How should an ECG be calibrated before each recording to ensure accurate voltage and timing measurements?

ECG Calibration Standards

Before each ECG recording, the device must display a standard calibration pulse (square-wave deflection) at 10 mm/mV amplitude and 25 mm/second paper speed, with these settings clearly marked on every tracing to ensure accurate voltage and timing measurements. 1

Standard Calibration Requirements

Voltage Calibration

• Set the standard gain to 10 mm/mV, where 1 mV produces a 10 mm deflection on the ECG paper 1, 2
• Each small box (1 mm) represents 0.1 mV and each large box (5 mm) represents 0.5 mV 1
• The calibration pulse must be visible at the beginning or end of each tracing to confirm proper recorder function before interpretation 1
• Voltage calibration must be accurate to ±2% to ensure reliable amplitude measurements 3

Paper Speed and Timing

• Standard paper speed is 25 mm/second, allowing accurate measurement of intervals critical for detecting conduction abnormalities and infarction patterns 2
• At this speed, each small box (1 mm) represents 0.04 seconds and each large box (5 mm) represents 0.20 seconds 1

Documentation Requirements

• Every printed or digitally saved ECG strip must show voltage (mm/mV) and paper-speed (mm/s) markings to ensure traceability 1
• When ECGs are stored or transferred within electronic health records, voltage standardization must remain visible to avoid misinterpretation of voltage-dependent criteria 1
• Always document if non-standard calibration (5 mm/mV or 20 mm/mV) is used, as this affects voltage-based diagnostic criteria 1

Technical Specifications for Accurate Measurements

Digital Sampling Requirements

• Minimum digital sampling rate of 500 samples per second is required to support accurate 150 Hz high-frequency filtering and keep amplitude-measurement error below 1% 1
• Data should be sampled at no less than 1,000 Hz and A/D converted with at least 12-bit precision for high-resolution applications 3
• All ECG leads must be recorded and converted concurrently 3

Frequency Response Standards

• For adults and adolescents: high-frequency cutoff must be ≥150 Hz to preserve waveform fidelity and prevent systematic underestimation of QRS amplitude 3, 1
• For infants and children: high-frequency cutoff must be 250 Hz to capture rapid deflections accurately 3, 1
• Low-frequency cutoff should be 0.05 Hz for routine filters, or may be relaxed to 0.67 Hz when using linear digital filters with zero-phase distortion 1
• The minimum band pass should be from 0.5 Hz to 250 Hz 3

Amplifier Requirements

• ECG signals should be recorded with a low-noise amplifier meeting American Heart Association standards for leakage current 3
• The range of linearity for input signals should not be less than ±2.5 mV 3
• Notch filters for power line interference should not be used, as they distort waveforms 3

Electrode Preparation and Placement

Skin Preparation

• Thoroughly cleanse the subject's skin with alcohol or another solvent and abrade to decrease impedance 3
• Ideally, impedance should be measured and be less than 1,000 Ω 3
• Silver-silver chloride electrodes have the lowest half-cell potential and are the electrodes of choice 3

Lead Placement Accuracy

• Precordial lead placement variability is a major source of measurement error—variation of as little as 2 cm can result in important diagnostic errors 3
• Fewer than two-thirds of routinely applied precordial electrodes are placed within 1.25 inch of the designated landmark 3
• Serial tracings in acute or subacute care settings should use skin marking to promote reproducibility of lead placement when electrodes cannot remain in place 3
• Periodic retraining in proper lead positioning should be routine for all personnel responsible for recording ECGs 3

Critical Pitfalls and Quality Assurance

Invalidating Technical Errors

• Employing a suboptimal high-frequency cutoff (e.g., 40 Hz) invalidates all amplitude-based diagnostic measurements, such as voltage criteria for left ventricular hypertrophy 1
• Inadequate high-frequency response systematically underestimates QRS amplitude and smooths critical features like Q waves, potentially leading to missed diagnoses of myocardial infarction 1, 2
• Poor frequency response causes smoothing of pathological Q waves and notched QRS components, leading to inaccurate diagnoses 2

Lead Misplacement Detection

• Medical personnel responsible for ECG recording should receive training on avoidance and recognition of lead switches 3
• Lead-switch detection algorithms should be incorporated into digital electrocardiographs with alarms for abnormally high lead impedance 3
• Suspected misplacements should be identified to the person recording the ECG in time to correct the problem 3
• Transposition of lead wires to V1 and V2 can cause reversal of R-wave progression that simulates anteroseptal wall infarction 3

Measurement Accuracy Standards

• Digital electrocardiographs must provide beat alignment that allows selective averaging or formation of a representative complex with adequate fidelity 3
• Noise levels should be reduced to <1 µV (root mean square) in the averaged signal for time-domain analysis 1
• Trigger jitter should be less than 1.0 msec and ideally 0.5 msec to prevent attenuation of high-frequency signals during signal averaging 3

Automated Alerts

• ECG devices should automatically generate alerts when suboptimal filter settings are detected and restore appropriate settings before the next recording 1
• A diagnostic statement alerting the reader to the presence of different types of lead switches should be incorporated into preliminary interpretive reports if not corrected before recording 3

Age-Specific Calibration Protocols

Pediatric Recordings

• Pediatric ECG acquisition must employ standard 10 mm/mV gain plus a 250 Hz high-frequency cutoff to avoid loss of rapid components in the QRS complex 1
• The higher frequency response is essential because children have more rapid deflections that would be smoothed by adult filter settings 3, 1

Adult Recordings

• For adults >35 years, use standard 10 mm/mV calibration with 150 Hz high-frequency cutoff 1
• For adolescents (16-35 years), use standard calibration but recognize that voltage criteria for left ventricular hypertrophy have lower accuracy in this age group 1

Serial Comparison Requirements

• Maintain consistent calibration across all ECGs to track evolution of ischemic changes or development of new infarction 2
• Compare current ECG with baseline using identical calibration settings to detect interval changes 2
• Use the largest-amplitude deflection in each lead as representative of that measurement, per ANSI/AAMI standards 1