What is the first step and systematic approach to obtain and interpret a standard 12‑lead electrocardiogram in a patient with suspected cardiac ischemia or arrhythmia?

ECG Acquisition and Interpretation in Suspected Cardiac Ischemia or Arrhythmia

First Step: Immediate ECG Acquisition

Obtain the 12-lead ECG as soon as possible after first patient contact—within minutes of presentation—particularly in patients with chest pain, syncope, dizziness, or symptoms predictive of sudden death or myocardial infarction. 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
• If an ECG was obtained in the prehospital setting, repeat it immediately on arrival to the emergency department, as serial ECGs combined with cardiac biomarkers improve diagnostic accuracy 1
• Consider serial ECGs every 10-15 minutes if the first ECG is non-diagnostic but clinical suspicion remains high—a single prehospital ECG identifies only 84.6% of STEMI patients, while three serial ECGs acquired over 25 minutes identify 100% 2

Technical Verification Before Interpretation

Before interpreting any ECG, verify proper electrode placement and adequate signal quality—misplaced precordial leads can significantly alter findings and lead to false diagnoses. 3, 1

• Confirm minimum high-frequency response of 150 Hz for adults (250 Hz for children) to maintain diagnostic precision 3, 1
• Check for baseline wander, electrical interference, and adequate electrode contact quality 3
• Apply appropriate low-frequency filters to reduce baseline drift while preserving ST-segment morphology 3
• Never accept computer interpretation without physician verification—automated systems produce frequent errors, particularly with arrhythmias (only 0-94% accuracy) 4, 3, 1

Systematic Interpretation Framework

Step 1: Rate and Rhythm Assessment

• 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 consecutive R waves 3, 1
• Identify the underlying rhythm by confirming a P wave before each QRS complex with consistent PR interval for sinus rhythm 3, 1
• Normal sinus rhythm: 60-100 bpm; bradycardia <60 bpm; tachycardia >100 bpm (though these may be normal variants in athletes or during physiological stress) 3, 1
• Note any irregularities such as premature beats, pauses, or completely irregular patterns suggesting atrial fibrillation 3, 1

Step 2: Intervals and Conduction

• Measure PR interval (normal: 120-200 ms) to assess AV conduction and identify AV blocks 3, 1
• Evaluate QRS duration (normal: <120 ms) to identify ventricular conduction delays or bundle branch blocks 3, 1
• Calculate corrected QT interval (QTc) using Bazett's formula; normal <450 ms for men, <460 ms for women 3, 1
• Measure QRS duration in a lead with an initial Q wave for accuracy 3

Step 3: Axis Determination

• Determine electrical axis using leads I and aVF: normal axis when both are positive (+90° to -30°) 3, 1
• Left axis deviation: lead I positive, aVF negative (-30° to -90°) 3, 1
• Right axis deviation: lead I negative, aVF positive (+90° to +180°) 3, 1
• Extreme axis deviation: both leads I and aVF negative (+180° to -90°) 3

Step 4: Chamber Enlargement

• Look for voltage criteria for left ventricular hypertrophy using Sokolow-Lyon criterion: S in V1 + R in V5 or V6 >3.5 mV 3
• Assess for atrial enlargement patterns 3
• Recognize that QRS voltage criteria decline with age and vary by population 3

Step 5: Ischemia, Injury, and Infarction (Critical for Emergency Management)

This is the most critical step for patients with suspected cardiac ischemia—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. 3, 1

• Identify pathological Q waves (>0.04 seconds or >25% of R wave amplitude) suggesting prior myocardial infarction 3, 1
• Assess T-wave abnormalities including inversion, hyperacute changes, or flattening 3, 1
• Note the location of abnormalities to determine affected coronary territory for catheterization lab activation decisions 3, 1
• Consider subtle ECG features suggestive of ischemia: use modified Sgarbossa criteria for acute MI in left bundle branch block or ventricular pacing, apply the 3- and 4-variable formula to differentiate normal ST elevation from subtle left anterior descending coronary artery occlusion, and use lead aVL for recognition of inferior MI 5

Integration with Clinical Context

Never interpret the ECG in isolation—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 must be interpreted in conjunction with clinical presentation for diagnosis and triage, including destination decisions and cardiac catheterization laboratory activation 1
• Compare with previous ECGs when available—failure to do so can miss important dynamic changes 3, 1
• Recognize that noncardiologists are more influenced by patient history when interpreting ECGs than cardiologists, making systematic approach even more critical 4, 1
• The same ECG pattern may occur in different pathophysiologic states, requiring skilled physician integration of clinical data and differential diagnosis 1

Critical Pitfalls to Avoid

• Do not over-rely on computer interpretation without physician verification—errors remain common, particularly with arrhythmias 4, 3, 1
• Do not misplace precordial electrodes—this significantly alters findings and leads to false diagnoses 3, 1
• Do not use inadequate filtering settings—this distorts waveforms and affects measurements 3
• Do not interpret findings without clinical context—this may lead to inappropriate management decisions and adverse outcomes 3, 1
• Do not fail to obtain serial ECGs in suspected STEMI with initially non-diagnostic ECG—15.4% of STEMIs are identified only on second or third ECG 2

Role of Non-Physician Interpretation in Emergency Settings

• Paramedics and nurses can identify STEMI independently as long as there is mandatory initial training and ongoing concurrent medical oversight of all ECG interpretations 1
• This allows for earlier catheterization lab activation and reduced door-to-balloon times 1
• Field-transmitted ECG for expert interpretation is reasonable if on-site interpretation is unavailable 1

Competency Considerations

• Residency training in internal medicine with Advanced Cardiac Life Support certification is sufficient for bedside interpretation in routine and emergency situations 4, 1
• Despite major interpretation errors being common (4-33%), adverse patient outcomes from these errors are rare, occurring in less than 1% of interpretations 4
• Continuing medical education through seminars or self-assessment programs is encouraged for physicians who read ECGs infrequently to maintain current skills 3, 1