Electrolyte shifts across the artificial lung in patients on extracorporeal membrane oxygenation: Interdependence between partial pressure of carbon dioxide and strong ion difference

Purpose: Partial pressure of carbon dioxide (Pco₂), strong ion difference (SID), and total amount of weak acids independently regulate pH. When blood passes through an extracorporeal membrane lung, Pco₂ decreases. Furthermore, changes in electrolytes, potentially affecting SID, were reported. We analyzed these phenomena according to Stewart's approach. Methods: Couples of measurements of blood entering (venous) and leaving (arterial) the extracorporeal membrane lung were analyzed in 20 patients. Changes in SID, Pco₂, and pH were computed and pH variations in the absence of measured SID variations calculated. Results: Passing from venous to arterial blood, Pco₂ was reduced (46.5 ± 7.7 vs 34.8 ± 7.4 mm Hg, P <.001), and hemoglobin saturation increased (78 ± 8 vs 100% ± 2%, P <.001). Chloride increased, and sodium decreased causing a reduction in SID (38.7 ± 5.0 vs 36.4 ± 5.1 mEq/L, P <.001). Analysis of quartiles of {increment}Pco₂ revealed progressive increases in chloride (P <.001), reductions in sodium (P <.001), and decreases in SID (P <.001), at constant hemoglobin saturation variation (P = .12). Actual pH variation was lower than pH variations in the absence of measured SID variations (0.09 ± 0.03 vs 0.12 ± 0.04, P <.001). Conclusions: When Pco₂ is reduced and oxygen added, several changes in electrolytes occur. These changes cause a Pco₂-dependent SID reduction that, by acidifying plasma, limits pH correction caused by carbon dioxide removal. In this particular setting, Pco₂ and SID are interdependent.