ECG Calibration Standards
Standard ECG calibration requires a paper speed of 25 mm/second and amplitude of 10 mm/mV (1 mV = 10 mm), which allows accurate measurement of intervals and voltages critical for detecting old myocardial infarction patterns. 1
Standard Calibration Settings
- Paper speed: 25 mm/second is the universal standard, where each small square (1 mm) represents 0.04 seconds and each large square (5 mm) represents 0.20 seconds 1
- Amplitude calibration: 10 mm/mV standardization means 1 mV produces a 10 mm deflection on the ECG paper 1
- Frequency response: Minimum 150 Hz for adults to preserve diagnostic accuracy and prevent systematic underestimation of signal amplitude 2
Critical Technical Factors Affecting Accuracy
Electrode Placement
- Limb leads: Improper placement alters the frontal plane axis and can mask or create false patterns of infarction 1
- Precordial leads: Incorrect positioning significantly changes QRS morphology and ST-segment interpretation, potentially causing false diagnoses of old MI 1, 2
Equipment-Related Artifacts
- Poor frequency response: Causes smoothing of critical features including pathological Q waves (the hallmark of old MI) and notched QRS components 1, 2
- Muscle tremor: Creates baseline artifact that obscures ST-segment and T-wave abnormalities 1
- Uneven paper transport: Distorts interval measurements, making PR, QRS, and QT assessments unreliable 1
Detecting Old Myocardial Infarction
Key ECG Features to Measure
- Pathological Q waves: Require accurate calibration to distinguish from normal septal Q waves; typically ≥0.04 seconds wide and ≥25% of R-wave amplitude 1
- QRS residuals: Persistent changes from prior infarction including loss of R-wave progression, fragmented QRS, or abnormal Q waves 1
- ST-segment changes: Persistent ST elevation may indicate ventricular aneurysm; proper calibration ensures accurate measurement of ≥1 mm elevation 1
Localization Requirements
- Anterior MI: Requires accurate V1-V4 lead placement and calibration to detect Q waves or poor R-wave progression 1
- Inferior MI: Demands proper limb lead calibration to identify pathological Q waves in leads II, III, aVF 1
- Lateral MI: Needs precise calibration in leads I, aVL, V5-V6 for Q-wave detection 1
- Posterior MI: May require additional posterior leads (V7-V9) with same calibration standards for optimal detection 3
Common Pitfalls in Cardiovascular Disease Monitoring
- Bundle branch blocks: Pre-existing LBBB or RBBB reduces reliability of Q-wave criteria for old MI; standard calibration remains essential but interpretation requires expertise 1
- Ventricular hypertrophy: LVH can mask or mimic infarction patterns; accurate voltage calibration is critical to apply diagnostic criteria 1
- Comparison with prior ECGs: Always compare current ECG with baseline using identical calibration settings to detect interval changes 1
- Computer interpretation limitations: Automated readings require physician verification as errors remain common, particularly for subtle old MI patterns 2
Special Considerations for Cardiovascular Disease Patients
- Serial monitoring: Maintain consistent calibration across all ECGs to track evolution of ischemic changes or development of new infarction 1
- Enhanced lead systems: Consider 16-lead (adding V4R, V5R, V8, V9) or 24-lead ECG with same calibration standards for improved MI detection sensitivity (37% vs 28% for standard 12-lead) 3
- Clinical correlation: Always interpret calibrated ECG findings in context of cardiac biomarkers, symptoms, and imaging studies 1, 4