Lack of correlation between central venous minus arterial PCO2 to arterial minus central venous O2 content ratio and respiratory quotient in patients with septic shock: A prospective observational study

Central venous-arterial PCO2 to arterial-central venous O2 content ratio (Pcv-aCO2/Ca-cvO2) is commonly used as a surrogate for respiratory quotient (RQ) and tissue oxygenation. Although Pcv-aCO2/Ca-cvO2 might be associated with hyperlactatemia and outcome, neither the interchangeability with RQ nor the correlation with conclusive variables of anaerobic metabolism has never been demonstrated in septic shock. Our goal was to compare Pcv-aCO2/Ca-cvO2 and RQ in patients with septic shock. Prospective, observational study. Two adult ICUs. Forty-seven patients with septic shock on mechanical ventilation with stable respiratory settings and vasopressor dose after initial resuscitation. None. We measured arterial and central venous gases, Hb, and O2Hb. Pcv-aCO2/Ca-cvO2 and the ratio of central venous-arterial CO2 content to arterial-central venous O2 content (Ccv-aCO2/Ca-cvO2) were calculated. RQ was determined by indirect calorimetry. Pcv-aCO2/Ca-cvO2 and Ccv-aCO2/Ca-cvO2 were not correlated with RQ (R2 = 0.01, P = 0.50 and R2 = 0.01, P = 0.58, respectively), showing large bias and wide 95 % limits of agreement with RQ (1.09, -1.10−3.27 and 0.42, -1.53−2.37). A multiple linear regression model showed Hb, and central venous PCO2 and O2Hb, but not RQ, as Pcv-aCO2/Ca-cvO2 determinants (R2 = 0.36, P = 0.0007). In patients with septic shock, Pcv-aCO2/Ca-cvO2 did not correlate with RQ and was mainly determined by factors that modify the dissociation of CO2 from Hb. Pcv-aCO2/Ca-cvO2 seems to be a poor surrogate for RQ; therefore, its values should be interpreted with caution. La relación entre la PCO2 venosa central-arterial y el contenido arterial-venoso central de O2 (Pvc-aCO2/Ca-vcO2) se emplea comúnmente como sustituto del cociente respiratorio (CR). Aunque la Pvc-aCO2/Ca-vcO2 puede asociarse a hiperlactacidemia y mayor mortalidad, la intercambiabilidad con CR y la correlación con variables definidas de metabolismo anaeróbico no se han demostrado en el shock séptico. Nuestro objetivo fue comparar Pvc-aCO2/Ca-vcO2 y CR en pacientes con shock séptico. Estudio observacional y prospectivo. Dos UCIs de adultos. Cuarenta y siete pacientes con shock séptico, ventilados mecánicamente, y con condiciones respiratorias y dosis de vasopresores estables, luego de la resucitación inicial. Ninguna. Se midieron gases arteriales y venosos centrales, Hb y O2Hb. Se calcularon Pcv-aCO2/Ca-cvO2 y la relación entre contenido venoso central-arterial de CO2 y el contenido arterial-venoso