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ECG Fundamentals

What is ECG?​

ECG (Electrocardiogram) is a test method used to record the heart’s electrical activity. Each heartbeat generates very weak electrical signals in heart muscle cells. These signals propagate through body tissues and can be detected on the body surface using electrodes.

Why measure ECG?​

ECG signals can reflect:

  • ❤️ Heart rate: the number of heartbeats per minute
  • 📊 Heart rhythm: whether the heartbeat is regular
  • ⚡ Cardiac conduction: the pathways the electrical signals travel through
  • 🩺 Myocardial health: whether there is ischemia, infarction, etc.
  • 🔍 Arrhythmias: various abnormal heartbeat patterns

ECG waveform details​

A complete cardiac cycle contains the following key waveforms:

         R
         |
         |
    P    |    T
    /\   |   /\
___/  \__|__/  \___
        QS

P wave​

  • Meaning: atrial depolarization (electrical activity before atrial contraction)
  • Normal duration: 0.08–0.11 seconds
  • Clinical significance: reflects atrial function

QRS complex​

  • Meaning: ventricular depolarization (electrical activity before ventricular contraction)
  • Normal duration: 0.06–0.10 seconds
  • Clinical significance: reflects ventricular function, and is the largest-amplitude waveform on the ECG

Composition of the QRS complex​

  • Q wave: the first downward deflection
  • R wave: the first upward deflection (usually the highest)
  • S wave: the first downward deflection after the R wave

T wave​

  • Meaning: ventricular repolarization (electrical activity before ventricular relaxation)
  • Clinical significance: reflects myocardial perfusion status

Important timing intervals​

Interval nameNormal rangeClinical significance
PR interval0.12–0.20 sAV conduction time
QT interval0.36–0.44 sTotal duration of ventricular electrical activity
RR interval0.6–1.0 sHeartbeat cycle time (used to calculate heart rate)

Heart rate calculation methods​

Method 1: Using the RR interval​

Heart rate (BPM) = 60 / RR interval (seconds)

Example: if the RR interval is 0.8 seconds

Heart rate = 60 / 0.8 = 75 BPM

Method 2: Counting method​

Heart rate (BPM) = (number of heartbeats × 60) / recording duration (seconds)

Standard 12-lead system​

An ECG typically uses 12 leads to observe the heart’s electrical activity from different perspectives:

Limb leads (6)​

  • Standard leads: I, II, III
  • Augmented leads: aVR, aVL, aVF

Precordial (chest) leads (6)​

  • V1–V6: observe the anterior wall, lateral wall, etc. from different chest positions

Common arrhythmia recognition​

1. Sinus rhythm (normal)​

Characteristics:
✅ P-wave shape is normal and appears regularly
✅ Every P-wave is followed by a QRS complex
✅ Heart rate 60–100 BPM
✅ RR interval is regular

2. Sinus tachycardia​

Characteristics:
⚡ Heart rate > 100 BPM
✅ Regular rhythm
📝 Common with exercise, stress, fever, etc.

3. Sinus bradycardia​

Characteristics:
🐌 Heart rate < 60 BPM
✅ Regular rhythm
📝 Common in athletes and during sleep

4. Atrial fibrillation (AF)​

Characteristics:
❌ No obvious P-waves
🌊 Baseline shows irregular fluctuations (f-waves)
❌ RR intervals are absolutely irregular
⚠️ Requires medical attention

5. Premature ventricular contractions (PVCs)​

Characteristics:
⚡ Premature, wide QRS complexes appearing early
❌ Abnormal QRS morphology (duration > 0.12 s)
❌ No preceding P-wave
📝 Occasional PVCs are common; frequent PVCs should be evaluated

6. Premature atrial contractions (PACs)​

Characteristics:
⚡ Premature QRS complexes appearing early
✅ QRS morphology is generally normal
🔄 The P-wave shape may differ from sinus rhythm
📝 Often benign

ECG signal quality assessment​

High-quality signal features​

✅ Baseline is stable with no drift
✅ Waveform is clear; P, QRS, and T waves are identifiable
✅ No obvious power-frequency interference (50/60 Hz)
✅ No noticeable muscle (EMG) artifacts

Common interference types​

1. Baseline drift​

Causes:
- Poor electrode contact
- Breathing movement
- Body movement

Solutions:
- Ensure electrodes are well attached to the skin
- Keep the body relaxed
- Use a high-pass filter (0.5–1 Hz)

2. Power-frequency interference (50/60 Hz)​

Appearance: regular high-frequency ripples are superimposed on the signal.

Causes:
- Interference from power lines
- Poor device grounding

Solutions:
- Check grounding
- Use a notch (trap) filter
- Keep away from power lines and electrical equipment

3. Muscle artifacts​

Appearance: irregular high-frequency noise.

Causes:
- Muscle tension
- Shivering due to cold

Solutions:
- Keep your body relaxed
- Ensure the environment is warm and comfortable

Sampling parameter explanation​

Sampling rate​

  • Definition: number of data points acquired per second, unit Hz
  • Common values
    • 250 Hz: basic diagnosis
    • 500 Hz: standard clinical applications (recommended)
    • 1000 Hz: high-precision research

Principle: according to the Nyquist theorem, the sampling rate should be at least 2 times the highest frequency present in the signal.

Signal amplitude​

  • Unit: mV
  • Typical range: -2 mV to +2 mV
  • R-wave amplitude: typically 0.5–2.5 mV

Gain settings​

  • Standard gain: 10 mm/mV
  • Meaning: a 1 mV signal corresponds to a 10 mm display height

Clinical application scenarios​

1. Daily health monitoring​

  • 💓 Heart rate monitoring
  • 📈 Heart rate variability (HRV) analysis
  • 🏃 Exercise heart rate tracking

2. Disease screening​

  • 🩺 Arrhythmia detection
  • ⚠️ Myocardial ischemia assessment
  • 🔍 Abnormal conduction evaluation

3. Telemedicine​

  • 📱 Mobile ECG monitoring
  • 🌐 Real-time data transmission
  • 👨‍⚕️ Remote expert diagnosis

4. Research applications​

  • 🧪 Cardiovascular drug research
  • 📊 Stress and emotional analysis
  • 🎯 Personalized health management

Recommendations for using our products​

New users​

  1. 📖 Start with a single-lead (e.g., Lead II)
  2. 🎯 Focus on basic parameters: heart rate and rhythm regularity
  3. 🔧 Use our visualization tools to view the signals intuitively
  4. 📚 Refer to our Quick Start guide

Advanced users​

  1. 📊 Use multi-lead analysis for a more comprehensive assessment of heart function
  2. ⚙️ Call advanced API endpoints to obtain detailed analysis results
  3. 🧮 Combine advanced metrics such as HRV
  4. 💡 Use our Advanced Tier API

Professional institutions​

  1. 🏥 Integrate into existing medical systems
  2. 🔒 Use our private deployment solution
  3. 📋 Comply with medical data security standards
  4. 👥 Batch processing and analysis of large-scale data

Important notice​

Medical advice
  • Our products provide signal analysis and reference suggestions.
  • They cannot replace professional medical diagnosis.
  • If you observe abnormalities, seek medical care promptly.
  • In emergencies, call local emergency services immediately.
Best practices
  • Maintain consistent measurement conditions (time, posture, etc.)
  • Build personal baseline data to enable comparison
  • Measure regularly and observe trends
  • Save raw data for later review and analysis

References​

  1. Wagner GS. Marriott's Practical Electrocardiography. 12th Edition.
  2. American Heart Association. ECG Database and Guidelines.
  3. MIT-BIH Arrhythmia Database.
  4. PhysioNet — ECG research resources