What are the basics of electrocardiogram (ECG) interpretation?

Basics of ECG Interpretation

A systematic approach to ECG interpretation is essential for accurate diagnosis of cardiac conditions, requiring careful analysis of rate, rhythm, axis, intervals, and waveform morphology. 1

Step-by-Step Approach to ECG Interpretation

1. Assess Heart Rate and Rhythm

• Calculate heart rate by counting the number of large squares between consecutive R waves or by counting QRS complexes in a 6-second strip and multiplying by 10 (normal: 60-100 beats per minute) 1, 2
• Evaluate rhythm regularity by examining R-R intervals for consistency to determine if the rhythm is regular or irregular 1
• Identify the underlying rhythm (sinus, atrial, junctional, ventricular) based on P wave morphology and relationship to QRS complexes 1

2. Analyze Intervals and Segments

• Measure PR interval (normal: 120-200 ms or 3-5 small squares) to assess AV conduction 1, 2
• Measure QRS duration (normal: <120 ms or <3 small squares) to evaluate ventricular conduction 1, 3
• Calculate QT interval and correct for heart rate (QTc, normal: <450 ms for men, <460 ms for women) to assess repolarization 1, 2
• Examine ST segments for elevation or depression that may indicate ischemia, injury, or other pathology 1

3. Determine Electrical Axis

• Examine leads I and aVF to quickly determine the quadrant of the axis (normal axis: between -30° and +90°) 1
• Left axis deviation is characterized by a positive deflection in lead I and a negative deflection in aVF 1
• Right axis deviation is characterized by a negative deflection in lead I and a positive deflection in aVF 1

4. Analyze P Waves, QRS Complexes, and T Waves

• Evaluate P wave morphology (normal: upright in I, II, aVF; biphasic in V1) and measurements (normal: <120 ms duration, <2.5 mm amplitude) 1
• Analyze QRS complex morphology for pathologic Q waves (>1 mm wide and >1/3 the height of the R wave) suggesting myocardial infarction 1
• Assess R wave progression across precordial leads (normally R wave amplitude increases from V1 to V4 and then decreases toward V6) 1
• Evaluate T wave morphology (normally upright in leads I, II, V3-V6, inverted in aVR, and variable in III, aVL, aVF, V1, and V2) 1

5. Look for Additional Features

• Identify U waves, which may indicate hypokalemia, bradycardia, or other conditions 1
• Assess for signs of chamber enlargement using P wave abnormalities and QRS voltage criteria 1
• Identify conduction abnormalities such as PR prolongation and QRS widening 1

Technical Considerations

• The fundamental frequency for the QRS complex at the body surface is 10 Hz, and most diagnostic information is contained below 100 Hz in adults 3
• Inadequate high-frequency response results in systematic underestimation of signal amplitude and smoothing of notches and Q waves 3
• Inadequate low-frequency response can result in important distortions of repolarization 3
• Avoid improper electrode placement, especially chest leads, which can significantly alter ECG interpretation and lead to false diagnoses 2
• Computerized ECG interpretations should always be verified by a qualified physician 1, 2

Special Considerations

• Age and gender affect normal ECG parameters (e.g., QT intervals are typically longer in women) 1
• Certain medications can affect ECG findings (e.g., antiarrhythmics, psychotropics) 1
• Always interpret ECG findings in the context of the patient's clinical presentation 2
• The QRS of infants often contains important components as high as 250 Hz, requiring different filtering parameters 3

Common Pitfalls to Avoid

• Over-reliance on computerized interpretation without physician verification 2
• Interpreting ECG findings in isolation without considering clinical context 2
• Inadequate filtering that can distort important waveform components 3
• Failure to recognize that interpretation varies even among expert electrocardiographers, emphasizing the need for ongoing education 2