What are the steps to reading an electrocardiogram (EKG)?

Systematic Approach to EKG Interpretation

Follow a structured 6-step approach when reading an EKG: assess rate and rhythm, measure intervals, determine axis, analyze waveform morphology, evaluate for ischemia/injury, and consider clinical context. 1, 2

Step 1: Calculate Heart Rate and Assess Rhythm

Rate Calculation:

• Count QRS complexes in a 6-second strip and multiply by 10, OR use the 300 method (divide 300 by the number of large boxes between consecutive R waves) 1, 2
• Normal sinus rhythm: 60-100 bpm in adults 2, 3
• Sinus bradycardia (<60 bpm) may be normal in athletes 2, 3

Rhythm Assessment:

• Identify if a P wave precedes each QRS complex with consistent PR interval (defines sinus rhythm) 2, 3
• Examine R-R intervals for regularity 1
• Note any irregularities suggesting premature beats, pauses, or atrial fibrillation 2

Step 2: Measure Critical Intervals

PR Interval (Normal: 120-200 ms or 3-5 small squares):

• Assesses AV conduction 1, 2
• Prolongation suggests AV block 2

QRS Duration (Normal: <120 ms or <3 small squares):

• Evaluates ventricular conduction 1, 2
• Widening indicates bundle branch block or ventricular conduction delay 2

QT Interval:

• Measure manually in lead II or V5 using the tangent method 3
• Correct for heart rate using Bazett's formula (QTc) 1, 2
• Normal QTc: <450 ms (men), <460 ms (women) 1, 2, 3
• Critical pitfall: QT intervals are longer in women; certain medications (antiarrhythmics, psychotropics, anti-infectives) prolong QTc 1, 2

Step 3: Determine Electrical Axis

Quick Quadrant Method using Leads I and aVF:

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

Clinical significance: Axis deviation indicates chamber enlargement, myocardial hypertrophy, or conduction abnormalities that directly impact morbidity and mortality 1

Step 4: Analyze Waveform Morphology

P Waves:

• Normal: Upright in I, II, aVF; biphasic in V1 1
• Duration <120 ms, amplitude <2.5 mm 1
• Abnormalities suggest atrial enlargement 1, 2

QRS Complexes:

• Pathologic Q waves: >1 mm wide (>0.04 seconds) AND >1/3 the height of R wave OR >25% of R wave amplitude 1, 2
• These suggest myocardial infarction 1, 2
• Assess R wave progression across V1-V6 (should increase V1→V4, then decrease toward V6) 1

Ventricular Hypertrophy Criteria:

• Left ventricular hypertrophy: S in V1 + R in V5 or V6 >3.5 mV (Sokolow-Lyon) 2
• Important caveat: QRS voltage criteria decline with age and vary by population 2

T Waves:

• Normal: Upright in I, II, V3-V6; inverted in aVR; variable in III, aVL, aVF, V1, V2 1
• Inversions, hyperacute changes, or flattening suggest ischemia 2
• In children >1 month, T waves often inverted in V1-V3 (normal variant) 2

Step 5: Evaluate ST Segments for Ischemia/Injury

ST Elevation:

• Measured at the J point 1
• Acute injury threshold: >0.1 mV (1 mm) in limb leads OR >0.15-0.2 mV (1.5-2 mm) in precordial leads 2
• With active symptoms indicates acute epicardial artery occlusion requiring emergent reperfusion 4

ST Depression:

• In leads other than V1-V3 suggests subendocardial ischemia from subocclusion, distal embolization, or supply-demand mismatch 4
• ST depression in V1-V3 may represent posterior MI 4
• Critical pitfall: ST changes can be nonischemic (LVH, cardiomyopathies, medications) 1, 4

Location determines coronary territory:

• Note which leads show abnormalities to identify affected vessel 2

Step 6: Technical and Clinical Context Integration

Technical Quality Assessment:

• Verify proper electrode placement (misplacement causes false diagnoses) 2
• Check for adequate filtering (inadequate settings distort waveforms) 2
• Ensure high-frequency response ≥150 Hz for adults to maintain diagnostic precision 2
• Identify artifacts from noise or movement 2

Clinical Context:

• Always interpret EKG with patient symptoms 2, 3, 5
• Compare with previous EKGs when available (failure to compare misses important changes) 2
• Consider age, gender, medications, and population-specific factors 1, 2

Computer Interpretation:

• Never rely solely on computer interpretation—physician verification is mandatory 1, 2, 3
• Computer errors remain common despite technological advances 2
• The American Heart Association states computer interpretation alone is not recognized as properly interpreted 2

Common Pitfalls to Avoid

• Electrode misplacement (especially precordial leads) significantly alters interpretation 2
• Over-reliance on automated readings without clinical correlation 2
• Ignoring medication effects (psychotropics, antiarrhythmics, antihypertensives affect EKG) 1, 3
• Interpreting in isolation without symptoms or previous tracings 2
• Missing nonischemic causes of ST changes (LVH, early repolarization, pericarditis) 4