Acid-Base, Fluid and Electrolyte Imbalances

Acid-Base Physiology and Diagnostic Evaluation

In the critical care unit, interpreting metabolic acid-base disorders requires looking beyond the arterial blood gas alone. The serum anion gap serves as an essential tool to differentiate causes of metabolic acidosis. Calculated as Sodium minus the sum of Chloride and Bicarbonate, a normal serum anion gap is typically between 8 and 12 mEq/L when potassium is excluded from the equation. An elevated anion gap suggests accumulation of unmeasured anions, including lactate, ketoacids, toxic alcohol metabolites, and inorganic anions such as sulfate and phosphate in advanced kidney failure; interpret the gap in light of the patient's albumin level. Conversely, a normal anion gap metabolic acidosis frequently stems from gastrointestinal bicarbonate loss, such as in severe diarrhea or pancreatic and small-bowel fistulas, or from rapid resuscitation with large volumes of 0.9% normal saline, which produces a hyperchloremic metabolic acidosis.

Systemic acid-base disturbances directly affect gas exchange and oxygen delivery at the tissue level. Metabolic acidosis shifts the oxyhemoglobin dissociation curve to the right, which facilitates oxygen unloading to ischemic tissues, although severe acidemia can depress myocardial contractility. In response to metabolic acidosis, respiratory compensation increases alveolar ventilation to lower PaCO2, manifesting as deep, rapid Kussmaul respirations when acidosis becomes severe. Clinicians also trend base excess; a value below −2 mEq/L indicates a base deficit that may reflect primary metabolic acidosis or compensation for respiratory alkalosis and must be interpreted alongside pH, PaCO2, lactate, and perfusion findings.

Metabolic alkalosis produces the opposite physiological challenge. It shifts the oxyhemoglobin dissociation curve to the left, which increases hemoglobin's affinity for oxygen and can impair oxygen unloading at the tissue level. Respiratory compensation for metabolic alkalosis involves hypoventilation to retain carbon dioxide, but this compensation is fundamentally limited by hypoxemia, which stimulates peripheral chemoreceptors to sustain alveolar ventilation. Furthermore, chloride-responsive metabolic alkalosis is frequently maintained by intravascular volume contraction, hypochloremia, and persistent hypokalemia. Iatrogenic contributions must also be recognized; the metabolism of citrate, whether used as regional anticoagulation during continuous renal replacement therapy or found in transfused blood products, generates bicarbonate and can exacerbate or drive metabolic alkalosis.

When managing severe lactic acidosis, administering sodium bicarbonate carries significant risks. Sodium bicarbonate can paradoxically worsen intracellular acidosis because the reaction generates carbon dioxide, which readily diffuses across cell membranes into cells while bicarbonate remains largely extracellular.