3. Central Venous Pressure and Cannulation

Question 1

Central Venous Pressure (CVP): The Waveform

Answer 1

Three upstrokes (the ‘a’, ‘c’ and ‘v’ waves) and
two descents (the ‘x’ and ‘y’) that relate to the cardiac cycle

Question 2

‘a’ wave

Answer 2

‘a’ wave: this occurs at the end of diastole and is caused by increased atrial pressure
as the atrium contracts (occurs at end-diastole)

Atria

Question 3

‘x’ (or ‘x”) descent:

Answer 3

‘x’ (or ‘x”) descent: this reflects the fall in atrial pressure as the atrium relaxes

relaX

Question 4

‘c’ wave:

Answer 4

‘c’ wave: this supervenes before full atrial relaxation, and is caused by the

bulging of the closed tricuspid valve into the atrium at the start of isovolumetric right ventricular contraction.

(closed contraction)

Question 5

‘x’ descent:

Answer 5

‘x’ descent:

this is a continuation of the ‘x” descent (interrupted by the ‘c’ wave) and
represents the pressure drop as the ventricle
and valve ‘screw’ downwards at the end of systole.

Question 6

‘v’ wave:

Answer 6

‘v’ wave: this is the increase in right atrial pressure as it is filled by the venous return
against a closed tricuspid valve.

Venous Valve

Question 7

‘y’ descent:

Answer 7

‘y’ descent: this reflects the drop in pressure as the right ventricle relaxes, the
tricuspid valve opens and the atrium empties into the ventricle.

Question 8

‘y’ descent:

Answer 8

‘y’ descent: this reflects the drop in pressure as the right ventricle relaxes, the
tricuspid valve opens and the atrium empties into the ventricle.

Question 9

Alterations

Answer 9

Any event that alters the normal relationship between these events will alter the
shape of the waveform.

For example, in atrial fibrillation the

‘a’ wave is lost;

in tricuspid incompetence,
a giant ‘v’ wave replaces the ‘c’ wave, the ‘x’ descent and the ‘v’ wave.

‘Cannon’ waves are seen when there is atrial contraction
against a closed tricuspid valve (as occurs at a regular interval if there is a
junctional rhythm, or at an irregular interval if there is complete atrioventricular
conduction block).

Question 10

Indications:

Answer 10

CVC catheters are used for the monitoring of central venous pressures,

for the insertion of pulmonary artery catheters

and to provide access for haemofiltration

transvenous cardiac pacing.

Central venous lines also allow the administration of
drugs that cannot be given peripherally,
such as inotropes and cytotoxic agents,
and the infusion of total parenteral nutrition.

It is suggested that they can be used to aspirate air from the right side of the heart after massive air embolism, although very few anaesthetists have ever used them for this
purpose

Question 11

Function of CVP monitoring – Volume

Answer 11

Function of CVP monitoring – intravascular volume:

the CVP is the hydrostatic pressure generated by the blood within the right atrium (RA) or the great veins of the thorax.

It provides an indication of volaemic status because the capacitance system,
including all the large veins of the thorax,
abdomen and proximal extremities,
forms a large compliant reservoir for around two-thirds of the total blood volume.

Hypovolaemia may be actual or effective, caused, for example,
by subarachnoid block or sepsis,

in which loss of venoconstrictor tone or venodilatation decreases venous return and reduces CVP.

A single reading
may be unhelpful, whereas trends are more useful, particularly when combined
with fluid challenges.

Question 12

Function of CVP monitoring – right ventricular function

Answer 12

Function of CVP monitoring – right ventricular function: CVP measurements
also provide an indication of right ventricular (RV) function.

Any impairment of RV function will be reflected by the higher filling pressures that are needed to maintain the same stroke volume (SV).

Question 13

Normal values

Answer 13

Normal values:

the normal range is 0–8 mmHg, measured at the level of the tricuspid valve.

The tip of the catheter should lie just above the RA in the superior vena cava.

Question 14

CVP decreases:

Answer 14

CVP decreases:

if the blood volume is unchanged,

then the CVP will alter with changes in cardiac output (CO).

It will fall as the CO rises because the rate at which
blood is removed from the venous reservoir also increases

Question 15

CVP increases:

Answer 15

CVP increases:

potential causes for an increase in CVP include a fall in CO
(the converse of the effect described previously).

Ventilatory modes may also cause the increase which is seen with IPPV, PEEP and CPAP.

The CVP also rises in response to volume overload,

if there is RV failure,
pulmonary embolus,
cardiac tamponade or
tension pneumothorax.

Rarer causes include obstruction of the superior vena cava
(assuming that the catheter tip lies proximally) and portal hypertension leading to
inferior vena caval backpressure.

Question 16

Complications of insertion

Answer 16

These are numerous and include arterial puncture
(carotid and subclavian), haemorrhage, air embolism, cardiac arrhythmias, pneumothorax, haemothorax, chylothorax, neurapraxia, cardiac tamponade and thoracic duct injury

Anatomically proximate structures such as the oesophagus and trachea can also
be damaged. Parts of catheters or entire guidewires can embolise into the circulation.
Ultrasound guidance can reduce complications associated with catheter insertion

Endocarditis and cardiac rupture have been reported. Venous thrombosis is common,
but the risk may be reduced by the use of heparin-bonded catheters. Infection is a
problem, and occurs in up to 12% of placements. Its risk is reduced by full aseptic
precautions, by the use of antiseptic- and antibiotic-coated catheters (in high-risk
patients) and by using the subclavian approach

approach. There is no definite evidence of benefit
for tunnelling, for prophylactic line changes or for the use of prophylactic antibiotics.
These complications can be fatal: the confidential maternal mortality reports alone
have documented three deaths caused by central catheterization

Question 17

CVP measurements are sometimes recorded as negative values.

Answer 17

If the CVP is measured from the accurate reference point of the tricuspid valve, then
a sustained negative intravascular pressure is impossible. Certainly, the negative
intrathoracic pressure during inspiration will be transmitted to the central veins,
and if there is respiratory obstruction this negative pressure will be high. It will,
however, be transient. If a mean CVP reading is consistently negative it can only be
because the transducer has been placed above the level of the right atrium

Question 18

CVP measurement and left ventricular function.

Answer 18

The right atrial pressure reflects the right ventricular end-diastolic pressure
(RVEDP), and it is frequently assumed that this also reflects the left ventricular
end-diastolic pressure (LVEDP).

This means that, for a given fluid load, the increase in SV of each
ventricle is identical, but the rise in filling pressure in the left ventricle exceeds that in
the right. This discrepancy is accentuated by LV dysfunction, and under these
circumstances accurate diagnostic information has to be obtained by other means.