Page 325 - Clinical Application of Mechanical Ventilation

Hemodynamic Monitoring 291

Verification of the Wedged Position

Since artifact or dampened waveform may occur during inflation of the balloon,
The wedged position of and it resembles that of a wedged pressure tracing, using the PCWP tracing alone
a pulmonary artery catheter
may be confirmed by: (1) PAP on the monitor to verify the wedged position may not be always reliable. Three
diastolic-PCWP gradient; methods are available to confirm a properly wedged pulmonary artery catheter:
(2) postcapillary-mixed
venous PO 2 gradient; and (1) PAP diastolic-PCWP gradient; (2) postcapillary-mixed venous PO gradient;
2
(3) postcapillary-mixed and (3) postcapillary-mixed venous O saturation gradient.
venous O 2 saturation gradient. 2
PAP Diastolic-PCWP Gradient. Under normal conditions, the PAP diastolic value is
about 1 to 4 mm Hg higher than the average wedge pressure of the same individual
(Daily et al., 1985). However, the PAP diastolic value may be lower than actual
with forceful spontaneous inspiratory efforts. The PCWP may be higher than
actual if there is significant downstream obstruction such as mitral valve disease
(McGrath, 1986). These factors must be taken into account when evaluating the
pressure gradient between PAP diastolic pressure and PCWP.
Postcapillary-Mixed Venous PO Gradient. The PO of a blood gas sample from the
2
2
distal opening of a properly wedged catheter should be at least 19 mm Hg higher
than that from a systemic artery. The PCO should be at least 11 mm Hg lower.
2
These differences are expected because a properly wedged catheter does not allow
mixing of shunted venous blood with the postcapillary (oxygenated) blood. This
procedure may not be feasible for a hypovolemic patient because up to 40 mL of
waste (mixed venous) blood sample may be required before reaching the postcapil-
lary blood sample (Morris et al., 1985).

Postcapillary-Mixed Venous O Saturation Gradient. If the pulmonary artery catheter
2
is capable of monitoring oxygen saturation by the oximetry method, the oxygen
saturation value of a properly wedged catheter should be about 20% higher than
the one recorded with the balloon deflated (Morris et al., 1985).

Cardiac Output and Cardiac Index

Another important value of the pulmonary artery catheter is its ability to measure
cardiac output by the thermodilution method. During cardiac output measurement,
a small amount (10 mL) of iced or room-temperature fluid (usually 5% dextrose in
water, D5W) is injected into the proximal port of the pulmonary artery catheter.
The temperature change of the blood flow is recorded as the flow passes by the
The normal cardiac thermistor at the catheter tip. This and other measurements are computed and the
output for an adult is from
4 to 8 L/min. flow rate through the heart is displayed as cardiac output. The normal cardiac out-
put for an adult is from 4–8 L/min.
Current pulmonary artery catheters are capable of monitoring cardiac output
continuously by thermodilution without injecting a bolus of room temperature or
iced injectate. This technology uses a thermal strip on the outside of the catheter
which is slightly heated by an electronic signal.
The normal cardiac index
is 2.5 to 3.5 L/min/m . 2 Since cardiac output normally varies from person to person depending on the
size of the individual, it is common to “index” the value by dividing cardiac output

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