How do I perform and interpret a standard 12‑lead electrocardiogram (ECG)?

How to Perform and Interpret a Standard 12-Lead ECG

Electrode Placement and Patient Preparation

The standard 12-lead ECG requires precise electrode placement with the patient supine, using proper skin preparation to ensure high-quality recordings. 1

Limb Lead Placement

• Place limb electrodes distal to the shoulders and hips (traditionally on wrists and ankles, though any distal location on the limbs is acceptable for standard recordings) 1
• Clean and gently abrade the skin before electrode application to reduce noise and improve signal quality 1
• Position the patient supine with a pillow under the head 1

Precordial Lead Placement (Critical for Accuracy)

• V1: Fourth intercostal space at the right sternal border 1
• V2: Fourth intercostal space at the left sternal border 1
• V3: Midway between V2 and V4 1
• V4: Fifth intercostal space in the midclavicular line 1
• V5: Horizontal plane of V4 at the anterior axillary line (or midway between V4 and V6 if the anterior axillary line is ambiguous) 1
• V6: Horizontal plane of V4 at the midaxillary line 1

Critical Pitfall to Avoid

Never use torso placement of limb electrodes (Mason-Likar position) for standard diagnostic ECGs, as these tracings are not equivalent to standard ECGs and can produce false-positive or false-negative infarction criteria. 1 Torso placement is only acceptable for exercise testing or continuous monitoring when clearly labeled as such. 1

Recording Standards

Technical Requirements

• Use simultaneous lead acquisition with digital electrocardiographs to ensure temporal alignment of waveforms (maximum misalignment ≤10 ms) 1
• Record at 25 mm/s paper speed with standard calibration 1
• The standard format presents four 2.5-second columns representing a continuous 10-second record 1

Lead Organization

• Column 1: Simultaneous leads I, II, III 1
• Column 2: Simultaneous aVR, aVL, aVF 1
• Column 3: Simultaneous V1, V2, V3 1
• Column 4: Simultaneous V4, V5, V6 1

Systematic Interpretation Approach

Step 1: Calculate Heart Rate

Count the number of large squares (5 mm) between consecutive R waves, or count QRS complexes in a 6-second strip and multiply by 10. 2 Normal range is 60-100 beats per minute.

Step 2: Assess Rhythm Regularity

• Examine R-R intervals for consistency to determine if the rhythm is regular or irregular 2
• Identify the underlying rhythm (sinus, atrial, junctional, or ventricular) based on P wave morphology and relationship to QRS complexes 2

Step 3: Measure Critical Intervals

• PR interval: Normal is 120-200 ms (3-5 small squares) to assess AV conduction 2
• QRS duration: Normal is <120 ms (<3 small squares) to evaluate ventricular conduction 2
• QTc interval: Correct for heart rate; normal is <450 ms for men, <460 ms for women 2

Step 4: Determine Electrical Axis

Examine leads I and aVF to quickly determine the axis quadrant: 2

• Normal axis (-30° to +90°): Positive in both I and aVF 2
• Left axis deviation: Positive in I, negative in aVF 2
• Right axis deviation: Negative in I, positive in aVF 2

Step 5: Analyze Waveform Morphology

P Waves

• Normal morphology: Upright in I, II, aVF; biphasic in V1 2
• Duration should be <120 ms and amplitude <2.5 mm 2

QRS Complexes

• Look for pathologic Q waves (>1 mm wide and >1/3 the height of the R wave), which suggest myocardial infarction 2
• Assess R wave progression across precordial leads (amplitude increases from V1 to V4, then decreases toward V6) 2

ST Segments and T Waves

• Measure ST segments at the J point for elevation or depression indicating ischemia or injury 2
• Normal T wave morphology: Upright in I, II, V3-V6; inverted in aVR; variable in III, aVL, aVF, V1, V2 2

Step 6: Identify Additional Features

• Look for U waves (may indicate hypokalemia or bradycardia) 2
• Assess for chamber enlargement using P wave abnormalities and QRS voltage criteria 2
• Identify conduction abnormalities such as PR prolongation or QRS widening 2

Essential Clinical Considerations

Patient-Specific Factors

• Age and gender affect normal parameters: QT intervals are typically longer in women 2
• Medications can alter ECG findings: Antiarrhythmics and psychotropics commonly affect the tracing 2

Quality Assurance

Always verify computerized ECG interpretations yourself, as automated readings require physician confirmation. 2 Compare current tracings with previous ECGs to identify changes. 3

Understanding Lead Independence

The 12-lead ECG actually contains only 8 independent pieces of information: 2 measured limb lead potential differences (from which the other 4 limb leads are mathematically derived) and 6 independent precordial leads. 1 Despite this redundancy, multiple leads facilitate spatial interpretation critical for diagnosis. 1