Differential Diagnosis for Acid-Base Abnormality

The patient's clinical presentation and laboratory values suggest an acid-base disturbance. The following differential diagnoses are considered:

• Single Most Likely Diagnosis

  • Metabolic alkalosis with respiratory compensation: The patient has a high bicarbonate level (HCO3 42 mEq/L), which indicates metabolic alkalosis. The elevated PCO2 (50 mm Hg) suggests respiratory compensation, as the body tries to retain more CO2 to buffer the excess bicarbonate. The clinical context of postoperative ileus, nasogastric tube in place, and orthostatic hypotension supports this diagnosis, as these conditions can lead to volume depletion and contraction alkalosis.

• Other Likely Diagnoses

  • Uncompensated metabolic alkalosis: Although the PCO2 is elevated, suggesting some degree of respiratory compensation, the primary disturbance appears to be metabolic alkalosis. However, the degree of compensation is not complete, making uncompensated metabolic alkalosis a possible consideration.
  • Respiratory acidosis with metabolic compensation: The elevated PCO2 could suggest a primary respiratory acidosis, but the high bicarbonate level and the clinical context make metabolic alkalosis a more likely primary disturbance.

• Do Not Miss Diagnoses

  • Severe volume depletion leading to lactic acidosis: Although not directly indicated by the provided laboratory values, severe volume depletion (suggested by orthostatic hypotension and diminished skin turgor) can lead to lactic acidosis, a condition that would significantly alter management and has a high mortality if missed.
  • Adrenal insufficiency: This condition can cause hypotension, volume depletion, and electrolyte imbalances, potentially contributing to or exacerbating the acid-base disturbance. It is crucial to consider adrenal insufficiency, especially in a postoperative patient with significant hypotension.

• Rare Diagnoses

  • Milk-alkali syndrome: This condition, caused by excessive ingestion of calcium and alkali, can lead to metabolic alkalosis. However, it is less likely in this postoperative context without a history of such ingestion.
  • Other rare causes of metabolic alkalosis, such as certain hormonal imbalances (e.g., hyperaldosteronism) or genetic disorders, are possible but less likely given the clinical scenario.