Heart Block USMLE notes & Mnemonics contains all the high-yield points you need to know.
Definition:
- Heart Block: A disorder in the heart’s rhythm due to a fault in the natural pacemaker system that controls heartbeats. (Ref)
Types:
- First-Degree Heart Block: Prolonged PR interval (>200 ms).
- Second-Degree Heart Block:
- Type I (Mobitz I/Wenckebach): Progressive lengthening of the PR interval until a beat is dropped.
- Type II (Mobitz II): Sudden dropped beat without a change in the preceding PR intervals.
- Third-Degree (Complete) Heart Block: Atria and ventricles beat independently.
Etiology:
- Ischemic Heart Disease (Ref)
- Cardiomyopathies
- Infectious (Lyme disease, Chagas disease)
- Drugs (Beta-blockers, Calcium channel blockers, Digoxin)
- Electrolyte Imbalances
Clinical Features:
- Asymptomatic: Especially in first-degree and Mobitz I.
- Bradycardia: Especially in Mobitz II and third-degree.
- Dizziness, Syncope: Due to decreased cardiac output.
- Heart Failure Symptoms: In severe cases.
Diagnosis:
- ECG: Key tool in diagnosis.
- History and Physical Exam: Assess for underlying causes and symptoms.
- Laboratory Tests: Check for electrolyte imbalances, cardiac enzymes.
Management:
- First-Degree and Mobitz I: Usually no treatment required unless symptomatic.
- Mobitz II and Third-Degree: Often require pacemaker insertion.
- Treat Underlying Cause: Adjust medications, manage heart disease, etc.
- Emergency Management: Transcutaneous pacing or atropine if symptomatic and unstable.
Mnemonic: “PR Longer, Longer, Longer, Drop; Then you have a Wenckebach”
- Helps remember the progressive PR interval prolongation in Mobitz I.
Chart: Heart Block Features
| Type | ECG Characteristics | Management |
|---|---|---|
| First-Degree | Prolonged PR interval (>200 ms) | Usually none |
| Second-Degree Type I | Progressive PR lengthening until a beat drops | Usually none unless symptomatic |
| Second-Degree Type II | Constant PR intervals with dropped beats | Pacemaker if symptomatic |
| Third-Degree | Atria and ventricles beat independently | Pacemaker, emergency pacing if unstable |
High-Yield Facts:
- Mobitz II and Third-Degree are more likely to progress to complete heart block.
- First-Degree and Mobitz I are often benign and discovered incidentally.
- Drug-Induced: Identify and discontinue offending medications.
- Lyme Disease: Consider in young patients with new heart block, particularly in endemic areas.
Note: On the USMLE, questions about heart block often involve interpretation of ECGs and correlating clinical scenarios. It is essential to recognize different types of heart blocks and understand their clinical implications and appropriate management strategies.
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