Electrophysiological Mechanisms of ST Segment and T‑Wave Changes

Authors, Journal, Affiliations, Type, DOI

• Praveen K. Neema, Nagarjuna Panidapu
• Department of Cardiac Anaesthesia, Amrita Institute for Medical Sciences, Kochi, Kerala, India
• Annals of Cardiac Anaesthesia 2026;29:169–79
• Narrative review article
• DOI: 10.4103/aca.aca_304_25

Overview

This narrative review systematically covers the electrophysiological basis of ST-T wave abnormalities on ECG, distinguishing primary (repolarization) from secondary (depolarization) causes. It provides mechanistic explanations grounded in ionic channel physiology — including how ischemia-induced partial depolarization reverses the normal repolarization gradient, and how injury currents produce apparent ST displacement. Electrolyte disturbances (hyperkalemia and hypokalemia) and secondary causes (LBBB, LVH, pre-excitation, PVCs, pacemaker) are covered with both mechanistic and clinical detail.

Keywords

ECG, electrocardiogram, hyperkalemia, hypokalemia, left ventricular hypertrophy, left-bundle branch block, ST-T changes, subendocardial myocardial ischemia, transmural myocardial ischemia

Key Takeaways

Normal ECG Morphology and Repolarization Direction

• The ST segment reflects plateau phase (phase II) of the ventricular AP; it is isoelectric because membrane potential remains nearly constant during this phase.
• Subepicardial cells have shorter APD than subendocardial cells → repolarization spreads from epicardium toward endocardium → T wave recorded as a positive deflection by overlying electrode (opposite to the depolarization direction).
• Normal measurement reference: J-point is the QRS-to-ST transition; ST deviation measured 40 ms after J-point vs. the PR-segment baseline. When J-point is not isoelectric (due to action potential dispersion), use J-60 or J-80 (standard in exercise stress testing).

Primary vs Secondary ST-T Changes

• Primary ST-T changes are repolarization abnormalities: myocardial ischemia, electrolyte disturbances, tachycardia, increased sympathetic activity, adverse drug effects.
• Secondary ST-T changes are activation (depolarization) abnormalities: LBBB, LVH, pre-excitation, premature ventricular ectopics, pacemaker-stimulated complexes. In all secondary causes, the common mechanism is delayed endocardium-to-epicardium depolarization → subendocardial myocytes repolarize earlier than subepicardial → reversed repolarization direction → discordant T-wave.

Myocardial Ischemia Mechanism

• Hypoxia → ↓ intracellular ATP → (1) ↓ Na⁺/K⁺ ATPase pump → ↑ extracellular K⁺; (2) ATP-sensitive K⁺ channels open → further K⁺ efflux. Combined effect initially causes hyperpolarization, then partial depolarization (resting membrane potential increases from −90 mV toward less negative values).
• Partial depolarization shortens APD → earlier repolarization of ischemic myocardium → reversal of the normal repolarization direction (now subendocardium → subepicardium instead of normal epicardium → endocardium).
• Subendocardial ischemia: Reversed repolarization causes T-wave inversion. Concurrently, during the TP/PQ period, ischemic subendocardium (higher resting voltage) → current flows toward overlying normal myocardium → elevates the baseline (TP segment); the ST segment, truly at its normal position, appears depressed relative to the elevated TP/PQ baseline.
• Transmural ischemia: During the TP/PQ period, current flows away from the ischemic zone → TP/PQ segment is depressed; ST segment, remaining at its normal position, appears elevated.
• ECG distinction: In both subtypes, the preceding QRS complex has normal morphology (key differentiator from secondary ST-T changes).

ACS Clinical Classification

• Partial coronary occlusion → subendocardial ischemia → NSTE-ACS: ST depression ≥0.5 mm (horizontal or downsloping) + T-wave inversion with pointed/arrow-shaped tip.
• Total coronary occlusion → transmural ischemia → STE-ACS: ST elevation ≥0.1 mV in limb leads or ≥0.2 mV in precordial leads (measured at J+40 ms).
• Exception — de Winter pattern (total proximal LAD occlusion): upsloping ST depression in precordial leads + prominent hyperacute T waves; may occur without ST elevation.
• T waves are hyperacute (tall and broad) in the earliest phase of transmural MI before ST elevation develops.
• Both STE-ACS and NSTE-ACS require urgent intervention (angioplasty or CABG).

Hypokalemia

• Inhibits Na⁺/K⁺ ATPase → hyperpolarization (resting potential becomes more negative); unlike ischemia, no partial depolarization because intracellular K⁺ remains relatively maintained.
• Inhibits delayed rectifier IKr current → reduces repolarization reserve → prolongs APD and QT interval.
• ECG: ST depression, T-wave inversion or flattening, prolonged QT interval, prominent U waves.
• Risk: Prolonged repolarization → transmural APD gradient widening → susceptibility to EAD-triggered arrhythmias.

Tachycardia and Sympathetic Activation

• ST depression and T-wave inversion can result from subendocardial ischemia secondary to demand-supply mismatch (increased O2 demand + reduced diastolic filling time + reduced coronary perfusion time).
• Resolves on rate/rhythm control — clinical differentiator from primary coronary artery disease.

Secondary ST-T Changes

Left Bundle Branch Block (LBBB)

• LV depolarized by RV-to-LV cell-to-cell spread (not Purkinje fibres) → slower endocardium-to-epicardium propagation → subendocardial myocytes repolarize earlier → reversed repolarization direction → T-wave inversion discordant to QRS.
• ECG: QRS >0.12 sec; deep and broad S in V1-V2; wide notched R in V5-V6.

Left Ventricular Hypertrophy (LVH)

• Mechanism not fully established: reversal of the normal endocardial/epicardial APD gradient (APD75 gradient reversal) → subendocardium repolarizes before subepicardium → reversed repolarization direction → T-wave inversion.
• Interstitial fibrosis in hypertrophied LV disrupts conduction pathways → heterogeneity of repolarization → further contributes to T-wave changes.
• ECG: Increased QRS amplitude (V5-6, aVL, lead I); R-wave peak time >50 ms (V5-6); S(V1) + R(V5) >35 mm; ST depression + T-wave inversion in I, aVL, V5-6 (strain pattern).

Pre-excitation (Wolff-Parkinson-White)

• Accessory pathway bypasses normal conduction → early ventricular activation → short PR (<0.12 sec), delta wave, prolonged QRS.
• Abnormal ventricular activation → secondary ST depression and T-wave inversion in leads with positive delta wave.

Premature Ventricular Contractions (PVCs)

• Ectopic ventricular focus → cell-to-cell spread → widened QRS + discordant ST-T changes + absent P wave.
• Compensatory pause follows as the next sinus impulse arrives on schedule → increased diastolic filling → increased stroke volume → perceived as palpitation.

Pacemaker-Stimulated Complexes (RV Pacing)

• Cell-to-cell spread similar to LBBB → QRS >120 ms + discordant ST-T changes; absent P waves or retrograde P waves.
• Discordant ST-T changes in paced rhythms are a normal finding, not a sign of ischemia.

Hyperkalemia ECG Progression

• Mild hyperkalemia: ↑ extracellular K⁺ → partial depolarization → shortened APD → exaggerated phase 2–3 slope → tall peaked T waves + shortened QT interval.
• Severe hyperkalemia: Na⁺ channel inactivation → slowed phase 0 (slower upstroke) → widened QRS + prolonged PR interval + flattened/absent P waves + sine-wave ECG pattern + conduction block + ST depression (mimicking MI).
• Represents impending cardiac arrest; requires emergency treatment: NaHCO₃, calcium, K⁺ binding resins, dialysis.

Limitations of the Document

• Narrative review — no systematic search strategy or PRISMA methodology; selection of cited evidence may reflect author perspective.
• No clinical validation data (sensitivity/specificity) for the ECG criteria described.
• Highly mechanistic focus; limited discussion of clinical overlap scenarios (e.g., LBBB + acute MI, LVH + ischemia) where ST-T interpretation is most challenging.
• References are not evenly weighted; some key claims cite textbooks rather than primary clinical studies.

Key Concepts Mentioned

• concepts/ST-T-Changes — central topic of the paper
• concepts/Cardiac-Action-Potential — ionic basis of AP phases; ischemia-induced AP changes; electrolyte effects
• concepts/Cardiac-Repolarization — transmural repolarization direction; injury current mechanism

Key Entities Mentioned

• entities/Atrial-Fibrillation — mentioned briefly in context of arrhythmia risk
• entities/HCM — LVH as a model for secondary ST-T changes

Wiki Pages Updated

• Created: wiki/sources/STT-mechanism-ACA-2026.md
• Created: wiki/concepts/ST-T-Changes.md
• Updated: wiki/concepts/Cardiac-Action-Potential.md (ischemia AP changes, hyperkalemia/hypokalemia AP effects)
• Updated: wiki/concepts/Cardiac-Repolarization.md (primary vs secondary ST-T classification, ischemia injury current)
• Updated: wiki/sourceindex.md
• Updated: wiki/wikiindex.md