Hyperchloremic Metabolic Acidosis: Renal Tubular Acidosis

Abstract

Renal tubular acidoses (RTAs) are discrete renal tubular disorders that are charac-terized by the inability to excrete H + in the urine, As a result, there is a positive H + balance, causing metabolic acidosis. The net acid excretion (NAE) is decreased, and some of the patients are unable to lower their urine pH < 5.5. Despite severe acido-sis, the anion gap (AG) remains normal because the decrease in serum [HCO 3 − ] is compensated for by a proportionate increase in serum [Cl − ]. There are five types of RTAs: 1. Distal RTA (classic or Type I RTA) 2. Proximal RTA (Type II RTA) 3. Incomplete RTA (Type III RTA) 4. Hyperkalemic Distal RTA (Type 4) with Variable Urine pH 5. Hyperkalemic Distal RTA with Alkaline Urine pH Hyperchloremic metabolic acidosis can also develop following large volume infu-sion of normal saline (dilutional acidosis) in patients with chronic kidney disease (CKD) (glomerular filtration rate (GFR) ∼ 50 ml/min), and during the recovery phase of diabetic ketoacidosis. NAE or urinary acidification is an important physiologic determinant that is use-ful in the assessment of metabolic acidosis. In a normal individual, NAE is in-creased in response to an acid load. Determination of NAE in a patient with hy-perchloremic metabolic acidosis can help determine the etiology of this disorder. Simple laboratory tests such as urine pH, urine anion gap (U AG), and urine osmolal gap are helpful during the workup of a patient with RTA. Of the three tests, the first two are routinely performed. Urine pH • Normal urine pH varies between 4.5 and 6.0, implying appropriate NAE • In patients with proximal RTA, urine pH can be acidic or alkaline (see further) A. S. Reddi, Fluid, Electrolyte and Acid-Base Disorders, DOI 10.1007/978-1-4614-9083-8_29, © Springer Science+Business Media New York 2014 348 29 Hyperchloremic Metabolic Acidosis: Renal Tubular Acidosis • In patients with distal RTA, urine pH is always > 6.5 • In type III RTA, urine pH is > 6.5 • In patients with hyperkalemic RTA with aldosterone deficiency, urine pH is < 5.5 • In hyperkalemic distal RTA with variable levels of aldosterone, urine pH is al-ways > 6.5 Urine Anion Gap (U AG)