An electrocardiogram (ECG) shows a positive deflection when electrical activity within the heart is moving towards the positive electrode of the ECG lead. This is due to the underlying principle of how ECGs measure electrical activity.
Understanding ECG Polarity
To fully understand why an ECG shows a positive deflection, it's essential to understand the basic principles of how electrical activity is measured:
- Depolarization: When heart muscle cells are stimulated, they become electrically active—a process called depolarization. This wave of depolarization spreads through the heart, causing muscle contraction.
- ECG Leads: ECGs use multiple electrodes (leads) placed on the body. Each lead represents a different viewpoint of the heart's electrical activity. Each lead consists of a positive (+) and negative (-) electrode.
The Cause of a Positive ECG Deflection
According to "ECG Basics," a positive deflection on an ECG is caused by a very specific situation:
A positive-going deflection will result when a wavefront of depolarization travels towards the + electrode and away from the electrode attached to the - terminal.
In simpler terms:
- Depolarization Wave: As the electrical wave of depolarization moves through the heart, it has a direction.
- Electrode Alignment: If this wave is moving towards the positive electrode of an ECG lead and away from the negative electrode, the ECG machine will register a positive deflection.
Example
Imagine a lead positioned on the left side of the chest. If the electrical wave of depolarization moves from the right to left, that is, towards this lead, it will record this as an upward or positive wave.
Summary
| Condition | Resulting ECG Deflection |
|---|---|
| Depolarization moves towards positive electrode | Positive |
| Depolarization moves away from positive electrode | Negative |
Key Points
- ECGs measure electrical activity.
- Positive deflections indicate that the depolarization wave is traveling toward the positive electrode of the lead being measured.
- The opposite movement of depolarization will result in a negative deflection.
