Marino The ICU Book 4e, IE

140 Hemodynamic Monitoring

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