Central venous pressure

The CVP is a pressure reading obtained from a central venous catheter whose tip is ideally situated in the vena cava at the level of the right atrium. Subclavian and internal jugular lines are usually employed for such measurements, and location placement (as well as ruling out pneumothorax) should be confirmed with a post-procedure chest X-ray (CXR). A pressure transducer is then attached such that readings of the CVP may be performed.

The CVP reflects the adjacent right atrial pressure (RAP). As blood is in continuity between the heart and the cava, the CVP reading is useful in helping gauge intra-vascular volume and cardiac filling. Intuitively, CVP is directly proportional to venous return and inversely proportional to cardiac contractility. It follows that CVP can be used to guide fluid replacement.

In order for the CVP to be an accurate guide for the management of fluid replacement, the patient's heart should be normal with RV function paralleling left ventricular (LV) function. Starling's law indicates that cardiac output is optimized as LV filling is optimized. As we cannot measure ventricular EDV, we rely on the ventricular end-diastolic pressure (VEDP). When the heart is normal, RV filling parallels that of the left. Furthermore, in a normal heart, right VEDP (RVEDP) equals RAP (as the tricuspid valve is open in diastole). Thus, RVEDP = RAP = CVP To summarize, in the normal heart, CVP can be used as a guide to resuscitation because it ultimately reflects left VEDP (LVEDP).

Figure 8.5. Floating a PA catheter. The catheter is introduced through a central vein and the balloon inflated. The flow of blood pulls the balloon and thus catheter via the right atrium and RV to its final resting position in the PA. As the catheter is floated, its position can be followed by evaluating pressure tracings on the monitor. Once the balloon has 'wedged' itself in the PA, the balloon should be deflated to avoid complications such as a ruptured PA or pulmonary infarct.

The problem is that the vast majority of critically ill patients do not have normal hearts. Thus, as an absolute value, CVP can be unreliable for a variety of reasons. CVP will be elevated in any situation where the intra-thoracic pressures are increased (mechanical ventilation). Pulmonary vascular disease, RV disease, LV failure, or valvular heart disease will also affect the CVP reading. Keeping such limitations in mind, looking at the dynamic changes in the CVP in response to fluid therapy can be very useful. If the CVP remains low in a hypotensive patient after receiving a fluid challenge, they may need more fluid. If the CVP rises rapidly in response to a fluid challenge, it implies the patient may already be appropriately filled.

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