endocrine and metabolic disorders
Acid Base Disorders
Diagnosis of Acid Base Disorders
Arterial blood gases (ABG) Serum electrolytes Anion gap calculated If metabolic acidosis is present, delta gap calculated and Winters formula applied Search for compensatory changes Evaluation is with ABG and serum electrolytes. The ABG directly measures arterial pH and Pco2. HCO3−level reported on the arterial blood gas panel is calculated using the Henderson-Hasselbalch equation. The HCO3−level on serum chemistry panel is directly measured. Directly measured HCO3−levels are considered more accurate in cases of discrepancy. Acid-base balance is most accurately assessed with measurement of pH and Pco2in an arterial blood sample. In cases of circulatory failure or during cardiopulmonary resuscitation, measurements from a sample of venous blood may more accurately reflect conditions at the tissue level and may be a more useful guide to bicarbonate administration and adequacy of ventilation. The pH establishes the primary process (acidosis or alkalosis), although pH moves toward the normal range with compensation. Changes in Pco2reflect the respiratory component, and changes in HCO3−reflect the metabolic component. show the expected compensation (see table Primary Changes and Compensations in Simple Acid-Base Disorders ). If not, then a second primary process should be suspected of causing the abnormal compensation. Interpretation must also consider clinical conditions (eg, chronic lung disease, renal failure, drug overdose). Calculation of the anion gap None Respiratory Acidosis is suggested by P Metabolic Alkalosis is suggested by HCO Respiratory Alkalosis is suggested by P Nomograms(acid-base maps) are an alternative way to diagnose mixed disorders, allowing for simultaneous plotting of pH, HCO3−, and Pco2. The anion gap is defined as serum sodium (Na) concentration minus the sum of serum chloride (Cl−) and serum bicarbonate (HCO3−) concentrations. Na+−(Cl−+HCO3−) The term “gap” is misleading, because the law of electroneutrality requires the same number of positive and negative charges in an open system; the gap appears on laboratory testing because certain cations (+) and anions (−) are not measured on routine laboratory chemistry panels. Thus, Na++unmeasured cations (UC)=Cl−+HCO3−+unmeasured anions (UA) and the anion gap, Na+−(Cl−+HCO3−)=UA−UC The predominant "unmeasured" anions are phosphate (PO43−), sulfate (SO4−), various negatively charged proteins, and some organic acids, accounting for 20 to 24 mEq/L (20 to 24 mmol/L). The predominant "unmeasured" extracellular cations are potassium (K+), calcium (Ca++), and magnesium (Mg++) and account for about 11 mEq/L (5.5 mmol/L). Thus, the typical anion gap is 23−11=12 mEq/L (12 mmol/L). The anion gap can be affected by increases or decreases in the UC or UA. is most commonly caused by Metabolic Acidosis in which negatively charged acids—mostly ketones, lactate, sulfates, or metabolites of methanol, ethylene glycol,or salicylate—consume (are buffered by) HCO is unrelated to metabolic acidosis but is caused by hypoalbuminemia (decreased anions); Hypercalcemia , occurs rarely as a laboratory artifact in severe cases of hypernatremia, Dyslipidemia , and bromide intoxication. ; elevation indicates the additional presence of a Metabolic Alkalosis . Example:A vomiting, ill-appearing patient with alcohol use disorder has laboratory results showing Na: 137 K: 3.8 Cl: 90 HCO3−: 22 pH: 7.40 Pco2: 41 Po2: 85 At first glance, the results appear unremarkable. However, calculations show elevation of the anion gap: 137−(90+22)=25 (normal, 10 to 12) indicating a metabolic acidosis. Respiratory compensation is evaluated by Winters formula: Predicted Pco2=1.5 (22)+8±2=41±2 Predicted=measured, so respiratory compensation is appropriate. Because there is metabolic acidosis, the delta gap is calculated, and the result is added to measured HCO3−: 25−10=15 15+22=37 The resulting corrected HCO3−is above the normal range for HCO3−, indicating a primary metabolic alkalosis is also present. Thus, the patient has a mixed acid-base disorder. Using clinical information, one could theorize a metabolic acidosis arising from alcoholic ketoacidosis combined with a metabolic alkalosis from recurrent vomiting with loss of acid (HCl) and volume. The management of this patient should include treatments to address each primary acid-base disorder. 