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ECG Interpretation for Medical Students

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1. The ECG Paper and Basic Setup

• Each small box = 1 mm = 40 ms (0.04 s)
• Each large box (5 small boxes) = 5 mm = 200 ms (0.20 s)
• Vertically: 1 mV = 10 mm at standard calibration

• Divide 300 by the number of large boxes between R-R intervals (e.g., 3 large boxes = 300/3 = 100 bpm)
• Or: divide 1500 by the number of small boxes between consecutive R waves

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2. The ECG Waveforms

P Wave

• Represents atrial depolarization
• Normal: upright in leads I and II, inverted in aVR
• The sinus node fires in the high right atrium; the depolarization vector moves downward-left, toward lead II (positive) and away from aVR (negative)
• In lead V1, the P wave may be biphasic - the initial positive component reflects right atrial depolarization; the terminal negative component reflects left atrial depolarization
• Normal P wave duration: <120 ms; amplitude <2.5 mm in lead II

PR Interval

• Measured from the onset of the P wave to the onset of the QRS complex
• Represents conduction through the atrial muscle, AV node, and His-Purkinje system - the AV node contributes most of the physiologic delay
• Normal: 120-200 ms (3-5 small boxes)
• PR >200 ms = first-degree AV block
• PR <120 ms = pre-excitation (e.g., WPW) or junctional rhythm

QRS Complex

• Represents ventricular depolarization
• Much larger in amplitude than P wave due to the greater ventricular muscle mass
• Terminology: the first negative deflection = Q wave; first positive deflection = R wave; negative deflection after the R wave = S wave
• Normal duration: 80-120 ms (2-3 small boxes)
• QRS >120 ms = bundle branch block or ventricular conduction delay

J Point and ST Segment

• The J point is the junction between the end of the QRS complex and the beginning of the ST segment
• The ST segment corresponds to the plateau (phase 2) of the action potential, when the membrane is depolarized but not yet repolarizing
• Normally isoelectric (neither elevated nor depressed)
• ST elevation suggests acute injury/infarction or pericarditis
• ST depression suggests ischemia or digoxin effect

T Wave

• Represents ventricular repolarization (phase 3 of action potential)
• Normally upright in leads I, II, V3-V6; inverted in aVR; variable in V1, III, aVL
• T waves are concordant with the QRS complex (same direction as the major QRS deflection) in most leads
• Normally asymmetric: gradual upstroke, rapid downstroke

U Wave

• A small deflection following the T wave
• Most prominent in V2-V3
• Often represents repolarization of the Purkinje system or mid-myocardial (M) cells
• Prominent U waves are seen in hypokalemia and with bradycardia
• Difficulty measuring QT: do not include the U wave in the QT measurement

QT Interval

• Measured from the start of the QRS complex to the end of the T wave
• Represents total ventricular depolarization + repolarization time
• Varies with heart rate - must be corrected using Bazett's formula:

QTc = QT / √RR (all values in seconds)

• Normal QTc: ≤450 ms in men, ≤460 ms in women
• Prolonged QTc (>500 ms is high risk): predisposes to torsades de pointes VT - caused by drugs (amiodarone, quinidine, macrolides, antipsychotics), hypocalcemia, hypokalemia, hypomagnesemia, congenital long QT syndromes
• Short QTc (<320-340 ms): rare short QT syndrome; also seen in hypercalcemia, digoxin effect

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3. The 12 ECG Leads

Frontal Plane Leads (Limb Leads) - 6 leads

• Lead II is best for rhythm analysis (most upright P wave)
• aVR is always negative in sinus rhythm; aVR positivity suggests dextrocardia or lead reversal

Horizontal Plane Leads (Precordial/Chest Leads) - 6 leads

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4. The Cardiac Axis

Normal Axis: -30° to +90°

Causes of Axis Deviation

• Left anterior fascicular block (most common)
• Inferior MI
• LVH
• LBBB
• Wolff-Parkinson-White (right-sided pathway)

• Right ventricular hypertrophy
• Left posterior fascicular block (diagnosis of exclusion)
• Lateral MI
• RBBB with LPFB (bifascicular block)
• Normal in children and tall thin adults
• Dextrocardia

• Ventricular tachycardia
• Severe RVH
• Hyperkalemia

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5. Conduction Intervals - Quick Reference Table

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6. A Systematic Approach to ECG Interpretation

1. Rate - Calculate bpm (300 ÷ large boxes between R waves)
2. Rhythm - Regular or irregular? P waves present? Each P followed by QRS?
3. Axis - Look at leads I and aVF
4. Intervals - PR, QRS, QT/QTc
5. P wave morphology - Shape, duration, axis (biphasic in V1?)
6. QRS morphology - Amplitude, R wave progression, Q waves, bundle branch block?
7. ST segment - Elevation or depression? Which leads?
8. T wave morphology - Upright, inverted, peaked, flat?
9. U wave - Present? Prominent?
10. Overall interpretation - Combine all findings

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7. Key Correlations: ECG Waves and Action Potentials

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