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Systematic ECG Interpretation - A Guide for Residents

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The 14-Parameter Systematic Approach (Harrison's, 22e)

1. Standardization & technical features
2. Rhythm
3. Heart rate
4. PR interval / AV conduction
5. QRS interval
6. QT / QTc interval
7. Mean QRS electrical axis
8. P waves
9. QRS voltages
10. Precordial R-wave progression
11. Abnormal Q waves
12. ST segments
13. T waves
14. U waves

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Step 1 - Standardization & Technical Features

• Paper speed: standard is 25 mm/s
• Gain/calibration: standard = 10 mm/mV (1 cm deflection = 1 mV). If the calibration box at the start of the strip is half-height, gain is halved - voltages will appear falsely low
• Lead placement errors: limb lead reversal (e.g., LA/RA swap) mimics dextrocardia; precordial misplacement distorts R-wave progression
• Artifact: muscle tremor (Parkinson's, shivering) mimics AF; AC interference gives a 60 Hz wobble

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Step 2 - Rate

• Small box = 0.04 s
• Large box = 0.20 s
• 5 small boxes = 1 large box
• 300 large boxes = 1 minute

• Bradycardia: HR < 60 bpm - think medications (beta-blockers, CCBs, digoxin, lithium), hypothyroidism, AV block, sick sinus syndrome, trained athletes
• Tachycardia: HR > 100 bpm - think pain, sepsis, PE, thyrotoxicosis, anticholinergic drugs, hypovolemia

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Step 3 - Rhythm

1. Is the rhythm regular or irregular?
   • Regular: measure R-R intervals (should all be equal)
   • Irregularly irregular (no pattern) = classic AF
   • Regularly irregular = consider Wenckebach, bigeminy
2. Is there a P wave before every QRS?
   • Normal sinus: upright P in leads I, II, aVF; inverted in aVR
3. Does every P wave conduct to a QRS?
   • If not: consider AV block

• Rate 60-100 bpm
• P wave before every QRS, constant PR interval
• P wave axis 0-75° (upright in I and II)

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Step 4 - PR Interval

• Normal: 120-200 ms (3-5 small boxes)
• Short PR (<120 ms): pre-excitation (WPW), junctional rhythm, ectopic atrial pacemaker
• Long PR (>200 ms): first-degree AV block

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Step 5 - QRS Duration

• Normal: <120 ms (< 3 small boxes)
• 120-150 ms: incomplete bundle branch block, or subtle intraventricular conduction delay
• >120 ms: complete bundle branch block (RBBB or LBBB), ventricular paced rhythm, hyperkalemia

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Step 6 - QT / QTc Interval

• Measured from start of QRS to end of T wave (best in lead II or V5)
• Must be corrected for heart rate because QT shortens as rate increases
• Bazett formula: QTc = QT / √RR (in seconds) - most common, used by ECG machines; overcorrects at fast rates
• Normal QTc: <440 ms in men, <460 ms in women
• Prolonged QTc (>500 ms): high risk for Torsades de Pointes (TdP)

• Drugs: antipsychotics, TCAs, amiodarone, sotalol, certain antibiotics (azithromycin, fluoroquinolones)
• Electrolytes: hypokalemia, hypomagnesemia, hypocalcemia
• Congenital long QT syndrome
• Bradycardia, hypothyroidism, acute myocardial infarction

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Step 7 - Electrical Axis

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Step 8 - P Waves

• Normal P: <120 ms wide, <2.5 mm tall, upright in I and II, biphasic or inverted in V1
• Peaked P in II (P pulmonale): >2.5 mm tall = right atrial enlargement (COPD, pulmonary HTN, tricuspid stenosis)
• Broad notched P (P mitrale): >120 ms with double hump in lead II = left atrial enlargement (mitral disease, LVH)
• Absent P waves: AF, sinoatrial block, junctional rhythm
• Retrograde P (inverted in II, III, aVF): junctional or low atrial rhythm

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Step 9 - QRS Voltages

• S in V1 + R in V5 or V6 >35 mm

• Dominant R in V1 (R:S ratio >1)
• RAD
• R in V1 + S in V5/V6 >11 mm

• Obesity, emphysema, pericardial effusion, hypothyroidism, amyloidosis

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Step 10 - Precordial R-wave Progression

• Normally, R waves grow from V1 (tiny) to V4-V5 (tallest), then decrease to V6
• Poor R-wave progression (PRWP): R waves fail to increase; consider anterior MI, LBBB, RVH, incorrect lead placement
• Transition zone (where R = S) normally at V3-V4; early transition (V1-V2) suggests posterior MI or RVH

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Step 11 - Pathological Q Waves

• Normal septal Q waves: small (<40 ms, <25% of R height) in I, aVL, V5, V6
• Pathological Q: >40 ms wide OR >25% of R height in ≥2 contiguous leads
• Indicate prior transmural myocardial infarction

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Step 12 - ST Segments

• Normal: isoelectric (at baseline), may have up to 1 mm variation
• ST elevation (STE): >1 mm in limb leads, >2 mm in precordial leads in 2+ contiguous leads

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Step 13 - T Waves

• Normal: upright in I, II, V3-V6; inverted in aVR; may be inverted in III, V1-V2
• Tall peaked T waves: hyperkalemia (early), hyperacute STEMI
• Deep symmetrical T inversion: ischemia (Wellens' syndrome in V2-V3 = critical LAD stenosis), PE (V1-V4), RVH
• Flattened T waves: non-specific but can indicate hypokalemia, digitalis effect, hypothyroidism

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Step 14 - U Waves

• Small positive deflection after T wave, best seen in V2-V3
• Prominent U waves: hypokalemia (most common), bradycardia, hypomagnesemia
• Inverted U waves: significant - associated with LVH, ischemia

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Key Clinical Patterns to Recognize Immediately

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Practical Tips for Residents

• Never read the computer interpretation uncritically - automated analysis has significant limitations and should always be reviewed
• Always get the clinical context first - a "normal ECG" in a patient with chest pain still warrants repeat ECGs and troponins
• Compare with old ECGs - a new LBBB or new Q wave changes everything
• Leads II, V1, V5 are your workhorses - rhythm in II, conduction in V1, ischemia in V5
• If the ECG looks "weird", check technical quality first - lead reversal and artifact account for many "abnormalities"

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