Metabolic alkalosis is due to a primary increase in serum [HCO3-], distinguished from chronic respiratory acidosiswith a compensatory increase in renal HCO3- reabsorptionby the associated increase in arterial pH (normal or decreased in chronic respiratory acidosis). Administered, exogenous alkali (HCO3-, acetate, citrate, or lactate) may cause alkalosis if the normal capacity to excrete HCO3- is reduced or if renal HCO3- reabsorption is enhanced. A recently resurgent problem is milk alkali syndrome, a triad of hypercalcemia, metabolic alkalosis, and acute renal failure due to ingested calcium carbonate, typically taken for the treatment or prevention of osteoporosis or for symptomatic relief of peptic ulcer disease.
Metabolic alkalosis is primarily caused by renal retention of HCO3- and is due to a variety of underlying mechanisms. Pts are typically separated into two major subtypes: Cl- responsive and Cl- resistant. Measurement of urine Cl- affords this separation in the clinical setting (Fig. 1-6). The quintessential causes of Cl--responsive alkalosis are GI induced from vomiting or gastric aspiration through a nasogastric tube, and renal induced from diuretic therapy. Hypovolemia, chloride deficiency, activation of the RAA axis, and hypokalemia play interrelated roles in the maintenance of this hypochloremic or contraction alkalosis. The various syndromes of true or apparent mineralocorticoid excess cause Cl--resistant metabolic alkalosis (Fig. 1-6); most of these pts are hypokalemic, volume expanded, and/or hypertensive.
Common forms of metabolic alkalosis are generally diagnosed from the history, physical examination, and/or basic laboratory tests. ABGs will help determine whether an elevated [HCO3-] is reflective of metabolic alkalosis or chronic respiratory acidosis; ABGs are required for the diagnosis of mixed acid-base disorders. Measurement of urinary electrolytes will aid in separating Cl--responsive and Cl--resistant forms. Urinary [Na+] may thus be >20 meq/L in Cl--responsive alkalosis despite the presence of hypovolemia; however, urinary [Cl-] will typically be very low, except in pts with severe hypokalemia. Notably, urinary [Cl-] may be variable in pts with diuretic-associated alkalosis, depending on the temporal relationship to diuretic administration. Other diagnostic testse.g., plasma renin, aldosterone, cortisolmay be appropriate in Cl--resistant forms with high urinary [Cl-] (Fig. 1-6).
Treatment: Metabolic Alkalosis The acid-base disorder in Cl--responsive alkalosis will typically respond to saline infusion; however, the associated hypokalemia should also be corrected. Pts with true or apparent mineralocorticoid excess require specific treatment of the underlying disorder. For example, hyperactive amiloride-sensitive ENaC channels cause Liddle's syndrome, which can respond to therapy with amiloride and related drugs; pts with hyperaldosteronism may in turn respond to blockade of the mineralocorticoid receptor with spironolactone or eplerenone. Finally, severe alkalosis in the critical care setting may require treatment with acidifying agents such as acetazolamide. |
Section 1. Care of the Hospitalized Patient