What is the approach to interpreting an emergency room electrocardiogram (ECG)?

Approach to Emergency Room ECG Interpretation

In the emergency room, a 12-lead ECG should be acquired and interpreted within 10 minutes of patient contact for suspected acute coronary syndrome, using a systematic approach that integrates clinical presentation with ECG findings, while recognizing that computer interpretations require mandatory physician verification. 1

Immediate Acquisition and Timing

• Obtain the 12-lead ECG as soon as possible after first patient contact, particularly in patients presenting with chest pain, syncope, dizziness, or symptoms predictive of sudden death or myocardial infarction. 1
• The ECG has a sensitivity of 76% and specificity of 88% for diagnosing acute cardiac ischemia in chest pain patients, with sensitivity of 68% and specificity of 97% for acute MI. 1
• Repeat the ECG on arrival if obtained in the prehospital setting, as serial ECGs combined with cardiac biomarkers improve diagnostic accuracy. 1

Systematic Interpretation Framework

Step 1: Technical Quality Assessment

• Verify proper electrode placement and adequate filtering settings before interpretation, as misplaced precordial leads can significantly alter findings and lead to false diagnoses. 2
• Ensure minimum high-frequency response of 150 Hz for adults to maintain diagnostic precision. 2

Step 2: 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. 2
• Identify the underlying rhythm by confirming a P wave before each QRS complex with consistent PR interval for sinus rhythm. 2
• Normal sinus rhythm: 60-100 bpm; bradycardia <60 bpm; tachycardia >100 bpm. 2
• Note any irregularities such as premature beats, pauses, or completely irregular patterns suggesting atrial fibrillation. 2

Step 3: Intervals and Conduction

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

Step 4: Axis Determination

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

Step 5: Acute Ischemia/Infarction Detection (Critical for ER)

• 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. 2, 3
• Look for ST depression in leads other than V1-V3, suggesting subendocardial ischemia from subocclusion, distal embolization, or supply-demand mismatch. 3
• Identify pathological Q waves (>0.04 seconds or >25% of R wave amplitude) suggesting prior myocardial infarction. 2
• Assess T-wave abnormalities including inversion, hyperacute changes, or flattening. 2
• Note the location of abnormalities to determine affected coronary territory for catheterization lab activation decisions. 2

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 ACS 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
• A skilled physician must integrate clinical data, provide differential diagnosis, and compare with previous tracings, as the same ECG pattern may occur in different pathophysiologic states. 1

Computer-Assisted Interpretation

• Computer interpretations must be verified by a qualified physician—they are helpful adjuncts but not substitutes for physician interpretation. 1, 2
• Computer programs provide accurate heart rate, intervals, and axes, but interpretations of rhythm disturbances, ischemia, or infarction require careful physician over-reading. 1
• Field-transmitted ECG for expert interpretation is reasonable if on-site interpretation is unavailable. 1

Nonphysician 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 in appropriate systems. 1

Critical Pitfalls to Avoid

• Do not over-rely on computer interpretation without physician verification, as errors remain common. 2
• Do not fail to compare with previous ECGs when available, as this can miss important dynamic changes. 2
• Do not misinterpret ST elevation from non-ischemic causes such as early repolarization, pericarditis, left ventricular hypertrophy, or bundle branch blocks. 3
• Do not interpret findings without clinical context, which may lead to inappropriate management decisions and adverse outcomes. 2
• Recognize that noncardiologists are more influenced by patient history than cardiologists when interpreting ECGs, making systematic approach even more critical. 1, 2

Competency Considerations

• Residency training with Advanced Cardiac Life Support certification is sufficient for bedside interpretation in routine and emergency situations. 1
• Continuing medical education through seminars or self-assessment programs is encouraged for physicians who read ECGs infrequently to maintain current skills. 1, 2