ECG Interpretation | Clinical Medicine

Introduction

This tutorial provides a systematic approach to ECG interpretation, essential for clinicians to accurately assess heart rhythms and identify potential abnormalities. By following these steps, you can confidently analyze an ECG and make informed clinical decisions.

Chapter 1: Approach to ECG Interpretation

1. Determine Heart Rate

   • Use the R-wave method:
     • Count the number of R waves in a 6-second strip.
     • Multiply by 10 for beats per minute.
   • Alternatively, use the box method:
     • Count the number of large boxes between R waves.
     • Divide 300 by the number of boxes to find the heart rate.

2. Assess Rhythm

   • Classify as:
     • Narrow regular
     • Narrow irregular
     • Wide regular
     • Wide irregular
   • This classification helps identify tachycardia or bradycardia and their underlying causes.

3. Evaluate Axis

   • Check leads I and aVF:
     • Positive in both: Normal axis
     • Positive in lead I and negative in aVF: Left axis deviation
     • Negative in lead I and positive in aVF: Right axis deviation
     • Negative in both: Extreme right axis deviation

4. Analyze Intervals

   • Focus on PR and QT intervals:
     • A prolonged PR interval can indicate AV blocks.
     • A QT interval greater than half the R-R interval suggests the risk of Torsades de Pointes.

5. Examine P Waves

   • Look for signs of atrial enlargement:
     • Left Atrial Enlargement: Bi-phasic P wave in lead II and a terminal component larger than the initial in lead V1.
     • Right Atrial Enlargement: Tall P waves in lead II and a dominant initial component in lead V1.

6. Review QRS Complex

   • Identify bundle branch blocks:
     • Left Bundle Branch Block: Deep S wave in V1/V2 and notched R wave in V5/V6.
     • Right Bundle Branch Block: RSR' pattern in V1/V2 and slurred S wave in V5/V6.
   • Assess for ventricular hypertrophy:
     • A deep S wave in V1/V2 combined with tall R waves in V5/V6 suggests left ventricular hypertrophy.

7. Assess ST Segment and T Waves

   • Check for ST segment elevation or depression:
     • Elevation greater than 1 mm in contiguous leads may indicate myocardial infarction.
     • Depression greater than 0.5 mm in V2/V3 or 1 mm in others can suggest ischemia.
   • T-wave inversions also indicate possible ischemia.

Chapter 2: Approach to Rate

1. Tachycardia Identification

   • Heart rate >100 bpm indicates tachycardia. Determine if QRS is narrow or wide:
     • Narrow QRS: Can be sinus tachycardia, atrial flutter, or paroxysmal supraventricular tachycardia.
     • Wide QRS: Consider ventricular tachycardia or other arrhythmias.

2. Bradycardia Identification

   • Heart rate <60 bpm indicates bradycardia. Check PR interval:
     • Normal PR interval with consistent P-QRS relationship suggests sinus bradycardia.
     • Prolonged PR interval indicates first-degree AV block.
     • Progressive prolongation with dropped QRS suggests second-degree AV block (Type I or II).
     • No correlation between P waves and QRS complexes indicates third-degree AV block.

Chapter 3: Approach to Axis

1. Normal Axis

   • Both lead I and aVF are positive.

2. Left Axis Deviation

   • Positive in lead I and negative in aVF. Confirm with lead II being negative.

3. Right Axis Deviation

   • Negative in lead I and positive in aVF.

4. Extreme Right Axis Deviation

   • Both leads show negative deflections.

Chapter 4: Approach to Intervals

1. PR Interval Assessment

   • Prolonged PR intervals (>200 ms) suggest AV blocks.
   • Short PR intervals may indicate conditions like Wolff-Parkinson-White syndrome.

2. QT Interval Assessment

   • A QT interval >500 ms increases the risk of Torsades de Pointes.
   • Monitor for medications that may prolong the QT interval.

Chapter 5: Approach to P Waves

1. Left Atrial Enlargement

   • Identify with bi-phasic P wave in lead II and larger terminal component in V1.

2. Right Atrial Enlargement

   • Tall P waves in lead II and dominant initial component in V1.

Chapter 6: Approach to QRS Complex

1. Bundle Branch Blocks

   • Assess V1/V2 for deep S waves and notched R waves in V5/V6 for left bundle branch block.
   • RSR' pattern in V1/V2 and slurred S wave in V5/V6 for right bundle branch block.

2. Ventricular Hypertrophy

   • Look for deep S waves in V1/V2 and tall R waves in V5/V6.

Chapter 7: Approach to ST-Segment and T Waves

1. Evaluate ST Segment

   • ST depression greater than 0.5 mm in V2/V3 or 1 mm in other leads can indicate ischemia.
   • ST elevation greater than 1 mm in contiguous leads signals potential myocardial infarction.

2. T-Wave Inversions

   • Assess for inversions which can suggest ischemia, especially in the context of chest pain.

Conclusion

Understanding and interpreting ECGs requires a systematic approach. By following these steps—assessing heart rate, rhythm, axis, intervals, P waves, QRS complex, and ST segments—you can accurately identify various cardiac conditions. Continuous practice and application of these principles will enhance your ECG interpretation skills, allowing for better patient care.