Central Venous Pressure for Nurses

Central Venous Pressure (CVP for short) is defined as the pressure of blood in the thoracic vena cava just before it (the blood) enters the right atrium of the heart. Normal CVP is 5 to 10 cm H2O.

CVP measurements are important in clinical cardiology because the CVP is a major determinant of the filling pressure of the right ventricle of the heart. The filling pressure of the right ventricle determines the stroke volume i.e. the amount of blood pumped with each contraction of the heart.

Background: Central Venous Pressure is an accurate indicator of the amount of blood returning to the heart from the head, body and limbs via the superior and inferior vena cava. If and when there is blood loss then the CVP reading will be altered (will fall) almost immediately as the amount of blood returning to the heart will have decreased. Central Venous Pressure is also an accurate indicator of the ability of the heart (myocardial pump strength) to pump out blood to maintain normal blood pressure and tissue perfusion. Last but not least, the CVP is an accurate indicator of right ventricular end diastolic volume. In most institutions CVP is measured in cm of water (H2O). On this scale the normal value of CVP is 5 to 10 cm H2O. Some, (very few) institutions measure CVP in mm. Hg (millimetres of mercury). On this scale the normal value is approximately 4 to 8 mm Hg.

Central Venous Pressure in measured using a sterile indwelling central venous catheter (CVC). One end of the CVC is attached to a manometer or an electronic transducer, computer and monitor. Ultrasound may be used to guide CVC insertion. In the facility where I used to work experienced practitioners went in blind. Usually they were successful in locating the desired blood vessels without difficulty.

Central Venous Pressure monitoring is more accurate then blood pressure monitoring because changes in circulating volume will be reflected in changes in CVP values as soon as there is blood loss. Nurses and Nursing students will already know that in the first stage of shock, following blood loss, the compensatory mechanisms “adjust the blood pressure to normal levels”. Consequently, blood pressure readings will remain within the normal range after blood loss but not the CVP.

When there is overloading of the circulatory system or there is heart failure the CVP rises. However, when there is dehydration (e.g. diabetes insipidus), fluid loss due to bleeding or shifting of fluids within the body compartments (e.g. shock) then the CVP will fall. We have studied that “fluid challenges” in the early stages of shock can prevent shock and subsequent death of a patient.

Generally, when the CVP is rising to unsafe (unhealthy) levels the patient may display difficulties with breathing. Conversely, when the CVP is falling there may be a decrease in urinary output and the patient may complain of feeling excessively thirsty. To correct over hydration, as illustrated by a rising CVP the physician may choose to restrict fluids or to administer a diuretic. To deal with a falling CVP the physician might choose to give the patient more fluids or blood as the case may be.

Most institutions have Policy and Procedure manuals which state the indications for the insertion of a CVP monitoring line.

Indications for Central Venous Pressure Lines

A central venous catheter is inserted by a qualified person into one of the central or peripheral veins and then threaded to the exterior of the right atrium where the superior vena cava meets the inferior vena cava. This is only done where medically indicated and qualified nursing staff is available to care, manage, and support the patient. Reading the CVP correctly is essential for the results to be useful therapeutically. The following are some of the general indications for inserting a CVP line.

1. Monitoring of Central Venous Pressure in the acutely ill patient. This allows the care giver to have an insight into the fluid balance status of the patient. High CVP would indicate fluid overload or a failing heart. Low CVP would indicate a degree of dehydration or blood loss. Exact fluid status can only be evaluated by correlating Hb, Cardiac Functioning and all other lab results and clinical history of the patient.
2. Total Parenteral Nutrition (TPN) Administration. When an acutely ill patient’s GI tract is not able to absorb nutrients then the treatment team may decide to give the patient nutrition. This is called TPN and TPN can be given safely only via a CVP line or a peripherally inserted central line (PICC). Generally TPN is administered via a central intravenous catheter which is inserted in the subclavian or jugular vein. In infants the Umbilical vein is used most frequently. The rationale for using big deep veins for the administration is the fact that TPN causes phlebitis in peripheral veins because it is contains many caustic components. Examples include Calcium Chloride and Potassium chloride.
3. Medication Administration. Certain medications can be given safely only via a central line. Hence a CVP may be inserted for this purpose. Drugs that are likely to cause phlebitis include Chemotherapeutic Agents used in the treatment and management of malignant conditions. Amiodarone is used extensively in the management and treatment of acute life-threatening arrhythmias as well as for the suppression of chronic arrhythmias. It is useful both for supraventricular and ventricular arrhythmias because it has a low incidence of pro-arrhythmic effects. In arrest trials amiodarone has shown to improve survival rates (when compared to placebo) in individuals who suffer a cardiac arrest.
4. Lack of peripheral access. In some acutely ill patients, when there is no peripheral venous access, then a CVP line may be inserted. This is usually done for the purposes of re-hydration, medication administration, administration of blood and blood products.

Standard Equipment

Prior to insertion of a CVC catheter it is good practice to clean and set up a trolley with the following equipment.

1. CVP catheter
2. Sterile gown for physician or practitioner
3. Sterile gloves and masks.
4. Local anaesthetic
5. Hypodermic needles
6. Silk sutures with needle.
7. CVP insertion kit (with drape, chlorohexidine).
8. Manometer or transducer with monitor and pressure bag
9. Normal saline or heparinised saline according to Hospital policy.
10. Sterile Scissors
11. Op site or other transparent dressing(s).
12. Writing (non-lead) pen
13. Patient’s chart

After the procedure has been explained to the patient and the patient has agreed to it draw the curtains to insure privacy. Push the equipment trolley to the patient’s bedside. When the physician has put on his mask and goggles he will need to wash his hands. When he is doing that the nurse can open the contents of the different packs and set up the trolley. There should be a sterile gown on the trolley or in the pack along with sterile towels for the physician to dry his hands. It is the Nurses’ responsibility to tie the back of the physician’s gown.

When the physician is inserting the CVC catheter the nurse may need to elevate the foot of the bed to increase venous distension so that the physician is able to visualise the blood vessel better. Further, the draped patient may need emotional support too because he may be anxious and it is not comfortable lying there with the face covered and the bright lights shining on the face.

The CVC insertion procedure is completed when the CVC is inserted, has been verified with a chest x-ray, adequately secured (stitched in placed) and covered with a sterile dressing. It is the physician’s responsibility to place all sharps in the sharps container.

Commonly Used Sites for CVP Insertion

As stated previously the CVC catheter is inserted peripherally into one of the big veins. The vein used is chosen according to the patient’s needs and suitability. Unit and Physician preference do play a minor role. The following vessels are used most frequently.

1. Internal jugular vein: This site is one of the most frequently chosen because it is easily located and has low probability of complications like pneumothorax. The internal jugular veins (left and right) are short, straight and large. Therefore they facilitate easy insertion of the CVC catheter. Catheter occlusion is the most complication and this is due to head movement. This may cause irritation in the conscious patient.
2. Subclavian veins, left and right. This is the site which is most often used. It is chosen because there are easily recognisable anatomical landmarks which make insertion of the CVC easier. Since the subclavian arteries are located beneath the clavicle the risk of pneumothorax is always present. Subclavian CVCs are most frequently recommended and inserted because they are considered to be more comfortable for the patients (Woodrow 2002).
3. Left and right Femoral veins. These sites are used because they provide rapid central
   access in an emergency. A cardiac arrest is one such example. However, as most nursing students know, the femoral veins are located in the groin. This site is associated with high bacterial counts and high infection rates. Additionally, the femoral veins are also considered to be uncomfortable for mobility purposes.

Measuring and Recording the Central Venous Pressure

Measuring and recording the CVP is a highly skilled procedure and must be done correctly. Incorrect readings and recordings can result in wrong medical diagnosis and wrong treatments and interventions with disastrous consequences for all care givers, the patient and family. In most institutions Registered Nurses who have had special training perform this procedure. In the acute setting CVP is measured and recorded each hour. Measuring the CVP is done by two methods depending upon availability of equipment. Both methods are accurate if done correctly and are outlined below.

Manometer Method

This is a low tech. method and is more frequently done in remote facilities and in poorer countries. All that is needed is a CVC line, fluid, manometer and a skilled team of doctors and nurses. The big disadvantage of this method is that continuous monitoring is impossible and therefore it is of limited value.

1. The procedure is explained to the conscious patient in simple language which he can understand. In instances where the patient is unable to understand the Institutional language an interpreter must be used. This constitutes informed consent and is an important nursing process.
2. Place the patient in a flat horizontal (supine) position. Use the spirit level indicator on the bed to insure that the patient is horizontal. Older beds may not be equipped with this facility. You may then use a manual spirit level or use your personal (visual) judgment to insure that the bed is truly horizontal. The rational for this is that fluids find their own level and spread out evenly when the patient is horizontal. Please see picture 1 which shows a patient in a horizontal position. Picture 2 shows the spirit level “bubble” in the middle to signify that the bed is perfectly horizontal.
   

Picture 1. Bed must be horizontal. Photograph by Ms. Kara Burns.

Picture 2. Modern hospital beds are equipped with spirit levels to indicate when they are horizontal. Photograph by Ms. Kara Burns.


Picture 3. Establish “Zero”. Use the fourth intercostal space and the mid axillary line as shown in this picture. Please note that this simulated patient is not lying horizontally.

CVC measurements can be taken with the patient in an upright or semi-upright position. The important point to remember is that all CVC measurements must be done with the patient in the same position for the readings to be comparable. It is for this reason that the supine position is the gold standard for recording CVP readings.

3. Flush the CVC to insure that the device is patent. Turn off or pause all fluids which are running through the CVC except medications like noradrenaline which may continue to run because the effective volumes are extremely small.

   Do not flush any medication lines. This is particularly important because the amount of medication in the line may be sufficient to raise the blood pressure if there is Noradrenaline in the line. The patient may become hypoglycaemic if there is insulin in the line.

4. The zero mark on the manometer is aligned with the phlebostatic axis. This is called “zeroing the CVP”. It is done with the aid of a spirit level or a tube containing a fluid with air and water. Zeroing the CVP must be done before every reading otherwise the readings will not be accurate.
5. Clamp off the fluid bag. Rotate the three way tap to connect the fluid and the manometer. Fill the manometer with fluid (Normal Saline should be use unless otherwise indicated) up to the 20 cm mark. Now rotate the three way clamp to connect the manometer to the patient.
6. The fluid level in the manometer falls to the same level as the patients CVP. If the CVP line is not dampened then the level of the CVP should rise and fall with the patient’s respirations. Please see CVP waveform for more details.
7. Read the CVP when the patient is taking a breath in (at the end of inspiration). This will be the lowest point during a swing cycle. If you ever end up working in the intensive care of a hospital where the patient is on a ventilator you may be able to note that patients who are on ventilators have a lower CVP because the intrathoracic pressure is higher – the CVP is elevated artificially.
8. Once a reading is obtained turn the three way tap to allow fluid to connect to patient. This will insure that the CVP line (CVC) remains patent for future use.
9. Document the CVP on the patient’s chart. Observe it with the previous reading and report changes and abnormal readings to the physician or treatment team.

Picture 4. Three way tap of manometer. Manometer is connected to CVP for recording the CVP. Fluids should not be administered when reading the CVP.

Picture 5. Triple Lumen CVP. Note that the CVP is connected to the proximal port.
The distal and medial ports are clamped off. Swabable IV connectors can be used to prevent occlusion of these ports. Photography by Ms. Kara Burns.

Picture 6. Manometer set to administer fluids to patient. The manometer limb is turned off. Photography by Ms. Kara Burns.

Picture 7. A Swabable IV connector. These devices are used to prevent CVP lines from becoming occluded by blood clots. Photography by Sarjeet S. Gill.

Electronic Transducer Method

This method is ‘high tech’ but is not necessarily more accurate than the manual manometer method described above. The equipment that is required include a CVC, pressured bag of normal saline, electronic transducer, monitor, connecting cables and a skilled team of nurses and doctors. The big advantage of this method is that continuous monitoring is possible and is often done in intensive care units.

1. The CVC is inserted by a duly qualified practitioner. The insertion procedure is the same as that for the manometer method. Informed consent must be obtained from the patient or legal custodian of the patient.
2. The procedure is explained to the conscious patient in simple language which he can understand. In instances where the patient is unable to understand the Institutional language an interpreter must be used. This constitutes informed consent and is an important nursing process.
3. A pressured bag of saline is hung up and connected to the CVC line, the transducer and the CVP lumen of the CVC line. This is generally the proximal lumen – most suitable for measuring the CVP.
4. The patient is placed in supine position (flat, horizontal). Use the spirit level indicator on the bed to insure that the patient is horizontal. Older beds may not be equipped with this facility. You may then use a manual spirit level or use your personal (visual) judgment to insure that the bed is truly horizontal. The rationale for this is that fluids find their own level and spread out evenly when the patient is horizontal.
5. The three-way tap is located and the CVP line is connected to the transducer. Most modern transducers give a continuous reading and are routine left in the “on” position for continuous readings.
6. CVC Catheters differ between manufacturers, however, the white or proximal lumen is the port which is used for measuring the CVP.
7. Turn the tap “off” to the patient. Open it to air by removing the cap from the three-way
   port by opening the system to the atmosphere. This will allow the “zeroing” process.
8. Press the zero button of the monitor and wait until calibration occurs. The time and date of calibration will be displayed. When ‘zeroed’ time is displayed on the monitor replace the cap of the three-way tap. Maintain sterility at all times. Finally turn the tap on to the patient.
9. Observe the CVP trace on the monitor. The waveform undulates as the right atrium contracts and relaxes, emptying and filling with blood.
10. Document the measurement and report any significant changes to the physician.
    Reading the CVP.

There are two methods for reading the CVP. The first is to read the high point of the “a” wave and then the low point of the “a”. Finally add the two numbers and obtain the average. The result is the mean CVP. Nowadays computers do this for the nurse. All the nurse has to do is to look at the computer and read the CVP off the screen.
When using a manual manometer reads the high point and the low point of a respiratory cycle. Add these two and obtain the arithmetic mean. This is the patient’s CVP. If there is no swinging of the meniscus with the inspiratory and expiratory cycle then the CVP is dampened and the reading is probably less accurate.

The second method consists of finding the Z-point which occurs mid to end QRS. The Z-point is read. Since the Z-point occurs just before the closure of the tricuspid valve it is a good indicator of CVP (right ventricular end diastolic pressure). The Z-point is useful when “A” waves are not visible. This often occurs when the patient is in atrial fibrillation. Again, if a transducer and monitor is being used the computer will do the calculations. The CVP will be displayed as a digital number and the waveform will be visible below the ECG tracing.

Common Causes of Altered Central Venous Pressure

The Central Venous Pressure of an individual patient can vary from normal the normal range of between 5 and 10 cm H2O due to a variety of reasons. Some of the common causes of raised (elevated) and lowered CVP are given below.

Causes of Elevated CVP Readings

1. Occlusion or kink in central venous catheter. These are the most common causes of wrong readings and all nurses should make sure that the meniscus rises and falls when a reading is being taken.
2. Heart failure. When a patient’s heat is failing it is unable to pump the blood which is being brought to it by the SVC and the IVC. The backlog of blood causes the CVP to rise. The back pressure is due to the failing heart which results in decreased cardiac output.
3. Therapeutic of accidental fluid overload. The pathophysiology is exactly the same as explained above in number 2. Essentially, the rise in CVP is due to the increased blood volume.
4. Pulmonary embolism. During this disease process the embolus restricts movement of blood through the lungs. The back pressure causes the CVP to rise.
5. Fluid overload. This may be the result of vasoconstriction due to medications or arteriosclerosis. These two factors cause the blood vessels to narrow and a backlog of fluid is produced. This fluid backlog causes CVP to rise.
6. Increased intra-thoracic pressure. This does not occur naturally but occurs when a patient is placed on a ventilator following respiratory failure. The causes of respiratory failure are not discussed here because they are beyond the scope of this article. IPPV and CPAP cause raised intrathoracic pressures which in turn cause the CVP to rise.
7. Dampening of the air filter (getting the air filter wet) can cause the CVP reading to rise because the wet filter does not allow the escape of air from the manometer. This is often referred to as erroneous reading and does not have anything to with the patient’s physiological processes.
8. Changing a patient’s position from standing to supine will cause the CVP to rise.
9. Valsalva manoeuvre (forced expiration) will cause the CVP to rise because there is a change in compliance.
10. Contractions of the abdominal and limb muscles alter compliance and circulating fluid returning to the heart. These contractions will generally cause the CVP to rise.

Causes of Lowered CVP Readings

1. Blood loss (haemorrhage) and fluid losses caus the CVP to fall. Burns and vomiting are common causes of fluid loss.
2. Excessive use of diuretics causes fluid loss too. This will cause the CVP reading to be low.
3. Vasodilation due to excessive heat, medications, sepsis and neurogenic shock will cause the CVP to fall.

CVP Waveforms

Diagram 1. CVP Waveform – Typical.

Typical Waveforms: The central venous waveforms seen on the cardiac monitor show the pressure changes in the SVC and IVC (at the location of the tip of the CVC). These are pressure variations which occur during the cardiac cycle. These pressure changes are transmitted in the form of characteristic waves. Typically, there are three positive waves. These are the a, c, and v waves. There are two negative waves. These are the x and y waves. These waves correlate with different phases of the cardiac cycle and EKG.

1. + a wave : This positive wave is due to the increased atrial pressure. During normal right atrial contraction it correlates with the P wave on an EKG.
2. + c wave : This is another positive wave. It is caused by a slight elevation of the tricuspid valve into the right atrium during normal early ventricular contraction. It correlates with the end of the QRS segment on an EKG. The QRS complex is due to ventricular contraction.
3. - x descent : This negative wave is less well understood. It is probably produced by the downward movement of the ventricles during systolic contractions. On the ECG, it occurs before the t-wave.
4. + v wave : This positive wave occurs from the pressure produced during the filling of the right atrium rises becomes it has to deal with the closed tricuspid valve. On the 12 lead ECG this wave correlates with the ending of the T wave. The T wave on the ECG is produced by the repolarisation of the myocardium.
5. - y descent : This is another negative wave. It is the result of the tricuspid valve opening in diastole and with blood flowing into the right ventricle. This wave occurs prior to the P wave on an ECG.

Cautionary Advice: Since the CVC line is always located in the central part of the thoracic cavity, central venous pressures and waveforms are influenced by changes in intrathoracic pressure during both spontaneous and assisted (ventilated) respirations. All of these changes are small however they can be seen clearly in well functioning CVC lines. As discussed elsewhere the CVP decreases slightly with spontaneous inspiration. It increases slightly with forced exhalation as well as positive pressure mechanical inspiration. A patient on PEEP (positive end expiratory pressure) will have an elevated CVP. These changes are most noticeable when the PEEP is greater than 7.5cm H2O.

Pathologic CVP Waveforms

Variations of the normal central venous waveform can provide information about cardiac pathology physiology e.g. in atrial fibrillation.

1. During AF: a waves will be absent, and in atrioventricular disassociation, a waves will be dramatically increased. When this happens these waves are called “cannon waves”. The cannon waves are produced because the atrium contracts when the tricuspid valve is closed.
2. During tricuspid regurgitation the c wave and the x wave will be negative. They will be replaced by large positive waves of regurgitation as the blood flows back into the right atrium during ventricular contraction. This generally elevates the average central venous pressure, but it is not an accurate measurement. A better way of obtaining an accurate CVP in similar cases would be by looking at the pressure between the regurgitation waves.
3. In cardiac tamponade, all pressure will be elevated, and the y descent will be nearly absent.

Correctly Interpreting the Patient’s CVP

The CVP does not measure blood volume or heart functioning directly. However, CVP readings are altered when blood volume alters (e.g. Blood loss or circulatory overload) or when there is heart muscle failure. CVP readings should be interpreted with other clinical data like heart rate, blood pressure, urine output, level of consciousness, and respiratory rate. The absolute value is less significant than serial measurements which reflect the changes in response to therapy. As stated before the normal value during spontaneous breathing is 5-10cm water cmH2O. During mechanical ventilation it rises to by 3-5cmH2O depending upon the PEEP (Positive end of expiration pressure) and ventilator pressure. In some situations the CVP measurement may be in the normal range even when there is Hypovolemia due to vaso/veno constriction. A summary guide to CVP interpretation is shown in below.

Table 1. Guide to interpreting CVP readings.

Prior to the insertion of a CVP line an accurate assessment of the patient must me made. A history of the patient may be obtained from the patient, family, notes of previous admissions and the authority who brought the patient to the Hospital. This may be the Police, ambulance crew, or a good citizen.

It is important to explain the procedure to the patient to obtain his or her history. Local anaesthetic should be used where the patient does not have an allergy to it. Placing the patient in a supine position is usually sufficient. In some cases where the patient is hypovolemic the trendelenberg position will help dilate the veins in the upper parts of the body and make it easier for the physician to insert the cannula.

Appropriate draping is important to maintain sterility and patient cooperation. In some situations it may be advisable to use a mild sedative to limit pain, awareness of the procedure and keep the patient calm.

After insertion of a CVP line it is usual to have a chest x-ray. This will confirm correct placement of the CVP line. It may also help in early diagnosis of pneumothorax if it happened during the procedure.

Questionable CVP Reading Reliability

CVP readings are used frequently to evaluate cardiac function and alterations in blood volume. CVP readings are accurate only in the absence of right ventricular disease and the presence of normal vascular resistance. Listed below are some situations when CVP readings should be considered as unreliable.

1. During Pulmonary Embolus and when the intrathoracic pressure is high (e.g. IPPV) there is high pulmonary vascular resistance even though left sided pressures and functions may be normal. A higher than normal CVP is needed to push the blood through the higher resistance in the lungs to the left side of the heart.
2. During left heart failure there is a rise in pulmonary venous pressure due to “back pressure”. This brings about right sided heart strain. In the initial stages the CVP may be normal but it will increase as the heart failure becomes significant.
3. During constrictive pericardial disease there is a paradoxical rise in CVP on inspiration and a fall on expiration. This is called paradoxical because it is the opposite of what happens in the normal healthy individual. The absolute level (CVP reading) will always be higher because the filling of the heart is impeded.
4. Occlusion of cotton wool in the manometer. This has been discussed above. This occurs because the moist cotton wool impedes the movement of air which interferes with the normal rise and fall of the CVP with inspiration and expiration.
5. During Complete heart block ‘Cannon waves’ may be seen during CVP readings. This reading will have a strong pulsatile element which will occur when the atrium contracts against a closed tricuspid valve. This sends the pressure wave back into the SVC which is picked up by sensitive transducers.
6. When the patient has tricuspid stenosis or regurgitation the CVP will not be reliable. This is because the mean CVP will be elevated.

Removal of the CVP Cannula

This is only done on the express directions of the treatment team. This is an aseptic technique and must be done after fully explaining the procedure to the patient. First remove all dressings and sutures. Then request the patient to take a deep breath and exhale completely. At this point remove the catheter by steadily pulling it out while the patient is holding his breath. Finally apply firm pressure to the puncture site and ask the patient to resume normal breathing. Keep the firm pressure for at least 5 minutes. This will stop the bleeding. Only a moderate amount of force will be needed to remove the catheter. If it does not come out request a physician to do it. An experienced physician will (most likely) try rotating the CVP catheter while pulling it out gently. If this intervention fails then cover the CVP insertion site with a sterile dressing and arrange for the surgical registrar to evaluate the removal.

Complications of Central Venous Pressure Monitoring.
CVP monitoring is not a hundred percent safe procedure. It has many complications. As nurses we must exercise more than due diligence when looking after patients who are having their CVPs monitored. Complications of CVP monitoring can be read at: http://healthmad.com/conditions-and-diseases/complications-of-central-venous-pressure-monitoring/ 

Conclusion

Central venous pressure measurements are often associated with Intensive and critical care settings of major treatment centres. However an ever increasing number of critically ill patients are being cared for on medical and surgical wards and in remote communities with limited resources. It is therefore essential that all nursing staff are able to help with the procedure and record central venous pressure measurement accurately and safely. They also need to be able to recognise normal and abnormal parameters. The normal parameters have been given in this article.