USMLE Guide: EKG Interpretation

Introduction

The electrocardiogram (EKG) is a crucial diagnostic tool used to evaluate the electrical activity of the heart. EKG interpretation is an essential skill for physicians, especially those preparing for the United States Medical Licensing Examination (USMLE). This guide aims to provide an overview of EKG interpretation and highlight key concepts that are commonly tested on the USMLE.

1. Understanding the EKG Waveforms

An EKG displays several important waveforms that represent different electrical events within the heart. These waveforms include:

• P Wave: Represents atrial depolarization (contraction).
• QRS Complex: Reflects ventricular depolarization (contraction) and atrial repolarization (relaxation).
• T Wave: Represents ventricular repolarization (relaxation).

2. Calculating Heart Rate

Heart rate calculation is a crucial skill in EKG interpretation. The most common method to calculate heart rate is the 300-150-100-75-60-50 method:

• Count the number of large squares between consecutive R waves.
• Divide 300 by the number of large squares to obtain the heart rate in beats per minute (bpm).

3. Determining Rhythm

EKG interpretation involves identifying the underlying rhythm. Common rhythms include:

• Normal Sinus Rhythm: Regular P waves, consistent PR interval, and a QRS complex within the normal range.
• Atrial Fibrillation: No discernible P waves, irregularly irregular R-R intervals, and an irregularly irregular rhythm.
• Atrial Flutter: Rapid and regular P waves, often referred to as "sawtooth" appearance.

4. Detecting Conduction Abnormalities

EKG interpretation also involves identifying conduction abnormalities. Key abnormalities include:

• First-Degree Atrioventricular (AV) Block: Prolonged PR interval (>0.20 seconds).
• Second-Degree av block Type I (Wenckebach): Progressive prolongation of PR interval until a QRS complex is dropped.
• Second-Degree AV Block Type II: Consistent PR interval with intermittent dropped QRS complexes.
• Third-Degree (Complete) AV Block: No association between P waves and QRS complexes.

5. Recognizing Ischemia and Infarction

EKG interpretation is essential in identifying signs of myocardial ischemia and infarction. Key findings include:

• ST-Segment Elevation: Indicates acute myocardial infarction (AMI) or injury.
• ST-Segment Depression: Often seen in myocardial ischemia.
• T-Wave Inversions: Can be indicative of myocardial ischemia or infarction.

6. Identifying Chamber Enlargement

EKG interpretation can help identify chamber enlargement, particularly of the atria and ventricles. Key findings include:

• Left Atrial Enlargement: Broad, notched P waves in lead II and/or aVL.
• Left Ventricular Hypertrophy: Increased QRS voltage, often with ST-T wave changes.
• Right Ventricular Hypertrophy: Right axis deviation, right atrial enlargement, and RV strain pattern.

7. Recognizing Electrolyte Abnormalities

EKG interpretation can provide clues regarding electrolyte abnormalities. Key findings include:

• Hyperkalemia: Peaked T waves, widened QRS complex, and prolonged PR interval.
• Hypokalemia: Flattened or inverted T waves, U waves, and ST-segment depression.

Conclusion

EKG interpretation is a vital skill for physicians preparing for the USMLE. Mastering the interpretation of waveforms, calculating heart rate, identifying rhythm abnormalities, detecting conduction abnormalities, and recognizing signs of ischemia, infarction, chamber enlargement, and electrolyte abnormalities will greatly enhance your ability to answer EKG-related questions on the exam. Regular practice and exposure to various EKG patterns are key to developing proficiency in EKG interpretation.