ECG Basics: A Simple Guide for Medical Students
Understand ECG waves, intervals, heart rate and a systematic approach to reading a 12-lead ECG.
ECG ya Electrocardiogram ek important investigation hai jo heart ki electrical activity ko record karta hai.
MBBS students ke liye ECG interpretation initially confusing lag sakta hai, especially jab multiple waves, intervals aur leads ek saath dekhne padte hain.
Lekin agar tum ECG ko ek systematic approach se padhna seekh jao, toh basics samajhna kaafi easy ho jata hai.
01. What is an ECG?
Electrocardiogram (ECG) heart ki electrical activity ka graphical recording hai.
Heart ke electrical impulses atria aur ventricles ko sequentially activate karte hain. ECG in electrical changes ko body surface electrodes ke through record karta hai.
Common indications:- Chest pain evaluation
- Palpitations
- Syncope
- Suspected arrhythmias
- Suspected myocardial ischemia or infarction
- Electrolyte-related cardiac changes
02. Understanding the 12 ECG Leads
Standard 12-lead ECG mein 10 electrodes use hote hain, jo 12 different electrical views provide karte hain.
A. Limb Leads – 6
- Lead I
- Lead II
- Lead III
- aVR
- aVL
- aVF
B. Chest Leads – 6
- V1 – V2: Septal region
- V3 – V4: Anterior region
- V5 – V6: Lateral region
Inferior leads → II, III, aVF
Lateral leads → I, aVL, V5, V6
Septal leads → V1, V2
Anterior leads → V3, V4
03. Chest Lead Placement
Correct electrode placement is essential for accurate ECG interpretation.
| Lead | Position |
|---|---|
| V1 | 4th intercostal space, right sternal border |
| V2 | 4th intercostal space, left sternal border |
| V3 | Midway between V2 and V4 |
| V4 | 5th intercostal space, left midclavicular line |
| V5 | Left anterior axillary line, horizontal with V4 |
| V6 | Left midaxillary line, horizontal with V4 |
04. Understanding ECG Paper
Standard ECG recording settings:
- Paper speed = 25 mm/second
- Gain = 10 mm/mV
| ECG Box | Time | Voltage |
|---|---|---|
| 1 small square | 0.04 sec | 0.1 mV |
| 1 large square | 0.20 sec | 0.5 mV |
| 5 large squares | 1 second | — |
Horizontal axis = Time
Vertical axis = Voltage
05. Important Waves of ECG
P Wave
- Represents atrial depolarization.
- Normally upright in lead II.
- Duration generally ≤ 0.12 sec.
QRS Complex
- Represents ventricular depolarization.
- Q = First negative deflection.
- R = First positive deflection.
- S = Negative deflection after R.
- Normal QRS duration is usually less than 0.12 sec.
T Wave
- Represents ventricular repolarization.
- Usually follows the QRS complex.
- Its morphology and direction vary across leads.
U Wave
- Small wave sometimes seen after the T wave.
- May become prominent in hypokalemia.
06. Important ECG Intervals
| Parameter | Normal Adult Value |
|---|---|
| PR interval | 0.12–0.20 sec |
| QRS duration | Usually < 0.12 sec |
| QT interval | Depends on heart rate |
| QTc | Interpret using sex-specific and method-specific reference limits |
PR Interval
Beginning of P wave to beginning of QRS complex. It represents atrial-to-ventricular conduction time.
ST Segment
End of QRS complex (J point) to beginning of T wave. ST elevation or depression may indicate ischemia, injury or other conditions; clinical context is essential.
QT Interval
Beginning of QRS complex to end of T wave. It represents total ventricular depolarization and repolarization time.
07. How to Calculate Heart Rate?
Method 1 – 300 Rule
Regular rhythm mein do consecutive R waves ke beech large squares count karo.
Heart Rate = 300 ÷ Number of Large Squares
Example: Agar 4 large squares hain:
300 ÷ 4 = 75 beats/minute.
Method 2 – 1500 Rule
Small squares between two R waves count karo.
Heart Rate = 1500 ÷ Number of Small Squares
Method 3 – 6 Second Method
6-second rhythm strip mein QRS complexes count karo aur 10 se multiply karo.
Example: 6 seconds mein 8 QRS complexes:
8 × 10 = 80 beats/minute.
08. What is Normal Sinus Rhythm?
Normal sinus rhythm generally has the following features:
- Heart rate approximately 60–100/min in adults.
- Regular rhythm.
- Normal sinus P-wave morphology.
- Each P wave followed by a QRS complex.
- PR interval usually 0.12–0.20 sec.
- QRS duration usually less than 0.12 sec.
Clinical context and the full ECG must always be considered.
09. Systematic Approach to ECG Interpretation
ECG dekhte waqt random interpretation mat karo. Ek fixed sequence follow karo.
| Step | What to Check? |
|---|---|
| 1 | Patient details and ECG calibration |
| 2 | Heart rate |
| 3 | Rhythm – regular or irregular? |
| 4 | Electrical axis |
| 5 | P waves and PR interval |
| 6 | QRS duration and morphology |
| 7 | ST segment |
| 8 | T waves and QT/QTc interval |
| 9 | Compare with previous ECG, if available |
10. Common ECG Abnormalities – Basic Overview
Sinus Bradycardia
Sinus rhythm with heart rate below 60/min in adults.
Sinus Tachycardia
Sinus rhythm with heart rate above 100/min in adults.
Atrial Fibrillation (AF)
Typically irregularly irregular ventricular rhythm with no distinct consistent P waves.
Ventricular Tachycardia (VT)
Typically a wide-complex tachycardia. It can be life-threatening and needs urgent assessment.
ST Elevation
Can occur in acute myocardial infarction and several other conditions. Interpret with symptoms and the complete ECG.
Hyperkalemia
May produce tall peaked T waves, PR prolongation, QRS widening and, in severe cases, a sine-wave pattern.
Hypokalemia
May produce ST depression, T-wave flattening and prominent U waves.
11. Common Mistakes While Reading ECG
- Ignoring ECG calibration and paper speed.
- Not checking patient details.
- Using only the machine-generated interpretation.
- Missing irregular rhythm.
- Ignoring ST-T changes.
- Not comparing previous ECGs.
- Interpreting ECG without clinical correlation.
12. Quick Revision – ECG High-Yield Points
- Standard paper speed → 25 mm/sec
- Standard gain → 10 mm/mV
- Small square → 0.04 sec
- Large square → 0.20 sec
- PR interval → 0.12–0.20 sec
- QRS duration → Usually < 0.12 sec
- Heart rate → 300 / Large squares
- Irregular rhythm → 6-second method
- P wave → Atrial depolarization
- QRS → Ventricular depolarization
- T wave → Ventricular repolarization
13. Test Your Knowledge – Quick MCQs
Q1. Standard ECG paper speed is:
A. 10 mm/sec
B. 25 mm/sec
C. 50 mm/sec
D. 100 mm/sec
Answer: B – 25 mm/sec
Q2. One large square on standard ECG paper represents:
A. 0.04 sec
B. 0.10 sec
C. 0.20 sec
D. 1 sec
Answer: C – 0.20 sec
Q3. P wave represents:
A. Ventricular depolarization
B. Atrial depolarization
C. Ventricular repolarization
D. Atrial repolarization only
Answer: B – Atrial depolarization
Q4. PR interval normal range is:
A. 0.04–0.08 sec
B. 0.12–0.20 sec
C. 0.20–0.30 sec
D. 0.30–0.40 sec
Answer: B – 0.12–0.20 sec
Q5. Heart rate by 300 rule with 5 large squares between R waves is:
A. 50/min
B. 60/min
C. 75/min
D. 100/min
Answer: B – 60/min
Final Words
ECG interpretation ek skill hai jo regular practice se improve hoti hai.
Start with normal ECG, understand waves and intervals, learn heart rate calculation and then move towards arrhythmias, ischemia and other abnormalities.
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