An electrocardiogram (ECG or EKG) is a non-invasive, rapid, and cost-effective diagnostic tool used to assess the electrical activity of the heart. It reflects the summation of cardiac action potentials as they propagate through the myocardium, captured by electrodes placed on the skin.
The ECG is essential in evaluating heart rhythm, detecting ischemia or infarction, diagnosing conduction abnormalities, and monitoring electrolyte imbalances or drug effects (e.g., digoxin, antiarrhythmics).
Principles of ECG Recording
Electrical Activity of the Heart
Cardiac myocytes generate action potentials that propagate through the conduction system:
- SA Node → Atria → AV Node → Bundle of His → Bundle Branches → Purkinje fibers → Ventricular myocardium
The propagation of depolarization and repolarization generates electric fields, which are transmitted through the body’s tissues and recorded at the skin’s surface using electrodes.
Deflection Principles
- A wave of depolarization moving toward a positive electrode = positive (upward) deflection
- A wave moving away = negative (downward) deflection
- A wave perpendicular to the electrode axis = isoelectric (flat) line
How to Obtain a 12-Lead ECG
A standard 12-lead ECG requires:
- 4 limb electrodes : Right Arm (RA), Left Arm (LA), Right Leg (RL - ground), Left Leg (LL)
- 6 chest (precordial) electrodes : V1–V6
Lead Types
- Limb leads (frontal plane) : I, II, III (bipolar); aVR, aVL, aVF (augmented unipolar)
- Chest leads (transverse plane) : V1–V6
Each lead provides a unique “view” of the heart’s electrical activity.
Understanding ECG Leads
Einthoven’s Triangle and Bipolar Limb Leads
Bipolar leads record voltage between two electrodes:
- Lead I : LA (+) ↔ RA (−) → 0°
- Lead II : LL (+) ↔ RA (−) → +60°
- Lead III : LL (+) ↔ LA (−) → +120°
These form an equilateral triangle around the heart called Einthoven’s Triangle .
Augmented Limb Leads (Unipolar)
These compare a single positive electrode to a central reference (Wilson’s Central Terminal):
- aVR : RA (+) → −150°
- aVL : LA (+) → −30°
- aVF : LL (+) → +90°
Precordial (Chest) Leads
Placed on the anterior chest wall and view the heart in the horizontal plane:
| Lead | Placement |
|---|---|
| V1 | 4th ICS, right sternal border |
| V2 | 4th ICS, left sternal border |
| V3 | Between V2 and V4 |
| V4 | 5th ICS, midclavicular line |
| V5 | Level with V4, anterior axillary line |
| V6 | Level with V4, midaxillary line |
ICS : Intercostal Space
ECG Paper and Calibration
ECG is usually recorded at:
- Speed : 25 mm/sec
- Voltage calibration : 10 mm = 1 mV
Grid Interpretation
- 1 small box (horizontal) = 0.04 sec
- 1 large box (5 small boxes) = 0.20 sec
- 1 large box (vertical) = 0.5 mV
ECG Waveform and Intervals
1. P Wave
- Represents : Atrial depolarization
- Normal duration : 0.08–0.12 sec (2–3 small boxes)
- Usually upright in leads I, II, aVF
- Biphasic in V1 (RA = first part; LA = second part)
2. PR Interval
- Represents : Conduction from atria to ventricles (SA → AV node)
- Normal : 0.12–0.20 sec (3–5 small boxes)
- Prolonged PR → 1° AV block
- Short PR → pre-excitation (e.g., WPW syndrome)
3. QRS Complex
- Represents : Ventricular depolarization
- Normal : < 0.12 sec (< 3 small boxes)
- Q wave : Initial negative deflection (can indicate old MI if >1 small box deep)
- R wave : First positive deflection
- S wave : Negative deflection after R
High-Yield:
- Wide QRS (>0.12 sec) → bundle branch block or ventricular origin
- Tall R in V5/V6 or deep S in V1 → LVH
4. ST Segment
- Represents : Early ventricular repolarization
- Normally isoelectric (flat)
- Elevation : STEMI
- Depression : Ischemia, reciprocal changes
5. T Wave
- Represents : Ventricular repolarization
- Usually upright in I, II, V3–V6
- Inverted → ischemia, hypokalemia
- Tall, peaked → hyperkalemia
6. QT Interval
- Represents : Total ventricular depolarization + repolarization
- Corrected QT (QTc) = QT / √RR
- Normal QTc : <440 ms (men), <460 ms (women)
- Prolonged QTc → risk of Torsades de Pointes (drug-induced, congenital)
High-Yield Notes
| Feature | Clinical Insight |
|---|---|
| ST elevation in ≥2 contiguous leads | STEMI (emergency!) |
| Peaked T waves | Hyperkalemia |
| U wave | Hypokalemia |
| Prolonged PR | 1° AV Block |
| Wide QRS | Bundle Branch Block |
| Delta wave + short PR | Wolff-Parkinson-White (WPW) Syndrome |
| Irregularly irregular rhythm | Atrial Fibrillation |
| Sawtooth P waves | Atrial Flutter |
| Wide QRS tachycardia | VTach (assume VT in unstable patients) |
