Normal values:
pH = 7.35-7.45
PCO2 = 35-45
HCO2 = 22-26
Anion gap < 16

Acid Base Disorder PaCO2 pH Primary Change Compensatory Change Etiology
Respiratory Acidosis Increased Decreased Increased CO2 Increased HCO3 Hypoventilation
Airway obstruction
Respiratory Alkalosis Decreased Increased Decreased CO2 Decreased HCO3 Anxiety
PE
Metabolic Acidosis Decreased Decreased Decreased HCO3 Decreased CO3 Lactic acidosis
DKA
Poisons
Metabolic Alkalosis Increased Increased Increased HCO3 Increased CO2 Vomiting (pyloric stenosis)
Diuretic therapy
Primary Disorder Expected Changes
Metabolic Acidosis PCO2 = 1.5 x HCO3 + (8 +/- 2)
Metabolic Alkalosis PCO2 = 0.7 x HCO3 + (21 +/- 2)
Acute Respiratory Acidosis Change in pH = 0.008 x (PCO2 - 40)
Chronic Respiratory Acidosis Change in pH = 0.003 x (PCO2 - 40)
Acute Respiratory Alkalosis Change in pH = 0.008 X (40 - PCO2)
Chronic Respiratory Alkalosis Change in pH = 0.017 x (40 - PCO2)

Caveats

Metabolic acid base problem exists if

  • pH is abnormal and pH and PCO2 change in same direction (both up or down)
  • Respiratory compensation is intact if PCO2 resembles last 2 digits of pH

Respiratory acid base problem exisits if:

  • PCO2 is abnormal
  • pH and PCO2 change in opposite directions

Mixed acid base problem exists if:

  • PCO2 is abnormal and pH has not changed as expected or normal
  • pH is abnormal and PCO2 has not changed as expected or is normal