How do you perform and interpret an ECG in a patient?

How to Perform and Interpret an ECG

Proper ECG Acquisition Technique

Accurate electrode placement is the single most critical technical factor—precordial lead misplacement of as little as 2 cm can result in diagnostic errors, particularly for anteroseptal infarction and ventricular hypertrophy. 1

Electrode Placement Standards

• Place V1 at the 4th intercostal space, right sternal border 1
• Place V2 at the 4th intercostal space, left sternal border 1
• Position V3 midway between V2 and V4 1
• Place V4 at the 5th intercostal space, midclavicular line 1
• Position V5 at the anterior axillary line, same horizontal level as V4 1
• Place V6 at the midaxillary line, same horizontal level as V4 and V5 1
• Common pitfall: V1 and V2 are frequently placed too high (superior misplacement in >50% of cases), and left precordial leads are often placed too low and leftward (>33% of cases) 1

Technical Recording Standards

• Use a minimum high-frequency response of 150 Hz for adults and adolescents, 250 Hz for children 2
• Set sampling rate to at least 500 Hz, preferably 1200 Hz for digital systems 2
• Apply appropriate low-frequency filters to reduce baseline drift while preserving ST-segment morphology 2
• Verify electrode contact quality before recording to minimize baseline wander and electrical interference 2
• For serial tracings in acute settings, use skin marking to ensure reproducible lead placement 1

Systematic ECG Interpretation Approach

Always interpret the ECG in the context of the patient's clinical presentation—the same ECG pattern may have different implications depending on symptoms. 1, 2

Step 1: Rate and Rhythm Analysis

• 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
• Normal sinus rhythm: rate 60-100 bpm in adults, P wave before each QRS with consistent PR interval 2
• Identify any irregularities suggesting premature beats, pauses, or atrial fibrillation 2

Step 2: Interval Measurements

• Measure PR interval (normal: 120-200 ms) to assess AV conduction 2
• Evaluate QRS duration (normal: <120 ms) to identify ventricular conduction delays 2
• Calculate QTc using Bazett's formula (normal: <450 ms for men, <460 ms for women) 2
• Measure QRS duration in a lead with an initial Q wave for accuracy 2

Step 3: Axis Determination

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

Step 4: Chamber Enlargement Assessment

• Apply Sokolow-Lyon criterion for left ventricular hypertrophy: S in V1 + R in V5 or V6 >3.5 mV 2
• Note that QRS voltage criteria decline with age and vary by population 2
• Assess for atrial enlargement patterns 2

Step 5: Ischemia and Infarction Evaluation

• Examine for ST-segment elevation (>0.1 mV in limb leads or >0.15-0.2 mV in precordial leads) indicating acute injury 2
• Look for T-wave abnormalities (inversion, hyperacute, flattening) 2
• Identify pathological Q waves (>0.04 seconds or >25% of R wave amplitude) suggesting myocardial infarction 2
• Note the location of abnormalities to determine affected coronary territory 2

Critical Interpretation Pitfalls to Avoid

Computer-generated ECG interpretations are not recognized as properly interpreted without qualified physician review—automated systems still produce frequent errors. 1, 2

Common Technical Errors

• Avoid misplacement of precordial electrodes, which can alter computer-based diagnostic statements in up to 6% of recordings 1
• Recognize that inadequate high-frequency response causes systematic underestimation of signal amplitude and smoothing of Q waves 2
• Do not accept computer interpretation without physician verification 2
• Always compare with previous ECGs when available to identify important changes 2

Interpretation Considerations

• Noncardiologists are more influenced by patient history when interpreting ECGs than cardiologists 1, 2
• ECG interpretation varies greatly even among expert electrocardiographers 1
• The same ECG finding may have different implications depending on clinical symptoms 1, 2
• Consider normal variants in athletes, such as sinus bradycardia ≥30 bpm 2
• In children >1 month, T waves are often inverted in leads V1, V2, and V3 2

Competency Requirements

Residency training in internal medicine with Advanced Cardiac Life Support instruction is sufficient for bedside interpretation of resting 12-lead ECGs in routine and emergency situations. 1

• Successful completion of residency, board certification, and ACLS training provides reasonable documentation of competency 1
• For interpreting ECGs when unfamiliar with the patient's clinical status, additional experience or standardized testing may be needed 1
• Continuing medical education through seminars or self-assessment programs is encouraged for physicians who read ECGs infrequently 1, 2
• Periodic retraining in proper lead positioning should be routine for all personnel recording ECGs 1