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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 HCO3loss.

    Aniongap=[Na+]([Cl]+[HCO3])Normalgap812mEq/L

  • Anion gap acidosis: gap >12 mEq/L

    • Caused by decrease in HCO3balanced 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 inHCO3 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

BEunexplained=*BEmeasuredBEexplained

*Get BEmeasured from blood gas

TABLE 22-1

Normal Values

CLINICAL APPROACH TO BLOOD GAS INTERPRETATION

  • Blood gases provide measurements of pH, PCO2, PO2, total hemoglobin, and derivation of indices such as ...

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