The central venous pressure, Also known by its acronym PVC, it is the pressure exerted by the blood at the level of the walls of the superior vena cava and the right atrium. It is an extremely important hemodynamic parameter, since it is the result of the combination of circulating blood volume in relation to the contraction force of the right ventricle..
Clinically, central venous pressure gives a very accurate idea of the patient's blood volume, as well as the force with which the right side of the heart contracts; in fact, the central venous pressure value represents in itself the preload of the right ventricle (filling volume of the ventricle at the end of diastole).
To obtain central venous pressure values, it is necessary to have a central venous access, either jugular or subclavian, with a catheter long enough so that the tip is located in the superior vena cava or the right atrium..
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The simplest way to describe central venous pressure is that it represents the amount of blood that returns to the heart through the systemic circulation (venous return)..
This blood exerts pressure on the walls of the inferior vena cava as well as on the right atrium, this being the value obtained when the PVC is measured..
However, the hemodynamic implications of this parameter go much further, since venous return in turn represents the filling volume of the right ventricle, that is, the amount of blood within it at the end of diastole..
In turn, this volume determines the intensity of the cardiac work, since according to the Frank-Starling mechanism, the greater the final diastolic volume of the ventricle (and therefore greater stretching of the cardiac muscle fibers), the greater the intensity of contraction of the myocardium.
Thus, central venous pressure makes it possible to indirectly estimate how the right heart is working..
To measure PVC, it is necessary to have a central venous access with a catheter whose length allows the tip to be positioned either in the superior vena cava or in the right atrium..
Once the catheter has been placed using the conventional central venous access technique, a chest radiograph should be performed to confirm the position of the catheter. In fact, under normal conditions the placement should be with the support of fluoroscopy in order to know at all times the position of the tip of the central line..
Once the central venous access is secured, the necessary material to measure PVC must be available..
The materials needed to take this measure are commonly used in hospitals. All of them must be sterile and handled with gloves to avoid contaminating the central venous access..
It is important that the connection lines are not excessively long, as this could lead to wrong values.
That said, the following material should be located:
- Male-male extension tube (K-50) .
- 3-way stopcock.
- Physiological solution (250 cc bottle).
- Infusion set (macro dripper).
- PVC ruler.
- Sterile gloves.
Once all the material is organized and at hand, the PVC can be measured, either using the manual or automated technique..
The manual technique is often used in critically ill patients who are treated in a trauma shock room, intermediate care room, and even hospital areas for critically ill patients, but where automated monitoring is not always available..
It is also an option to validate the results of the automatic method when there are doubts about it..
First, the patient's head should be positioned at a 15 degree inclination on the horizontal plane; ideally, the legs should remain parallel to this plane.
Once the patient is positioned, one end of the male-male extender should be connected to the central line. The other end will connect with a 3-way tap.
Subsequently, the PVC rule is connected to the 3-way valve. Simultaneously an assistant places the infusion set (macro dripper) in the physiological solution and purges the line.
Once this is done, the last free terminal of the three-way stopcock can be connected to the solution.
When all the elements of the system are connected and in position, the PVC screed is primed. This is done by placing the 3-way cock in the following position:
- Central line (to the patient) closed.
- Open physiological solution.
- Open PVC ruler.
Physiological solution is allowed to flow through the system until it begins to flow out of the free (upper) end of the PVC ruler, and the infusion set is then closed..
The PVC ruler is then positioned next to the patient's chest at the level of the Louis angle, perpendicular to the horizontal to proceed to open the 3-way valve in the following position:
- Central line (to the patient) open.
- Closed physiological solution.
- Open PVC ruler.
Once this is done, the solution located on the PVC ruler will begin to pass through the central line to the patient until it reaches a point where it is no longer infused. This position is known as the oscillating top and represents the central venous pressure value..
When the procedure is complete, all systems are closed with their safety clips and the PVC value is recorded. Nothing needs to be disconnected as central venous pressure is usually measured periodically.
Therefore, once the system is connected, it can be used repeatedly. The important thing in successive shots is not to forget to prime the PVC ruler before each measurement in order to obtain reliable measurements..
The automated technique is very similar to the manual technique, the only difference being that instead of using the PVC rule, a pressure transducer is used that is connected to the multiparameter monitor..
So the connection is as follows:
- One end of the 3-way tap connected to the central track.
- Other end connected to infusion set.
- The last connection is with the pressure transducer of the multiparameter monitor.
When all the connections have been made, all the lines must be primed and then open the connection to the central line.
Once this is done, the pressure transducer will pass the information to the multi-parameter monitor, which will show the pressure value on the screen either in millimeters of mercury or centimeters of water (it all depends on the configuration of the equipment).
When the automated technique is used, it is not necessary to close the connections once the PVC has begun to be monitored, since with this methodology it can be measured continuously and in real time..
Also, if the connections are attached to the patient's arm so that they are at the level of the right atrium, it is not necessary to elevate the patient's head..
Central venous pressure is very useful to evaluate two very relevant parameters in the management of critically ill patients:
- Volume level.
- Right ventricular function.
The PVC value directly correlates with the circulating blood volume. Thus, the lower the PVC, the less fluid is available in the intravascular space..
On the other hand, when the right ventricle does not function properly, the central venous pressure tends to rise well above normal, since the right heart is not able to adequately evacuate the final diastolic volume, causing blood to accumulate in the large venous vessels.
To differentiate between volume overload and right ventricular systolic dysfunction, the CVP value must be correlated with diuresis.
Thus, if the diuresis is preserved (1 cc / Kg / hour on average), the increased PVC values indicate right ventricular dysfunction, while if the diuresis is increased, a high PVC indicates fluid overload..
Normal PVC values should be between 5 and 12 cm of water.
When using automated equipment that reports PVC in millimeters of mercury, the normal value should be between 4 and 9 mmHg.
In the event that measurements of the same patient must be compared in cm of H20 and mmHg, it should be considered that 1 mmHg = 1.36 cm of H20.
Thus, to go from cm H20 to mmHg, the value of centimeters of water must be divided by 1.36. On the other hand, to go from mmHg to cm of H20, the value to be transformed is multiplied by 1.36.
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