Central venous pressure is considered
a direct measurement of the blood pressure in the right atrium and vena cava.
It is acquired by threading a central venous catheter (subclavian double lumen
central line shown) into any of several large veins. It is threaded so that
the tip of the catheter rests in the lower third of the superior vena cava.
The pressure monitoring assembly is attached to the distal port of a multilumen
central vein catheter.
Assisting with CVP placement
The CVP catheter is an important tool used to assess right ventricular function and systemic fluid status.
The CVP catheter is also an important treatment tool which allows for:
There are two ways to read a CVP waveform:
1. Find the mean of the A wave.
- read the high point of the A wave
- read the low point of the A wave
- add the high point to the low point
- divide the sum by 2
- the result is the mean CVP
The A wave starts just after the P wave ends and represents the atrial contraction. The high point of the A wave is the atrial pressure at maximum contraction. During the A wave the atrial pressure is greater than the ventricular diastolic pressure. At that point, the atrium is contracted, the tricuspid is open. Therefore, the high point of the A wave closely parallels the right ventricular end diastolic pressure. Remember, when the tricuspid valve is open and the right ventricle is full, the ventricle, atrium and vena cavae are all connected. Therefore, that point is the CVP.
2. Find the Z-point.
- Find the Z-point which occurs mid to end QRS
- Read the Z-point
The Z-point coincides with the middle to end of the QRS wave. It occurs just before closure of the tricuspid valve. Therefore, it is a good indicator of right ventricular end diastolic pressure. The Z-point is useful when A waves are not visible, as in atrial fibrillation. (The c-wave occurs at closure of the tricuspid valve. The crest of the c-wave is the atrial pressure increase caused by the tricuspid valve bulging back into the atrium.)