Introduction. - According to the Stewart approach of acid-base regulation, chloride from either volume replacement or cardiopulmonary bypass (CPB) priming solution may induce metabolic acidosis. The alternative hypothesis stands in volume dilution with solutions free of bicarbonate. Objectives. - Evaluate the acid-base status of patients undergoing cardiac surgery with CPB priming containing chloride and bicarbonate. Material and methods. - Prospective study. Methods. - Twenty-eight patients were prospectively included. Priming of CPB contained 47.4 mmol/l of bicarbonate and 97.7 mmol/l of chloride. Arterial blood samples were taken at 3 timings: prior (T1) and after (T2) CPB, and on arrival in the ICU (T3). Following measurements were performed: Na+, K+, Cl-, Mg++, Ca++, phosphates, albumin, lactate and arterial blood gases. Results. - After CPB respiratory acidosis was observed. There was a significant increase of chloride with a decrease in apparent strong ion difference (SIDa). At the same time bicarbonate and base excess (BE) remained constant. A significant but weak correlation between BE and SIDa existed (r(2) = 0.06, p = 0.024). On the contrary, no correlation was found between variations of BE and SIDa. However, the correlation was stronger between values and variations of bicarbonate and BE (respectively r(2) = 0.605, p < 0.0001 and r(2)-0.495, p < 0.0001). Conclusion. - No metabolic acidosis occurred after cardiac surgery when CPB was primed with bicarbonate. Therefore, it appears that chloride administration is not the main mechanism being involved in the acid-base regulation. This reinforces the hypothesis that metabolic acidosis during CPB may mainly be due to dilution of bicarbonate. (c) 2006 Elsevier Masson SAS. Tons droits reserves.