Can venous-to-arterial carbon dioxide differences reflect microcirculatory alterations in patients with septic shock?

PURPOSE: Septic shock has been associated with microvascular alterations and these in turn with the development of organ dysfunction. Despite advances in video microscopic techniques, evaluation of microcirculation at the bedside is still limited. Venous-to-arterial carbon dioxide difference (Pv-aCO(2)) may be increased even when venous O(2) saturation (SvO(2)) and cardiac output look normal, which could suggests microvascular derangements. We sought to evaluate whether Pv-aCO(2) can reflect the adequacy of microvascular perfusion during the early stages of resuscitation of septic shock. METHODS: Prospective observational study including 75 patients with septic shock in a 60-bed mixed ICU. Arterial and mixed-venous blood gases and hemodynamic variables were obtained at catheter insertion (T0) and 6 h after (T6). Using a sidestream dark-field device, we simultaneously acquired sublingual microcirculatory images for blinded semiquantitative analysis. Pv-aCO(2) was defined as the difference between mixed-venous and arterial CO(2) partial pressures. RESULTS: Progressively lower percentages of small perfused vessels (PPV), lower functional capillary density, and higher heterogeneity of microvascular blood flow were observed at higher Pv-aCO(2) values at both T0 and T6. Pv-aCO(2) was significantly correlated to PPV (T0: coefficient −5.35, 95 % CI −6.41 to −4.29, p < 0.001; T6: coefficient, −3.49, 95 % CI −4.43 to −2.55, p < 0.001) and changes in Pv-aCO(2) between T0 and T6 were significantly related to changes in PPV (R(2) = 0.42, p < 0.001). Absolute values and changes in Pv-aCO(2) were not related to global hemodynamic variables. Good agreement between venous-to-arterial CO(2) and PPV was maintained even after corrections for the Haldane effect. CONCLUSIONS: During early phases of resuscitation of septic shock, Pv-aCO(2) could reflect the adequacy of microvascular blood flow. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (doi:10.1007/s00134-015-4133-2) contains supplementary material, which is available to authorized users.