Anaesthesiology

CVP Interpretation Quiz

A normal CVP reading in a spontaneously breathing patient is 5-10 cmH₂O. However, as stated earlier, its value is 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. Therefore an isolated CVP reading is of limited value. Trends in the CVP reading, and its response to therapy such as a fluid challenge, are much more important than a single reading.

Your patient is tachycardic, hypotensive and has a low urine output and poor capillary refill. You therefore suspect he is hypovolaemic and you insert a CVP line to aid your diagnosis and management. The initial CVP reading is 6 cmH₂O.

Which of the following statements is correct?

	A.	No fluid replacement is needed as the CVP reading is within the normal range.
	B.	Fluid replacement is justified but must be stopped as soon as the CVP reading starts to rise.
	C.	Fluid replacement should continue until there is a sustained rise in the CVP reading.

A. False.

Your patient has clear clinical evidence of hypovolaemia and this requires treatment with fluid replacement. A hypovolaemic patient may have a normal CVP reading due to venoconstriction (remember the CVP is affected by other factors not just intravascular volume and should therefore be interpreted in the context of the clinical signs). Treat your patient and not the absolute CVP value. The trend in the CVP readings following fluid challenges can guide your fluid replacement for this patient as described below (see 'How to give a fluid challenge')

B. False.

Fluid replacement is necessary but should not be stopped at the first sign of a rise in CVP value. A transient CVP rise in response to a fluid challenge indicates that more volume is required. Adequate fluid replacement should produce a sustained rise in CVP value together with an improvement in heart rate, blood pressure and urine output etc (remember - don't just look at the CVP in isolation; it should be interpreted in the context of the clinical signs). However, if the CVP rises and stays up but the blood pressure and urine output do not improve, then consider using inotropes to support the circulation. (See below 'How to give a fluid challenge')

C. Correct.

Fluid replacement should not be stopped at the first sign of a rise in CVP value. A transient CVP rise in response to a fluid challenge indicates that more volume is required. Adequate fluid replacement should produce a sustained rise in CVP value together with an improvement in heart rate, blood pressure and urine output etc (remember - don't just look at the CVP in isolation; it should be interpreted in the context of the clinical signs). However, if the CVP rises and stays up but the blood pressure and urine output do not improve, then consider using inotropes to support the circulation. (See below 'How to give a fluid challenge')

Your patient returns to the ward from the operating theatre following major abdominal surgery. He has a Right Internal Jugular CVP line in place. You notice that the reading is high and that the patient is hypotensive and very short of breath.

Which of the following statements are correct? (may be more than one)

	A.	The patient has congestive cardiac failure.
	B.	The patient received too much intravenous fluid during surgery.
	C.	The patient should be given oxygen supplementation.
	D.	The patient should be given a diuretic.

A, B, D. False.

This patient needs oxygen and a rapid assessment to determine the underlying problem. It is possible that this patient is suffering from congestive cardiac failure and fluid overload. However a number of other conditions can also cause the clinical scenario of a raised CVP reading, shortness of breath and hypotension – eg. tension pneumothorax (possibly caused by the CVP insertion itself!), pulmonary embolism and cardiac tamponade.

Before you can state the diagnosis with certainty you must take a quick history and examination of the patient looking for evidence to back up your diagnosis:

Cardiac Failure	pink frothy sputum, bi-basal crackles on chest auscultation, peripheral oedema, enlarged tender liver, suggestive history
Tension Pneumothorax	assymetrical chest movement, decreased air entry unilaterally, chest resonant to percussion unilaterally, tracheal deviation
Pulmonary Embolus	respiratory examination normal, haemodynamic instability, gallop rhythm, pleuritic chest pain, haemoptysis
Cardiac Tamponade	muffled heart sounds

C. Correct.

Which of the following conditions will cause the CVP reading to be inaccurate?

	A.	Blocked cotton wool at manometer top
	B.	Left ventricular failure
	C.	Tricusped regurgitation
	D.	CVP 'zeroed' at the level of the sternal notch
	E.	Constrictive pericardial disease
	F.	Respiration

A. True.

If the cotton wool is blocked then fluid will not move in the tube to give a correct reading .

B. False.

The CVP reading (i.e. the pressure measured in the right atrium) will be accurate but its interpretation may not be, since with pure left ventricular failure the CVP may be normal, leading the unwary to believe that the cardiac function is normal.

In normal patients the CVP is assumed to be a reflection of left ventricular preload (CVP (RAP) α PCWP α LAP α LVEDP α LVEDV). However in patients with cardiac and/or pulmonary disease the right and left ventricles may function independently, and in these situations the CVP cannot accurately indicate left ventricular filling pressures. 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.

C. False.

Again the CVP reading (i.e. the pressure measured in the right atrium) will be accurate but its interpretation may not be.

With TR, during ventricular systole blood flows back into the RA through the incompetent valve increasing RAP and therefore increasing the CVP reading. The end result is an increase in the size of the CVP v wave (see notes on CVP waveform) and an increased mean CVP value which may be misinterpreted as fluid overload.

D. True.

The CVP should be 'zeroed' at the level of the right atrium. When the patient is lying supine, this is approximately level with the point at the mid-axillary line in the 4th intercostal space. Measurements should be taken in the same position each time.

E. False.

Again the CVP reading (i.e. the pressure measured in the right atrium) will be accurate but its interpretation may not be.

With constrictive pericardial disease the pressures within the heart chambers will increase, resulting in an increased CVP reading which may be interpreted as fluid overload.

F. False.

The CVP will accurately measure the RAP. However ventilation, either spontaneous or controlled, does have a significant effect on the CVP. During spontaneous ventilation, the pleural and pericardial pressures will drop during inspiration resulting in a fall in CVP reading. Conversely the CVP reading will rise (usually be 3-5 cmH2O) during inspiration due to positive pressure ventilation. When taking the CVP reading we should account for this by always taking the mean value of the CVP wave at the end of expiration to be the CVP value.

Which of the following statements, regarding CVP use in septic patients, are correct? (may be more than one)

	A.	The CVP reading may be high
	B.	The CVP reading may be normal
	C.	The CVP reading may be low
	D.	If a patient develops Adult Respiratory Distress Syndrome (ARDS) their CVP reading is likely to be high
	E.	The CVP line should be removed when its insertion site becomes red and inflamed

A, B, C. True.

In septic patients the CVP reading may be low, normal or high depending on a number of factors such as, cardiac output, venous return, venous tone, the use of inotropes, right myocardial function, myocardial compliance. As before the trend in the CVP value in response to therapy (often fluid resuscitation and/or inotropic support) is a much more useful guide than a single CVP reading.

D. True.

ARDS causes, among other problems, non-cardiogenic pulmonary oedema with a subsequent rise in right atrial pressure as measured by the CVP. The unwary may therefore be wrongly led to believe that the patient is suffering from heart failure in this situation. However this is one example of a diseased lung resulting in dissociation of right and left heart function. Placement of a pulmonary artery catheter (PAC) and measurement of the pulmonary capillary wedge pressure (PCWP) in these patients will show a normal left atrial filling pressure and is a much better guide to cardiac function than the CVP in this situation.

E. True.

CVP lines can be a source of infection in all patients, not just septic ones. Different units have different policies regarding the timing of removal of lines, in an attempt to avoid line-related infection. Some units will change lines regularly every few days whether or not the lines appear inflamed; other units will change lines only when there is clinical evidence indicating line inflammation or infection. In either case good aseptic technique during insertion and subsequent care of the line, can help prevent line infection from developing.

Interpreting the CVP waveform

When the CVP is displayed via electronic transduction, the waveform showing how the pressure changes throughout the cardiac cycle can be clearly seen. This can give a lot of useful information, and the presence of several disease conditions may be confirmed by examination of the CVP waveform. The classical CVP waveform is shown below.

a wave = atrial contraction

c wave = bulging of tricuspid valve into atrium at start of ventricular contraction

x descent = atrium relaxes and tricuspid valve is pulled downwards

v wave = passive filling of right atrium and vena cavae when tricuspid valve closes

y descent = tricuspid valve opens and blood flows into right ventricle

Disease patterns apparent in CVP waveform

Atrial fibrillation - no a wave

Heart block - cannon waves
(large a waves due to atrium contracting against a closed tricuspid valve)

Nodal rhythm - cannon waves
(large a waves due to atrium contracting against a closed tricuspid valve)

Tricuspid regurgitation- large c and v waves, loss of the x descent

Tricuspid stenosis - prominent a waves, muted y descent

How to give a fluid challenge?

To treat hypovolaemia/hypotension give repeated boluses (250-500 mls) of intravenous fluid (eg. 0.9% saline / Ringer's lactate / gelofusine). Observe the effect on CVP, blood pressure, pulse, urine output, capillary refill etc. If the CVP rises following the fluid bolus wait 5 – 10 mins and then repeat the CVP reading to see if the rise is sustained. The fluid challenges should be repeated until the CVP shows a sustained rise and/or there is an improvement in the other cardiovascular parameters.

In patients who are known or suspected to be at high risk of developing congestive cardiac failure, the above technique for giving fluid challenges should still be followed. However it is sensible in these patients to use smaller fluid boluses (50-100 mls), and to titrate the total amount of fluid delivered to the patient's response as described.

In situations when the CVP rises and stays up but the blood pressure and urine output do not improve, then inotropes may be considered to support the circulation. Any patient who may require inotropic support or who has received two reasonable volumes of fluid challenge without improvement, should be reviewed by an experienced doctor.