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Urea Excretion Rate as a Contributor to Trimethoprim-induced Hyperkalemia
- Martin Schreiber, MD; and
- Mitchell L. Halperin, MD
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TO THE EDITOR:
Velazquez and colleagues [1] and Greenberg and associates [2] have made valuable contributions to our understanding of the incidence and mechanisms of trimethoprim-induced hyperkalemia in patients with the acquired immunodeficiency syndrome (AIDS) and Pneumocystis carinii pneumonia.
What remains unclear, however, is why hyperkalemia develops in only a subset of patients who are exposed to equivalent doses of trimethoprim. Other obvious contributing factors to hyperkalemia were not identified.
We suggest the following potential mechanism. Renal potassium (K+) excretion depends not only on the ability to have a high K+ concentration in the lumen of the distal nephron, but also on the volume delivery to this part of the nephron. This delivery in turn depends on the number of osmoles delivered to K+ secretory sites. If the antidiuretic hormone is acting, the osmolality of tubular fluid at this site is nearly equal to that of plasma [3]. Thus, to increase the rate of excretion of K+ at a given urine K+ concentration, volume delivery to K+ secretory sites must be high and requires an increased delivery of osmoles [4].
The major urinary osmoles are urea and sodium salts. Malnutrition or cachexia, common in patients with advanced human immunodeficiency virus (HIV) infection, lead to low rates of urea excretion. A low urea excretion rate might diminish the delivery of tubular fluid to K+ secretory sites and thus exacerbate the impact of the trimethoprim-mediated blockade of K (+) secretion. Can the authors provide data on urea excretion rates for patients in whom hyperkalemia did and did not develop? If our speculation is correct, patients at risk for hyperkalemia could be identified and provided with a larger osmole excretion through an increased protein intake or use of loop diuretics.
Martin Schreiber, MD
Mitchell L. Halperin, MD
St. Michael's Hospital; University of Toronto; 38 Shuter Street; Toronto M5B 1A6, Ontario
The Editors welcome submissions for possible publication in the Letters section. Authors of letters should:
•Include no more than 300 words of text, three authors, and five references
•Type with double-spacing
•Send three copies of the letter, an authors' form signed by all authors, and a cover letter describing any conflicts of interest related to the contents of the letter.
Letters commenting on an Annals article will be considered if they are received within 6 weeks of the time the article was published. Only some of the letters received can be published. Published letters are edited and may be shortened; tables and figures are included only selectively. Authors will be notified that the letter has been received. If the letter is selected for publication, the author will be notified about 3 weeks before the publication date. Unpublished letters cannot be returned.
Annals welcomes electronically submitted letters.
- Copyright ©2004 by the American College of Physicians
