The A Wave X Descent C Wave and X Descent

1. When the P wave of the ECG occurs, what atrial event results?

ANS: The right atrium contracts and produces a rise in right atrial pressures. Rises in atrial pressure are not named. When the right atrium relaxes, the right atrial pressure falls. The fall in pressure, when it is named, is called the X descent. (Not all cardiology texts give this descent a name [1].) The rise and fall produces a wave universally known as the A wave.

On the right atrial pressure curve, the rise in pressure resulting from atrial depolarization is not named.

Atrial relaxation produces the drop in pressure known as the X descent.

The rise in right atrial pressure due to bulging upward of the tricuspid valves into the right atrium has no name because it is not yet a wave.

2. What atrial event is initiated by the QRS?

ANS: The QRS results in RV contraction that pushes up the tricuspid valve and raises the right atrial pressure slightly. This slight rise in pressure is not named.

3. Does the RV contract by approximation of its free wall to the septum, i.e., by movement of these structures toward each other? Does the apex move up toward the base?

ANS: During systole the septum does not move toward the free wall of the RV and the apex does not move up. Normal RV ejection is due to two movements: The base moves downward and the free wall moves inward toward the septum.

Note: Left ventricular (LV) ejection differs from RV ejection in that the septum and free wall move toward each other and the apex moves up slightly. In the RV the septum and free wall move in the same direction so that the RV depends much more on the descent of the base and the inward movement of the free wall.

The broken lines represent the four possible inward movements of the RV that could eject its blood. Only 1 and 4 occur normally.

4. What happens to right atrial pressure when the floor of the atrium (base of ventricle) is pulled down during RV systole?

ANS: Its pressure falls. This produces the major movement of the jugular pulse known as the X' descent. (This also produces a small wave known as the C wave.)

Note: Only a minority of cardiologists name this descent, and most of those who do name it call it X, i.e., they give it the same name as that given to atrial relaxation, thus leading to great confusion [1].

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