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Introduction  Our aims in this study were to report changes in the ratio of alveolar dead space to tidal volume (VD alv /V T ) in the prone position (PP) and to test whether changes in partial pressure of arterial CO 2  (PaCO 2 ) may be more relevant than changes in the ratio of partial pressure of arterial O 2  to fraction of inspired O 2  (PaO 2 /FiO 2 ) in defining the respiratory response to PP. We also aimed to validate a recently proposed method of estimation of the physiological dead space (VD physiol /V T ) without measurement of expired CO 2 .    Methods  Thirteen patients with a PaO 2 /FiO 2  ratio < 100 mmHg were included in the study. Plateau pressure (Pplat), positive end-expiratory pressure (PEEP), blood gas analysis and expiratory CO 2  were recorded with patients in the supine position and after 3, 6, 9, 12 and 15 hours in the PP. Responders to PP were defined after 15 hours of PP either by an increase in PaO 2 /FiO 2  ratio > 20 mmHg or by a decrease in PaCO 2  > 2 mmHg. Estimated and measured VD physiol /V T  ratios were compared.    Results  PP induced a decrease in Pplat, PaCO 2  and VD alv /V T  ratio and increases in PaO 2 /FiO 2  ratios and compliance of the respiratory system (Crs). Maximal changes were observed after six to nine hours. Changes in VD alv /V T  were correlated with changes in Crs, but not with changes in PaO 2 /FiO 2  ratios. When the response was defined by PaO 2 /FiO 2  ratio, no significant differences in Pplat, PaCO 2  or VD alv /V T  alterations between responders ( n  = 7) and nonresponders ( n  = 6) were observed. When the response was defined by PaCO 2 , four patients were differently classified, and responders ( n  = 7) had a greater decrease in VD alv /V T  ratio and in Pplat and a greater increase in PaO 2 /FiO 2  ratio and in Crs than nonresponders ( n  = 6). Estimated VD physiol /V T  ratios significantly underestimated measured VD physiol /V T  ratios (concordance correlation coefficient 0.19 (interquartile ranges 0.091 to 0.28)), whereas changes during PP were more reliable (concordance correlation coefficient 0.51 (0.32 to 0.66)).    Conclusions  PP induced a decrease in VD alv /V T  ratio and an improvement in respiratory mechanics. The respiratory response to PP appeared more relevant when PaCO 2  rather than the PaO 2 /FiO 2  ratio was used. Estimated VD physiol /V T  ratios systematically underestimated measured VD physiol /V T  ratios.

critical care

PaCO2 and alveolar dead space are more relevant than PaO2/FiO2 ratio in monitoring the respiratory response to prone position in ARDS patients: a physiological study