Optimum End-Expiratory Airway Pressure in Patients with Acute Pulmonary Failure

Abstract

To determine whether in the management of pulmonary failure, the maximum compliance produced by positive end-expiratory pressure coincides with optimum lung function, 15 normovolemic patients requiring mechanical ventilation for acute pulmonary failure were studied. The end-expiratory pressure resulting in maximum oxygen transport (cardiac output times arterial oxygen content) and the lowest dead-space fraction both resulted in the greatest total static compliance. This end-expiratory pressure varied between 0 and 15 cm of water and correlated inversely with functional residual capacity at zero end-expiratory pressure (r equal -0.72, p less than or equal to 0.005). Mixed venous oxygen tension increased between zero end-expiratory pressure and the end-expiratory pressure resulting in maximum oxygen transport, but then decreased at higher end-expiratory pressures. When measurements of cardiac output or of true mixed venous blood are not available, compliance may be used to indicate the end-expiratory pressure likely to result in optimum cardiopulmonary function.

Early severe acute respiratory distress syndrome: What’s going on? Part II: controlled vs. spontaneous ventilation?

“…High PEEP, when adequate [76], allows the lung to operate on the part of the P−V curve (incremental limb [116]; decremental [49]) with the highest slope [114] ( Fig. 2 in [114]: "best compliance" [117] in "safe window": Figure 1 in [115]). Given this high slope, a small increment in pressure generated by low PS generates in turn a large change in volume.…”

Early severe acute respiratory distress syndrome: What’s going on? Part I: pathophysiology

“…Although this group [24] bundles together cardiac output optimization with low stretch strategy, the 2 issues are 2 separate issues: a circulatory optimization may well be combined with recruitment of alveoli and high PEEP [40,149,162,163,171] Each group has its own definition of the "best", "optimal", "right" PEEP, etc. Briefly, the titration of PEEP was based on lowest intrapulmonary shunt [39], "best" compliance and O 2 transport [126], physiological measurements (Pflex on the incremental [127] vs. decremental [3] limb of the P-V curve), trial PEEP, end-expiratory transpulmonary pressure [10], and lastly end-inspiratory transpulmonary pressure [11] 1) Open-lung approach: Resetting PEEP from 0 up to 30 cm H 2 0 increased P/F from 48 ± 14 (severe ARDS) to 220 ± 98 (mild ARDS). [AEThus, very high PEEP "cured" the oxygenation defect over a ≈3 h challenge (i.e., arbitrarily P/F>150, irrespective of PEEP: see Table 2, part II)¨].…”

“…The "best" PEEP (mean≈8 cm; range: 0−15 cm H 2 O) combines maximum O 2 transport, lowest dead-space (Vd), and highest slope on P-V curve ("best compliance"); recruitment superseded overdistension [126]. Translating this into practice is difficult a) circulatory optimization ( § I A) is mandatory b) dead space is not a concern in the setting of early ARDS [64].…”

“…The more FRC is lowered, the more efficacious the PEEP is. Patients with emphysema and a high FRC do not benefit from PEEP in ARDS [126] …”

“…From a chronological point of view, i) PEEP was increased to very high levels to lower the functional intrapulmonary shunt [39] or to almost completely reduce atelectasis on a CT scan [40,82]. ii) an adequate CO was combined to the highest slope on the P-V curve representing a compromise between optimized circulation ("avoid low PvO 2 effect" [24,42]) and ventilation ("best compliance" [126]); alveolar recruitment of a "penumbra" area). iii) PEEP was set above the inflexion point of the incremental (Pflex, Pinf ) [127] (Fig.…”