ECG Grid Measurements: Standard Dimensions
On a standard electrocardiogram (ECG), one small square measures 1 mm × 1 mm, which equals 0.1 mV vertically and 0.04 seconds (40 ms) horizontally. 1
Standard ECG Calibration and Measurements
Vertical Axis (Amplitude)
- 1 small square (1 mm) = 0.1 mV
- 5 small squares (5 mm) = 0.5 mV
- 10 small squares (10 mm) = 1.0 mV
Horizontal Axis (Time)
- 1 small square (1 mm) = 0.04 seconds (40 ms)
- 5 small squares (5 mm) = 0.20 seconds (200 ms)
- Large square (5 mm × 5 mm) = 0.20 seconds (200 ms)
Paper Speed
- Standard paper speed: 25 mm/second 2
- This standardization ensures consistent time measurements across different ECG machines
Clinical Significance of ECG Grid Measurements
Voltage Measurements
- QRS voltage criteria for left ventricular hypertrophy (LVH) depend on accurate measurement of amplitudes 1
- For example, Sokolow-Lyon criterion uses the sum of SV1 and RV5 or RV6 amplitudes
- Cornell voltage criterion uses the sum of SV3 and RaVL
Interval Measurements
- Normal PR interval: 0.12-0.20 seconds (3-5 small squares) 2
- Normal QRS duration: ≤0.10 seconds (≤2.5 small squares) 2
- QT interval varies with heart rate: typically ≤0.44 seconds in men and ≤0.46 seconds in women 2
ST-Segment Elevation Criteria
- ST elevation thresholds for myocardial infarction diagnosis 1:
- Men ≥40 years: ≥0.2 mV (2 mm) in V2-V3, ≥0.1 mV (1 mm) in other leads
- Men <40 years: ≥0.25 mV (2.5 mm) in V2-V3
- Women: ≥0.15 mV (1.5 mm) in V2-V3, ≥0.1 mV (1 mm) in other leads
Technical Considerations
Filtering Effects
- Excessive filtering can distort important diagnostic features 2
- High-pass filters set too high (>0.5 Hz) may distort ST segments
- Low-pass filters set too low (<100 Hz) can reduce QRS amplitude
Digitization and Measurement
- Modern ECG machines convert analog signals to digital form before processing 1
- Initial sampling rates in modern systems can be as high as 10,000-15,000 samples per second 1
- This oversampling improves detection of narrow features like pacemaker spikes
Common Pitfalls to Avoid
- Using non-standard calibration settings without documentation
- Failing to recognize when ECG machines are not set to standard calibration (sometimes used for very large QRS complexes)
- Misinterpreting ECG measurements when paper speed is different from standard (e.g., 50 mm/second instead of 25 mm/second)
- Overlooking the effects of digital filtering on ECG morphology
Understanding these standard measurements is essential for accurate ECG interpretation and diagnosis of cardiac conditions that depend on precise measurement of intervals, durations, and amplitudes.