ECG Dual Calibration: Standard Settings and Performance
Standard ECG calibration is universally set at 10 mm/mV amplitude and 25 mm/s paper speed, where 1 mV produces a 10 mm vertical deflection and each 1 mm horizontal box represents 0.04 seconds. 1
Standard Calibration Parameters
Amplitude (Voltage) Calibration
- Standard gain: 10 mm/mV where each small box (1 mm) represents 0.1 mV and each large box (5 mm) represents 0.5 mV 1
- This setting is universally recommended for routine ECG recordings across all age groups 1
- Alternative calibrations (5 mm/mV for high-amplitude signals or 20 mm/mV for low-amplitude signals) must be clearly documented on the tracing, as they invalidate standard voltage-based diagnostic criteria 1
Paper Speed Calibration
- Standard paper speed: 25 mm/s where each small box (1 mm) represents 0.04 seconds and each large box (5 mm) represents 0.20 seconds 1
- At this speed, accurate temporal measurements of intervals (PR, QRS, QT) can be performed 1
How Dual Calibration is Performed
Calibration Signal Display
- A standardization mark (calibration pulse) should be visible on the ECG tracing, typically appearing as a square wave deflection at the beginning or end of the recording 2
- This calibration pulse verifies that the machine is properly calibrated before interpretation begins 2
- When obtaining waveform strips for documentation, verify that voltage standardization and paper speed are visible on the strip 2
Technical Requirements for Accurate Calibration
Digital Sampling and Filtering Standards:
- Minimum sampling rate of 500 samples per second is required to allow accurate 150 Hz filtering and reduce amplitude measurement errors to <1% 2, 1
- A/D conversion requires minimum 12-bit precision for adequate signal resolution 1
Frequency Response Requirements:
- For adults and adolescents: high-frequency cutoff of at least 150 Hz 2, 1
- For infants and children: high-frequency cutoff of 250 Hz to capture rapid deflections accurately 2, 1
- Low-frequency cutoff of 0.05 Hz for routine filters, or relaxed to 0.67 Hz for linear digital filters with zero phase distortion 2, 1
Critical Pitfalls to Avoid
Technical Errors That Invalidate Calibration
- Suboptimal high-frequency cutoff (such as 40 Hz used for noise reduction) invalidates all amplitude measurements used for diagnostic classification 2
- Electrocardiographs should automatically alert users when suboptimal settings are used and restore proper settings between recordings 2
- Inadequate high-frequency response systematically underestimates QRS amplitude and smooths important features like Q waves 2
Documentation Requirements
- Always verify the calibration settings are displayed on printed or saved ECG strips 2
- Non-standard calibration settings must be explicitly documented to prevent misinterpretation of voltage-dependent criteria (such as LVH criteria) 1
- When using electronic health records, ensure voltage standardization is visible when waveforms are scanned or electronically transferred 2
Age-Specific Calibration Considerations
- Adults >35 years: Standard 10 mm/mV calibration with 150 Hz high-frequency cutoff 1
- Adolescents (16-35 years): Standard calibration, but recognize voltage criteria for LVH have lower accuracy in this age group 1
- Children and infants: Standard 10 mm/mV calibration but requires 250 Hz high-frequency cutoff to capture rapid deflections accurately 2, 1
Quality Assurance Measures
- Use the largest-amplitude deflection in each lead as representative of that measurement, per ANSI/AAMI standards 1
- Noise levels should be reduced to <1 µV (root mean square) in the averaged signal 1
- Digital electrocardiographs improve reproducibility by creating representative templates from accurately aligned complexes 2
- Verify electrode placement accuracy before attributing low voltage to pathology, as misplacement is the most common cause of apparent calibration errors 3