Changes in urinary electrolytes during acute respiratory acid-base modifications

Abstract

Introduction The renal system compensates respiratory disorders of acid‐base equilibrium by modifying the urinary electrolyte composition [1]. Such a condition mainly involves changes in urinary ammonium excretion as a reaction to prolonged periods of acid‐base disequilibrium, leaving the renal response to acute derangements unexplored. We aimed to determine the acute variations of urinary ammonium [NH4+]u, sodium [Na+]u, potassium [K+]u and chloride [Cl‐]u concentrations following controlled minimal respiratory acid‐ base imbalances, and to further investigate whether urinary anion gap ([AG]u = [Na+]u + [K+]u ‐ [Cl‐]u) and sodium and chloride difference ([Na+]u ‐ [Cl‐]u) might unveil an early activation of the renal response. Methods Patients admitted to the ICU after major surgery were enrolled, during intubation and sedation. Patients with chronic renal failure were excluded. A urinary catheter was connected to the quasicontinuous urinary analyzer KING®, measuring [NH4+]u, [Na+]u, [K+]u and [Cl‐]u over time. Based upon the arterial pH (pHa) at entrance, patients were randomly assigned to controlled hypoventilation (30% reduction of minute ventilation if pHa ≥7.40) or hyperventilation (30% increase of minute ventilation if pHa <7.40) for 2 hours. Samples for blood gas analysis were collected every 30 minutes. Results Thirty patients were enrolled; 20 were hypoventilated, 10 hyperventilated. At 2 hours from ventilation change, pHa was respectively decreased from 7.44 ± 0.02 to 7.34 ± 0.02 and increased from 7.37 ± 0.03 to 7.44 ± 0.02 (P <0.001) in the two groups. Mean [NH4+]u rose by 2.6 ± 3.3 mEq/l in hypoventilated patients and fell by 2.5 ± 2.4 mEq/l in the hyperventilated (P <0.001). No difference in mean [K+]u was observed at any time, while [Na+]u and [Cl‐]u progressively decreased in both groups (P <0.05). [Na+]u was reduced by 48 ± 43 mEq/l during hypoventilation and by 32 ± 41 mEq/l during hyperventilation (P = 0.14), while [Cl‐]u decreased by 29 ± 69 and by 46 ± 66 mEq/l respectively (P = 0.93). Whereas [AG]u did not differ between groups, [Na+]u ‐ [Cl‐]u variation of hyperventilated patients was greater than that of hypoventilated (17 ± 34 vs. ‐18 ± 54 mEq/l, P <0.05). Conclusion During acute respiratory modifications, changes in urinary ammonium can be observed within the first 2 hours even while maintaining arterial pH within a physiologic range. This response seems to be better associated with changes in the difference between urinary sodium and chloride rather than anion gap.