Page 323 - Clinical Application of Mechanical Ventilation

Hemodynamic Monitoring 289

Positive Pressure Ventilation (without PEEP)

Increase in Intrathoracic Pressure

Decreased Venous Return

Lower Right Ventricular Output

Lower Blood Volume (Pressure) in the Pulmonary Artery
Note: When PEEP is used in conjunction with positive pressure © Cengage Learning 2014
ventilation, the PAP may not show a decrease because PEEP tends
to increase the PAP by compressing the pulmonary vessels.
Figure 10-10 Effects of positive pressure ventilation.

In the absence of compensation by increasing the heart rate, decrease of right and left
ventricular stroke volumes generally leads to a decreased cardiac output.
Pulmonary Capillary Wedge Pressure

The pulmonary artery catheter is also used to measure the pulmonary capillary
wedge pressure (PCWP) (also called pulmonary artery wedge pressure). PCWP
PCWP reflects the left
ventricular preload. is measured by slowly inflating the balloon via the balloon inflation port on the
pulmonary artery catheter. As the balloon inflates, the pulmonary arterial waveform
on the monitor will change to the wedged pressure waveform. Proper inflation of the
balloon usually requires no more than 1.5 mL (0.75 to 1.5 mL depending on size of
balloon) of air. The balloon is deflated as soon as the reading of PCWP is obtained.
The PCWP reading is typ-
ically taken at end-expiration The PCWP reading is typically taken at end-expiration for both spontaneous
for both spontaneous breathing and mechanically ventilated patients (Ahrens, 1991; Campbell et al.,
breathing and mechanically
ventilated patients. 1988). This practice should be done consistently for consistent PCWP measurements
and meaningful interpretation of hemodynamic data.
Components of Pulmonary Capillary Wedge

Pressure Waveform

The components of the PCWP waveform are similar to the CVP or right atrial wave-
form. When all of the components are present, the a wave of the PCWP waveform
reflects left atrial contraction and x downslope represents the decrease in left atrial
pressure following contraction. The c wave, if present, is seen along the x downslope,
and it occurs during closure of the mitral valve. The v wave indicates left ventricular
contraction and passive atrial filling. The y downslope is due to the decrease in blood
volume and pressure following the opening of the mitral valve (Figure 10-11).
Abnormal Pulmonary Capillary Wedge Pressure Waveform. Increased PCWP measure-
ments are often observed in conditions where partial obstruction or excessive blood
flow is present in the left heart (Schriner, 1989). Two common changes in the PCWP
waveform are the a and v waves.

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