ECG Changes in 10 Common Clinical Conditions

1. Normal Sinus Rhythm (Reference Baseline)

• Regular P wave before every QRS
• PR interval: 120-200 ms
• QRS duration: <120 ms
• QT interval varies with heart rate (QTc <440 ms in men, <460 ms in women)
• Upright T waves in lateral leads, inverted in aVR

2. STEMI - ST-Elevation Myocardial Infarction

1. Hyperacute T waves - earliest sign; tall, peaked, broad T waves
2. ST elevation - convex ("tombstone") upsloping ST elevation ≥1 mm in ≥2 contiguous limb leads or ≥2 mm in ≥2 precordial leads
3. Reciprocal ST depression - in leads facing the opposite wall
4. Pathological Q waves - develop over hours to days; width ≥40 ms, depth ≥¼ R wave = necrosis marker
5. T-wave inversion - follows ST elevation as infarct evolves

Anterior STEMI - ECG sequence (acute top row, evolving bottom row):

Inferior STEMI - ECG sequence:

Subendocardial vs. transmural ischemia - current of injury diagram:

Left: subendocardial ischemia → ST vector directed inward → overlying leads show ST depression. Right: transmural/epicardial ischemia → ST vector directed outward → overlying leads show ST elevation. - Harrison's Principles of Internal Medicine 22E, Fig. 247-11

3. NSTEMI / Unstable Angina (Subendocardial Ischemia)

• ST depression ≥0.5 mm (horizontal or downsloping) in ≥2 contiguous leads
• T-wave inversions - symmetric, deep; in precordial leads V1-V4 with high-grade LAD stenosis = Wellens sign
• No Q waves, no ST elevation
• A normal ECG does not exclude NSTEMI

"Patients who present with deep T-wave inversions in multiple precordial leads (V1-V4) typically have severe obstruction in the left anterior descending coronary artery." - Harrison's Principles of Internal Medicine 22E, p. 1916

4. Atrial Fibrillation (AF)

• No P waves - replaced by low-amplitude fibrillatory (f) waves, best seen in V1
• Irregularly irregular RR intervals - the hallmark; no two consecutive RR intervals are equal
• QRS complexes are narrow (unless aberrant conduction or bundle branch block)
• Ventricular rate typically 100-160 bpm if untreated

"In the ECG, one can see either no P waves or only a fine, high-frequency, very low voltage wave. The QRS-T complexes are normal unless there is some pathology of the ventricles, but their timing is irregular." - Guyton & Hall Textbook of Medical Physiology, Fig. 13.20

5. Atrial Flutter

• Sawtooth flutter (F) waves at 250-350/min, best seen in leads II, III, aVF and V1
• Regular atrial rate; ventricular rate typically 150 bpm (2:1) or 100 bpm (3:1)
• No discrete P waves
• Classic pearl: any regular tachycardia at exactly 150 bpm - suspect atrial flutter with 2:1 block

6. Ventricular Tachycardia (VT)

• Wide QRS complexes ≥120 ms (usually ≥140 ms)
• Rate 100-250 bpm, typically 140-200 bpm
• AV dissociation - P waves march through at their own rate, independent of QRS
• Capture beats - occasional narrow QRS when a sinus impulse successfully captures the ventricle
• Fusion beats - hybrid QRS between sinus beat and VT beat
• Concordance - all precordial leads with same QRS polarity; negative concordance (all negative) strongly suggests VT
• Left axis deviation common

"For wide-QRS complex tachycardias, the 12-lead ECG is useful in distinguishing SVT (with aberrancy) from ventricular tachycardia. The presence of fusion beats, capture beats, and AV dissociation all favor VT." - Goldman-Cecil Medicine, Electrocardiography section

7. Complete Heart Block (3rd-Degree AV Block)

• P waves and QRS complexes are completely independent (AV dissociation)
• P-P intervals are regular; RR intervals are regular - but at different rates
• Atrial rate: 60-100/min; ventricular escape rate: 40-60/min (junctional, narrow QRS) or 20-40/min (ventricular, wide QRS)
• The PR interval varies constantly - no fixed relationship between P and QRS
• Classic pearl: more P waves than QRS complexes on the strip

8. Left Ventricular Hypertrophy (LVH)

• Tall R waves in lateral leads (V5, V6, I, aVL)
• Deep S waves in right precordial leads (V1, V2)
• Sokolow-Lyon criteria: S in V1 + R in V5 or V6 >35 mm (adults)
• Cornell criteria: R in aVL + S in V3 >28 mm (men), >20 mm (women)
• LV strain pattern: ST depression + T-wave inversion in leads I, aVL, V4-V6 (asymmetric, downsloping)
• Left axis deviation, prolonged QRS

LVH vs. RVH vs. Normal in V1 and V6:

"LVH increases the amplitude of electrical forces directed to the left and posteriorly. Repolarization abnormalities may cause ST-segment depression and T-wave inversion in leads with a prominent R wave." - Harrison's Principles of Internal Medicine 22E, Fig. 247-9

9. Right Atrial / Left Atrial Enlargement (P-wave Changes)

Right Atrial Enlargement (P pulmonale):

• Tall, peaked P waves ≥2.5 mm in leads II, III, aVF
• P wave axis rightward

Left Atrial Enlargement (P mitrale):

• Broad, notched P waves ≥120 ms in limb leads (bifid "m-shaped" P wave)
• Biphasic P wave in V1 with prominent negative component (representing delayed LA depolarization)
• P-wave axis leftward

"Right atrial overload may cause tall, peaked P waves in the limb or precordial leads. Left atrial abnormality may cause broad, often notched P waves in the limb leads and a biphasic P wave in lead V1 with a prominent negative component." - Harrison's Principles of Internal Medicine 22E, Fig. 247-8

10. Hyperkalemia

"ECG changes characteristically progress from symmetrically peaked T waves, often with a shortened QT interval, to widening of the QRS complex, prolongation of the PR interval, loss of the P wave... to an ECG that resembles a sine wave before final progression into ventricular fibrillation or asystole." - Morgan & Mikhail's Clinical Anesthesiology 7e, Fig. 49-7

Bonus: Pulmonary Embolism (PE)

• Sinus tachycardia - most common finding
• S1Q3T3 pattern: deep S wave in lead I, Q wave + T-wave inversion in lead III
• Right axis deviation
• New right bundle branch block (complete or incomplete)
• T-wave inversions in V1-V4 (right heart strain pattern)
• Atrial fibrillation or flutter may occur

"Acute cor pulmonale due to pulmonary thromboembolism may be associated with sinus tachycardia, right axis deviation, the S₁Q₃T₃ pattern, slow R-wave progression, and ST-T abnormalities in V1 to V4 simulating acute anterior infarction." - Harrison's Principles of Internal Medicine 22E, p. 1915

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