Chapter 22: Blood Gases and Acid-Base Disorders

DEFINITIONS

• pH = power of Hydrogen defined by the Henderson-Hasselbalch equation:

  $pH=pK+log([HCO3−]/0.03PCO2)$

• Acidosis = decrease in arterial pH

  • Respiratory acidosis – caused by CO₂ retention. This increases the denominator in the Henderson-Hasselbalch equation and depresses the pH.

    • Hypoventilation and ventilatory failure can cause respiratory acidosis.

  • Metabolic acidosis – reduced pH not explained by increased PCO₂. Caused by a primary fall in the numerator [$HCO3−$] of the Henderson-Hasselbalch equation.

    • It is usually associated with an increased anion gap (see later).

• Alkalosis = increase in arterial pH

  • Respiratory alkalosis – the decreased PCO₂ explains the increased pH.

    • Seen in alveolar hyperventilation.

  • Metabolic alkalosis – raised pH out of proportion to changes in PCO₂.

    • It is associated with hypokalemia, exogenous alkali administration, or volume contraction (e.g., severe prolonged vomiting) when the plasma bicarbonate concentration rises.

• Anion gap: helps differentiate between acid gain and $HCO3−$loss.

  $Aniongap=[Na+]−([Cl−]+[HCO3−])Normalgap8−12mEq/L$

• See Figure 22-1.

• Anion gap acidosis: gap >12 mEq/L

  • Caused by decrease in $HCO3−$balanced by an increase in unmeasured acid ions, not by an increase in chloride.

  • Causes include salicylates, methanol, paraldehyde, ethylene glycol, lactic acidosis, ketoacidosis (from diabetes or starvation), and uremia.

• Non–anion gap acidosis: gap 8 to 12 mEq/L

  • Caused by a decrease in$HCO3−$ balanced by an increase in chloride.

  • Causes include renal tubular acidosis, carbonic anhydrase inhibitor, and diarrhea.

• Table 22-1 gives normal values for infants and children.

• Fencl-Stewart approach to understanding acid-base balance: looks for unmeasured anions, which could contribute to a pH disturbance.

  $BEexplained=H2O+Cl−+albumin$

• H₂O contribution = 0.3 (Na⁻ + 140)

• Cl⁻ contribution = 102 − (Cl⁻ × 140/Na)

• Albumin contribution = 3.4 (4.5-albumin)

• BE: base excess

*Get BE_measured from blood gas

CLINICAL APPROACH TO BLOOD GAS INTERPRETATION

• Blood gases provide measurements of pH, PCO₂, PO₂, total hemoglobin, ...