The respiratory response to PP appeared more relevant when PaCO 2 rather than the PaO 2/FiO 2 ratio was used.
PP induced a decrease in VD alv/V T ratio and an improvement in respiratory mechanics. 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)). 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). 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. Changes in VD alv/V T were correlated with changes in Crs, but not with changes in PaO 2/FiO 2 ratios. Maximal changes were observed after six to nine hours. 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). Estimated and measured VD physiol/V T ratios were compared. Thirteen patients with a PaO 2/FiO 2 ratio 20 mmHg or by a decrease in PaCO 2 > 2 mmHg. 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.
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.
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