Wedge PressureTracing
The wedge pressure represents the venous pressure on the left side of the
heart, and the magnified section of the wedge pressure in Figure 8.3
shows a typical venous contour that is similar to the venous pressure on
the right side of the heart. The
a
wave is produced by left atrial contrac-
tion, the
c
wave is produced by closure of the mitral valve (during iso-
metric contraction of the left ventricle), and the
v
wave is produced by
systolic contraction of the left ventricle against a closed mitral valve.
These components are often difficult to distinguish, but prominent
v
waves are readily apparent in patients with mitral regurgitation.
Principle of theWedge Pressure
The principle of the wedge pressure is illustrated in Figure 8.4. When the
balloon on the PA catheter is inflated to obstruct flow (Q
=
0), there is a
static column of blood between the tip of the catheter and the left atrium,
and the wedge pressure at the tip of the catheter (P
W
) is equivalent to the
pulmonary capillary pressure (P
c
) and the pressure in the left atrium
(P
LA
). To summarize: if Q
=
0, then P
W
=
P
c
=
P
LA
. If the mitral valve is
behaving normally, the left atrial pressure (wedge pressure) will be
equivalent to the end-diastolic pressure (the filling pressure) of the left
138
Hemodynamic Monitoring
30
20
10
0
30
20
10
0
30
20
10
0
30
20
10
0
1
2
3
4
Right Atrium
Wedged
Pulmonary Artery
Right Ventricle
Pulmonary Artery
FIGURE 8.2
The pressure waveforms at different points along the normal course of a
pulmonary artery catheter.These waveforms are used to identify the location of the
catheter tip as it is advanced.
