Regional venous-to-arterial carbon dioxide pressure and content differences during endotoxemic shock: influence of hydrogen ion accumulation vs. Haldane effect

Background The relationship between carbon dioxide pressures (PCO 2 ) and contents (CCO 2 ) is linked to the Haldane effect. Nevertheless, under shock conditions, hydrogen ion accumulation might strongly influence the discrepancies between PCO 2 and CCO 2 . This study aims to evaluate the impact of hydrogen ion accumulation and hemoglobin oxygen saturation (Haldane effect) on PCO 2 :CCO 2 relationships during induction and resuscitation of endotoxemic shock. Methods 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 PCO 2 were measured at baseline, time of shock, and then, every hour for 6 h, while their respective CCO 2 were computed using the Douglas equation. Mesenteric venous-to-arterial PCO 2 and CCO 2 differences (i.e., ΔPCO 2 and ΔCCO 2 ), and then, their absolute arithmetic differences (i.e., [|ΔPCO 2 – ΔCCO 2 |]) were calculated. Discrepancies in [|ΔPCO 2 – ΔCCO 2 |] between adjacent measurement time points (i.e., ∆–[|ΔPCO 2 – ΔCCO 2 |]) were compared with the variations in mesenteric venous O 2 saturation (∆–S vmes O 2 ) and arterial-to-mesenteric venous pH (∆–pH a-vmes ). In addition, arterial and venous CCO 2 values were recalculated, maintaining baseline pH (Def pH ) or SO 2 values (Def SO2 ) to then quantify the impact of pH and S vmes O 2 on the PCO 2 :CCO 2 relationship. Results Variations in ∆–[|∆PCO 2 – ∆CCO 2 |]) were paralleled by ∆–pH a-vmes (R 2 = 0.56, p < 0.001), while poorly correlated with ∆–S vmes O 2 (R 2 = 0.15, p < 0.001). When variations in pH were not included in CCO 2 calculations (i.e., Def pH –CCO 2 ), both arterial and mesenteric venous CCO 2 disagreed in ranges from 21.8 to 50.4% and 15.3 to 47.6%, respectively. Conversely, overestimation of CCO 2 was almost null when variations in SvmesO 2 were not assumed (Def SvmesO2 ). Calculations under Def pH –CCO 2 conditions revealed an almost linear relationship between PCO 2 and CCO 2 , contrasting with a non-linear relationship when pH variations were acknowledged. Conclusions Regional splanchnic PCO 2 :CCO 2 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 PCO 2 :CCO 2 dissociation curve during endotoxemic shock could have important implications when interpreting ΔPCO 2 and its combination with arterial-to-venous oxygen differences in vasodilated shock conditions.