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

Table 2 Changes in respiratory mechanics, blood gas analysis and VD_alv in PP

Parameters	Supine	PP H3	PP H6	PP H9	PP H12	PP H15	P value
PaO₂/FiO₂, mmHg	70 (51 to 77)	91 (81 to 103)	87 (73 to 139)	90 (81 to 111)	93 (83 to 137)	99 (83 to 139)	< 0.0001
PaCO₂, mmHg	58 (52 to 60)	54 (51 to 58)	54 (45 to 59)	50 (47 to 59)	54 (47 to 56)	52 (47 to 56)	0.04
Pplat, cmH₂O	27 (26 to 28)	25 (23 to 27)	25 (22 to 26)	25 (23 to 26)	25 (21 to 26)	25 (24 to 26)	0.0004
Crs, mL/cmH₂O	16 (13 to 30)	18 (14 to 36)	17 (15 to 40)	18 (15 to 38)	19 (15 to 38)	18 (15 to 30)	0.02
VD_alv/V_T	0.42 (0.35 to 0.47)	0.31 (0.28 to 0.41)	0.35 (0.22 to 0.39)	0.35 (0.26 to 0.39)	0.39 (0.28 to 0.44)	0.40 (0.26 to 0.45)	0.007

^aCrs: compliance of the respiratory system; PP: prone position, Pplat: plateau pressure, VD_alv/V_T: ratio of alveolar dead space to tidal volume. H3, H6, H9, H12 and H15: 3, 6, 9, 12 and 15 hours of PP, respectively. P value is between supine position and PP. Data are expressed as medians (1^st to 3^rd interquartile range).

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