1.Draw & label various waves in ECG lead II mbbs 1st year physiology
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ECG Lead II - Waves, Intervals & Segments (MBBS 1st Year Physiology)
The Diagram (from Costanzo Physiology, Fig. 4.17 - Lead II)
Why Lead II?
Lead II records between the right arm (negative) and the left leg (positive). The direction of cardiac depolarization (from SA node downward and leftward through the ventricles) is roughly parallel to the Lead II axis, so all waves are normally upright (positive) and well-defined - making it the classic teaching lead for ECG morphology.
Labeled Waves
1. P Wave
- Represents: Atrial depolarization (SA node firing spreads across both atria)
- Shape: Small, smooth, rounded, upright in Lead II
- Duration: 80-100 ms (< 0.12 sec)
- Amplitude: < 2.5 mm
- What is NOT seen: Atrial repolarization - it is buried within the QRS complex (cancelled out by larger ventricular currents)
2. QRS Complex (three components)
The ventricles are much larger than the atria, so the QRS complex is taller. Despite the larger muscle mass, the total duration is similar to the P wave because the His-Purkinje system conducts much faster than atrial conduction pathways.
| Wave | Direction | Represents |
|---|---|---|
| Q wave | Downward (negative) | Initial septal depolarization (left to right) |
| R wave | Upward (positive) - tallest peak | Main ventricular depolarization (apex to base) |
| S wave | Downward (negative) after R | Basal ventricular depolarization |
- Duration: 60-100 ms (< 0.12 sec)
- Represents: Ventricular depolarization - just before ventricular contraction
3. T Wave
- Represents: Ventricular repolarization (recovery of ventricles)
- Shape: Broad, asymmetric, upright in Lead II - rises slowly and descends quickly
- Direction: Upright (same as QRS) - because repolarization proceeds from epicardium → endocardium (opposite direction to depolarization), which reverses the polarity and yields a net positive deflection
- Duration: ~160 ms
- Note: This is a repolarization wave, NOT a depolarization wave
Labeled Intervals & Segments
4. PR Interval
- From: Start of P wave → start of QRS
- Includes: P wave + PR segment (isoelectric flat line = AV node conduction delay)
- Normal: 120-200 ms (0.12-0.20 sec)
- Clinical note: Prolonged PR = first-degree AV block; shortened PR = pre-excitation (WPW)
- The PR segment alone represents the time the impulse pauses at the AV node (this is the physiologic delay that allows atria to fill ventricles before they contract)
5. ST Segment
- From: End of S wave → start of T wave
- Normal: Isoelectric (flat, at baseline)
- Corresponds to: Plateau phase (Phase 2) of the ventricular action potential - entire ventricular myocardium is depolarized, no current flows
- Clinical note: ST elevation = STEMI (myocardial infarction); ST depression = ischemia or NSTEMI
6. QT Interval
- From: Start of QRS → end of T wave
- Represents: Total ventricular electrical systole (depolarization + repolarization)
- Normal: < 440 ms in men, < 460 ms in women (corrected for heart rate using Bazett's formula: QTc = QT / √RR)
- Clinical note: Prolonged QT predisposes to Torsades de Pointes (a dangerous arrhythmia)
7. RR Interval
- From: One R peak → next R peak
- Used to calculate heart rate: Heart rate = 60 / RR interval (in seconds)
- Normal resting HR: 60-100 bpm
Summary Table
| Component | Origin | Type | Normal Duration |
|---|---|---|---|
| P wave | Atrial depolarization | Depolarization wave | < 0.12 sec |
| PR interval | Atrial depol. + AV node delay | Interval | 0.12-0.20 sec |
| QRS complex | Ventricular depolarization | Depolarization wave | < 0.12 sec |
| ST segment | Ventricular plateau | Isoelectric segment | ~0.08-0.12 sec |
| T wave | Ventricular repolarization | Repolarization wave | ~0.16 sec |
| QT interval | Total ventricular electrical activity | Interval | < 0.44 sec |
Key Exam Points (1st Year Physiology)
- No atrial T wave on normal ECG - buried within QRS complex
- QRS same duration as P wave - because His-Purkinje velocity is much faster than atrial pathways
- T wave is upright in Lead II - because ventricular repolarization goes epicardium to endocardium (opposite to depolarization direction), net result is positive deflection
- PR segment = AV nodal delay - physiologically important to allow ventricular filling
- Einthoven's Law: Lead I + Lead III = Lead II (applies to any instantaneous potential)
Sources: Guyton & Hall Textbook of Medical Physiology (Chapter 11, Figs. 11.1-11.3) | Costanzo Physiology 7th Edition (Chapter 4, Fig. 4.17 - ECG from Lead II)
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