This is the point where the superior vena cava meets the inferior vena cava prior to entry into the right atrium of the heart. It is usually measured by placing a catheter in one of the veins and then threading it to this point. This procedure is usually done under aseptic techniques in a ward, operating theatre, or the intensive care unit of a well equipped and staffed hospital. A qualified medical practitioner is authorised to perform this procedure.
In normal health the amount of blood coming to the heart is balanced precisely by the amount of blood leaving the heart. The cardiovascular system will be viewed as a closed system for the purposed of this discussion. When there is blood loss or other alteration in the cardiovascular system's homeostatic mechanism it will first be reflected in changes of the venous pressure at this point. As a rule of thumb the central venous pressure is a good indicator of the amount of blood returning to the heart from the systemic circulation. Further, it (CVP) is a good indicator of the pumping ability of the right atrium and the right ventricle. When the right atrium or ventricle is failing (e.g. following right atrial or ventricular MI) the CVP will be one of the first indicators to rise. The rising CVP indicates that the atria and/or ventricle are failing.
Diagram 1. Major blood vessels of the Heart showing blood flow.
Fluid Challenge:
Diagnoses of right ventricular failure and/or hypo volaemia are difficult without the use of invasive procedures. Sometimes, indirect means are used to evaluate hypo volaemia. The patient may be given a fluid challenge of 250 to 500 ml. This will cause the CVP to rise. CVP rise which is not sustained for more than 10 minutes suggests hypo volaemia. Serial readings are of greater use clinically because they more accurately predict the trend than single readings.
Normal CVP values.
In homeostasis CVP is from 0 to 8 cm of water. The Hospital where I work normally accepts 5 to 10 cm of water as normal. CVP is usually measured in cm of water but in some institutions it is measured in mm of mercury; when using the mercury scale 2 to 6 mm of mercury is considered normal.
Conditions which cause the CVP to rise.
Several conditions can cause the CVP to rise. Some situations which cause the CVP to rise are as follows:
- Increased intra thoracic pressure will cause the CVP to be higher than normal. This will happen when a patient has been intubated and is being ventilated artificially.
- Whenever there is impaired cardiac function (right sided heart failure, tamponade) the CVP will rise
- Hypervolemia happens when a patient has been given an excess amount of IV fluids. This will cause the CVP to rise. It is for this reason that physicians often prescribe fruosemide to patients when giving packed red blood cells. The RBCs increase circulating volume and the fruosemide decreases it. Homeostasis is maintained.
- Obstruction of the superior vena cava will cause the CVP to rise
- Pulmonary artery stenosis which limit venous outflow and lead to venous congestion cause the CVP to rise
- Straining, forced exhalation, tension pneumothorax and pleural effusion will cause CVP to rise
Conditions which cause the CVP to decrease.
Some conditions will cause the CVP to fall. Stated below are some conditions which will cause the CVP to fall.
- Hypovolaemia is a decrease in circulating volume. These conditions include blood loss and excessive diuresis.
- Reduced intra thoracic pressure as seen during inspiration
Measuring CVP.
After washing one's hands the procedure is explained to the patient. A verbal consent is obtained from the patient. Then the head of the bed is lowered to a horizontal position. The manometer is zeroed in line with the patient's mid-axillary line. After the manometer has been zeroed the CVP line is connected to the manometer. The point to which the saline level rises is your CVP reading. Always read at the lower end of the meniscus. The level should swing with inspiration and expiration. Once you have read the CVP connect the saline line to the CVP so that the CVP line remains patent.
Zeroing the Electronic Transducer.
New methods of CVP measurement do not use a manual manometer. The CVP catheter is inserted by a Doctor. It is hooked to a pressure bag which is pressurized to 200 cm water. This pressure allows the infusion of about 3ml of fluid each hour but keeps the line patent. With this method the transducer is generally taped to the arm at a level which is in line with the heart. Transducer zeroing is done at the start of each shift and subsequently if the need arises. This method allows continuous monitoring of the CVP but since the patient is in the recumbent position it is less accurate.
Right Sided Pump (heart) Failure:
The CVP rises because the amount of blood returning from the systemic circulation remains unchanged. Following myocardial infarction (the damaged right atria, right ventricle, or valve failure) interfere with the ability of this pump to pump away the same amount of blood that arrives at this point. This leads to increased back pressure and a rise in the CVP readings.
The old method of measuring CVP consisted of passing a central venous catheter through one of the peripheral veins in the arm or the leg and threading it to the “trigone” (meeting point of the superior vena cava, opening of the right atrium and the inferior vena cava). After zeroing the manometer the central pressure can be read off using a manual manometer. The level of the central part of the heart is considered the zero point. This is often done using a plumber's spirit level.
The new method consists of the “plumbing described” above. However, the CVP catheter is hooked up to an electronic transducer, a bag of saline under pressure and a sterile tubing system. In this system the CVP reading is displayed on a monitor. Also, in this system an alarm system can be activated to sound when CVP values become outside of acceptable parameters. Some systems allow a continuous tracing of the ECG waveform and recording of all alarms.
