ECG Interpretation Essentials

• Cardiac Action Potential and Ionic Mechanisms: Myocardial cells maintain resting membrane potential of approximately −80 mV through sodium-potassium pump activity. Upon stimulation, rapid sodium influx through L-type calcium channels depolarizes the cell from −80 to +20 mV during Phase 0 (rapid depolarization). Calcium influx during Phase 1-2 (plateau phase) maintains positive membrane potential, while Phase 3 (repolarization) occurs through potassium efflux via delayed rectifier channels. Phase 4 represents the resting potential; in pacemaker cells (SA and AV nodes), the membrane potential spontaneously drifts toward threshold, generating automaticity. The rate of depolarization depends on the steepness of Phase 0, which is determined primarily by sodium channel availability and integrity.

• Electrical Conduction System and Vector Generation: Normal cardiac impulse originates at the sinoatrial (SA) node (60-100 bpm), propagates through atrial myocardium (generating P wave), then traverses the atrioventricular (AV) node (conduction delay creating PR interval), enters the His bundle, and bifurcates into right and left bundle branches. The left bundle divides into anterior and posterior fascicles, and impulse spreads across ventricular myocardium (generating QRS complex). The mean electrical axis of ventricular depolarization (QRS axis) is determined by the summation of all instantaneous electrical vectors and normally ranges from −30° to +90° (left axis deviation <−30°; right axis deviation >+90°). Repolarization (T wave) normally occurs opposite to depolarization direction due to differences in myocardial refractoriness. The PR interval (0.12-0.20 sec) reflects atrial depolarization and AV node conduction; the QRS duration (0.06-0.10 sec) reflects ventricular depolarization; the QT interval (corrected QTc <0.44 sec in males, <0.46 sec in females) reflects total ventricular electrical systole.

• Lead Positioning and Spatial Representation: The ECG uses 12 leads to record cardiac electrical activity from different spatial perspectives. Limb leads (I, II, III, aVR, aVL, aVF) view the heart in the frontal plane: Lead I views the lateral wall (positive vector toward left arm), Lead II views the inferior wall (positive vector toward left leg), and Lead III views the inferior wall (positive vector toward left leg); augmented leads (aVR, aVL, aVF) are amplified unipolar leads. Precordial leads (V1-V6) view the heart in the horizontal plane: V1-V2 (right ventricle), V3-V4 (interventricular septum and anterior wall), and V5-V6 (left ventricle lateral wall). A positive deflection indicates electrical activity moving toward the lead; negative deflection indicates activity moving away from the lead. This spatial arrangement allows localization of myocardial ischemia, infarction, and conduction abnormalities.

• Acute Coronary Syndromes: ST-elevation myocardial infarction (STEMI) produces characteristic ST segment elevation ≥2 mm (≥1 mm in lead aVR or V1) in contiguous leads with reciprocal ST depression; the location indicates the culprit vessel (anterior wall: LAD; inferior wall: RCA or LCx; posterior wall: RCA). Non-ST elevation myocardial infarction (NSTEMI) typically demonstrates ST depression and/or T wave inversion without ST elevation. Unstable angina may show transient ST changes or T wave inversions during symptoms that resolve with relief.

• Cardiac Arrhythmias: Abnormal automaticity, triggered activity (early afterdepolarizations with prolonged QT; delayed afterdepolarizations with digoxin toxicity), and reentry mechanisms produce atrial fibrillation, atrial flutter, supraventricular tachycardia, ventricular tachycardia, and bradyarrhythmias. The underlying etiology includes structural heart disease (cardiomyopathy, valvular disease, HF), electrolyte abnormalities, metabolic disorders, infiltrative diseases, and medication effects.

• Structural and Congenital Abnormalities: Ventricular hypertrophy from chronic hypertension or aortic stenosis produces characteristic ECG patterns; left ventricular hypertrophy (LVH) causes increased QRS voltages (Sokolow-Lyon: S in V1 + R in V5 or V6 >35 mm), leftward axis deviation, and strain pattern (ST depression and T wave inversion in lateral leads). Right ventricular hypertrophy (RVH) from pulmonary hypertension or chronic lung disease produces rightward axis deviation, increased R wave in V1, and S wave in V5-V6. Atrial enlargement from volume or pressure overload generates abnormal P wave morphology (biphasic in V1 for left atrial enlargement; tall and peaked in inferior leads for right atrial enlargement). Congenital conditions include pre-excitation syndromes (Wolff-Parkinson-White), long QT syndrome, Brugada syndrome, and hypertrophic cardiomyopathy.

• Pulmonary Pathology: Acute pulmonary embolism classically produces S1Q3T3 pattern (S wave in I, Q wave and inverted T in III) with right axis deviation, but more commonly shows sinus tachycardia with nonspecific changes; chronic pulmonary disease causes cor pulmonale with RVH and atrial fibrillation. Pneumothorax may cause axis shift and electrical alternans.

• Metabolic and Electrolyte Derangements: Hyperkalemia produces progressive ECG changes: peaked T waves (tall, narrow), QRS widening, PR prolongation, and eventual loss of P waves (eventually bradycardia and cardiac standstill). Hypokalemia causes U waves (deflection after T wave), ST depression, and prolonged QT interval. Hypercalcemia shortens QT interval; hypocalcemia prolongs it. Hypomagnesemia and hypothermia produce characteristic findings.

• Inflammatory and Infiltrative Conditions: Myocarditis (viral, autoimmune, toxic) typically shows diffuse ST elevation (concave upward) with PR depression without reciprocal changes, following atrial involvement. Pericarditis produces similar but more diffuse changes. Cardiac amyloidosis, sarcoidosis, and Chagas disease cause distinctive patterns. Acute intracranial hemorrhage may produce dramatic ECG changes including diffuse T wave inversions and prolonged QT (subarachnoid hemorrhage pattern).

• Presentation of Underlying Conditions: Patients presenting with acute chest pain warrant immediate ECG to evaluate for STEMI or NSTEMI; palpitations suggest arrhythmia detection (regular vs. irregular rhythm, tachycardia vs. bradycardia); syncope or presyncope may result from detected arrhythmias (long QT, Brugada pattern, severe bradycardia); dyspnea may accompany atrial fibrillation or signs of pulmonary hypertension; constitutional symptoms in myocarditis or pericarditis context.

• ECG Findings Without Symptoms: Many ECG abnormalities are discovered incidentally during routine screening or evaluation for other conditions, including asymptomatic LVH, asymptomatic atrial fibrillation, silent ischemia in diabetic patients, and congenital long QT syndrome identified during family screening.

• Risk Factors and Clinical Context: Age >40 years, hypertension, diabetes, smoking, hyperlipidemia, family history of premature CAD, and prior cardiac events increase the pretest probability of finding significant ECG abnormalities. Physical examination findings—such as irregular irregular rhythm (atrial fibrillation), pulse deficits (conduction block), murmurs (structural disease)—correlate with expected ECG patterns.

• Rate Determination: Count the number of QRS complexes in a 6-second strip (multiply by 10) or use the 300-150-100-75-60-50 rule (distance between consecutive R waves inversely proportional to heart rate). Normal sinus rhythm: 60-100 bpm. Bradycardia (<60 bpm) may be physiologic (athletes) or pathologic (beta-blocker use, heart block, hypothyroidism). Tachycardia (>100 bpm) reflects sympathetic activation, fever, hyperthyroidism, or primary arrhythmia.

• Rhythm Analysis: Determine whether rhythm is regular or irregular. Regular rhythms include sinus rhythm, atrial flutter with fixed AV conduction, ventricular tachycardia. Irregular rhythms include atrial fibrillation ("irregularly irregular"), atrial flutter with variable conduction, frequent premature beats, and sinus arrhythmia. Identify P wave presence and morphology: Normal SA nodal P waves are upright in leads I, II, aVF (negative in aVR). Abnormal P waves suggest ectopic atrial rhythm or atrial enlargement. AV relationship: PR interval (0.12-0.20 sec) assesses AV conduction; prolonged PR suggests AV block.

• Axis Determination: Examine QRS deflection in leads I and aVF. Normal axis (−30° to +90°): positive QRS in both I and aVF. Left axis deviation (<−30°): positive I, negative aVF (associated with inferior MI, LVH, left anterior fascicular block, obesity, pregnancy). Right axis deviation (>+90°): negative I, positive aVF (associated with lateral MI, chronic lung disease, RVH, left posterior fascicular block). Extreme axis deviation (−90° to ±180°, "northwest axis"): negative I and aVF (associated with emphysema, lead reversal, ventricular tachycardia, artificial pacemaker).

• Interval Measurements:
• PR interval (0.12-0.20 sec): Represents atrial depolarization and AV node conduction. Prolongation indicates first-degree AV block or effects of AV-nodal blocking drugs.
• QRS duration (0.06-0.10 sec): Represents ventricular depolarization. Prolongation (≥0.12 sec) indicates bundle branch block, ventricular rhythms, or drug effects (tricyclic antidepressants, antiarrhythmics).
• QT interval (measured from QRS onset to T wave end; corrected for rate using Bazett formula: QTc = QT/√RR interval). Normal QTc: males <0.44 sec, females <0.46 sec. Prolonged QTc (>0.46 sec males, >0.48 sec females) increases risk of torsades de pointes; associated with medications (antiarrhythmics, antipsychotics, macrolide antibiotics), electrolyte abnormalities (hypokalemia, hypocalcemia, hypomagnesemia), and congenital long QT syndromes.

• Segment and Wave Analysis:
• ST Segment (from QRS end to T wave onset): Normally isoelectric (on baseline). ST elevation ≥2 mm in contiguous leads (≥1 mm in aVR or V1) indicates STEMI, pericarditis, Takotsubo cardiomyopathy, or early repolarization (benign variant with concave upward ST elevation in V2-V4 and prominent J waves). ST depression ≥1 mm in contiguous leads indicates NSTEMI, ischemia, or reciprocal changes opposite to STEMI zone.
• T Wave: Normally upright in leads I, II, aVL, aVF, V2-V6; negative in aVR. T wave inversions in anatomically contiguous leads indicate ischemia, infarction, or pulmonary embolism; isolated T wave inversion in aVR is nonspecific. Peaked T waves (tall, narrow base) suggest hyperkalemia. Flattened T waves occur with hypokalemia or chronic ischemia.
• U Wave: Small deflection after T wave (best seen in V2-V3 with slow heart rates). Prominent U waves indicate hypokalemia or hypomagnesemia.

• QRS Morphology:
• Normal QRS: Narrow (<0.12 sec), positive in leads I and aVF, with characteristic progression in precordial leads (small R waves V1-V2, increasing through V3-V4, then decreasing in V5-V6).
• Q Waves: Small normal Q waves in I, aVL, aVF, V5-V6 represent septal activation. Pathologic Q waves (>0.04 sec duration or >1/3 QRS height) indicate prior myocardial infarction; location indicates culprit vessel.
• Right Bundle Branch Block (RBBB): QRS ≥0.12 sec with rsR' ("M" or "rabbit ears") pattern in V1-V2 and wide S wave in I, aVL, V5-V6.
• Left Bundle Branch Block (LBBB): QRS ≥0.12 sec with broad, notched R wave in I, aVL, V5-V6 and absent Q waves in these leads; associated with LBBB obscures ST analysis for ischemia detection.
• Left Anterior Fascicular Block: Left axis deviation (−45° to −60°), qR pattern in aVL, rS pattern in III.
• Left Posterior Fascicular Block: Right axis deviation (>90°), rS pattern in I, qR pattern in aVF (rare, often requires careful exclusion of lateral MI or RVH).

• Diagnostic Criteria for Specific Conditions:

Acute Myocardial Infarction

• STEMI: ST elevation ≥2 mm (≥1 mm in aVR or V1) in ≥2 contiguous leads with reciprocal ST depression; may include elevated troponin (not required for diagnosis), elevated CK-MB, elevated myoglobin
• Inferior STEMI: ST elevation in II, III, aVF; reciprocal changes in I, aVL; often associated with right ventricular involvement (check V4R—ST elevation in V4R)
• Anterior STEMI: ST elevation in V1-V4; extent determines LAD occlusion level (proximal with extensive elevation and rightward axis shift; distal with limited elevation)
• Lateral STEMI: ST elevation in I, aVL, V5-V6
• Posterior STEMI: Tall R wave and ST depression in V1-V2 (reciprocal to posterior ST elevation not directly visible)
• NSTEMI: ST depression ≥

Immediate stabilisation

• Obtain and read the tracing first: the ACC/AHA 2021 Chest Pain guideline recommends a 12-lead ECG within 10 minutes of presentation for suspected acute coronary syndrome, with serial tracings if the first is non-diagnostic.
• Pulseless patient: per AHA ACLS, sort the rhythm into shockable (ventricular fibrillation / pulseless VT → immediate defibrillation) versus non-shockable (asystole, PEA → high-quality CPR plus vasopressor, epinephrine 1 mg IV every 3–5 min). Rhythm identification, not the monitor's alarm, drives the shock decision.
• Unstable tachyarrhythmia (hypotension, ischemia, shock): synchronized cardioversion. Symptomatic bradycardia or high-grade AV block: antimuscarinic, atropine 0.5 mg IV, then transcutaneous pacing or a chronotropic infusion (dopamine, epinephrine), per the ACC/AHA/HRS 2018 bradycardia guideline.

Pattern-directed first-line therapy

• STEMI: reperfusion — primary PCI when timely, fibrinolysis when PCI is not available within guideline timeframes — plus aspirin, a P2Y12 inhibitor (ticagrelor), and anticoagulation (ACC/AHA).
• Regular narrow-complex SVT, stable: vagal maneuvers, then adenosine 6 mg IV push (ACC/AHA/HRS SVT guideline).
• Atrial fibrillation: rate control with a beta blocker (metoprolol) or non-dihydropyridine calcium channel blocker (diltiazem), and anticoagulation by CHA₂DS₂-VASc risk per the ACC/AHA/HRS 2023 AF guideline.
• Torsades de pointes / long QT: IV magnesium sulfate, stop the offending QT-prolonging drug, replete K⁺ and Mg²⁺; overdrive pacing or isoproterenol for pause-dependent recurrences (AHA/ACC/HRS ventricular arrhythmia guideline).
• Hyperkalemic ECG changes: IV calcium (calcium gluconate) first to stabilise the membrane, then insulin with glucose to shift K⁺, then definitive removal (dialysis).

Contraindicated

• AV-nodal blockers (adenosine, verapamil, digoxin) in pre-excited atrial fibrillation — use procainamide or cardioversion.
• Verapamil for undifferentiated wide-complex tachycardia.
• Class I sodium-channel blockers in Brugada syndrome; QT-prolonging agents in long QT.

Complications of misinterpretation (the dominant hazard)

• Missed STEMI behind a wide QRS: LBBB or a ventricular paced rhythm reverses normal repolarization and mimics or masks injury current; apply Sgarbossa criteria (concordant ST elevation, concordant ST depression in V1–V3, excessively discordant ST elevation) rather than dismissing the tracing. Emergency.
• Missed posterior infarction: the standard 12 leads have no posterior electrode, so tall R waves with ST depression in V1–V2 are the only clue; failure to obtain posterior leads (V7–V9) delays reperfusion. Emergency.
• Lead misplacement/reversal: right-left arm reversal produces a globally negative lead I with negative P waves — mimicking extreme axis deviation or dextrocardia and prompting unnecessary workup. Repeat the tracing before acting.
• Overcalling benign variants: early repolarization, athlete's T-wave changes, and LVH strain are frequently read as ischemia, leading to avoidable catheterisation, anticoagulation, or admission.

Complications of the conditions detected

• Malignant ventricular arrhythmia: prolonged QTc permits early afterdepolarizations → torsades de pointes → VF; signalled by QTc creep, macroscopic T-wave alternans, and R-on-T ectopy. Emergency.
• Complete AV block and asystole in hyperkalemia: progressive QRS widening merging into a sine wave precedes arrest. Emergency.
• Bradyasystolic arrest in inferior MI: RCA occlusion compromises the AV nodal artery and vagal tone; look for new PR prolongation with inferior ST elevation.
• Cardioembolic stroke in atrial fibrillation: loss of organized atrial systole with stasis; an irregularly irregular rhythm with absent P waves is the finding that mandates stroke-risk scoring.
• Degeneration of pre-excited AF to VF when an accessory pathway conducts rapidly. Emergency.

Complications of treatment

• Fibrinolysis: intracranial hemorrhage — new headache or neurologic deficit.
• Adenosine: transient sinus arrest, AV block, flushing, bronchospasm; can accelerate conduction over an accessory pathway.
• Defibrillation/cardioversion: skin burns, post-shock myocardial stunning, thromboembolism if AF is cardioverted without adequate anticoagulation.
• Device therapy: pneumothorax and lead dislodgement acutely, later infection; ICDs cause inappropriate shocks when supraventricular rates cross the detection zone.

• Single best next step for chest pain: a 12-lead ECG within 10 minutes (ACC/AHA 2021 Chest Pain guideline) — before troponin, before imaging, before analgesia. Repeat it when symptoms change or the first tracing is non-diagnostic.
• The shockable pair is ventricular fibrillation / pulseless VT: defibrillate immediately. Asystole and PEA are not shockable — CPR plus epinephrine. This is the most commonly missed ACLS discrimination.
• Hyperkalemia marches in a fixed order: peaked T waves → PR prolongation and P-wave loss → QRS widening → sine wave → arrest. The best first drug is IV calcium (membrane stabilisation), not insulin — insulin shifts K⁺ but does nothing for the myocyte threshold in the first minutes.
• Inferior ST elevation obliges two extra moves: obtain a right-sided lead (V4R) for RV infarction, and check V1–V2 for tall R waves with ST depression indicating posterior extension. In RV infarction the ventricle is preload-dependent — nitrates cause profound hypotension; give fluids.
• Electrical alternans plus sinus tachycardia and low voltage = pericardial tamponade from a swinging heart; the next step is bedside echocardiography, then pericardiocentesis, not diuresis.
• **Short PR with a *delta wave* = Wolff-Parkinson-White. If such a patient develops irregular pre-excited atrial fibrillation, AV-nodal blockade (adenosine, verapamil, digoxin) can precipitate VF — use procainamide** or cardioversion. Definitive therapy is accessory-pathway ablation.
• Classic buzzword pairings: rabbit ears (rsR′) in V1 = RBBB; U waves = hypokalemia; S1Q3T3 = pulmonary embolism (though sinus tachycardia is far more common); Osborn J waves = hypothermia; coved ST elevation in V1–V2 with RBBB-like morphology = Brugada.
• Common distractor: high QRS voltage with lateral ST depression is LVH strain, not NSTEMI; likewise concave early repolarization ST elevation in young patients is benign. Conversely, do not accept "nonspecific changes" in a patient with ongoing symptoms — serial ECGs, not reassurance.