A comparison of methods to identify open-lung PEEP

Abstract

Purpose: Many methods exist in the literature for identifying PEEP to set in ARDS patients following a lung recruitment maneuver (RM). We compared ten published parameters for setting PEEP following a RM.

Methods: Lung injury was induced by bilateral lung lavage in 14 female Dorset sheep, yielding a PaO(2) 100-150 mmHg at F(I)O(2) 1.0 and PEEP 5 cmH(2)O. A quasi-static P-V curve was then performed using the supersyringe method; PEEP was set to 20 cmH(2)O and a RM performed with pressure control ventilation (inspiratory pressure set to 40-50 cmH(2)O), until PaO(2) + PaCO(2) > 400 mmHg. Following the RM, a decremental PEEP trial was performed. The PEEP was decreased in 1 cmH(2)O steps every 5 min until 15 cmH(2)O was reached. Parameters measured during the decremental PEEP trial were compared with parameters obtained from the P-V curve.

Results: For setting PEEP, maximum dynamic tidal respiratory compliance, maximum PaO(2), maximum PaO(2) + PaCO(2), and minimum shunt calculated during the decremental PEEP trial, and the lower Pflex and point of maximal compliance increase on the inflation limb of the P-V curve (Pmci,i) were statistically indistinguishable. The PEEP value obtained using the deflation upper Pflex and the point of maximal compliance decrease on the deflation limb were significantly higher, and the true inflection point on the inflation limb and minimum PaCO(2) were significantly lower than the other variables.

Conclusion: In this animal model of ARDS, dynamic tidal respiratory compliance, maximum PaO(2), maximum PaO(2) + PaCO(2), minimum shunt, inflation lower Pflex and Pmci,i yield similar values for PEEP following a recruitment maneuver.

Fig. 1

Examples of different points on the P–V curve that have been proposed for setting PEEP. The lower inflection point of the inflation limb (lower Pflex,i) is identified by the intersection of two lines, one drawn through a region of low compliance at the beginning of inflation and one through the region of highest compliance. A similar method is done for the upper inflection point of the deflation limb (upper Pflex,d). The point of maximum compliance increase on the inflation limb (Pmci,i), point of maximum compliance decrease on the deflation limb (Pmcd,d) and true inflection point of the deflation limb (Pinf,d) are calculated from the curve-fitting parameters of the equation V = a + b/(1 + e^{−(P − c)/d}) [28, 29]

Fig. 2

Protocol for study: Baseline, Lung Injury, Pressure–volume (P–V) curve, and the Recruitment Strategy performed under pressure controlled ventilation with PEEP 20 cmH₂O, Peak Pressure of 40, 45, and 50 cmH₂O (2 min each) alternating with a 5-min stabilization period with a peak pressure of 35 cmH₂O. When the PaO₂ + PaCO₂ > 400 mmHg was achieved, PEEP Titration was performed from 20 to 15 cmH₂O (5 min each) in all animals

Fig. 3

Open-lung PEEP identified by ten different methods: Compliance, PaO₂ + PaCO₂, PaO₂, PaCO₂, shunt, inflation lower Pflex (Lower Pflex,i), deflation upper Pflex (Upper Pflex,d), point of maximum compliance increase on inflation (Pmci,i), point of maximum compliance decrease on deflation (Pmcd,d), and true inflection point of the deflation limb (Pinf,d). The bars represents mean values, the open circles are actual values for each animal. *P < 0.05 compared to all other variables