguous leads, a Q wave with a duration of ≥ 0.03 s (≥ 1 mm on paper at 25 mm/s) and a depth of ≥ 1 mm; for V2–V3 specifically, any Q wave of duration ≥ 0.02 s or a QS complex; the "Q ≥ 1/4 R" criterion is no longer used independently due to high false-positive rates
- Subacute stage (days to weeks): gradual return of ST elevation toward the isoelectric line; the T wave becomes inverted and symmetrical
- Subacute stage: the Q wave may persist, decrease, or in some cases disappear — the presence or dynamics of the Q wave are not reliable markers of infarct size or myocardial viability
- Scar stage (weeks and beyond): ST segment on the isoelectric line, Q wave usually stable (though may be absent), T wave either positive or remaining mildly inverted
- Reciprocal changes (ST depression in leads opposite to the infarction zone) are most characteristic of the hyperacute and acute stages during occlusion, but their absence does not rule out MI; an example of localization dependence is ST depression in V1–V3 with a tall R wave as a sign of posterior occlusion
What it looks like on ECG
The classic four-stage progression describes typical STEMI with ongoing occlusion of a single coronary artery and sequential changes in the leads of its vascular bed. In practice, evolution is often atypical: early reperfusion, incomplete occlusion, spontaneous thrombus lysis, collateral circulation, re-infarction, and non-ST-elevation ACS can alter the sequence or accelerate stage transitions. A pathological Q wave may not form at all, while ST elevation or T wave inversion may persist longer than expected.
| Grade | Timing | T wave | ST segment | Q wave |
|---|---|---|---|---|
| Hyperacute (ischemia) | minutes to hours | may be tall, pointed, symmetrical | elevation, often merges with T wave | absent |
| Acute (injury) | hours to days | merges with ST or becomes inverted | maximal elevation, then begins to decline | may develop and progress |
| Subacute | days to weeks | symmetrical inversion | returns toward the isoelectric line | may persist, diminish, or be absent |
| Scar | weeks and beyond | may normalize or remain mildly inverted | usually on the isoelectric line | often persistent, but may be absent |
In leads opposite to the infarction zone, reciprocal ST depression is frequently visible during the hyperacute and acute stages — its absence does not exclude MI. R-wave amplitude in the infarct zone often decreases concurrently with Q-wave growth, reflecting the loss of electrically active myocardium. A pathological Q wave may fail to develop in any MI, including a hemodynamically significant one: the absence of a Q wave does not by itself indicate a small infarct size or early recanalization — assessment of myocardial injury requires clinical correlation, serial troponins, ECG dynamics, and imaging (echocardiography, MRI).
Mechanism
The underlying mechanism follows the sequence ischemia → injury → necrosis → scar, but the pathophysiology of each component is more complex than a single vector. ST elevation in acute ischemia is driven by injury currents — a potential difference between ischemic and healthy myocardium caused by alterations in the resting membrane potential and shortening of the action potential duration within the ischemic zone; hyperacute T waves reflect early repolarization changes in the ischemic myocardium and are not always captured. If blood flow is not restored, cardiomyocytes die and cease to depolarize — the necrotic zone loses normal depolarization vectors, and vectors directed away from it begin to predominate in the facing leads, recorded as a pathological Q wave. As the necrosis organizes into a fibrous scar (connective tissue is electrically inactive), the ST segment returns to the isoelectric line, while the inverted T wave reflects prolonged repolarization abnormalities in the peri-infarcted zone and may partially normalize over time.
Causes
- Acute thrombotic coronary occlusion (atherothrombosis) — Type 1 MI
- Spontaneous coronary artery dissection (SCAD)
- Coronary artery embolism
- Severe coronary spasm (Prinzmetal angina) progressing to infarction
- Supply-demand mismatch without acute atherothrombosis — Type 2 MI; potential causes include tachyarrhythmia, bradyarrhythmia, anemia, hypoxemia, hypotension or hypertension, severe coronary stenosis
Differential diagnosis
- Pericarditis: diffuse concave ST elevation, often accompanied by PR depression and reciprocal ST depression primarily in aVR (frequently in V1 as well); no Q-wave evolution; regional and atypical variants are possible, requiring clinical and echocardiographic evaluation
- Early repolarization syndrome: stable, unchanging ST elevation over years, concave upward, without progression to Q waves or T wave inversion
- Left ventricular aneurysm: persistent ST elevation lasting months to years in the presence of an already established Q wave — requires ruling out a new acute occlusion rather than being automatically dismissed as a chronic finding
- Left bundle branch block and LV hypertrophy: can mimic pathological Q waves and discordant ST segments without typical temporal evolution; Sgarbossa criteria are used for MI diagnosis in LBBB
- Takotsubo cardiomyopathy: ST-T changes often extend beyond a single coronary territory and are usually transient, though pathological Q waves and ST elevation can occur; the ECG alone cannot differentiate Takotsubo from ACS — management protocols for suspected acute coronary occlusion (coronary angiography/imaging) must be followed until verified.
Clinical significance and management
Staging via ECG can tentatively help estimate the approximate time of an event when the history is unclear (unconscious patient, incidental finding), but it does not replace clinical data and serial ECGs. ECG signs of acute coronary occlusion are divided into two groups with distinct management strategies. When ST elevation or its direct occlusion equivalents are present (de Winter waves, posterior MI with ST depression in V1–V3 and tall R wave, "tombstone" ST elevation), immediate STEMI management is initiated: primary PCI is preferred if achievable within recommended timeframes; fibrinolysis is considered only if timely PCI is unavailable, within the appropriate therapeutic window, and in the absence of contraindications. ST elevation in aVR accompanied by diffuse ST depression in other leads carries a different significance: it indicates severe subendocardial ischemia and a high likelihood of left main coronary artery disease or multivessel disease, but it is not considered an independent STEMI equivalent and does not prove acute coronary occlusion. This pattern is interpreted within the clinical and hemodynamic context as high-risk ACS and requires urgent invasive evaluation, though without automatically triggering the door-to-balloon primary PCI protocol. Decision-making is not based solely on the ECG "stage" — it is determined by clinical presentation, symptom onset time, and diagnostic signs of acute occlusion; a repeat ECG in 15–30 minutes helps clarify ambiguous presentations. In the subacute stage, persistent ST elevation with a pathological Q wave may indicate developing LV aneurysm and warrants echocardiography. In the scar stage, a pathological Q wave without pain or acute ECG evolution is a sign of prior MI, which is crucial for risk stratification and indications for further testing (stress tests, myocardial viability imaging).
Common interpretation errors
- Attempting to precisely date a myocardial infarction from a single tracing without comparison to prior ECGs and clinical context
- In the presence of persistent ST elevation and suspected LV aneurysm, deferring urgent evaluation without ruling out re-infarction, stent thrombosis, or a new occlusion: comparing prior ECGs, assessing symptoms, obtaining serial troponins, performing urgent echocardiography, and consulting according to the ACS protocol are required
- Equating ST-elevation in aVR with diffuse ST-depression to a standalone STEMI equivalent without considering clinical presentation and hemodynamics
- Failing to account for the fact that with successful early reperfusion and in a portion of other myocardial infarctions, the Q wave may not form at all
- Confusing a normal septal Q wave (narrow, low amplitude in V5–V6, I, aVL) with a pathological infarction Q wave
In occlusion MI, dynamic ST-T and Q-wave changes are possible, but their absence or regression does not rule out infarction
1. Which ECG feature is most characteristic of the hyperacute stage of myocardial infarction?
ST elevation (sometimes accompanied by hyperacute T waves) without a Q wave is the typical pattern of the earliest stage of acute coronary occlusion, prior to the development of necrosis.
2. What distinguishes the scar stage of infarction from the acute stage?
In the scar stage, the ST segment is usually at the isoelectric line, and the pathological Q wave, if formed, may persist as an electrical footprint of necrosis.
3. Which finding requires ruling out reinfarction or a new occlusion rather than simply being regarded as a chronic left ventricular aneurysm?
In the presence of a stable, known pattern without acute dynamics, a chronic aneurysm is more likely, but new symptoms, new ECG changes, or the absence of a baseline tracing for comparison require ruling out reinfarction or a new occlusion.
Frequently asked questions
How to determine from an ECG how much time has passed since a myocardial infarction?
Only approximately, based on a combination of features: hyperacute T without Q — minutes to hours; maximal ST with a progressing Q — hours to days; declining ST with a deep negative T — days to weeks; isoelectric ST with a persistent Q — weeks and later. Exact dating from a single ECG is impossible — history and a series of tracings are needed, and the evolution may differ from the classical scheme.
Can the Q wave disappear over time?
Yes. A pathological Q wave may decrease or disappear, especially after early successful reperfusion, but also in some cases of established MI. The presence or absence of a Q wave is not a reliable marker of infarct size — clinical presentation, troponin, and imaging (echocardiography, MRI) are used to assess it.
Is the subacute stage of infarction dangerous?
Yes, in the subacute stage, the risk of rhythm disturbances, left ventricular aneurysm formation, and early post-infarction pericarditis persists, although the risk of myocardial rupture and life-threatening arrhythmias is lower than in the hyperacute and acute stages. Dressler's syndrome is a distinct late autoimmune post-infarction syndrome that usually develops after weeks and is not specifically characteristic of the early subacute ECG pattern.
How does persistent ST elevation in an LV aneurysm differ from the acute stage of infarction?
The main difference is time and dynamics: ST elevation in an aneurysm persists for months and years against the background of an already formed pathological Q wave without acute changes, whereas in the acute stage, ST elevation appears for the first time and is accompanied by evolution over the subsequent hours and days. When in doubt, comparison with previous ECGs, clinical presentation, and serial troponins are needed, since a patient with an aneurysm can also experience a reinfarction.
Is it necessary to wait for a pathological Q wave to start reperfusion therapy?
No. When there is clinical suspicion of acute coronary occlusion, treatment is initiated immediately according to protocol regardless of the presence or absence of a Q wave — the decision is based on symptoms, ST elevation or its equivalents, and time from symptom onset, rather than whether the pattern fits the "hyperacute stage". The presence of a Q wave also does not rule out the benefit of urgent reperfusion in ongoing ischemia.
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Related terms
More in the section "Ischemia and Myocardial Infarction"
Sources
- Goldberger's Clinical Electrocardiography, 9th ed.
- Surawicz, Knilans. Chou's Electrocardiography in Clinical Practice, 6th ed.
- Orlov V.N. Guide to Electrocardiography
- ESC Guidelines for the management of acute coronary syndromes, 2023
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.