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 (P-CO2), strong ion difference (SID), and total amount of weak acids independently regulate pH. When blood passes through an extracorporeal membrane lung, P-CO2 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, P-CO2, 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 Delta P-CO2 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 P-CO2 is reduced and oxygen added, several changes in electrolytes occur. These changes cause a P-CO2-dependent SID reduction that, by acidifying plasma, limits pH correction caused by carbon dioxide removal. In this particular setting, P-CO2 and SID are interdependent.