Urinary carbon dioxide tension in lithium carbonate-treated patients

Renal acidification was studied in 12 lithium carbonate-treated psychiatric patients. The urinary Pco2 response to oral sodium bicarbonate loading, a qualitative index of distal hydrogen ion secretion, was evaluated in all patients and the results were compared with those obtained in 10 control subjects. The average maximal urine to arterial blood Pco2 difference (U-A Pco2) in the psychiatric patients [26 +/- 3 (S.E.) mm Hg] was significantly lower (P less than .001) than that of control subjects (51 +/- 3 mm Hg) and only three patients had values greater than 31 mm Hg (2 S.D. below the mean control value). Eight of these patients were also evaluated with NH4CL acid loading. Seven of eight patients had a minimal urine pH less than 5.30 after NH4CL administration; only one of the seven had a normal U-APco2 after bicarbonate loading. Three patients were evaluated prior to treatment and after 2 weeks of lithium administration. Pretreatment U-APco2 values were normal. After therapy the values were lower in all three patients becoming definitely abnormal in two. The present investigation, in concert with previous animal studies, demonstrates that chronic lithium carbonate therapy in man may result in decreased U-A Pco2.