ST-T Changes
Definition
ST-T abnormalities are deviations of the ST segment or T-wave morphology from normal on the surface ECG. They are classified as primary (repolarization abnormalities — caused by ischemia, electrolytes, drugs, tachycardia) or secondary (depolarization abnormalities — caused by LBBB, LVH, pre-excitation, PVCs, pacemaker). This distinction is clinically essential because primary changes imply a primary myocardial process, while secondary changes are a downstream consequence of abnormal depolarization.
Key Concepts
Measurement
- ST deviation is measured as the vertical distance from the PR-segment baseline to the J-point, taken 40 ms after the J-point (J+40).
- When the baseline is unclear or J-point is non-isoelectric (due to AP dispersion), measure at J-60 or J-80 — standard in exercise stress testing.
- Significant ST depression: >0.5 mm horizontal or downsloping below baseline.
- Significant ST elevation: >0.1 mV in limb leads or >0.2 mV in precordial leads. (sources/STT-mechanism-ACA-2026, rating: medium)
Primary ST-T Changes — Myocardial Ischemia
- Hypoxia → ↓ ATP → ↓ Na⁺/K⁺ ATPase + ATP-sensitive K⁺ channel opening → ↑ extracellular K⁺ → partial depolarization (resting membrane potential less negative than −90 mV).
- Partial depolarization shortens APD → ischemic myocardium repolarizes earlier than normal → reversal of the normal epicardium-to-endocardium repolarization direction.
- Subendocardial ischemia (NSTE-ACS): Partial coronary occlusion; during TP/PQ segment, ischemic subendocardium (higher resting voltage) drives current toward overlying normal myocardium → TP/PQ baseline is elevated; ST segment, which remains at its true normal position during phase II of AP, appears depressed relative to the elevated baseline. T-wave becomes inverted (arrow-shaped tip) due to reversal of repolarization direction.
- Transmural ischemia (STE-ACS): Total coronary occlusion; during TP/PQ segment, current flows away from ischemic zone toward surrounding normal myocardium → TP/PQ baseline is depressed; ST segment appears elevated relative to the depressed baseline.
- Key ECG rule: In ischemia-related ST-T changes, the preceding QRS complex has normal morphology — this is the critical distinguisher from secondary ST-T changes. (sources/STT-mechanism-ACA-2026)
- de Winter pattern (exception): Total proximal LAD occlusion may present with upsloping ST depression + prominent T waves rather than classic ST elevation. (sources/STT-mechanism-ACA-2026)
Primary ST-T Changes — Electrolyte Disturbances
Hypokalemia
- Inhibits Na⁺/K⁺ ATPase → hyperpolarization (resting potential more negative); unlike ischemia, no partial depolarization.
- Inhibits IKr (delayed rectifier) → reduced repolarization reserve → prolonged APD.
- ECG: ST depression, T-wave inversion or flattening, prolonged QT interval, prominent U waves.
- Mechanism of arrhythmia risk: wide transmural APD gradient → EAD-triggered tachyarrhythmias; treatment by IV KCl infusion. (sources/STT-mechanism-ACA-2026)
Hyperkalemia
- ↑ Extracellular K⁺ → partial depolarization → shortened APD → exaggerated phase 2–3 slope → fast repolarization.
- Mild hyperkalemia ECG: Tall, peaked ("tented") T waves + shortened QT interval.
- Severe hyperkalemia ECG: Na⁺ channel inactivation → slowed phase 0 → widened QRS + prolonged PR + flattened/absent P waves + sine-wave pattern + conduction block + ST depression (may mimic MI).
- Represents impending cardiac arrest; emergency management: NaHCO₃, IV calcium (membrane stabilisation), K⁺ binding resins, dialysis. (sources/STT-mechanism-ACA-2026)
Primary ST-T Changes — Tachycardia and Sympathetic Activation
- ST depression and T-wave inversion can result from subendocardial ischemia secondary to demand-supply mismatch: increased myocardial O₂ demand + reduced diastolic filling time + reduced coronary perfusion time.
- Resolves on rate/rhythm control — clinical differentiator from coronary artery disease. (sources/STT-mechanism-ACA-2026)
Secondary ST-T Changes
Common mechanism across all secondary causes: abnormal (slow, cell-to-cell) endocardium-to-epicardium depolarization → subendocardium repolarizes before subepicardium → reversed repolarization → discordant ST-T changes relative to QRS complex.
| Condition | Key ECG Feature | ST-T Change |
|---|---|---|
| LBBB | QRS >120 ms; broad S in V1-2; notched R in V5-6 | T-wave inversion discordant to QRS |
| LVH | Increased QRS voltage; R-wave peak time >50 ms (V5-6); SV1+RV5 >35 mm | ST depression + T inversion (strain pattern) in I, aVL, V5-6 |
| Pre-excitation (WPW) | Short PR (<120 ms); delta wave; prolonged QRS | ST depression + T inversion in leads with positive delta wave |
| PVCs | Widened QRS; absent P wave | Discordant ST-T; T opposite to QRS direction |
| RV Pacing | Pacing spike; QRS >120 ms; retrograde or absent P | Discordant ST-T (normal finding in paced rhythm) |
| Hyperkalemia (severe) | Sine-wave pattern; absent P waves | ST depression (may mimic MI) |
(sources/STT-mechanism-ACA-2026)
LBBB Mechanism
LV depolarized by RV-to-LV cell-to-cell spread (bypassing Purkinje) → slower spread → subendocardial myocytes repolarize before subepicardial → repolarization direction reversed → discordant T-wave. (sources/STT-mechanism-ACA-2026)
LVH Mechanism
Reversal of the normal endocardial/epicardial APD gradient (APD75 reversal); interstitial fibrosis disrupts conduction → repolarization heterogeneity → T-wave inversion. Mechanism not fully established. (sources/STT-mechanism-ACA-2026)
OMI ECG Patterns Beyond STEMI Criteria
Standard STEMI ECG criteria (≥1–2.5 mm STE at J-point in ≥2 contiguous leads) have only 43% sensitivity for acute coronary occlusion by meta-analysis and miss 38% of total LAD occlusions (TIMI-0 flow) on all serial ECGs. The following OMI ECG findings, when identified by expert or AI interpretation, achieve 100% sensitivity for LAD OMI where STEMI criteria fail: (sources/failure-stemi-criteria-lad-omi-ehjacc-2025, rating: high)
| OMI ECG Finding | Prevalence in Subtle LAD OMI |
|---|---|
| Subtle STE (<1 mm, not meeting criteria) | 85% |
| Hyperacute T-waves (incl. de Winter pattern) | 85% |
| Pathologic Q-waves (associated with subtle STE) | 70% |
| Reciprocal STD and/or T-wave inversion | 50% |
| Terminal QRS distortion | 20% |
| Inferior STE + aVL STD/T-wave inversion | 20% |
- Hyperacute T-waves are not defined by amplitude alone: the T/R amplitude ratio is the key discriminator from benign early repolarization; excessive symmetry and increased T-wave "bulk" are additional features. See concepts/Hyperacute-T-waves.
- Terminal QRS distortion: absence of both J-wave and S-wave in a lead where an S-wave would be expected, preceding any subtle STE.
- de Winter pattern: upsloping STD in V1–V6 with peaked T-waves ± STE in aVR; signifies proximal LAD occlusion; subset of the hyperacute T-wave category.
- Modified Sgarbossa criteria (concordant STE ≥1 mm, concordant STD ≥1 mm in V1–V3, discordant STE:S ratio ≥0.25): apply when LBBB or ventricular paced rhythm are present.
- Critically, serial ECGs do not reliably evolve from subtle patterns to STEMI criteria in total LAD occlusion: in 16/16 patients with serial ECGs, hyperacute T-waves never progressed to diagnostic STE. (sources/failure-stemi-criteria-lad-omi-ehjacc-2025, rating: high)
- For the full OMI/NOMI framework see concepts/OMI-NOMI-Paradigm.
Q-Wave Formation and Regression in Myocardial Infarction
- Pathological Q waves (ESC/ACCF/AHA/WHF criteria: ≥0.02 s in V2–V3; ≥0.03 s and ≥0.1 mV in I, aVL, V4–V6, II, III, aVF) develop within hours of STEMI onset and represent transmural myocardial loss large enough to generate depolarization vectors directed away from the recording electrode.
- Presence of Q waves is determined primarily by infarct size (endocardial extent), not transmurality: in reperfused STEMI patients, 23% have non-diagnostic ECGs at 1 week despite confirmed LGE-CMR infarction; 44% are non-diagnostic by 5 years due to infarct shrinkage post-PCI. (sources/qwave-mri-jacc-imaging-2012, rating: high)
- A relative infarct size ≥6.2% of LV mass at 1 year is the optimal threshold for Q-wave presence (sensitivity 89%, specificity 74%); anterior MIs are reliably detected, while nonanterior (inferior/lateral) MIs are frequently electrically silent. (sources/qwave-mri-jacc-imaging-2012, rating: high)
- Q-wave regression represents pseudo-normalization: LGE-CMR confirms persistent irreversible scar even when Q waves have disappeared. Electrically silent MIs carry equivalent prognosis to overt Q-wave MIs. (sources/qwave-mri-jacc-imaging-2012, rating: high)
- See concepts/Q-Wave-Remodeling for the full framework on post-infarct ECG limitations.
Contradictions / Open Questions
- LVH strain pattern vs. ischemia overlap: The strain pattern (ST depression + T inversion in lateral leads) in LVH is mechanistically secondary, but clinically it can coexist with or mask underlying subendocardial ischemia in hypertensive or HCM patients — no validated ECG criteria reliably differentiate the two. (sources/STT-mechanism-ACA-2026)
- LBBB + suspected MI (Sgarbossa criteria): When LBBB is present, classic ST elevation criteria for ACS lose sensitivity and specificity. The Sgarbossa criteria (concordant ST elevation ≥1 mm, concordant ST depression ≥1 mm in V1-V3, discordant ST elevation ≥5 mm) attempt to address this but have moderate accuracy — an ongoing clinical challenge not addressed by this review. (sources/STT-mechanism-ACA-2026)
- Injury current theory completeness: The injury current / diastolic current model of ST changes (TP/PQ baseline shift) is the dominant teaching model, but its quantitative accuracy in human ischemia has been questioned; alternative models exist. (sources/STT-mechanism-ACA-2026)
- STEMI criteria inadequacy for occlusion MI: Standard STEMI STE criteria have only 43% sensitivity for acute coronary occlusion by meta-analysis and miss 38% of total LAD TIMI-0 occlusions on all serial ECGs. Serial ECGs do not reliably evolve to STEMI criteria in subtle OMI. Expert interpretation and AI (PMCardio Queen of Hearts) both achieved 100% sensitivity on the first ECG for all LAD TIMI-0 cases, highlighting that STEMI criteria alone are insufficient gatekeepers for reperfusion. See concepts/OMI-NOMI-Paradigm. (sources/failure-stemi-criteria-lad-omi-ehjacc-2025, rating: high)
- Hyperacute T-wave definition: No validated quantitative threshold exists for hyperacute T-waves — the most prevalent OMI ECG finding in subtle cases (85%). The T/R ratio approach has empirical support but no guideline-endorsed cutoff. (sources/failure-stemi-criteria-lad-omi-ehjacc-2025, rating: high)
Connections
- Related to concepts/Cardiac-Action-Potential
- Related to concepts/Cardiac-Repolarization
- Related to concepts/Torsades-de-Pointes
- Related to entities/Atrial-Fibrillation
- Related to entities/HCM
- Related to entities/Long-QT-Syndrome
- Related to concepts/OMI-NOMI-Paradigm — OMI ECG findings beyond STEMI criteria; paradigm shift
- Related to concepts/Hyperacute-T-waves — most common OMI ECG finding in subtle cases
- Related to entities/Acute-Coronary-Syndrome — clinical context for ischemia-related ST-T changes
- Related to sources/STT-mechanism-ACA-2026
- Related to concepts/Q-Wave-Remodeling
- Related to sources/qwave-mri-jacc-imaging-2012
- Related to sources/failure-stemi-criteria-lad-omi-ehjacc-2025