An electrocardiogram (ECG or EKG) is one of the most essential diagnostic tools in modern clinical medicine. It is a painless, non-invasive, and quick test that records the heart’s electrical activity through electrodes placed on the skin. An ECG plays a vital role in identifying heart rhythm disorders (arrhythmias), myocardial infarctions (heart attacks), electrolyte disturbances (like hyperkalemia or hypokalemia), conduction system abnormalities (like bundle branch blocks), and even structural heart diseases such as hypertrophy or ischemia.
ECG interpretation is considered a fundamental skill for all healthcare professionals. Whether you are a medical student, nurse, paramedic, or physician, the ability to understand ECGs can directly affect patient outcomes, especially in emergency settings. A timely and accurate ECG reading can be the difference between immediate intervention and critical delay.
What Is a 12-Lead ECG and How Does It Work?
A 12-lead ECG does not mean 12 wires or electrodes, but rather 12 different “views” or electrical perspectives of the heart. This is achieved using 10 physical electrodes, which together produce 12 leads (tracings). These leads are classified into:
- 6 limb leads: I, II, III, aVR, aVL, aVF — providing frontal plane views
- 6 chest (precordial) leads: V1 to V6 — providing horizontal (transverse) plane views
Each lead detects the heart’s electrical activity from a different angle, allowing a complete picture of the electrical conduction across all regions of the myocardium. The combination of leads gives insights into the anterior, septal, lateral, and inferior walls of the heart.
- Inferior leads: II, III, aVF
- Lateral leads: I, aVL, V5, V6
- Anterior leads: V3, V4
- Septal leads: V1, V2
This detailed mapping helps localize cardiac problems like myocardial infarction based on which leads show abnormalities.
Step-by-Step Approach to Reading an ECG
Proper ECG interpretation begins with a structured approach. Below is a systematic method to analyze any ECG:
1. Check the Heart Rate
- Count the number of large boxes between two R waves (peak of QRS) and divide 300 by that number.
- Example: 300 ÷ 4 = 75 beats per minute.
2. Evaluate Rhythm
- Look for regularity between R-R intervals.
- Check if each P wave is followed by a QRS complex (suggesting a sinus rhythm).
3. Analyze the P Wave
- Represents atrial depolarization (contraction of atria).
- Should be upright in leads I and II, and less than 0.12 seconds (3 small boxes) in duration.
4. Measure the PR Interval
- Distance from the start of the P wave to the start of the QRS complex.
- Normal range: 0.12 to 0.20 seconds (3–5 small boxes).
- A prolonged PR interval may indicate first-degree AV block.
5. Inspect the QRS Complex
- Reflects ventricular depolarization.
- Should be narrow: < 0.12 seconds.
- Wide QRS may suggest bundle branch block, ventricular rhythm, or hyperkalemia.
6. Assess the ST Segment
- Should be flat and at the isoelectric line.
- ST elevation can indicate acute myocardial infarction.
- ST depression may be seen in ischemia, digitalis effect, or posterior MI.
7. Evaluate the T Wave
- Represents ventricular repolarization.
- Should be upright in most leads.
- Inverted, tall, or symmetrical T waves may suggest ischemia, hyperkalemia, or CNS pathology.
8. Check the QT Interval
- Represents total ventricular activity (depolarization + repolarization).
- QT should be corrected for heart rate (QTc).
Normal QTc: < 440 ms in men, < 460 ms in women. - Prolonged QT can lead to dangerous arrhythmias like Torsades de Pointes.
Proper Lead Placement: Why It Matters
Incorrect electrode placement is one of the most common causes of ECG misinterpretation. Here’s how chest leads should be placed:
- V1: 4th intercostal space, right of sternum
- V2: 4th intercostal space, left of sternum
- V3: Midway between V2 and V4
- V4: 5th intercostal space, midclavicular line
- V5: Same level as V4, anterior axillary line
- V6: Same level as V4, midaxillary line
Limb leads are usually placed on wrists and ankles or proximally on shoulders and hips in clinical settings.
Accurate placement ensures the ECG reflects the true electrical behavior of the heart. Misplaced leads can mimic pathologies like myocardial infarction or axis deviation.
Localizing Myocardial Infarction Using Leads
Understanding which leads correspond to specific regions of the heart helps localize infarction:
| Region | Leads Affected |
|---|---|
| Inferior | II, III, aVF |
| Lateral | I, aVL, V5, V6 |
| Anterior | V3, V4 |
| Septal | V1, V2 |
| Extensive Anterior | V1–V6, I, aVL |
| Posterior | V1 (reciprocal changes), V7–V9 (posterior leads) |
For example:
- ST elevation in II, III, aVF → Inferior MI
- ST elevation in V2 to V4 → Anterior MI
- ST depression in V1–V3 + tall R waves → Posterior MI
Common ECG Abnormalities to Know
- Sinus Tachycardia: HR > 100 bpm with normal P waves
- Sinus Bradycardia: HR < 60 bpm, may be normal in athletes
- Atrial Fibrillation: No P waves, irregularly irregular rhythm
- Ventricular Tachycardia: Wide QRS, rapid rate, dangerous
- STEMI: ST elevation in 2 or more contiguous leads
- NSTEMI/Unstable Angina: ST depression, T wave inversion
Final Thoughts
ECG interpretation may seem overwhelming at first, but with consistent practice and a structured approach, it becomes an intuitive process. Remember, ECGs are snapshots of the heart’s electrical function — they don’t diagnose alone, but when interpreted in the clinical context, they provide powerful insights.
Whether in emergency care or routine check-ups, being confident in reading ECGs allows healthcare providers to make faster, more accurate decisions that can truly save lives.
