Marino The ICU Book 4e, IE

139

The Pulmonary Artery Catheter

30

a

c

v

20

Balloon Inflation

10 mm Hg

Wedge Pressure

0

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 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

Q = 0

If Q = 0

P LA

= P c

= P LA

Then P W

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 ).