NMS. Surgery

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Part I ♦ Foundations II. Metabolic acidosis: Results either from HCO 3

− loss or accumulation of

strong anions (measured or non-measured) or weak acids. A. Causes 1. Weak acid accumulation (anion gap): Loss of HCO 3 − . a. Acid accumulation can occur because of renal failure and the inability to clear acid by-products of metabolism. b. Lactic acidosis: Inadequate tissue perfusion and anaerobic metabolism. c. Diabetic ketoacidosis: Acetoacetate and beta-hydroxybutyrate are weak acids. d. Toxins (polyethylene glycol, methanol): Methanol is metabolized to formaldehyde and then formic acid. 2. Strong anion accumulation: Normal anion gap; in hyperchloremic acidosis, excess chloride induces water to dissociate and pH to drop. B. Treatment: Correct the underlying metabolic disorder. Bicarbonate administration should rarely be used unless pH is dangerously low (<7.2). Alkalosis I. Respiratory alkalosis A. Causes 1. Spontaneously breathing patient: Caused by alveolar ventilation increase and subsequent reduction in CO 2 levels (anxiety, pain, shock, sepsis, toxic substances [salicylate poisoning], or central nervous system dysfunction). 2. Mechanically ventilated patient: Iatrogenic overventilation is common. B. Treatment: Decrease minute ventilation. Most cases are self-limited. II. Metabolic alkalosis: pH increases to >7.45, and HCO 3 − is >26 mEq/L. A. Causes 1. Most common non-iatrogenic cause is loss of gastric contents (HCl and large volumes of water are lost). 2. Drugs that limit renal excretion of HCO 3 − (e.g., steroids and diuretics). 3. Over-administration of alkali (e.g., in ulcer therapy), acetate in TPN that is used to replace other anions, and citrate in transfused blood that is converted to CO 2 and water and then to HCO 3 − by the kidneys. B. Treatment: Stop the loss of chloride and replace the water and chloride with isotonic sodium chloride and potassium supplementation. For other causes, stopping the offending agent is usually sufficient. Diagnosing Acid–Base Disorders (Table 1-4) Quick Cuts • Acidosis is a pH-lowering process; acidemia is a low blood pH. • Paradoxical aciduria: The kidney exchanges H for Na, so the urine may be acidotic when the patient is alkalotic. 3. Loss of bicarbonate: Normal anion gap. a. Excess renal excretion of bicarbonate. b. Diarrhea.