Acid Base Disorders

Arterial blood gas analysis is used to determine the adequacy of oxygenation and ventilation, assess respiratory function and determine the acid–base balance. These data provide information regarding potential primary and compensatory processes that affect the body’s acid–base buffering system.

Interpret the ABGs in a stepwise manner:

  1. Determine the adequacy of oxygenation (PaO2)
    • Normal range: 80–100 mmHg (10.6–13.3 kPa)
  2. Determine pH status
    • Normal pH range: 7.35–7.45 (H+ 35–45 nmol/L)
    • pH <7.35: Acidosis is an abnormal process that increases the serum hydrogen ion concentration, lowers the pH and results in acidaemia.
    • pH >7.45: Alkalosis is an abnormal process that decreases the hydrogen ion concentration and results in alkalaemia.
  3. Determine the respiratory component (PaCO2)
  4. Primary respiratory acidosis (hypoventilation) if pH <7.35 and HCO3– normal.
    • Normal range: PaCO2 35–45 mmHg (4.7–6.0 kPa)
    • PaCO2 >45 mmHg (> 6.0 kPa): Respiratory compensation for metabolic alkalosis if pH >7.45 and HCO3– (increased).
    • PaCO2 <35 mmHg (4.7 kPa): Primary respiratory alkalosis (hyperventilation) if pH >7.45 and HCO3– normal. Respiratory compensation for metabolic acidosis if pH <7.35 and HCO3– (decreased).
  5. Determine the metabolic component (HCO3–)
    • Normal HCO3– range 22–26 mmol/L
    • HCO3 <22 mmol/L: Primary metabolic acidosis if pH <7.35. Renal compensation for respiratory alkalosis if pH >7.45.
    • HCO3 >26 mmol/L: Primary metabolic alkalosis if pH >7.45. Renal compensation for respiratory acidosis if pH <7.35.

Additional definitions

  • Osmolar Gap
    • Use: Screening test for detecting abnormal low MW solutes (e.g. ethanol, methanol & ethylene glycol [Reference])
    • An elevated osmolar gap (>10) provides indirect evidence for the presence of an abnormal solute which is present in significant amounts [Reference]
    • Osmolar gap = Osmolality – Osmolarity
    • Osmolality (measured)
      • Units: mOsm/kg
      • Measured in laboratory and returned as the plasma osmolality
    • Osmolarity (calculated)
      • Units: mOsm/l
      • Osmolarity = (1.86 x [Na+]) + [glucose] + [urea] + 9  (using values measured in mmol/l)
      • Osmolarity = (1.86 x [Na+]) + glucose/18 + BUN/2.8 + 9 (using US units of mg/dl)
    • NOTE: even though the units of measured (mOsm/kg) and calculated (mOsm/l) are different [Reference], strictly they cannot be subtracted from one another… However, the value of the difference is clinically useful so the problem is usually overlooked!

Rules and Resources

Arterial Blood Gas Interpretation Chart

Arterial Blood Gas (ABG) Interpretation Chart

1 2 3 4 5 Rule

Simple table to calculate metabolic compensation in respiratory acidosis and alkalosis (aka the 1-2-3-4-5 rule)

Simple calculation to predict changes in HCO3– from PaCO2

(Baseline 24 mmol/L)

Every 10 mmHg change in PaCO2
from baseline 40 mmHg









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