Central Venous Pressure Measurement
Anatomy of the CVP transducer
Influence of position on CVP measurement
In a supine patient, there is a reasonably equal distribution of central venous blood throughout the central venous volume. Not so if your patient is upright. The inferior vena cava ends up at the bottom of a column of blood, and the superior vena cava ends up at the top.
If you get your patient to stand up, the venous pressure in the feet ends up about 90mmHg. The neck veins collapse under the influence of atmospheric pressure; the dural sinuses inside the skull cannot collapse, and a negative pressure exists there (around -10mmHg).
The only point in the body where the venous pressure remains unchanged by changes in position is the tricuspid valve; each time the right ventricular pressure increases the right ventricle experiences increased preload and ejects more blood, thereby decreasing the pressure. Thus, the tricuspid valve pressure never varies more than 1-2mmHg in spite of differences in posture.
It is a generally accepted rule that the CVP can be measured with the patient at 45 degrees head-up elevation, in spite of the venous blood sloshing around and confounding things.
Influence of CVC insertion point and CVP measurement
It is a general rule that CVP measurement is performed with the tip of the catheter in the proximal part of the superior vena cava. However, at least one study has confirmed that femoral (or common iliac) venous pressure usually differs by no more than 1mmHg fro the right atrial pressure, provided the patient is supine.
The caveat is that high intra-abdominal pressure will result in a more abnormal measurement if you measure from the femoral vessels, as the pressure in the abdomen will be transmitted to the iliac veins.
Influence of catheter tip position on CVP monitoring
In actual fact, the catheter tip position does not matter too much. The measurement is a difference between the pressure in the fluid-filled tubing and the atmospheric pressure (which is the "zero" reference pressure).
Because the tubing is filled with fluid (which itself experiences hydrostatic pressure) shoving the catheter deeper will not change the pressure inside the system. If the transducer is still at the same position, all it will measure is the pressure from the column of fluid above its zero point. The pressure of the fluid inside the submerged catheter will counteract (and equal) the pressure of the fluid pushing on the catheter.
Thus, only changing the transducer height will influence the CVP measurement.
What if I am infusing something though the CVP measurement line?
This will result in bizarre waveforms and pressures, which reflect the pressure generated by the infusion pump.
If you run the infusion through another lumen, provided the infusion is running slowly the CVP measurement is not affected. At rapid rates of infusion you may see some difference.
The influence of PEEP on CVP measurements
You are measuring the difference in pressure between the atmosphere (where you have zeroed your transducer) and the central vein your line is resting in. However, the actual pressure you are interested in is the transmural pressure: the difference between right atrial pressure and intrathoracic pressure.
Intravascular pressures will be equal to transmural pressures when the thoracic pressure equals zero (i.e. when it equalizes with atmospheric pressure). Thus, CVP should be measured at the end of expiration.
The influence of thoracic pressure on CVP measurements
If there is PEEP, you are never going to get a "zero" intrathoracic pressure. Indeed both PEEP and Auto-PEEP will cause the intrathoracic pressure to be raised at the end of expiration.
Because the transducer is zeroed to atmospheric pressure, and the "atmospheric pressure" inside the chest cavity is the PEEP, the CVP in a mechanically ventilated patient will be slightly higher (because the PEEP puts a little extra pressure on the catheter tip)
About half of the PEEP is transmitted to the heart chambers. Less so in people with stiff diseased lungs.
Thus, a PEEP of 10 increases the CVP by only ~ 3mmHg.