What is the proper method for reading an electrocardiogram (ECG)?

How to Read an ECG Properly

Essential Principle

All ECG interpretations must be verified by a qualified physician, as computer algorithms alone are insufficient and can produce erroneous information that may lead to incorrect diagnoses with significant medical and legal consequences. 1

Systematic Approach to ECG Interpretation

Step 1: Verify Technical Quality

• Ensure proper electrode placement and minimize artifacts, as recording quality directly affects interpretation accuracy 2
• Confirm standard calibration (10 mm = 1 mV) and paper speed (25 mm/sec) 1
• Check that the ECG was recorded using appropriate bandwidth (0.05-150 Hz for diagnostic quality) to avoid distortion of high-frequency components like Q waves and low-frequency components like ST segments 1

Step 2: Assess Rate and Rhythm

• Calculate heart rate: Normal is 60-100 bpm in adults 2
• Identify rhythm: Confirm normal sinus rhythm by verifying a P wave precedes each QRS complex with consistent PR intervals 2
• Note that sinus bradycardia (<60 bpm) may be normal in athletes 2
• Verify regular R-R intervals for rhythm regularity 2

Step 3: Measure Intervals Systematically

• PR interval: Should be 120-200 ms (3-5 small squares); values >200 ms indicate first-degree AV block 2
• QRS duration: Should be <120 ms (<3 small squares); prolongation suggests bundle branch block or ventricular conduction delay 2
• QT interval: Measure in lead II or V5 using the tangent method, then correct for heart rate (QTc) using Bazett's formula 2
  • Normal QTc: <450 ms (men), <460 ms (women) 2
• Use global measurements from temporally aligned multiple leads rather than single leads, as this detects the earliest onset and latest offset of waveforms more accurately 1

Step 4: Evaluate Axis and Morphology

• Determine electrical axis using leads I and aVF
• Assess QRS morphology for bundle branch blocks, hypertrophy patterns, or Q waves suggesting infarction 1
• Examine ST segments for elevation or depression relative to the baseline 1
• Analyze T wave configuration for inversion, flattening, or peaked appearance 1

Step 5: Integrate Clinical Context

• Critical: The ECG records electrical activity only; diagnoses of structural changes (infarction, hypertrophy) or pathophysiologic states (electrolyte disturbances, drug effects) are made by inference and are subject to error 1
• Compare with previous ECGs to identify significant changes 1
• Consider the clinical presentation: chest pain, dyspnea, syncope, or palpitations warrant immediate ECG interpretation 1
• Recognize that the same ECG pattern may occur in different structural and pathophysiologic states, explaining the frequent low specificity of ECG for determining etiology 1

Critical Pitfalls to Avoid

Computer Interpretation Errors

• Never rely solely on computer interpretations, particularly for rhythm disturbances, ischemia, or infarction, as these require careful physician over-reading 1
• Computer algorithms can provide accurate heart rate, intervals, and axes, but all interpretations must be verified 1
• Computerized systems commonly fail in pathological signals, especially with arrhythmias like atrial fibrillation or premature ventricular contractions 3

Technical Artifacts

• Low-frequency cutoff at 0.5 Hz (common in rhythm monitors) can cause marked ST-segment distortion mimicking ischemia 1
• Inadequate high-frequency response leads to underestimation of QRS amplitude and smoothing of Q waves and notches 1
• Baseline wander from respiration can distort measurements if not properly filtered 1

Clinical Misinterpretation

• ST elevation may be secondary to non-ischemic causes (early repolarization, pericarditis, left ventricular hypertrophy) 4
• ST depression can result from subendocardial ischemia, left ventricular hypertrophy, or cardiomyopathies 4
• T-wave inversion has multiple etiologies including cerebrovascular accidents and cardiomyopathy, not just ischemia 5

Special Considerations

When to Obtain Serial ECGs

• If initial ECG is equivocal or doesn't support clinical suspicion of myocardial ischemia 5
• When patients develop new symptoms: syncope, changed chest pain pattern, new dyspnea, extreme fatigue, or palpitations 2
• For patients on medications affecting cardiac conduction (psychotropic agents, anti-infectives, antihypertensives, heart failure medications) 2

High-Risk Patterns Requiring Urgent Action

• Wellens syndrome: Deeply inverted or biphasic T waves in precordial leads indicating critical proximal LAD stenosis requiring urgent intervention within 120 minutes 5
• ST elevation with active symptoms indicating acute arterial occlusion requiring emergent reperfusion 4
• New conduction abnormalities or significant rhythm changes 1

Comparison with Previous ECGs

• Use the same type of ECG recorder and standardized acquisition procedures for accurate comparison 2
• Significant changes include: new rhythm diagnosis, heart rate change ≥20 bpm crossing 50 or 100 bpm threshold, new ischemia or infarction, new conduction abnormality, or QTc change ≥60 ms 1

Understanding Sensitivity and Specificity

• Each ECG waveform (P, QRS, ST, T) has its own sensitivity and specificity 1
• Different pathologic and pathophysiologic factors influence each component differently 1
• The ECG serves as an independent marker of myocardial disease and reflects electrophysiologic, anatomic, metabolic, and hemodynamic alterations 1
• It remains the only practical, noninvasive method for recording and analyzing cardiac rhythm abnormalities 1