USMLE/COMLEX 2 - ECG

Here are key facts for USMLE Step 2 CK & COMLEX-USA Level 2 from the Electrocardiogram/ECG, EKG tutorial, focusing on clinical application, interpretation, and management that are essential for these exams. See the tutorial notes for further details and relevant links.

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1. Basic Components & Clinical Relevance:

• Waves: P (atrial depolarization), QRS (ventricular depolarization), T (ventricular repolarization), U (occasionally present)
• Segments: PR, ST (clinically significant for diagnosing ischemia/infarction)
• Intervals: PR, QT (critical for assessing conduction and repolarization abnormalities)

2. Normal Parameters & Deviations:

• PR interval: 0.12-0.20 seconds (prolongation indicates AV conduction block)
• QRS duration: <0.12 seconds (widening suggests bundle branch block or ventricular origin)
• QT interval: Heart rate dependent (prolongation associated with increased risk of arrhythmias)

3. Conduction Pathway Disorders:

• SA node dysfunction presents as inappropriate bradycardia or sinus pauses
• AV nodal disease manifests as varying degrees of heart block
• Bundle branch pathology produces characteristic wide QRS morphologies

4. Heart Rate Determination:

• Calculate using PP or RR intervals
• Clinical significance of bradycardia (<60 bpm) and tachycardia (>100 bpm)

5. Wave Morphology Interpretation: Changes in wave appearance provide diagnostic clues to underlying pathology. 1. Normal Conduction Sequence:

• Sinoatrial (SA) node → atria → AV node → bundle of His → bundle branches → Purkinje fibers → ventricular myocardium
• Functional significance: Ensures coordinated contraction from apex to base

2. Rhythm Assessment:

• Regularity: Consistent RR intervals indicate regular rhythm
• Rate: Normal (60-100 bpm), bradycardia (<60 bpm), tachycardia (>100 bpm)
• P wave: Present before each QRS in normal sinus rhythm
• PR interval: Consistent in normal conduction

3. Sinus Rhythms:

• Normal sinus rhythm: Regular P waves with normal PR interval and rate
• Sinus tachycardia: Rate >100 bpm with normal P wave morphology
• Sinus bradycardia: Rate <60 bpm with normal P wave morphology

4. Heart Blocks:

• First-degree: Prolonged PR interval (>0.20 seconds)
• Second-degree: Intermittent failure of AV conduction (Mobitz I/Wenckebach or Mobitz II)
• Third-degree (complete): No relationship between P waves and QRS complexes

5. Bundle Branch Blocks: Produce wide QRS (>0.12 seconds) with characteristic morphologies in precordial leads. 1. Ischemic Changes:

• ST segment depression: Subendocardial ischemia
• T wave inversion: Myocardial ischemia or strain
• ST segment elevation: Transmural injury/infarction

2. Evolution of Myocardial Infarction:

• Hyperacute T waves → ST elevation → Q wave development → T wave inversion → resolution of ST elevation
• Timeline of changes helps determine infarct age

3. Localization of Infarction:

• Anterior: V1-V4 (left anterior descending artery)
• Lateral: I, aVL, V5-V6 (left circumflex artery)
• Inferior: II, III, aVF (right coronary artery)
• Posterior: Tall R waves and ST depression in V1-V2 (reciprocal changes)

4. Right Ventricular Infarction: ST elevation in right-sided leads (V4R) 5. Non-ST Elevation MI: ST depression or T wave inversions without ST elevation, diagnosed with cardiac biomarkers. 1. Potassium Abnormalities:

• Hyperkalemia: Tall, peaked T waves → widened QRS → sine wave pattern
• Hypokalemia: U wave prominence, ST depression, flattened T waves

2. Calcium Abnormalities:

• Hypercalcemia: Shortened QT interval
• Hypocalcemia: Prolonged QT interval

3. Magnesium Abnormalities:

• Hypomagnesemia: Prolonged QT, U waves, increased risk of torsades de pointes

4. Drug Effects:

• Digoxin: "Scooped" ST segments (Salvador Dali mustache), shortened QT
• Antiarrhythmics: Various effects on intervals and repolarization
• Tricyclic antidepressants: Prolonged QRS, rightward axis, terminal R wave in aVR

5. Prominent U Wave: May indicate hypokalemia, bradycardia, ischemia, or effects of antiarrhythmic drugs.

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1. Rate & Rhythm Assessment:

• Calculate heart rate (300 ÷ number of large boxes between consecutive R waves)
• Assess regularity of rhythm (consistent vs. variable RR intervals)
• Identify P waves and their relationship to QRS complexes

2. Interval Measurement & Interpretation:

• PR interval: Normal (0.12-0.20 sec), prolonged (>0.20 sec), shortened (<0.12 sec)
• QRS duration: Normal (<0.12 sec), prolonged (>0.12 sec)
• QT interval: Corrected for heart rate (QTc)

3. Axis Determination:

• Normal axis: +90° to -30°
• Left axis deviation: -30° to -90°
• Right axis deviation: +90° to +180°
• Clinical significance and associated conditions

4. Morphology Analysis:

• P wave: Size, shape, and orientation
• QRS complex: Configuration in different leads
• ST segment: Elevation, depression, or normal
• T wave: Normal vs. abnormal morphology

5. Systematic Lead Examination: Evaluating all 12 leads for diagnostic patterns.

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Below is information not explicitly contained within the tutorial but important for USMLE Step 2 CK & COMLEX Level 2. 1. Pediatric ECG Differences: Age-specific normal values, lead placement, and common abnormalities. 2. Geriatric Considerations: Higher prevalence of conduction abnormalities, effects of comorbidities, and medication interactions. 3. Pregnancy-Related Changes: Physiologic left axis deviation, increased heart rate, and occasional benign arrhythmias. 4. Athletes: Physiologic bradycardia, early repolarization, and increased QRS voltage mimicking pathology. 5. Specific Disease States: Characteristic ECG findings in hypertrophic cardiomyopathy, infiltrative diseases, and pulmonary hypertension. 1. Pre-operative Risk Assessment: ECG abnormalities that increase perioperative risk and require further evaluation. 2. Continuous Monitoring Indications: When to use continuous ECG monitoring vs. intermittent assessment. 3. Post-cardiac Surgery Patterns: Expected changes after CABG, valve replacement, and transplantation. 4. Mechanical Ventilation Effects: Impact of positive pressure ventilation on ECG appearance. 5. Extracorporeal Support: ECG considerations during ECMO and other mechanical support devices. 1. Emergent Interventions:

• ST-elevation MI: Immediate reperfusion (PCI or thrombolytics)
• Unstable bradyarrhythmias: Temporary pacing
• Hemodynamically unstable tachyarrhythmias: Cardioversion

2. Urgent Evaluation:

• Non-ST elevation MI: Antiplatelet therapy, anticoagulation, risk stratification
• Prolonged QT interval: Correction of underlying causes, avoidance of QT-prolonging medications

3. Monitoring Requirements:

• First-degree AV block: Usually requires observation only
• Second-degree Mobitz I: Monitoring for progression
• Second-degree Mobitz II and third-degree block: Often require pacemaker

4. Preventive Strategies:

• Rate control for atrial fibrillation
• Antiarrhythmic therapy for recurrent symptomatic arrhythmias

5. Chronic Management: Long-term therapies based on underlying causes and risk assessment. 1. Antiarrhythmic Drug Selection: Using ECG features to guide appropriate pharmacologic therapy. 2. Cardioversion Protocols: Indications, contraindications, and procedural management. 3. Temporary Pacing Indications: When to implement transcutaneous, transvenous, or epicardial pacing. 4. Permanent Pacemaker Programming: Rate responsiveness, sensing parameters, and mode selection based on ECG findings. 5. Ablation Therapy Decision-Making: Identifying arrhythmia mechanisms amenable to catheter ablation. 1. Echocardiographic Correlation: Integrating ECG findings with structural and functional echocardiographic data. 2. Cardiac Biomarker Integration: Combining ECG changes with troponin and other biomarkers in ACS assessment. 3. Exercise Stress Testing: Interpreting stress-induced ECG changes and their clinical significance. 4. CT and MRI Correlation: Relating ECG abnormalities to advanced imaging findings. 5. Nuclear Imaging Integration: Using ECG to guide interpretation of perfusion studies. 1. Mimics of Ischemia: Conditions that can produce ST-T changes simulating ischemia (LVH, BBB, electrolyte abnormalities). 2. Technical Factors Affecting Interpretation: Lead misplacement, artifact, and improper calibration. 3. Critical Value Communication: Protocols for urgent notification of life-threatening ECG findings. 4. Documentation Standards: Requirements for complete and accurate ECG interpretation documentation. 5. Quality Improvement Strategies: Measures to improve ECG interpretation accuracy and reduce diagnostic errors.