Systematic Approach to EKG Interpretation
Follow a structured, step-by-step method when interpreting an EKG, beginning with rate and rhythm assessment, followed by interval measurements, axis determination, and morphologic analysis of waveforms—this systematic approach is essential for accurate cardiac diagnosis. 1
Step 1: Assess Heart Rate and Rhythm
- Calculate heart rate by counting the number of large squares between consecutive R waves (divide 300 by the number of large squares), or count QRS complexes in a 6-second strip and multiply by 10 1
- Evaluate rhythm regularity by examining R-R intervals for consistency across the tracing 1
- Identify the underlying rhythm (sinus, atrial, junctional, or ventricular) based on P wave morphology and its relationship to QRS complexes 1
Step 2: Measure Critical Intervals
- PR interval: Measure from the beginning of the P wave to the beginning of the QRS complex; normal is 120-200 ms (3-5 small squares) to assess AV conduction 1
- QRS duration: Measure the width of the QRS complex; normal is <120 ms (<3 small squares) to evaluate ventricular conduction 1
- QT interval: Measure from the beginning of the QRS to the end of the T wave and correct for heart rate (QTc); normal is <450 ms for men and <460 ms for women 1
Step 3: Determine Electrical Axis
- Quick quadrant method: Examine leads I and aVF simultaneously 1
Step 4: Analyze Waveform Morphology
P Waves
- Assess P wave morphology: Should be upright in leads I, II, and aVF; biphasic in V1 1
- Measure P wave duration and amplitude: Normal is <120 ms and <2.5 mm 1
QRS Complexes
- Look for pathologic Q waves: >1 mm wide and >1/3 the height of the R wave suggests myocardial infarction 1
- Evaluate R wave progression: R wave amplitude should increase from V1 to V4, then decrease toward V6 1
ST Segments and T Waves
- Examine ST segments for elevation or depression measured at the J point, which may indicate ischemia or injury 1
- Assess T wave morphology: Normally upright in leads I, II, V3-V6; inverted in aVR; variable in III, aVL, aVF, V1, and V2 1
Step 5: Identify Additional Features
- Search for U waves, which may indicate hypokalemia, bradycardia, or other conditions 1
- Assess for chamber enlargement using P wave abnormalities and QRS voltage criteria 1
- Identify conduction abnormalities such as PR prolongation or QRS widening 1
Critical Technical Considerations
Electrode Placement Accuracy
- Proper precordial lead positioning is essential: Variation in lead placement of as little as 2 cm can result in important diagnostic errors, particularly for anteroseptal infarction and ventricular hypertrophy 2
- Lead misplacement alters computer-based diagnostic statements in up to 6% of recordings 2
- Use skin marking for serial tracings in acute or subacute care settings to promote reproducibility when electrodes cannot remain in place 2
Computer Interpretation Limitations
- Always verify computerized ECG interpretations with physician overreading—automated algorithms can be the source of erroneous information 1, 3
- Computer diagnostic accuracy depends heavily on proper signal processing, filtering (bandwidth 1-150 Hz for adults, up to 250 Hz for children), and template formation 2
Patient-Specific Factors
- Consider age and gender: QT intervals are typically longer in women, and normal parameters vary with age 1
- Account for medications: Antiarrhythmics and psychotropics can significantly affect ECG findings 1
- Body position changes can cause QRS and ST-T changes that may trigger false alarms during monitoring 4
Common Pitfalls to Avoid
- Inadequate high-frequency response results in systematic underestimation of signal amplitude and smoothing of notches and Q waves 2
- Inadequate low-frequency response produces important distortions of repolarization 2
- Reproducibility of amplitude measurements is generally worse than duration measurements, particularly with variable lead placement 2
- Personnel require periodic retraining in proper lead positioning to maintain diagnostic accuracy 2