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

PURPOSE: To analyze the effects of positive end-expiratory pressure (PEEP) on Bohr's dead space (VDBohr/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). VDBohr/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 VDBohr/VT only in the ∆P˃15% group (r = 0.74, p < .001). CONCLUSIONS: Increases in PEEP result in higher VDBohr/VT only when associated with an increase in driving pressure.

Influence of FIO2 on PaCO2 during noninvasive ventilation in patients with COPD.

BACKGROUND: The administration of a high FIO2 to COPD patients breathing spontaneously may result in hypercapnia, due to reversal of preexisting regional hypoxic pulmonary vasoconstriction, resulting in a greater dead space. Arterial blood gas trends have not been reported in these patients. In a 31-bed medical ICU in a teaching hospital we prospectively investigated the response of 17 CO2-retaining COPD patients, after acute respiratory crisis stabilization with noninvasive ventilation, to an FIO2 of 1.0 for 40 min, after having been noninvasively ventilated with an FIO2 of ≤ 0.50 for 40 min. RESULTS: The mean ± SD baseline findings were: PaO2 101.4 ± 21.7 mm Hg, PaCO2 52.6 ± 10.4 mm Hg, breathing frequency 17.8 ± 3.7 breaths/min, tidal volume 601 ± 8 mL, and Glasgow coma score of 14.8 ± 0.3. PaO2 significantly increased (P < .001) when FIO2 was increased to 1.0, but there was no significant change in PaCO2, breathing frequency, tidal volume, or Glasgow coma score. CONCLUSIONS: During noninvasive ventilation with an FIO2 sufficient to maintain a normal PaO2, a further increase in FIO2 did not increase PaCO2 in our CO2-retaining COPD patients.