How to interpret an electrocardiogram (ECG) in emergency cases?

How to Interpret ECG in Emergency Cases

Obtain a 12-lead ECG immediately upon patient contact—within minutes of arrival—for any patient presenting with chest pain, syncope, dyspnea, or suspected cardiac emergency, and use a systematic five-step approach to identify life-threatening conditions that require immediate intervention. 1, 2

Immediate Acquisition and Timing

• Acquire the prehospital 12-lead ECG as early as possible for patients with possible acute coronary syndrome, as this is a Class I recommendation that can reduce time to treatment 1
• Repeat the ECG on hospital arrival even if obtained in the prehospital setting, since serial ECGs combined with cardiac biomarkers significantly improve diagnostic accuracy 2
• The ECG demonstrates 76% sensitivity and 88% specificity for acute cardiac ischemia in chest pain patients, with 68% sensitivity and 97% specificity specifically for acute myocardial infarction 2
• Activate the catheterization laboratory based on prehospital ECG findings when STEMI is recognized, as prehospital notification reduces door-to-balloon time 1

Pre-Interpretation Technical Verification

• Verify proper electrode placement before interpretation, particularly the precordial leads, as misplacement can create false diagnoses and lead to catastrophic errors 2, 3
• Confirm adequate filtering settings with minimum high-frequency response of 150 Hz for adults to maintain diagnostic precision 2, 3
• Assess the overall quality of the ECG recording and identify technical artifacts (muscle tremor, electrical interference, baseline wander) that may compromise interpretation 3
• Ensure the paper speed is standard (25 mm/sec) and calibration is correct (10 mm/mV) before proceeding 4

Systematic Five-Step Interpretation Framework

Step 1: Calculate Heart Rate and Identify Rhythm

• 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, 3
• Identify the underlying rhythm by confirming a P wave before each QRS complex with consistent PR interval for sinus rhythm 2, 3
• Normal sinus rhythm: 60-100 bpm; sinus bradycardia <60 bpm; sinus tachycardia >100 bpm 2, 3
• Note any irregularities such as premature beats, pauses, or completely irregular patterns suggesting atrial fibrillation 2, 3

Step 2: Measure Critical Intervals

• Measure PR interval (normal: 120-200 ms) to assess AV conduction and identify heart blocks 2, 3
• Evaluate QRS duration (normal: <120 ms) to identify ventricular conduction delays, bundle branch blocks, or ventricular rhythms 2, 3
• Calculate corrected QT interval (QTc) using Bazett's formula; normal <450 ms for men, <460 ms for women, as prolongation increases risk of torsades de pointes 2, 3

Step 3: Determine Electrical Axis

• Determine electrical axis using leads I and aVF: normal axis when both are positive (+90° to -30°) 2, 3
• Left axis deviation: lead I positive, aVF negative (-30° to -90°) suggests left anterior fascicular block or left ventricular hypertrophy 2, 3
• Right axis deviation: lead I negative, aVF positive (+90° to +180°) suggests right ventricular hypertrophy or left posterior fascicular block 2, 3

Step 4: Identify Life-Threatening ST-Segment and T-Wave Changes

• 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 therapy 2, 3
• Activate the catheterization laboratory emergently for out-of-hospital cardiac arrest patients with suspected cardiac etiology and ST elevation on ECG (Class I recommendation) 1
• Identify pathological Q waves (>0.04 seconds or >25% of R wave amplitude) suggesting prior or evolving myocardial infarction 2, 3
• Assess T-wave abnormalities including inversion, hyperacute changes (tall peaked T waves in early MI), or flattening 2, 3
• Note the anatomic location of abnormalities to determine affected coronary territory: anterior (V1-V4), lateral (I, aVL, V5-V6), inferior (II, III, aVF), or posterior changes 2

Step 5: Assess for Chamber Enlargement and Conduction Abnormalities

• Look for voltage criteria for left ventricular hypertrophy: S wave in V1 + R wave in V5 or V6 >3.5 mV 3
• Identify bundle branch blocks, AV blocks, or pre-excitation patterns that may affect management 3

Integration with Clinical Context—Critical for Accuracy

• Never interpret the ECG in isolation—clinical signs and symptoms alone have insufficient sensitivity (35-38%) and specificity (28-91%) to rule in or rule out acute coronary syndrome without ECG and biomarkers 2
• The ECG must be interpreted in conjunction with clinical presentation for diagnosis, triage decisions, destination hospital selection, and catheterization laboratory activation 1, 2
• Compare with previous ECGs when available, as dynamic changes (new ST elevation, new Q waves, evolving T-wave inversions) are more specific for acute events than isolated findings 2
• Recognize that the same ECG pattern may occur in different pathophysiologic states (ST elevation in early repolarization vs. acute MI, T-wave inversion in Wellens' syndrome vs. normal variant) 2

Computer-Assisted Interpretation—Use with Caution

• Computer interpretations are helpful adjuncts but never substitutes for physician interpretation—errors in computer analysis remain common, particularly for rhythm disturbances and ischemia detection 1, 2
• The 2015 American Heart Association guidelines specifically recommend against using computer-assisted ECG interpretation as the sole means to diagnose STEMI (Class III: Harm) due to high false-negative rates 1
• Computer programs provide accurate heart rate, intervals, and axes, but rhythm and ischemia interpretations require careful physician over-reading 1, 2
• Computer-assisted interpretation may be used in conjunction with physician or trained provider interpretation to recognize STEMI (Class IIb) 1

Nonphysician Interpretation in Emergency Settings

• Trained paramedics and emergency nurses can identify STEMI independently when there is mandatory initial training and ongoing concurrent medical oversight of all ECG interpretations 2
• Field-transmitted ECG for expert physician interpretation is reasonable if on-site interpretation is unavailable, improving positive predictive value and therapeutic decision-making 1
• This approach allows earlier catheterization lab activation and reduced door-to-balloon times in appropriate systems 2

Critical Pitfalls to Avoid in Emergency ECG Interpretation

• Do not over-rely on computer interpretation without physician verification, as this leads to missed STEMIs and inappropriate management 1, 2
• Do not fail to obtain serial ECGs—a single normal ECG does not exclude acute coronary syndrome, and dynamic changes over 15-30 minutes may reveal evolving injury 2
• Do not interpret findings without clinical context, as noncardiologists are more influenced by patient history than cardiologists when interpreting ECGs, making systematic approach critical 1, 2
• Do not miss electrode misplacement, particularly reversed limb leads or incorrect precordial placement, which creates false patterns mimicking infarction or conduction abnormalities 2, 3
• Do not overlook subtle ST elevation in leads with small QRS complexes, or fail to recognize ST depression in posterior MI (requires posterior leads V7-V9) 2
• Do not delay reperfusion therapy while waiting for cardiac biomarkers in patients with clear STEMI on ECG, as time is myocardium 1

Competency Requirements for Emergency ECG Interpretation

• Residency training completion with Advanced Cardiac Life Support (ACLS) certification demonstrates basic competency to interpret resting 12-lead ECGs at the bedside 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
• Physicians must understand the pathophysiology of electrocardiographic abnormalities, recognize common normal and abnormal patterns, and apply this knowledge in bedside clinical decision-making 1