Effect of PEEP on Dead Space in an Experimental Model of ARDS

BACKGROUND: The difference between Bohr and Enghoff dead space are not well described in ARDS patients. We aimed to analyze the effect of PEEP on the Bohr and Enghoff dead spaces in a model of ARDS. METHODS: 10 pigs submitted to randomized PEEP steps of 0, 5, 10, 15, 20, 25 and 30 cm H2O were evaluated with the use of lung ultrasound images, alveolar-arterial oxygen difference (P(A-a)O2), transpulmonary mechanics, and volumetric capnography at each PEEP step. RESULTS: At PEEP >= 15 cm H2O, atelectasis and P(A-a)O2 progressively decreased while end-inspiratory transpulmonary pressure (P-L), end-expiratory P-L, and driving P-L increased (all P < .001). Bohr dead space (V-DBohr/V-T), airway dead space (V-Daw/V-T), and alveolar dead space (V-Dalv/V-Talv), reached their highest values at PEEP 30 cm H2O (0.69 +/- 0.10, 0.53 +/- 0.13 and 0.35 +/- 0.06, respectively). At PEEP <15 cm H2O, the increases in atelectasis and P(A-a)O2 were associated with negative end-expiratory P-L and highest driving P-L. V-DBohr/V-T and V-Daw /V-Talv showed the lowest values at PEEP 0 cm H2O (0.51 +/- 0.08 and 032 +/- 0.08, respectively), whereas V-Daw/V(Talv )increased to 0.27 +/- 0.05. Enghoff dead space and its derived V-Dalv/V-Talv showed high values at low PEEPs (0.86 +/- 0.02 and 0.79 +/- 0.04, respectively) and at high PEEPs (0.84 +/- 0.04 and 0.65 +/- 0.12), with the lowest values at 15 cm H2O (0.77 +/- 0.05 and 0.61 +/- 0.11, respectively: all P < .001). CONCLUSIONS: Bohr dead space was associated with lung stress, whereas Enghoff dead space was partially affected by the shunt effect.