What is Central Venous Pressure?
The central venous pressure (CVP) is the pressure measured in the central veins close to the heart. It indicates mean right atrial pressure and is frequently used as an estimate of right ventricular preload. The CVP does not measure blood volume directly, although it is often used to estimate this. In reality the CVP value is determined by the pressure of venous blood in the vena cava and by the function of the right heart, and it is therefore influenced not only by intravascular volume and venous return, but also by venous tone and intrathoracic pressure, along with right heart function and myocardial compliance.
Underfilling or overdistention of the venous collecting system can be recognised by CVP measurements before clinical signs have become apparent. Under normal circumstances an increased venous return results in an augmented cardiac output, without significant changes in CVP. However with poor right ventricular function, or an obstructed pulmonary circulation, the right atrial pressure rises, therefore causing a resultant rise in measured CVP. Similarly, although it is possible for a patient with hypovolaemia to exhibit a CVP reading in the normal range due to venoconstriction, loss of blood volume or widespread vasodilation will result in reduced venous return and a fall in right atrial pressure and CVP.
In a normal patient the mean right atrial pressure measured by the CVP closely resembles the mean left atrial pressure (LAP). At end diastole left atrial pressure is assumed to equal left ventricular end diastolic pressure (LVEDP), which in turn is assumed to reflect left ventricular end diastolic volume (LVEDV). Thus, in normal patients, CVP is assumed to be a reflection of left ventricular preload. However, in patients with cardiac or pulmonary disease the right and left ventricles may function independently. In these cases left ventricular preload should be estimated by measuring the pulmonary capillary 'wedge' pressure, using a pulmonary artery catheter (PAC), as this is a better guide to the venous return to the left side of the heart than CVP. The PAC may also be connected to a computer to calculate the cardiac output using a thermodilution technique and further guide patient management.
PAC are therefore sometimes used to measure left atrial pressure in patients with significant right sided valve disease, right heart failure or lung disease as the CVP may be unreliable in predicting the left atrial pressure in these cases.
When should CVP be measured?
The CVP can be measured either manually using a manometer (Diagram 1) or electronically using a transducer (Diagram 4). In either case the CVP must be ‘zeroed’ at the level of the right atrium. This is usually taken to be the level of the 4th intercostal space in the mid-axillary line while the patient is lying supine. Each measurement of CVP should be taken at this same zero position. Trends in the serial measurement of CVP are much more informative than single readings. However if the CVP is measured at a different level each time then this renders the trend in measurement inaccurate.
1. Using the manometer
A 3-way tap is used to connect the manometer to an intravenous drip set on one side, and, via extension tubing filled with intravenous fluid, to the patient on the other (Diagram 1). It is important to ensure that there are no air bubbles in the tubing, to avoid administering an air embolus to the patient. You should also check that the CVP catheter tubing is not kinked or blocked, that intravenous fluid can easily be flushed in and that blood can easily be aspirated from the line. The 3-way tap is then turned so that it is open to the fluid bag and the manometer but closed to the patient, allowing the manometer column to fill with fluid (Diagram 2). It is important not to overfill the manometer, so preventing the cotton wool bung at the manometer tip from getting wet. Once the manometer has filled adequately the 3-way tap is turned again – this time so it is open to the patient and the manometer, but closed to the fluid bag (Diagram 3). The fluid level within the manometer column will fall to the level of the CVP, the value of which can be read on the manometer scale which is marked in centimetres, therefore giving a value for the CVP in centimetres of water (cmH2O). The fluid level will continue to rise and fall slightly with respiration and the average reading should be recorded.
2. Using the transducer
The transducer is fixed at the level of the right atrium and connected to the patient's CVP catheter via fluid filled extension tubing. Similar care should be taken to avoid bubbles and kinks etc as mentioned above. The transducer is then 'zeroed' to atmospheric pressure by turning its 3-way tap so that it is open to the transducer and to room air, but closed to the patient. The 3-way tap is then turned so that it is now closed to room air and open between the patient and the transducer. A continuous CVP reading, measured in mmHg rather than cmH2O, can be obtained. (Diagram 4)
(Back to text)
(Back to text)