Healthcare and Medicine Reference
In-Depth Information
Improving the CO of critically ill patients improves oxygen deliv-
ery to the tissues and organs. In some studies, particularly in septic
shock, this has been shown to decrease morbidity and mortality.
converts one form of energy into another. In this case, mechanical
energy in the form of the central venous pressure is converted to
electrical energy which is displayed as a waveform on the monitor.
The measuring system must be zeroed to atmospheric pressure
before use. The transducer is usually placed at the level of the right
atrium, as described above. If the patient's position is altered by
raising or lowering the bed or operating table, the transducer must
be moved with it to the new level of the mid-axillary line (repeating
the zero each time is unnecessary).
Measurement of central venous pressure
The CVP is the pressure within the superior vena cava (SVC)
just above the right atrium. It is impossible without imaging to
determine the precise position of the SVC-atrial boundary and
the position of the end of the central venous tip in each patient,
and then to relate this to the body surface. Therefore it is standard
practice to take all measurements at the same level in all patients.
In the supine patient the pressure is measured from the fourth
intercostal space in the mid-axillary line which is taken to be the
level of the right atrium.
The normal range of CVP is 3-10 cmH 2 O. Previously, this
pressure was measured by attaching the central line to a water-
fi lled manometer but this has been superseded by electronic pres-
sure transducers that are able to display the CVP waveform in
real-time.
CVP waveform
The central venous pulsation is a complex waveform which differs
in many respects from the arterial pulsation. It is described as hav-
ing three positive defl ections ('a', 'c' and 'v') and two negative defl ec-
tions ('x' and 'y'). Figure 19.3 explains what causes these defl ections
and how they are related to the ECG.
The CVP waveform displays abnormal morphology during vari-
ous pathological states (Box 19.1). The point on the CVP wave-
form which most accurately refl ects cardiac preload is just before
the 'c' wave. This is the point just before the tricuspid valve closes
and before ventricular systole begins. The pressure at this point
Pressure transducers
Pressure transducers consist of a length of tubing with a trans-
ducer situated at the midpoint between the patient and a bag of
fl uid under pressure (Figure 19.2). At the patient end, the giving
set is attached to the central venous catheter and the other end is
attached to a bag of pressurised saline. Saline fl ows down the tub-
ing, limited by a fl ow regulator within the transducer that allows a
fl ow of 3 mL per hour. This prevents the formation of blood clots
within the catheter.
In order to display the CVP waveform on a monitor, the pres-
sure wave has to be converted into an electrical signal. A transducer
R
T
P
ECG
tracing
Q
S
c
a
v
y
x
Jugular
tracing
Figure 19.3 +a wave : due to right atrial contraction and is not seen in
patients with atrial fi brillation. It correlates with the P wave on an ECG.
+c wave : a result of closure and bulging of the tricuspid valve during
isovolumetric contraction of the right ventricle. It correlates with the end of
the QRS segment on an ECG.
-x descent : due to atrial relaxation and descent of the tricuspid annulus
during ventricular contraction. It occurs before the T wave on an ECG.
+v wave : result of continuing fi lling of the right atrium against the closed
tricuspid valve. It occurs as the T wave is ending on an ECG.
-y descent : due to the tricuspid valve opening and rapid ventricular fi lling.
It occurs before the P wave on an ECG.
Box 19.1 Abnormal CVP waveforms
Cannon waves
AV dissociation/junctional rhythm/VT/pacing
Large 'a' waves
Tricuspid stenosis/pulmonary stenosis/
pulmonary hypertension/right ventricular
failure/right atrial myxoma
Large 'v' waves
Tricuspid incompetence
Rapid 'x' descent Cardiac tamponade/constrictive pericarditis
Rapid 'y' descent Constrictive pericarditis
Figure 19.2 A photograph of a pressure transducer set up.
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