How should an electrocardiogram (ECG) be interpreted in patients with varying clinical presentations and medical histories?

ECG Interpretation Framework

ECG interpretation requires a systematic approach that integrates technical accuracy, clinical context, and physician verification of any automated readings to optimize diagnostic accuracy and patient outcomes. 1

Immediate Technical Verification

Before interpreting any ECG, verify these critical technical parameters:

• Confirm proper electrode placement, particularly precordial leads, as misplacement significantly alters findings and causes false diagnoses 1, 2
• Ensure adequate filtering settings with minimum high-frequency response of 150 Hz for adults (250 Hz for children) to maintain diagnostic precision 1, 2
• Check signal quality for baseline wander, electrical interference, and artifacts that compromise interpretation 2
• Verify standard 12-lead configuration is used, with additional leads (V3R, V4R, V7) when evaluating specific coronary territories 2

Systematic Interpretation Algorithm

Step 1: Rate and Rhythm Analysis

• Calculate heart rate by counting QRS complexes in a 6-second strip and multiplying by 10, or using 300 divided by the number of large boxes between R waves 1, 2
• Normal sinus rhythm: 60-100 bpm with P wave before each QRS and consistent PR interval 1, 2
• Identify bradycardia (<60 bpm) or tachycardia (>100 bpm), recognizing these may be normal variants in athletes or physiological stress 2
• Note any irregularities including premature beats, pauses, or completely irregular patterns suggesting atrial fibrillation 1, 2

Step 2: Intervals and Conduction

• Measure PR interval (normal: 120-200 ms) to assess AV conduction 1, 2
• Evaluate QRS duration (normal: <120 ms) to identify ventricular conduction delays or bundle branch blocks 1, 2
• Calculate corrected QT interval (QTc) using Bazett's formula; normal <450 ms for men, <460 ms for women 1, 2
• Identify conduction abnormalities such as AV blocks, bundle branch blocks, or pre-excitation 2

Step 3: Axis Determination

• Determine electrical axis using leads I and aVF 1, 2:
  • Normal axis: both positive (+90° to -30°)
  • Left axis deviation: lead I positive, aVF negative (-30° to -90°)
  • Right axis deviation: lead I negative, aVF positive (+90° to +180°)
  • Extreme axis deviation: both negative (+180° to -90°)

Step 4: Chamber Enlargement

• Assess for left ventricular hypertrophy using Sokolow-Lyon criterion (S in V1 + R in V5 or V6 >3.5 mV) or Cornell voltage (S in V3 + R in aVL) 2
• Recognize that QRS voltage criteria decline with age and vary by population 2
• Look for atrial enlargement patterns 2

Step 5: Ischemia, Injury, and Infarction

• Examine for ST-segment elevation (>0.1 mV in limb leads or >0.15-0.2 mV in precordial leads) indicating acute injury requiring emergent reperfusion 1, 2
• Identify pathological Q waves (>0.04 seconds or >25% of R wave amplitude) suggesting prior myocardial infarction 1, 2
• Assess T-wave abnormalities including inversion, hyperacute changes, or flattening 1, 2
• Note the location of abnormalities to determine affected coronary territory for catheterization lab activation decisions 1, 2
• Recognize that ST depression in leads other than V1-V3 usually indicates subendocardial ischemia from subocclusion, distal embolization, or supply/demand mismatch 3

Integration with Clinical Context

Never interpret the ECG in isolation—this is the most critical principle for avoiding diagnostic errors:

• Clinical signs and symptoms alone lack sufficient sensitivity (35-38%) and specificity (28-91%) to rule in or rule out acute coronary syndrome without ECG and biomarkers 1
• The ECG has 76% sensitivity and 88% specificity for diagnosing acute cardiac ischemia in chest pain patients, with 68% sensitivity and 97% specificity for acute MI 1
• Always correlate ECG findings with clinical presentation, as the same ECG pattern may occur in different pathophysiologic states 1, 2
• Compare with previous ECGs when available, as dynamic changes provide crucial diagnostic information 1, 2
• Obtain serial ECGs in patients with ongoing symptoms, as repeat tracings combined with cardiac biomarkers improve diagnostic accuracy 1

Clinical Context Considerations

• Noncardiologists are more influenced by patient history than cardiologists when interpreting ECGs, making systematic approach even more critical 4, 2
• Cardiologists demonstrate greater accuracy than noncardiologists in ECG interpretation during standardized examinations when minimal history is given 4
• ST segment elevation with active symptoms usually indicates acute occlusion of an epicardial artery requiring emergent reperfusion 3
• ST segment elevation in asymptomatic patients may be secondary to nonischemic causes and requires different management 3

Computer-Assisted Interpretation

Computer interpretations must be verified by a qualified physician—they are helpful adjuncts but not substitutes for physician interpretation:

• Computer programs provide accurate heart rate, intervals, and axes, but interpretations of rhythm disturbances, ischemia, or infarction require careful physician over-reading 1, 2
• Automated interpretation statements are very specific but not satisfactorily sensitive—sensitivity for detecting STEMI is only 62.9% despite 95.6% specificity 5
• Patients without automated interpretation statements of acute MI/ischemia should still be considered for significant underlying coronary disease based on clinical context 5
• GPs demonstrate higher specificity (85.7%) but lower sensitivity (69.8%) compared to automated interpretation (75.6% specificity, 84.4% sensitivity) 6
• Pay special attention to automated diagnoses of ST-segment deviation, T-wave inversion, or Q-waves to avoid reclassifying true-positive findings as false-negatives 6

Emergency Department Specific Considerations

• Obtain the 12-lead ECG as soon as possible after first patient contact, particularly in patients with chest pain, syncope, dizziness, or symptoms predictive of sudden death or myocardial infarction 1
• Repeat the ECG on arrival if obtained in the prehospital setting 1
• Field-transmitted ECG for expert interpretation is reasonable if on-site interpretation is unavailable 1
• Paramedics and nurses can identify STEMI independently with mandatory initial training and ongoing concurrent medical oversight, allowing earlier catheterization lab activation 1

Critical Pitfalls to Avoid

• Do not over-rely on computer interpretation without physician verification, as errors remain common 1, 2
• Do not fail to compare with previous ECGs when available, as this can miss important dynamic changes 1, 2
• Do not interpret findings without clinical context, which may lead to inappropriate management decisions and adverse outcomes 1, 2
• Do not accept inadequate high-frequency response, which results in systematic underestimation of signal amplitude and smoothing of important features like Q waves 2
• Do not ignore electrode misplacement, particularly precordial leads, which significantly alter interpretation 1, 2

Competency and Maintenance

Residency training with board certification and Advanced Cardiac Life Support certification is sufficient for bedside interpretation in routine and emergency situations 4:

• Recommended training includes interpretation and review of 800 ECGs within a 3-year training period under experienced faculty supervision, reflecting a wide variety of clinical situations 4
• Continuing medical education through seminars or self-assessment programs is encouraged for physicians who read ECGs infrequently to maintain current skills 4, 2
• For interpreting ECGs when clinical history is unknown, documentation of proficiency beyond residency training may be desirable, based on experience or standardized testing 4
• No specific yearly minimum number of ECG interpretations has been established to maintain competency, but periodic self-study is recommended for those with infrequent exposure 4