Central venous-to-arterial carbon dioxide difference combined with arterial-to-venous oxygen content difference (PcvaCO2/CavO2) reflects microcirculatory oxygenation alterations in early septic shock.

PURPOSE: To explore the relationship between central venous-to-arterial carbon dioxide difference (PcvaCO2), PcvaCO2/arterial-venous oxygen content difference ratio (PcvaCO2/CavO2) and the microcirculatory status, evaluated by using near-infrared spectroscopy, in septic shock patients. METHODS: Observational study in a 30-bed mixed ICU. Fifty septic shock patients within the first 24 h of ICU admission were studied. After restoration of mean arterial pressure, hemodynamic, metabolic and microcirculatory parameters were simultaneously evaluated. Local tissue oxygen saturation (StO2), and local hemoglobin index (THI) were measured on the thenar eminence by means of near-infrared spectroscopy. A transient vascular occlusion test was performed in order to obtain StO2 deoxygenation rate (DeO2), local oxygen consumption (nirVO2), and reoxgenation rate (ReO2). RESULTS: At inclusion, increased PcvaCO2 values were associated with lower StO2 and THI, whereas increased PcvaCO2/CavO2 values were associated with lower DeO2, nirVO2, and ReO2. Multiple regression models confirmed the association between PcvaCO2/CavO2 and nirVO2, while PcvaCO2 was only related to CI, and not to microcirculatory parameters. CONCLUSIONS: In a population of early septic shock patients, increases in PcvaCO2 and PcvaCO2/CavO2 reflected different alterations at the microcirculatory level. While PcvaCO2 was related to global flow, the PcvaCO2/CavO2 ratio was associated to impaired local oxygen utilization and diminished microvascular reactivity.

Respiratory quotient estimations as additional prognostic tools in early septic shock.

Central venous-to-arterial carbon dioxide difference (PcvaCO2), and its correction by the arterial-to-venous oxygen content difference (PcvaCO2/CavO2) have been proposed as additional tools to evaluate tissue hypoxia. Since the relationship between pressure and content of CO2 (CCO2) might be affected by several factors, some authors advocate for the use of CcvaCO2/CavO2. The aim of the present study was to explore the factors that might intervene in the difference between PcvaCO2/CavO2 and CcvaCO2/CavO2, and to analyze their association with mortality. Observational study in a 30-bed mixed ICU. Fifty-two septic shock patients within the first 24 h of ICU admission were studied. After restoration of mean arterial pressure, hemodynamic and metabolic parameters were evaluated. A total of 110 sets of measurements were performed. Simultaneous PcvaCO2/CavO2 and CcvaCO2/CavO2 values were correlated, but agreement analysis showed a significant proportional bias. The difference between PcvaCO2/CavO2 and CcvaCO2/CavO2 was independently associated with pH, ScvO2, baseline CcvaCO2/CavO2 and hemoglobin. A stepwise regression analysis showed that pH was the single best predictor for the magnitude of such difference, with very limited effect of other variables. At inclusion, variables associated with ICU-mortality were lactate, pH, PcvaCO2/CavO2, and the difference between PcvaCO2/CavO2 and CcvaCO2/CavO2. Initial ScvO2, PcvaCO2, CcvaCO2/CavO2, and cardiac index were not different in survivors and non-survivors. In a population of early septic shock patients, simultaneous values of PcvaCO2/CavO2 and CcvaCO2/CavO2 were not equivalent, and the main determinant of the magnitude of the difference between these two parameters was pH. The PcvaCO2/CavO2 ratio was associated with ICU mortality, whereas CcvaCO2/CavO2 was not.

Central venous-to-arterial carbon dioxide difference and the effect of venous hyperoxia: A limiting factor, or an additional marker of severity in shock?

Central venous-to-arterial carbon dioxide difference (PcvaCO2) has demonstrated its prognostic value in critically ill patients suffering from shock, and current expert recommendations advocate for further resuscitation interventions when PcvaCO2 is elevated. PcvaCO2 combination with arterial-venous oxygen content difference (PcvaCO2/CavO2) seems to enhance its performance when assessing anaerobic metabolism. However, the fact that PCO2 values might be altered by changes in blood O2 content (the Haldane effect), has been presented as a limitation of PCO2-derived variables. The present study aimed at exploring the impact of hyperoxia on PcvaCO2 and PcvaCO2/CavO2 during the early phase of shock. Prospective interventional study. Ventilated patients suffering from shock within the first 24 h of ICU admission. Patients requiring FiO2 ≥ 0.5 were excluded. At inclusion, simultaneous arterial and central venous blood samples were collected. Patients underwent a hyperoxygenation test (5 min of FiO2 100%), and arterial and central venous blood samples were repeated. Oxygenation and CO2 variables were calculated at both time points. Twenty patients were studied. The main cause of shock was septic shock (70%). The hyperoxygenation trial increased oxygenation parameters in arterial and venous blood, whereas PCO2 only changed at the venous site. Resulting PcvaCO2 and PcvaCO2/CavO2 significantly increased [6.8 (4.9, 8.1) vs. 7.6 (6.7, 8.5) mmHg, p 0.001; and 1.9 (1.4, 2.2) vs. 2.3 (1.8, 3), p < 0.001, respectively]. Baseline PcvaCO2, PcvaCO2/CavO2 and ScvO2 correlated with the magnitude of PO2 augmentation at the venous site within the trial (ρ -0.46, p 0.04; ρ 0.6, p < 0.01; and ρ 0.7, p < 0.001, respectively). Increased PcvaCO2/CavO2 values were associated with higher mortality in our sample [1.46 (1.21, 1.89) survivors vs. 2.23 (1.86, 2.8) non-survivors, p < 0.01]. PcvaCO2 and PcvaCO2/CavO2 are influenced by oxygenation changes not related to flow. Elevated PcvaCO2 and PcvaCO2/CavO2 values might not only derive from cardiac output inadequacy, but also from venous hyperoxia. Elevated PcvaCO2/CavO2 values were associated with higher PO2 transmission to the venous compartment, suggesting higher shunting phenomena.