Critical illness is an uncommon but potentially devastating complication of pregnancy. Th e fi rst priority for the intensivist is to stabilize the mother, with the understanding that what is good for the mother is good for the fetus. Th e intensivist must be aware of the physiologic changes associated with pregnancy and their timing. Critical care interventions are similar to those for the non-pregnant patient; however, adjustment of physiologic targets for metabolic, pulmonary, and hemodynamic control may be necessary. Th e majority of acquired pregnancy-related diseases, such as PET, AFLP, and cardiomyopathy, are abrogated by delivery. However, timing and the state of fetal maturation are crucial to decision making. PaCO(2 )and alveolar dead space are more relevant than PaO(2)/FiO(2 )ratio in monitoring the respiratory response to prone position in ARDS patients: a physiological study 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. Since its first description in 1967 [1] , it has been accepted that acute respiratory distress syndrome (ARDS) includes a number of lung injuries of various origins whose consequences are decreased lung capacity available for ventilation, leading to the concept of "baby lung" [2] . Considerable progress has been made over the past decade in the ventilatory management of patients with ARDS. In particular, a strict limitation of tidal volume (V T ) and plateau pressure (Pplat) below 30 cmH 2 O reduces mortality [3] . The application of positive end-expiratory pressure (PEEP) is recognized to recruit the lung and to restore functional residual capacity [4] , but its optimum level is still widely debated [5] .