catheter is below the level of the left atrium, or posterior to the left atri-
um in the supine position. Most PA catheters enter dependent lung
regions naturally (because the blood flow is highest in these regions), and
lateral chest x-rays are rarely obtained to verify catheter tip position.
Respiratory variations in the wedge pressure suggest that the catheter tip
is in a region where alveolar pressure exceeds capillary pressure (7). In
this situation, the wedge pressure should be measured at the end of expi-
ration, when the alveolar pressure is closest to atmospheric (zero) pres-
sure. The influence of intrathoracic pressure on cardiac filling pressures
is described in more detail in Chapter 9.
SpontaneousVariations
In addition to respiratory variations, the CVP and wedge pressures can
vary spontaneously, independent of any change in the factors that influ-
ence these pressures. The spontaneous variation in wedge pressure is
≤
4 mm Hg in 60% of patients, but it can be as high as 7 mm Hg (8). In
general,
a change in the wedge pressure should exceed 4 mm Hg to be consid-
ered a clinically significant change.
Wedge vs. Hydrostatic Pressure
The wedge pressure is often mistaken as the hydrostatic pressure in the
pulmonary capillaries, but this is not the case (9,10). The wedge pressure
is measured in the absence of blood flow. When the balloon is deflated
and flow resumes, the pressure in the pulmonary capillaries (P
c
) will be
higher than the pressure in the left atrium (P
LA
), and the difference in
pressures will be dependent on the flow rate (Q) and the resistance to
flow in the pulmonary veins (R
V
); i.e.,
P
c
– P
LA
=
Q
×
R
V
(8.1)
Since the wedge pressure is equivalent to left atrial pressure, Equation 8.1
can be restated using the wedge pressure (P
W
) as a substitute for left atri-
al pressure (P
LA
).
P
c
– P
W
=
Q
×
R
V
(8.2)
Therefore t
he wedge pressure and capillary hydrostatic pressure must be differ-
ent to create a pressure gradient for venous flow to the left side of the heart.
The
magnitude of this difference is unclear because it is not possible to deter-
mine R
V
. However, the discrepancy between wedge and capillary hydro-
static pressures may be magnified in ICU patients because conditions
that promote pulmonary venoconstriction (i.e., increase R
V
), such as
hypoxemia, endotoxemia, and the acute respiratory distress syndrome
(11,12), are common in these patients.
Wedge Pressure in ARDS
The wedge pressure is used to differentiate hydrostatic pulmonary
edema from the acute respiratory distress syndrome (ARDS); a normal
wedge pressure is considered evidence of ARDS (13). However, since the
140
Hemodynamic Monitoring
