Pressure support ventilation in patients with acute lung injury

Objectives: To assess the success rate of pressure support ventilation (PSV) in acute lung injury patients undergoing continuous positive pressure ventilation (CPPV), to study physiologic changes after the transition from CPPV to PSV, and to investigate differences between patients who succeed and patients who fail PSV according to predetermined criteria. Design: Observational study. Setting: General intensive care unit in a teaching hospital. Subjects: We studied 48 patients having acute lung injury, as defined by a PaO2/FIO2 80 mm Hg, at positive end-expiratory pressure of 50 mm Hg) during CPPV, the (V) over dot(E) increase was higher than in normocapnic patients. In the latter patients, PaCO2 and pH did not change significantly going from CPPV to PSV. A total of 38 patients (79%) were allocated to Group S and the remaining 10 patients were included in Group F. In Group S, positive end-expiratory pressure of 9.4 +/- 2.9 cm H2O (range, 3-14 cm H2O) and a PSV level of 14.9 +/- 3.8 cm H2O (range, 9-22 cm H2O) were applied. In Group F, positive end-expiratory pressure of 8.9 +/- 3.1 cm H2O (range, 5-15 cm H2O) acid a PSV level of 21.6 +/- 4.6 cm H2O (range, 16-31 cm H2O) were adopted. Compared with Group S, Group F had a longer duration of intubation (20.2 +/- 19.2 days vs. 9.2 +/- 13.5 days), a lower static compliance of the respiratory system (30.4 +/- 16.5 mL/cm H2O vs. 41.7 +/- 15.0 mL/cm H2O), and a higher V-D/V-T (0.70 +/- 0.09 vs. 0.52 +/- 0.10), but similar oxygenation and positive end-expiratory pressure, (V) over dot(E) was higher in Group F during both CPPV and PSV. Conclusions: In a relatively high proportion of the investigated patients, PSV was successful. The institution of PSV led to no major changes in oxygenation or in hemodynamics, PSV was associated with increases in (V) over dot(E), and respiratory frequency. In patients who had been hypercapnic during CPPV, PaCO2 decreased despite a compensated pH. Compared with PSV success patients, patients who failed PSV appeared to be sicker, as shown by the higher duration of respiratory support, increased ventilatory needs, and decreased respiratory system compliance, despite similar arterial oxygenation and positive end-expiratory pressure.