[Longitudinal distribution of pulmonary vascular resistance in patients with acute respiratory insufficiency]

Effective pulmonary capillary pressure (Pc) is a major factor determining transvascular fluid filtration in the lung. It may easily be estimated from the pressure decay after rapid pulmonary artery occlusion. If Pc is known, the longitudinal distribution of pulmonary vascular resistance (PVR) can be evaluated. The present study was performed to address the following questions: (a) whether the severity of acute lung injury influences Pc and the longitudinal distribution of PVR; and (b) whether pulmonary artery occlusion (PAOP) or wedge pressure represents effective Pc during acute respiratory failure. PATIENTS AND METHODS. The investigation was performed in 45 mechanically ventilated patients. According to Murray's criteria 13 patients showed no lung injury, 19 had mild to moderate lung injury, and 13 had severe lung injury (adult respiratory distress syndrome, ARDS). As described by Holloway, effective Pc was evaluated from the pressure decay after rapid occlusion of the pulmonary artery (Figs. 1 and 2). The precapillary pressure gradient was determined as the difference between mean pulmonary artery pressure and Pc, the postcapillary pressure gradient as the difference between Pc and PAOP. Three measurements were performed and Pc determined as their mean value. The Kruskal-Wallis test and Mann-Whitney U test were performed to check statistically significant differences between groups. A Bonferroni correction was performed for multiple testing; P < 0.05 was accepted. RESULTS. Effective Pc was significantly different between patients with severe lung injury (20 +/- 3 mm Hg) and patients with mild to moderate lung injury (16 +/- 3 mm Hg), and between the latter group and patients without lung injury (12 +/- 3 mm Hg). The postcapillary pressure gradient and the relative amount of pulmonary venous vascular resistance, as well, were significantly influenced by the severity of the lung injury. In patients with ARDS the postcapillary pressure gradient was 4 +/- 1 mm Hg, whereas in patients with mild to moderate and without lung injury the postcapillary pressure gradients were 3 +/- 1 mm Hg and 2 +/- 1 mm Hg, respectively. Two ARDS patients had a postcapillary pressure gradient of 7 mm Hg (Pc 22 mm Hg and 19 mm Hg, PAOP 15 mm Hg and 12 mm Hg). One patient with severe lung injury had a postcapillary pressure gradient of 9 mm Hg (Pc 22 mm Hg, PAOP 13 mm Hg). In patients with severe lung injury 28 +/- 7% of the PVR was located in the postcapillary vascular system, whereas in patients with mild to moderate and without lung injury 22 +/- 7% and 16 +/- 6% of PVR was located in the pulmonary venous system. CONCLUSIONS. The longitudinal distribution of PVR is influenced by the severity of lung injury. PAOP, therefore, may not represent changes in Pc in patients with acute respiratory failure. The routine use of Pc measurement, however, can not be recommended until it has proven more useful than determination of PAOP when managing critically ill patients.