Pressure support ventilation in adult respiratory distress syndrome: Short-term effects of a servocontrolled mode

Abstract

Purpose : To assess the short-term effects of pressure support ventilation in adult respiratory distress syndrome (ARDS), we studied 17 patients with moderate to severe ARDS using mandatory rate ventilation (MRV), a servocontrolled mode of PSV having respiratory rate as the targeted parameter. Materials and Methods : Based on the duration of ARDS, the patients were divided into two groups: Group 1, early ARDS (duration up to 1 week), 10 patients; Group 2, intermediate ARDS (duration between 1 and 2 weeks). The patients were initially ventilated with assisted mechanical ventilation then with MRV, and finally with controlled mechanical ventilation. After a 20-minute period allowed for stabilization in each mode, ventilatory variables, gas exchange, hemodynamics, and patient's inspiratory effort were evaluated. Results : During MRV blood gases, airway pressures and hemodynamic variables remained within acceptable limits in all patients. Compared with assisted mechanical ventilation, during MRV, patients of group 1 decreased their V t and V̇ (from 0.64 ± 0.04 to 0.42 ± 0.03 L/sec) and increased their T i/T t (from 0.39 ± 0.03 to 0.52 ± 0.03). f did not change. P AO 2 - Pa O 2 and Q̇ s/Q̇ t decreased (from 306 ± 16 to 269 ± 15 mm Hg, and from 20.2 ± 1.4 to 17.5 ± 1.1, respectively), while Pa CO 2 increased (from 44 ± 3 to 50 ± 3 mm Hg). On the contrary, patients of group 2 increased their V t (from 0.69 ± 0.02 to 0.92 ± 0.09 L), decreased their f (from 22.3 ± 0.5 to 19.3 ± 0.3 b/min), although they did not change their V̇ and T i/T t. P AO 2 - Pa O 2 and Q̇ s/Q̇ t remained stable. Pa CO 2 diminished (from 39 ± 3 to 34 ± 3 mm Hg). Pressure support level was higher in group 2 than in group 1 (29.4 ± 3.0 v 19.8 ± 2.9 cm H 2O). Conclusions : We conclude that (1) PSV delivered by MRV may adequately ventilate patients with moderate to severe ARDS, preserving gas exchange and hemodynamics, at least for the short period tested; (2) early and intermediate ARDS respond in a different manner to MRV in terms of breathing pattern, gas exchange, and level of pressure assistance; and (3) patients with early ARDS are those who have an improvement in intrapulmonary oxygenation probably due, at least in part, to alveolar recruitment augmented by active diaphragmatic contraction.