The Pulmonary Artery Catheter
139
ventricle. Therefore,
in the absence of mitral valve disease, the wedge pressure
is a measure of left ventricular filling pressure
.
Influence of Alveolar Pressure
The wedge pressure will reflect left atrial pressure only if the pulmonary
capillary pressure is greater than the alveolar pressure (P
c
>
P
A
in Figure
8.4); otherwise the wedge pressure will reflect the alveolar pressure.
Capillary pressure exceeds alveolar pressure when the tip of the PA
20
30
10
0
mm Hg
Balloon
Inflation
Wedge Pressure
a
c
v
FIGURE 8.3
Pressure tracing showing the transition from a pulsatile pulmonary artery
pressure to a balloon occlusion (wedge) pressure.The magnified area shows the compo-
nents of the wedge pressure:
a
wave (atrial contraction),
c
wave (mitral valve closure),
and
v
wave (ventricular contraction).
Alveolus
P
c
> P
A
P
W
P
c
P
LA
Q = 0
If Q = 0
Then P
W
= P
c
= P
LA
FIGURE 8.4
The principle of the wedge pressure measurement.When flow ceases
because of balloon inflation (Q
=
0), the wedge pressure (P
W
) is equivalent to the pul-
monary capillary pressure (P
c
) and the pressure in the left atrium (P
LA
).This occurs only
in the most dependent lung region, where the pulmonary capillary pressure (P
c
) is
greater than the alveolar pressure (P
A
).
