What are the proper protocols and applications for a basic electrocardiogram (ECG)?

Basic ECG: Proper Protocols and Applications

Standard Electrode Placement and Lead Configuration

The standard 12-lead ECG must be recorded with the patient supine using specific anatomical landmarks: limb electrodes on the wrists and ankles, and precordial electrodes at precise chest positions (V1-V6), as this is the only configuration that produces diagnostically equivalent tracings for serial comparison. 1, 2

Limb Lead Placement

• Place arm electrodes on the wrists in the infraclavicular fossae medial to the deltoid insertions 1
• Position leg electrodes on the ankles or lower legs 1
• Never use torso placement of limb electrodes for standard diagnostic ECGs, as these produce non-equivalent tracings that cannot be compared serially and may cause false-positive or false-negative infarction criteria 1, 2

Precordial Lead Placement

• V1: 4th intercostal space, right sternal border 2
• V2: 4th intercostal space, left sternal border 2
• V3: Midway between V2 and V4 2
• V4: 5th intercostal space, midclavicular line 2
• V5: Same horizontal level as V4, anterior axillary line 2
• V6: Same horizontal level as V4, midaxillary line 2

Technical Standards for Signal Acquisition

Digital electrocardiographs must maintain a minimum bandwidth of 0.5 Hz to 250 Hz for adults, with high-frequency response of at least 150 Hz for adults and 250 Hz for children to preserve diagnostic accuracy. 1, 2, 3

Critical Bandwidth Requirements

• Filtering between 1-30 Hz produces artifact-free tracings but is unacceptable for diagnostic recording because it distorts both high and low-frequency components 1, 3
• Inadequate high-frequency response causes systematic underestimation of QRS amplitude and smoothing of Q waves and notched components 1, 3
• Inadequate low-frequency response produces significant repolarization distortions 1, 3

Signal Quality Optimization

• Prepare skin by cleaning with alcohol and gentle abrasion to reduce impedance below 5000 Ω (ideally <1000 Ω) 3
• Use silver-silver chloride electrodes with adhesive gel for optimal signal quality 3
• Verify temporal alignment of multiple leads with maximum misalignment no more than 10 ms 1
• Keep the patient relaxed in a warm environment to minimize movement artifacts 3

Systematic ECG Interpretation Protocol

Always interpret the ECG using a systematic approach that evaluates rate, rhythm, axis, intervals, and morphology in sequence, and never accept computer interpretation without physician verification. 2, 4

Step 1: Rate and Rhythm Assessment

• Calculate heart rate by counting QRS complexes in a 6-second strip and multiplying by 10, or use 300 divided by the number of large boxes between consecutive R waves 2, 4
• Normal sinus rhythm: 60-100 bpm with P wave before each QRS and consistent PR interval 2
• Evaluate R-R interval regularity to identify arrhythmias 4

Step 2: Interval Measurements

• PR interval: Normal 120-200 ms (3-5 small squares); prolongation indicates AV conduction delay 2, 4
• QRS duration: Normal <120 ms (<3 small squares); widening suggests ventricular conduction abnormalities 2, 4
• QTc interval: Calculate using Bazett's formula; normal <450 ms for men, <460 ms for women 2, 4

Step 3: Axis Determination

• Examine leads I and aVF to determine quadrant 2, 4
• Normal axis: -30° to +90° (positive in both I and aVF) 2
• Left axis deviation: -30° to -90° (positive in I, negative in aVF) 2
• Right axis deviation: +90° to +180° (negative in I, positive in aVF) 2

Step 4: Chamber Enlargement and Hypertrophy

• Left ventricular hypertrophy: S in V1 + R in V5 or V6 >3.5 mV (Sokolow-Lyon criteria) 2, 4
• Note that QRS voltage criteria decline with age and vary by population 4

Step 5: Ischemia and Infarction Evaluation

• ST elevation: >0.1 mV in limb leads or >0.15-0.2 mV in precordial leads indicates acute injury 2
• Pathological Q waves: >0.04 seconds or >25% of R wave amplitude suggests myocardial infarction 2
• Identify the coronary territory based on lead distribution of abnormalities 2

Critical Pitfalls to Avoid

Electrode misplacement, particularly of precordial leads, is the most common technical error that significantly alters ECG interpretation and leads to false diagnoses. 2

Common Technical Errors

• Never interchange torso-placed limb lead ECGs with standard ECGs for serial comparison, as they are not equivalent 1, 2
• Avoid inadequate filtering settings that distort waveforms and affect measurements 2
• Replace electrode cables approximately every year to reduce electrical interference 3
• Verify signal quality before interpretation by checking for artifacts, baseline stability, and electrical interference 2, 3

Interpretation Errors

• Computer-generated interpretations are not recognized as properly interpreted ECGs without qualified physician review 1, 2, 4
• Always interpret ECG findings in conjunction with clinical presentation, as the same pattern may have different implications depending on symptoms 2, 4
• Compare with previous ECGs when available to identify important changes 2
• Recognize that interpretation varies even among expert electrocardiographers, emphasizing the need for ongoing education 2

Special Clinical Applications

Exercise and Ambulatory Monitoring

• Torso placement of limb electrodes (Mason-Likar position) is acceptable only for exercise testing and ambulatory monitoring to reduce motion artifact 1
• These tracings must be clearly labeled as using modified lead placement and cannot be used for serial comparison with standard ECGs 1
• Rhythm diagnosis is not adversely affected by monitoring lead placement, but QRS morphology and infarction criteria may be altered 1

Pediatric Considerations

• Use minimum high-frequency response of 250 Hz for children, as QRS complexes contain important components up to 250 Hz 1
• Normal axis in neonates ranges 55-200° at birth, decreasing to ≤160° by 1 month 2
• T waves are often inverted in leads V1, V2, and V3 in children older than 1 month 2