ECG Review

Always approach ECGs in the same order:

1. Rate.
2. Rhythm.
3. Axis.
4. Intervals (PR, QRS, QT).
5. Hypertrophy.
6. Conduction blocks.
7. ST-segment and T-wave changes.
8. Clinical correlation.

Four rhythm questions

When analyzing any rhythm strip, ask:

1. Are P waves present?
2. Is the QRS narrow (<120 ms) or wide (>120 ms)?
3. What is the relationship between P waves and QRS complexes?
4. Is the rhythm regular or irregular?

Pacemaker rates

Important associations

Left axis deviation: left anterior fascicular block and left ventricular hypertrophy.

Right axis deviation: right ventricular hypertrophy, pulmonary hypertension, and left posterior fascicular block.

Atrial fibrillation

• No distinct P waves.
• Irregularly irregular rhythm.
• Increased stroke risk.

Management: rate control and anticoagulation when indicated.

Atrial flutter

• Sawtooth flutter waves.
• Atrial rate approximately 300 bpm.
• Commonly 2:1 conduction.

Supraventricular tachycardia (SVT)

• Regular narrow-complex tachycardia.
• Treated with vagal maneuvers and adenosine.

Ventricular tachycardia

• Wide-complex tachycardia.
• Treat unstable patients with synchronized cardioversion.

First-degree AV block

• PR interval >200 ms.
• Every P wave conducts.

Causes: increased vagal tone, beta blockers, and digoxin.

Mobitz I (Wenckebach)

• Progressive PR prolongation.
• Dropped QRS complex.

Common causes: inferior MI and digoxin toxicity.

Mobitz II

• Fixed PR interval.
• Sudden dropped beat.
• Often progresses to complete heart block.
• Requires a pacemaker.

Third-degree AV block

• Complete AV dissociation.
• P waves and QRS complexes are independent.

Treatment: permanent pacemaker.

Right bundle branch block (RBBB)

ECG findings: wide QRS, M-shaped QRS in V1, and W-shaped QRS in V6.

Mnemonic: “M in V1, W in V6.”

Left bundle branch block (LBBB)

ECG findings: wide QRS, W-shaped QRS in V1, and M-shaped QRS in V6.

Mnemonic: “W in V1, M in V6.”

Evolution of acute MI

1. Hyperacute T waves.
2. ST elevation.
3. T-wave inversion.
4. Pathologic Q waves.

Inferior MI

Leads: II, III, aVF.

Usually due to right coronary artery occlusion. Associated findings: bradycardia and AV block.

Anterior MI

Leads: V1–V4.

Usually due to LAD occlusion.

Lateral MI

Leads: I, aVL, V5, V6.

Usually due to left circumflex occlusion.

Posterior MI

Findings: ST depression in V1–V3 and tall R waves in V1–V3.

Criteria:

• Width >0.04 seconds.
• Depth >one-third of R-wave height.

Suggests prior myocardial infarction.

Small Q waves in lateral leads may be normal.

Seen in young healthy adults and athletes.

Characteristics: J-point elevation, a stable ECG finding, and no symptoms.

Hypokalemia

ECG findings: flattened T waves and prominent U waves.

Clinical significance: increases the risk of digoxin toxicity.

Hyperkalemia

ECG findings: peaked T waves, widened QRS, and a sine-wave pattern in severe cases.

Treatment: calcium gluconate.

Symptoms: nausea, vomiting, visual disturbances, and arrhythmias.

ECG findings: AV block, junctional rhythms, and atrial tachycardia with block.

Risk factors: hypokalemia and renal failure.

ECG findings: short PR interval, delta wave, and wide QRS.

Clinical importance: predisposes to AV reentrant tachycardia; can cause dangerous AF with rapid ventricular response.

Top 10 USMLE ECG diagnoses

1. Atrial fibrillation.
2. Atrial flutter.
3. Supraventricular tachycardia.
4. Ventricular tachycardia.
5. First-degree AV block.
6. Mobitz I AV block.
7. Mobitz II AV block.
8. Complete heart block.
9. STEMI localization.
10. Wolff–Parkinson–White syndrome.

For every ECG

1. Determine the rate.
2. Determine whether the rhythm is regular or irregular.
3. Look for P waves.
4. Determine whether the QRS is narrow or wide.
5. Evaluate ST segments and T waves.
6. Correlate findings with the clinical presentation.