Dead space analysis at different levels of positive end-expiratory pressure in acute respiratory distress syndrome patients

Abstract

Purpose: To analyze the effects of positive end-expiratory pressure (PEEP) on Bohr's dead space (VD_Bohr/VT) in patients with acute respiratory distress syndrome (ARDS).

Material and methods: Fourteen ARDS patients under lung protective ventilation settings were submitted to 4 different levels of PEEP (0, 6, 10, 16 cmH₂O). Respiratory mechanics, hemodynamics and volumetric capnography were recorded at each protocol step.

Results: Two groups of patients responded differently to PEEP when comparing baseline with 16-PEEP: those in which driving pressure increased > 15% (∆P_˃15%, n = 7, p = .016) and those in which the change was ≤15% (∆P_≤15%, n = 7, p = .700). VD_Bohr/VT was higher in ∆P_≤15% than in ∆P_≤15% patients at baseline ventilation [0.58 (0.49-0.60) vs 0.46 (0.43-0.46) p = .018], at 0-PEEP [0.50 (0.47-0.54) vs 0.41 (0.40-0.43) p = .012], at 6-PEEP [0.55 (0.49-0.57) vs 0.44 (0.42-0.45) p = .008], at 10-PEEP [0.59 (0.51-0.59) vs 0.45 (0.44-0.46) p = .006] and at 16-PEEP [0.61 (0.56-0.65) vs 0.47 (0.45-0.48) p = .001]. We found a good correlation between ∆P and VD_Bohr/VT only in the ∆P_˃15% group (r = 0.74, p < .001).

Conclusions: Increases in PEEP result in higher VD_Bohr/VT only when associated with an increase in driving pressure.

Keywords: ARDS; Carbon dioxide; Dead space; PACO(2); PEEP; Volumetric capnography.