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

Abstract

IntroductionOur aims in this study were to report changes in the ratio of alveolar dead space to tidal volume (VDalv/VT) in the prone position (PP) and to test whether changes in partial pressure of arterial CO2 (PaCO2) may be more relevant than changes in the ratio of partial pressure of arterial O2 to fraction of inspired O2 (PaO2/FiO2) in defining the respiratory response to PP. We also aimed to validate a recently proposed method of estimation of the physiological dead space (VDphysiol/VT) without measurement of expired CO2.MethodsThirteen patients with a PaO2/FiO2 ratio < 100 mmHg were included in the study. Plateau pressure (Pplat), positive end-expiratory pressure (PEEP), blood gas analysis and expiratory CO2 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 PaO2/FiO2 ratio > 20 mmHg or by a decrease in PaCO2 > 2 mmHg. Estimated and measured VDphysiol/VT ratios were compared.ResultsPP induced a decrease in Pplat, PaCO2 and VDalv/VT ratio and increases in PaO2/FiO2 ratios and compliance of the respiratory system (Crs). Maximal changes were observed after six to nine hours. Changes in VDalv/VT were correlated with changes in Crs, but not with changes in PaO2/FiO2 ratios. When the response was defined by PaO2/FiO2 ratio, no significant differences in Pplat, PaCO2 or VDalv/VT alterations between responders (n = 7) and nonresponders (n = 6) were observed. When the response was defined by PaCO2, four patients were differently classified, and responders (n = 7) had a greater decrease in VDalv/VT ratio and in Pplat and a greater increase in PaO2/FiO2 ratio and in Crs than nonresponders (n = 6). Estimated VDphysiol/VT ratios significantly underestimated measured VDphysiol/VT 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)).ConclusionsPP induced a decrease in VDalv/VT ratio and an improvement in respiratory mechanics. The respiratory response to PP appeared more relevant when PaCO2 rather than the PaO2/FiO2 ratio was used. Estimated VDphysiol/VT ratios systematically underestimated measured VDphysiol/VT ratios.

Monitoring Dead Space in Mechanically Ventilated Children: Volumetric Capnography Versus Time-Based Capnography

“…[2][3][4][5]27 Larger dead space is associated with mortality in children with acute hypoxemic respiratory failure and longer duration of mechanical ventilation in neonates with congenital heart disease. 7,28 Increased alveolar V D can be due to decreased pulmonary perfusion (microvascular thrombosis, low cardiac output, pulmonary hypertension) or alveolar overdistention.…”

The Physiology of Ventilation

“…63 Gattinoni et al 64 also reported improved prognosis in subjects in whom P aCO 2 declined after an initial prone position session. Charron et al 65 showed that prone positioning induced a decrease in plateau pressure, P aCO 2 , and alveolar V D /V T ratio and an increase in P aO 2 /F IO 2 and C RS ; these changes peaked after 6 -9 h. In fact, the respiratory response to prone positioning appeared more relevant when P aCO 2 rather than P aO 2 /F IO 2 was used. Protti et al 66 investigated the gas exchange response to prone positioning as a function of lung recruitability, measured by computed tomography in a supine position.…”

Application of dead space fraction to titrate optimal positive end-expiratory pressure in an ARDS swine model

“…Additionally, it will also increase the risk of hemodynamic abnormalities as well as the lung injury induced by ventilation (7,8). Numerous studies have attempted to define the optimal PEEP level on the basis of a variety of methods during a recruitment maneuver (RM) with decreasing PEEP (9)(10)(11).…”

“…A number of studies have applied the VD/VT method to assess the effects of lung recruitment and PEEP titration in patients with severe ARDS (9)(10)(11). VD/VT is a specific value based on the relatively high diffusibility of CO 2 across tissue membranes (12), and the exchange of CO 2 depends strictly on alveolar ventilation volume (13).…”