Overview
This lecture explains how an electrocardiogram (ECG) records the electrical activity of the heart through electrodes, describing the setup, lead types, and interpretation principles.
ECG Basics
- Electrical activity of the heart can be detected on the skin using electrodes.
- An ECG machine graphically records changes in the heart's electrical potential over time during a cardiac cycle.
- The 12-lead ECG provides 12 views of the heart's electrical activity from different angles.
Electrode Placement and Lead Types
- Ten electrodes are used in a standard 12-lead ECG: one on each limb and six across the chest.
- Six limb leads and six chest (precordial) leads are used.
- Limb leads view the heart in the vertical plane; chest leads view it in the horizontal plane.
- Right leg electrode functions as an earth (ground) electrode.
Limb Leads
- Six limb leads include three bipolar (I, II, III) and three unipolar (aVR, aVL, aVF) leads.
- Bipolar leads measure voltage between two limb electrodes:
- Lead I: right arm (negative) to left arm (positive) — views heart from the left.
- Lead II: right arm (negative) to left leg (positive) — inferior left view.
- Lead III: left arm (negative) to left leg (positive) — inferior right view.
- Unipolar (augmented) leads use one positive limb electrode and average the other two as reference:
- aVR: upper right heart view; aVL: upper left heart view; aVF: inferior wall view.
Chest (Precordial) Leads
- Chest leads are unipolar and placed on the chest to view the heart in the horizontal plane.
- All chest leads use the same negative reference (average of limb electrode inputs); each chest electrode is a positive pole.
Interpreting ECG Deflections
- Depolarization toward a lead causes a positive wave (upward deflection).
- Depolarization away from a lead causes a negative wave (downward deflection).
- Repolarization produces the reverse effect; wave direction varies by lead viewpoint.
Key Terms & Definitions
- ECG (Electrocardiogram) — A graphical recording of heart's electrical activity.
- Lead — An electrical view of the heart formed by electrodes.
- Bipolar Lead — Measures voltage between two electrodes.
- Unipolar Lead — Uses one positive electrode and a reference from other electrodes.
- Depolarization — The process of electrical activation of the heart muscle.
- Repolarization — The process of the heart muscle returning to its resting state.
Action Items / Next Steps
- Review the placement of electrodes for the 12-lead ECG.
- Memorize the orientation and viewpoint of each ECG lead.