Chapter 207. Acid–Base Abnormalities

Three essential questions to answer in a critically ill patient with acid–base disorder:

• What disorder does the patient have?
• How severe is the disorder?
• What is the underlying etiology?

Knowing the clinical scenario is important for a correct interpretation of acid–base abnormalities.

There are two main approaches.

• Henderson–Hasselbalch approach (traditional approach)—relates pH to PCO2 and HCO3:
  

  Does not take into account changes in non-bicarbonate buffers such as phosphate, albumin, etc.

• Stewart approach (physicochemical or quantitative approach)—the Stewart approach takes into account the other components of the extracellular fluid. It is mostly useful in critically ill patients:
  • Strong ions are completely dissociated at physiologic pH:
    • The strong ion difference (SID) = (Na + K + Ca + Mg) −(Cl − other strong anions) = 40–44 mEq/L
  • SID is balanced by an equivalent amount of “buffer base,” mostly HCO3, albumin, and phosphate:
    • Albumin and phosphate are weak acids and are measured as ATOT
    • While usually not significant, they can affect the acid–base balance in critically ill patients:
      • Hypoalbuminemia results from malnutrition, hepatic failure, and/or hemodilution
      • Hypophosphatemia can result from malnutrition, refeeding, and hemodilution
        • Hypoalbuminemia and hypophosphatemia will result in metabolic alkalosis
      • Hyperphosphatemia occurs from renal failure and will worsen metabolic acidosis
  • The major source of acid in the body is CO2. Most of the H+ produced by the dissociation of H2CO3 (CO2 binding to water) is buffered by hemoglobin
  • Two principles apply:
    • Electrical neutrality: all positively charges = all negatively charges
    • Mass conservation, taking into account strong ions, buffer bases, and PCO2

Five steps (see Figure 207-1):

1. Determine arterial pH (acidemia or alkalemia)

2. Identify the primary acid–base abnormality (metabolic/respiratory)

3. If respiratory disturbance, is ...