A Q wave appears as a negative deflection on an electrocardiogram (ECG) because of the direction of the electrical forces during early ventricular depolarization relative to the lead being observed.
Understanding Q Waves
A Q wave is defined as the first negative deflection that occurs within the QRS complex on an ECG. Here's why it's negative:
- Electrical Direction: The Q wave represents the initial direction of electrical activity (depolarization) as it spreads through the ventricles.
- Lead Perspective: ECG leads are strategically placed on the body to view electrical activity from different angles. The Q wave is negative when the electrical forces of ventricular depolarization are moving away from the positive pole of the lead being observed. This is further explained in the reference which states, "a Q wave indicates that the net direction of early ventricular depolarization (QRS) electrical forces projects toward the negative pole of the lead axis in question."
- Initial Depolarization: Typically, ventricular depolarization starts in the septum, which often spreads from left to right, or back towards the left side. As such, leads located on the left side or in the anterior direction will register the activity as a negative force because the electrical current is traveling away from the leads positive pole.
How It Works
- Think of it like a flashlight. If the light is shining towards your eye (the positive lead), it registers as positive; if the light is moving away, it appears negative.
- The QRS complex as a whole represents the whole ventricular depolarization which is the main event of electrical excitation in the heart, therefore each part including the Q wave, is a segment of that process.
Example Scenarios
| Scenario | Direction of Depolarization | Effect on ECG Lead |
|---|---|---|
| Early Ventricular Depolarization | Away from positive lead | Negative (Q wave) |
Key Takeaway
In short, a Q wave appears negative because the electrical current is initially moving away from the lead's positive pole. It is not due to a specific pathology, but rather a result of the initial forces of depolarization which travel away from positive leads due to the orientation of the cardiac muscle fibers.
