How do I interpret a standard 12‑lead ECG recorded at 25 mm/s and 10 mm/mV?

How to Read a Standard 12-Lead ECG

Interpret a 12-lead ECG using a systematic five-step approach that evaluates rate, rhythm, axis, intervals, and morphology—always verifying that the recording meets standard calibration of 25 mm/s paper speed and 10 mm/mV gain before beginning your analysis. 1

Verify Calibration First

Before interpreting any ECG, you must confirm proper recording standards to ensure accurate measurements:

• Check the calibration pulse: It should produce a 10 mm vertical deflection, confirming the standard 10 mm/mV gain 1
• Verify paper speed: Standard recording at 25 mm/s means each small (1 mm) box = 0.04 seconds and each large (5 mm) box = 0.2 seconds 1
• Without proper calibration, all voltage-based criteria (ST-segment changes, LVH criteria, Q-wave depth) become unreliable 1

This is a critical but commonly overlooked step—many interpretation errors stem from analyzing ECGs recorded at non-standard settings 2.

Systematic Interpretation Framework

The American Heart Association endorses a structured approach to ECG analysis that includes five key processing steps 3:

1. Rate Calculation

• Count the number of large boxes between R waves and divide into 300, or count R waves in 6 seconds and multiply by 10
• Normal: 60-100 bpm

2. Rhythm Assessment

• Identify P waves before each QRS complex
• Assess regularity of R-R intervals
• Classify complexes as "dominant" (normal sinus) versus "nondominant" (ectopic) types 3

3. Axis Determination

• Use leads I and aVF to determine the cardiac axis quadrant
• Normal axis: -30° to +90°

4. Interval Measurements (Feature Extraction)

• PR interval: 0.12-0.20 seconds (3-5 small boxes)
• QRS duration: <0.12 seconds (3 small boxes)
• QT interval: Varies with heart rate; use Bazett's formula for correction

5. Waveform Morphology Analysis

• Examine each lead systematically for P wave, QRS complex, ST segment, and T wave abnormalities
• The 12 leads provide different spatial views of cardiac electrical activity, with each lead representing a specific anatomical perspective 4

Critical Measurement Standards

ST-segment analysis requires strict adherence to calibration standards because diagnostic thresholds are voltage-based 1:

• STEMI criteria: ≥2 mm (0.2 mV) elevation in V1-V3 for men ≥40 years; ≥1 mm (0.1 mV) in other leads 1
• At 10 mm/mV gain, 1 mm of ST elevation = 0.1 mV 1
• The J point (junction between QRS and ST segment) must be identified accurately, with acceptable error <0.1 mV for ischemia detection 5

Lead-Specific Interpretation

The standard 12-lead ECG consists of 3:

• Limb leads (I, II, III, aVR, aVL, aVF): View the heart in the frontal plane
• Precordial leads (V1-V6): View the heart in the horizontal plane
  • V1-V2: Right ventricle and septum
  • V3-V4: Anterior wall
  • V5-V6: Lateral wall

Electrode placement is critical—even one intercostal space displacement can alter QRS amplitude by >200 μV and change diagnostic classifications in up to 8% of cases 6.

Common Pitfalls to Avoid

Electrode Misplacement

• Limb electrode position variations, particularly of the left arm electrode, significantly alter the ECG 3
• Precordial electrode displacement, especially longitudinal (up/down), causes the most diagnostic error 6, 2
• Always verify proper electrode placement: V1 and V2 at 4th intercostal space, V4 at 5th intercostal space in midclavicular line 3

Technical Artifacts

• Inadequate skin preparation increases noise and baseline wander 3
• Improper filtering can distort high-frequency signals like pacemaker spikes and Q waves 3
• Patient position changes (elevation, rotation) alter recorded amplitudes and axes 3

Computer Interpretation Reliance

• Physician overreading of computer-generated interpretations is mandatory—automated systems have improving but imperfect sensitivity and specificity 3
• Computer algorithms may use different diagnostic criteria, leading to interpretation variability 3

Diagnostic Classification Approach

After waveform recognition and feature extraction, diagnostic classification can be either 3:

• Heuristic (rule-based): Uses experience-based diagnostic criteria
• Statistical: Compares to databases of known ECG patterns 7

Both approaches require adherence to standardized measurement methodology to ensure consistent interpretation across different machines and institutions 1.