Regional venous-to-arterial carbon dioxide pressure and content differences during endotoxemic shock: influence of...

The relationship between carbon dioxide pressures (PCO₂) and contents (CCO₂) is linked to the Haldane effect. Nevertheless, under shock conditions, hydrogen ion accumulation might strongly influence the discrepancies between PCO₂ and CCO₂. This study aims to evaluate the impact of hydrogen ion accumulation and hemoglobin oxygen saturation (Haldane effect) on PCO₂:CCO₂ relationships during induction and resuscitation of endotoxemic shock.

Shock was induced by an escalating dose of lipopolysaccharide in 12 female Landrace pigs. Norepinephrine was then started to maintain mean arterial pressure ≥ 75 mmHg, while successive fluid boluses were administered targeting arterial lactate < 2.0 mmol·L^-1 or decreases > 10% per 30 min. Mesenteric venous and arterial PCO₂ were measured at baseline, time of shock, and then, every hour for 6 h, while their respective CCO₂ were computed using the Douglas equation. Mesenteric venous-to-arterial PCO₂ and CCO₂ differences (i.e., ΔPCO₂ and ΔCCO₂), and then, their absolute arithmetic differences (i.e., [|ΔPCO₂ - ΔCCO₂|]) were calculated. Discrepancies in [|ΔPCO₂ - ΔCCO₂|] between adjacent measurement time points (i.e., ∆-[|ΔPCO₂ - ΔCCO₂|]) were compared with the variations in mesenteric venous O₂ saturation (∆-S_vmesO₂) and arterial-to-mesenteric venous pH (∆-pH_a-vmes). In addition, arterial and venous CCO₂ values were recalculated, maintaining baseline pH (Def_pH) or SO₂ values (Def_SO2) to then quantify the impact of pH and S_vmesO₂ on the PCO₂:CCO₂ relationship.

Variations in ∆-[|∆PCO₂ - ∆CCO₂|]) were paralleled by ∆-pH_a-vmes (R² = 0.56, p < 0.001), while poorly correlated with ∆-S_vmesO₂ (R² = 0.15, p < 0.001). When variations in pH were not included in CCO₂ calculations (i.e., Def_pH-CCO₂), both arterial and mesenteric venous CCO₂ disagreed in ranges from 21.8 to 50.4% and 15.3 to 47.6%, respectively. Conversely, overestimation of CCO₂ was almost null when variations in SvmesO₂ were not assumed (Def_SvmesO2). Calculations under Def_pH-CCO₂ conditions revealed an almost linear relationship between PCO₂ and CCO₂, contrasting with a non-linear relationship when pH variations were acknowledged.

Regional splanchnic PCO₂:CCO₂ relationship was mostly influenced by hydrogen ion accumulation rather than the Haldane effect during development and resuscitation of endotoxemic shock. Predominant influence of hydrogen ion accumulation on PCO₂:CCO₂ dissociation curve during endotoxemic shock could have important implications when interpreting ΔPCO₂ and its combination with arterial-to-venous oxygen differences in vasodilated shock conditions.