Overview
This lecture covers the acquisition and interpretation of ECGs, including basic electrophysiology, electrode placement, and rhythm analysis using a five-step method.
Introduction to ECGs
- An ECG (electrocardiogram) records the heart's electrical activity using electrodes on the skin.
- The electrical conduction system of the heart initiates and transmits impulses that can be graphically represented.
Basic Electrophysiology and ECG Components
- Upward ECG deflection: electrical current towards positive electrode; downward: away from it.
- The isoelectric line is the ECG baseline with no deflections.
- ECG calibration: 1 small box = 0.04 seconds (horizontal), 0.1 mV (vertical); 1 large box = 0.20 seconds, 0.5 mV.
ECG Waves and Segments
- P Wave: atrial depolarization (contraction); should be upright, uniform, one per QRS complex.
- PR Interval (PRI): time from start of atrial to start of ventricular depolarization (0.12–0.20 seconds).
- QRS Complex: ventricular depolarization (contraction), normal duration <0.12 seconds.
- T Wave: ventricular repolarization (relaxation).
- ST Segment: period between ventricular depolarization and repolarization; elevation/depression suggests myocardial injury.
ECG Acquisition and Electrode Placement
- Good ECG quality requires clean, dry skin and correct, symmetrical electrode placement.
- Limb leads (RA, LA, RL, LL) are placed equally spaced on/near the torso or limbs for 3/12 lead ECG.
- For 12 lead ECG, add chest (precordial) leads V1–V6 at specified intercostal spaces and anatomical landmarks.
12 Lead ECG Interpretation
- 12 lead ECG uses 10 electrodes to provide 12 heart views: limb leads (vertical plane) and chest leads (horizontal plane).
- Each lead views a specific heart territory (e.g., septal, anterior, lateral, inferior).
- aVR should be primarily negative; a positive aVR suggests reversed limb leads.
- Multiple 12 lead ECGs (serial ECGs) aid in monitoring evolving cardiac changes.
Five-Step Method for ECG Rhythm Interpretation
- Assess: Regularity (R-R interval), Rate (bpm), P wave (presence/appearance), PRI (duration/consistency), QRS (duration/appearance).
- Use Lead II for rhythm strip interpretation.
- Methods for rate: 1500 Rule (regular), 6-second x10 (any rhythm), 300 Rule (regular).
Common Cardiac Rhythms
- Normal Sinus Rhythm: regular, 60–100 bpm, upright P waves, normal PRI and QRS.
- Sinus Tachycardia: regular, >100 bpm.
- Sinus Bradycardia: regular, <60 bpm.
- Ventricular Fibrillation (VF): chaotic, no discernible waves/complexes.
- Ventricular Tachycardia (VT): regular, >150 bpm, wide QRS.
- Asystole: total electrical silence.
- SVT: regular, >150 bpm, P waves hidden.
- Atrial Fibrillation (AF): irregularly irregular, no P waves, replaced by f waves.
- Pulseless Electrical Activity (PEA): ECG shows rhythm but no pulse.
Key Terms & Definitions
- ECG — Electrocardiogram, records heart's electrical activity
- Dysrhythmia/Arrhythmia — Abnormal heart rhythm
- Infarction — Localized heart muscle tissue death
- Ischaemia — Reduced blood supply causing potential damage
- Myocardium — Heart muscle
- STEMI/Non-STEMI — Types of myocardial infarction based on ECG findings
- Artefact — Electrical interference on ECG
- PRI — Time interval from P wave start to QRS start (0.12–0.20s)
- QRS Complex — Electrical representation of ventricular contraction (<0.12s)
Action Items / Next Steps
- Review skill sheets for 3 and 12 lead ECG acquisition.
- Practice the five-step method with sample ECG strips.
- Refer to Clinical Procedures and Guidelines for further details.
- Complete quiz associated with this module.