Can you interpret the electrocardiogram (EKG)?

ECG Interpretation: A Systematic Approach

Yes, I can interpret an electrocardiogram using a standardized, systematic approach endorsed by the American Heart Association and American College of Cardiology that ensures accurate diagnosis of life-threatening conditions including acute coronary syndromes, arrhythmias, and conduction abnormalities. 1

Essential Pre-Interpretation Requirements

Before analyzing any ECG, verify you have the following clinical data, as interpretation accuracy depends critically on this information 2:

• Patient demographics: Age and gender (affects normal QT intervals and voltage criteria) 3
• Current medications: Antiarrhythmics and psychotropics alter ECG findings 3
• Electrolyte values: Particularly potassium and calcium levels 1
• Clinical indication: Chest pain, dyspnea, palpitations, syncope 4
• Cardiovascular history: Prior MI, heart failure, hypertension 2

Step 1: Verify Technical Quality

Always check electrode placement first - this is the most frequent cause of diagnostic errors 5:

• Confirm V1 and V2 are in the 4th intercostal space (superior misplacement reduces R-wave amplitude by 0.1 mV per interspace, creating false poor R-wave progression) 5
• Check for lead wire transposition (V1 with V2 reversal simulates anteroseptal infarction) 5
• Verify left precordial leads are not placed inferior-leftward (occurs in >33% of routine ECGs) 5

Step 2: Calculate Heart Rate

Use either method recommended by the American College of Cardiology 3:

• Large square method: Count large squares between consecutive R waves, divide 300 by this number
• 6-second method: Count QRS complexes in 6 seconds, multiply by 10

Step 3: Assess Rhythm Regularity

• Measure R-R intervals for consistency across the tracing 3
• Identify the underlying rhythm based on P wave morphology and relationship to QRS complexes (sinus, atrial, junctional, or ventricular origin) 3

Step 4: Measure Critical Intervals

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

• <120 ms suggests pre-excitation or junctional rhythm

• 200 ms indicates first-degree AV block

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

• ≥120 ms indicates bundle branch block or ventricular conduction delay
• Complete LBBB alters R-wave progression and may indicate structural heart disease 5

QT Interval (corrected for heart rate) 3:

• Normal QTc: <450 ms (men), <460 ms (women)
• Prolonged QTc increases risk of torsades de pointes

Step 5: Determine Electrical Axis

Quick quadrant method using leads I and aVF 3:

• Normal axis (-30° to +90°): Positive in both I and aVF 3
• Left axis deviation: Positive in I, negative in aVF 3
• Right axis deviation: Negative in I, positive in aVF 3
• Extreme axis deviation: Negative in both I and aVF

Clinical significance: Axis deviation reflects chamber enlargement, myocardial hypertrophy, and conduction abnormalities that directly impact morbidity and mortality 3

Step 6: Analyze P Waves

Morphology assessment 3:

• Normal: Upright in I, II, aVF; biphasic in V1
• Duration: <120 ms 3
• Amplitude: <2.5 mm 3

Abnormal P waves suggest atrial enlargement or ectopic atrial rhythms.

Step 7: Evaluate QRS Complexes

Pathologic Q waves (critical for MI diagnosis) 3, 5:

• Width >1 mm (40 ms) AND depth >1/3 the R wave height 3
• Q/R ratio ≥0.25 in two contiguous leads 5
• Sensitivity 85%, specificity 71% for anterior MI when combined with poor R-wave progression 5

R-wave progression (V1 through V5) 3, 5:

• Normal: R-wave amplitude increases from V1 to V4, then decreases toward V6 3
• Poor R-wave progression differential diagnosis:
  • First, exclude electrode misplacement (most common cause) 5
  • Prior anterior MI (especially with pathologic Q waves) 5
  • LVH (increased posterior forces) 5
  • RVH (rightward/anterior QRS vector shift) 5
  • Normal variant (positive predictive value only 7.3% for CAD in general population) 5

Step 8: Assess ST Segments

Measure at the J point 3:

• ST elevation: Suggests acute MI, pericarditis, or early repolarization
• ST depression: Indicates ischemia, reciprocal changes, or digitalis effect
• Context matters: Chest pain with ST elevation requires immediate reperfusion therapy

Step 9: Examine T Waves

Normal morphology 3:

• Upright in I, II, V3-V6
• Inverted in aVR
• Variable in III, aVL, aVF, V1, V2

Inverted T waves suggest ischemia, strain pattern, or electrolyte abnormalities.

Step 10: Look for Additional Features

• U waves: May indicate hypokalemia, bradycardia 3
• Chamber enlargement: Assess voltage criteria and P-wave abnormalities 3
• Conduction abnormalities: PR prolongation, QRS widening 3

Critical Clinical Integration

The ECG is indispensable for 1:

• Acute coronary syndromes: Immediate diagnosis enables prompt reperfusion therapy
• Arrhythmias: Most accurate diagnostic method
• Conduction disturbances: Definitive identification of bundle branch blocks
• Electrolyte abnormalities: Particularly potassium and calcium
• Genetic cardiac conditions: Long QT syndrome, Brugada syndrome, hypertrophic cardiomyopathy

Mandatory Verification Step

Never rely solely on computerized interpretations - they contain imprecise and overlapping terms that generate diagnostic errors 1, 3. A qualified physician must verify all computer-generated readings by integrating clinical data and comparing with previous tracings 3, 5.

High-Risk Features Requiring Immediate Action

Patients with these ECG findings combined with symptoms require urgent cardiac evaluation 5:

• Chest pain, dyspnea, or syncope
• Pathologic Q waves with ST-segment changes
• New bundle branch block
• Ventricular arrhythmias

Risk stratification: Major ECG abnormalities predict all-cause mortality (HR 1.8), cardiovascular mortality (HR 3.3), and coronary heart disease mortality (HR 2.3) 5

Common Pitfalls to Avoid

• Lead placement errors: As little as 2 cm variability causes diagnostic errors for anteroseptal infarction and ventricular hypertrophy 5
• Ignoring clinical context: The same ECG pattern occurs in different pathophysiologic states, explaining low specificity 3
• Overlooking serial changes: Compare with previous ECGs to assess progression or regression of abnormalities 3
• Misinterpreting normal variants: Voltage criteria for LVH/RVH in athletes represent physiologic adaptation when isolated 5