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Changes in urinary electrolytes during acute respiratory acid-base modifications


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.


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 quasi- continuous 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.


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 [Clļu 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).


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.


  1. Ramadoss J, et al.: Can J Physiol Pharmacol. 2011, 89: 227-231. 10.1139/Y11-008

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Ferrari, M. Changes in urinary electrolytes during acute respiratory acid-base modifications. Crit Care 18 (Suppl 1), P430 (2014).

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