Objective: To investigate the relationship of stress index and positive end-expiratory pressure (PEEP) in patients with acute respiratory distress syndrome (ARDS) and to determine the relationship of stress index, lung recruitment, oxygenation and respiration mechanics.
Methods: Fourteen ARDS patients were enrolled. During volume control ventilation with constant inspiratory flow, the pressure-time (P-t) curve was fitted to a power equation: P = a x time(b) + c, where coefficient b (stress index) describes the shape of curve: b = 1, straight curve; b < 1, progressive increase in slope; and b > 1, progressive decrease in slope. Positive end-expiratory pressure (PEEP) was set to obtain b value between 0.9 and 1.1 after application of a recruiting maneuver (RM). PEEP was changed to obtain 0.6 < b < 0.8 and 1.1 < b < 1.3. The sequence of experimental conditions was random. Recruited volume (RV) was measured by static pressure-volume curve method. Hemodynamics, pulmonary mechanics and gas exchange were observed simultaneously.
Results: PEEP at b < 1, b = 1 and b > 1 were (8.3 +/- 1.5) cm H2O, (15.0 +/- 1.9) cm H2O and (18.4 +/- 1.9) cm H2O respectively with a significant difference (P < 0.001). At b = 1 and b > 1, the partial arterial oxygen tension (PaO2/FiO2) [(350 +/- 113) mm Hg, (338 +/- 124 mm Hg)] were higher than that [165 +/- 60) mm Hg] of pre-RM (P < 0.05). the plateau pressure (Pplat) at b = 1 [(29 +/- 4) cm H2O] and b > 1 [(33 +/- 7) cm H2O] post-RM were significantly higher than that at b < 1 [(22 +/- 4) cm H2O] (P < 0.05); the Pplat at b > 1 was higher than that [(25 +/- 16) cm H2O] pre-RM (P < 0.05). Compared with the static pulmonary compliance (Cst) at b = 1 [(39 +/- 11) ml/cm H2O], Cst at b > 1 [(26 +/- 7) ml/cm H2O] decreased significantly (P < 0.05). The RV at b = 1 and b > 1 [(402 +/- 204) ml, (588 +/- 269) ml] were significantly higher than that at pre-RM and b < 1 [(136 +/- 111) ml, (175 +/- 122) ml] (P < 0.05). At pre-RM, b < 1, b = 1 and b > 1, HR, mean arterial pressure (MAP) and lactate showed no significant difference (P > 0.05).
Conclusion: Stress index at post-RM can be an excellent method of PEEP titration for ARDS patients.