Tell me about ecg abnormalities
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I now have comprehensive material from Harrison's Principles of Internal Medicine (22nd Ed., 2025) and Frameworks for Internal Medicine. Here is a thorough overview of ECG abnormalities:
ECG Abnormalities - A Comprehensive Overview
1. Axis Deviations
The normal QRS axis ranges from -30° to +100° in the frontal plane.
- Left axis deviation (axis < -30°): seen with left ventricular hypertrophy, left anterior fascicular block (hemiblock), or inferior MI.
- Right axis deviation (axis > +90° to +100°): seen in right ventricular overload, lateral infarction, dextrocardia, left pneumothorax, left posterior fascicular block, or as a normal variant in young patients.
2. Atrial Abnormalities
| Abnormality | ECG Finding |
|---|---|
| Right atrial overload ("P-pulmonale") | Tall, peaked P waves ≥2.5 mm in lead II or V1 |
| Left atrial abnormality ("P-mitrale") | Broad (≥120 ms), notched P wave in limb leads; biphasic P in V1 with prominent negative component |
- Harrison's Principles of Internal Medicine 22E, p. 1914
3. Ventricular Hypertrophy
Right Ventricular Hypertrophy (RVH):
- Relatively tall R wave in V1 (R ≥ S), with right axis deviation
- qR pattern in V1 or V3R
- ST depression and T-wave inversion in right to mid-precordial leads (right ventricular "strain")
- Prominent S waves in left lateral precordial leads
- Associated with: pulmonic stenosis, pulmonary arterial hypertension, ASD (often with RBBB)
Left Ventricular Hypertrophy (LVH):
-
Tall left precordial R waves and deep right precordial S waves (e.g., SV1 + RV5 or RV6 >35 mm; RaVL >20 mm in women, >28 mm in men)
-
ST depression with T-wave inversions in leads with prominent R waves ("strain" pattern)
-
May progress to incomplete or complete LBBB
-
LVH on ECG is a marker of increased cardiovascular morbidity and sudden cardiac death risk
-
Harrison's Principles of Internal Medicine 22E, p. 1914-1915
4. Bundle Branch Blocks and Intraventricular Conduction Defects
Complete bundle branch blocks have QRS duration ≥120 ms; incomplete blocks show QRS of 110-120 ms.
| Block | ECG Pattern |
|---|---|
| Right bundle branch block (RBBB) | rSR' in V1 (terminal R'), qRS (wide S) in V6; terminal QRS vector rightward and anterior |
| Left bundle branch block (LBBB) | Broad notched R in V5-V6, I, aVL; QS or rS in V1; the entire early and late depolarization is altered |
| Left anterior fascicular block | Left axis deviation, small Q in I/aVL, small R in II/III/aVF |
| Left posterior fascicular block | Right axis deviation, small R in I/aVL, small Q in II/III/aVF |
| Bifascicular block | RBBB + left anterior or posterior fascicular block |
- Harrison's Principles of Internal Medicine 22E, p. 1915
5. Myocardial Ischemia and Infarction
Subendocardial ischemia: ST vector directed inward → ST depression in overlying leads.
Transmural/epicardial ischemia: ST vector directed outward → ST elevation in overlying leads.
Regional localization of ischemia:
| Territory | ECG Leads Affected |
|---|---|
| Anterior/apical/lateral | V1-V6, I, aVL |
| Inferior | II, III, aVF |
| Posterior (indirect) | Reciprocal ST depression V1-V3 |
| Right ventricle | Right-sided leads (V3R, V4R) |
Temporal evolution of MI:
- ST elevation (earliest sign, hours)
- T-wave inversions (hours to days)
- Q-wave development (hours to days)
Wellens T-wave sign: Deep T-wave inversions in V1-V4 (± I, aVL) indicate high-grade left anterior descending artery stenosis.
Abnormal Q waves: ≥40 ms wide or >25% depth of R wave - indicate zones of necrosis (though Q waves are NOT exclusively transmural - subendocardial MI can produce Q waves too).
- Harrison's Principles of Internal Medicine 22E, p. 1916
6. AV Conduction Blocks (Heart Block)
| Type | ECG Finding | Clinical Note |
|---|---|---|
| 1st-degree AV block | PR interval >200 ms; all P waves conduct | Generally benign |
| 2nd-degree Mobitz I (Wenckebach) | Progressive PR prolongation until a P wave is dropped; next PR resets to shortest | Block typically at AV node; associated with RCA territory MI |
| 2nd-degree Mobitz II | Constant PR interval with sudden, unpredictable dropped QRS | Block in His-Purkinje system; more dangerous, may require pacing |
| 3rd-degree (complete heart block) | AV dissociation - P waves and QRS complexes beat independently | Emergent; junctional or ventricular escape rhythm |
- Frameworks for Internal Medicine, p. 1460-1498
7. Metabolic and Drug-Induced Changes
| Cause | ECG Effect |
|---|---|
| Hyperkalemia | Tall peaked (tented) T waves → PR prolongation → P-wave loss → wide QRS → sine-wave pattern → asystole |
| Hypokalemia | Prominent U waves, QT prolongation, ST depression |
| Hypocalcemia | Prolonged QT interval (ST segment elongated) |
| Hypercalcemia | Shortened QT interval |
| Digitalis | Shortened QT, "scooped" ST-T complex (digitalis effect); toxicity causes AV blocks and arrhythmias |
| Class IA/III antiarrhythmics (quinidine, amiodarone, sotalol) | QT prolongation, prominent U waves |
| Hypothermia | Bradycardia, Osborn wave (J-point elevation), prolonged repolarization |
| Intracranial bleeds (SAH) | Deep wide T-wave inversions, prolonged QT ("CVA T-wave" pattern) |
- Harrison's Principles of Internal Medicine 22E, p. 1917
8. Other Notable ECG Abnormalities
| Condition | ECG Pattern |
|---|---|
| Acute pulmonary embolism | Sinus tachycardia (most common); S1Q3T3 pattern; right axis shift; RBBB; ST changes V1-V4 simulating anterior MI |
| Chronic cor pulmonale (COPD) | Low voltage; slow R-wave progression; rightward axis |
| Wolff-Parkinson-White (WPW) | Short PR, delta wave, wide QRS |
| Long QT syndrome | QTc >440-460 ms; risk of torsades de pointes |
| Brugada pattern | Coved or saddle-back ST elevation in V1-V2; right bundle branch morphology |
| Pericarditis | Diffuse saddle-shaped ST elevation with PR depression (all leads except aVR/V1) |
| Dextrocardia | Negative P/QRS/T in lead I; R-wave regression V1-V6 |
9. Interval Reference Summary
| Interval | Normal | Abnormal |
|---|---|---|
| PR | 120-200 ms | >200 ms = 1st-degree block |
| QRS | <120 ms | ≥120 ms = complete BBB |
| QTc | 370-440 ms | >450 ms (men), >460 ms (women) = prolonged |
| P-wave duration | <120 ms | ≥120 ms = left atrial abnormality |
| P-wave amplitude | <2.5 mm | ≥2.5 mm = right atrial overload |
ECG abnormalities rarely occur in isolation - clinical context (symptoms, history, biomarkers) is always needed for proper interpretation. Findings like LBBB or LVH predict increased cardiovascular morbidity; complete heart block and Mobitz II may require urgent pacing.
Sources: Harrison's Principles of Internal Medicine 22E (2025), Frameworks for Internal Medicine, The Harriet Lane Handbook 23rd Ed.
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