ECG Fundamentals
What is ECG?
ECG (Electrocardiogram) is a method of recording the heart’s electrical activity. With each heartbeat, cardiac muscle cells generate small electrical signals that propagate through body tissues and can be detected on the skin with electrodes.
Why measure ECG?
Electrical signals can reflect:
- ❤️ Heart rate: beats per minute
- 📊 Rhythm: whether the heartbeat is regular
- ⚡ Conduction: how electrical signals travel through the heart
- 🩺 Myocardial health: ischemia, infarction, etc.
- 🔍 Arrhythmias: abnormal heartbeat patterns
ECG waveforms explained
A complete cardiac cycle includes these key waves:
R
|
|
P | T
/\ | /\
___/ \__|__/ \___
QS
P wave
- Meaning: atrial depolarization (electrical activity before atrial contraction)
- Normal duration: 0.08–0.11 s
- Clinical significance: reflects atrial function
QRS complex
- Meaning: ventricular depolarization (electrical activity before ventricular contraction)
- Normal duration: 0.06–0.10 s
- Clinical significance: reflects ventricular function; largest amplitude on the ECG
QRS components:
- Q wave: first downward deflection
- R wave: first upward deflection (usually tallest)
- S wave: first downward deflection after the R wave
T wave
- Meaning: ventricular repolarization (electrical activity before ventricular relaxation)
- Clinical significance: reflects myocardial perfusion
Important intervals
| Interval | Normal range | Clinical meaning |
|---|---|---|
| PR interval | 0.12–0.20 s | AV conduction time |
| QT interval | 0.36–0.44 s | total ventricular electrical activity |
| RR interval | 0.6–1.0 s | cardiac cycle (used to compute heart rate) |
How to calculate heart rate
Method 1: RR interval
Heart rate (BPM) = 60 / RR interval (seconds)
Example: if RR interval is 0.8 s
Heart rate = 60 / 0.8 = 75 BPM
Method 2: count method
Heart rate (BPM) = (number of beats × 60) / recording duration (seconds)
Standard 12-lead system
ECG usually uses 12 leads to view cardiac electrical activity from different angles:
Limb leads (6)
- Standard leads: I, II, III
- Augmented leads: aVR, aVL, aVF
Precordial leads (6)
- V1–V6: anterior, lateral, etc., from different positions
Common arrhythmias
1. Sinus rhythm (normal)
Features:
✅ Normal P-wave morphology, regular
✅ QRS follows every P wave
✅ Heart rate 60–100 BPM
✅ Regular RR intervals
2. Sinus tachycardia
Features:
⚡ Heart rate > 100 BPM
✅ Regular rhythm
📝 Common with exercise, stress, fever, etc.
3. Sinus bradycardia
Features:
🐌 Heart rate < 60 BPM
✅ Regular rhythm
📝 Common in athletes and during sleep
4. Atrial fibrillation (AF)
Features:
❌ No clear P waves
🌊 Irregular baseline (f waves)
❌ RR intervals absolutely irregular
⚠️ Requires medical attention
5. Premature ventricular contraction (PVC)
Features:
⚡ Early wide QRS complex
❌ Abnormal QRS morphology (duration > 0.12 s)
❌ No preceding P wave
📝 Occasional PVCs are common; frequent PVCs need follow-up
6. Premature atrial contraction (PAC)
Features:
⚡ Early QRS complex
✅ QRS morphology mostly normal
🔄 P-wave shape may differ from sinus
📝 Usually benign
ECG signal quality
High-quality signal
✅ Stable baseline, no drift
✅ Clear P, QRS, T waves
✅ No obvious power-line interference (50/60 Hz)
✅ No significant muscle artifact
Common artifacts
1. Baseline wander
Causes:
- Poor electrode contact
- Respiration
- Body movement
Mitigation:
- Ensure good skin contact
- Stay relaxed
- Use high-pass filtering (0.5–1 Hz)
2. Power-line interference (50/60 Hz)
Appearance: regular high-frequency ripple on the signal
Causes:
- AC mains coupling
- Poor equipment grounding
Mitigation:
- Check grounding
- Use notch filters
- Stay away from power cords and appliances
3. Muscle artifact
Appearance: irregular high-frequency noise
Causes:
- Muscle tension
- Shivering
Mitigation:
- Relax
- Keep the environment warm
Sampling parameters
Sampling rate
- Definition: samples per second (Hz)
- Common values:
- 250 Hz: basic diagnostics
- 500 Hz: standard clinical use (recommended)
- 1000 Hz: high-precision research
Principle: per Nyquist, sampling rate should be at least twice the highest signal frequency.
Signal amplitude
- Unit: mV
- Typical range: −2 mV to +2 mV
- R-wave amplitude: usually 0.5–2.5 mV
Gain
- Standard gain: 10 mm/mV
- Meaning: 1 mV corresponds to 10 mm vertical deflection on the display
Clinical use cases
1. Routine wellness monitoring
- 💓 Heart rate monitoring
- 📈 Heart rate variability (HRV)
- 🏃 Exercise heart rate tracking
2. Screening
- 🩺 Arrhythmia detection
- ⚠️ Myocardial ischemia assessment
- 🔍 Conduction abnormalities
3. Telemedicine
- 📱 Mobile ECG monitoring
- 🌐 Real-time data transfer
- 👨⚕️ Remote specialist review
4. Research
- 🧪 Cardiovascular drug studies
- 📊 Stress and emotion analysis
- 🎯 Personalized health management
Tips for using our products
Beginners
- 📖 Start with a single lead (e.g. lead II)
- 🎯 Focus on basics: rate and rhythm regularity
- 🔧 Use our visualization tools to view signals
- 📚 See our quick start guide
Advanced users
- 📊 Use multi-lead analysis for a fuller picture
- ⚙️ Call advanced APIs for detailed metrics
- 🧮 Combine HRV and other advanced indices
- 💡 Use our advanced API
Professional organizations
- 🏥 Integrate with existing clinical systems
- 🔒 Consider private deployment
- 📋 Meet healthcare data security standards
- 👥 Batch processing and analytics
Important notices
Medical advice
- ⚕️ Our products provide signal analysis and reference information
- 🚫 They do not replace professional medical diagnosis
- 🩺 If you find abnormalities, seek medical care promptly
- 📞 In emergencies, call emergency services immediately
Best practices
- 📏 Keep measurement conditions consistent (time, posture, etc.)
- 📊 Establish a personal baseline for comparison
- 🔄 Measure regularly and watch trends
- 💾 Keep raw data for later review
Related resources
References
- Wagner GS. Marriott's Practical Electrocardiography. 12th Edition.
- American Heart Association. ECG Database and Guidelines.
- MIT-BIH Arrhythmia Database.
- PhysioNet – ECG research resources