Blood Gas Analysis

A Systematic Approach

What’s the pH?

• pH < 7.35 ⇒ acidaemia
• pH > 7.45 ⇒ alkalaemia
• pH 7.35 – 7.45 ⇒ either:
  • no acid-base disorder
  • fully compensated acid-base disorder (over-compensation never occurs!)
  • mixed acid-base disorder

What’s the primary process?

• Check pCO2 and HCO3:
  • If pH < 7.35 and pCO2 > 44 mmHg ⇒ respiratory acidosis
  • If pH > 7.45 and pCO2 < 40 mmHg ⇒ respiratory alkalosis
  • If pH < 7.35 and HCO3 < 22 mmol/L ⇒ metabolic acidosis
  • If pH > 7.45 and HCO3 > 26 mmol/L ⇒ metabolic alkalosis

Is there any compensation?

• Rules for respiratory acid-base disorders:

  • Acute respiratory acidosis: 1 for 10 rule
    • Expected HCO3 = 24 + (Measured pCO2 – 40)/10
    • HCO3 increases by 1 mmol/L for every 10 mmHg elevation in pCO2 above 40 mmHg
    • In general, HCO3 cannot rise above 30 mmol/L
  • Chronic respiratory acidosis: 4 for 10 rule
    • Expected HCO3 = 24 + 4(Measured pCO2 – 40)/10
    • HCO3 increases by 4 mmol/L for every 10 mmHg elevation in pCO2 above 40 mmHg
    • In general, HCO3 may rise above 55 mmol/L
  • Acute respiratory alkalosis: 2 for 10 rule
    • Expected HCO3 = 24 – 2(40 – Measured pCO2)/10
    • HCO3 decreases by 2 mmol/L for every 10 mmHg decrease in pCO2 below 40 mmHg
    • In general, pCO2 cannot fall below 16 mmHg
  • Chronic respiratory alkalosis: 5 for 10 rule
    • Expected HCO3 = 24 – 5(40 – Measured pCO2)/10
    • HCO3 decreases by 5 mmol/L for every 10 mmHg decrease in pCO2 below 40 mmHg
    • In general, pCO2 cannot fall below 16 mmHg

• Rules for metabolic acid-base disorders:

  • Metabolic acidosis: 1.5 plus 8 rule
    • Expected pCO2 = 1.5 x HCO3 + 8 (±2)
    • Winter’s formula
    • In general, pCO2 cannot fall below 10 mmHg
  • Metabolic alkalosis: 0.7 plus 20 rule
    • Expected pCO2 = 0.7 x HCO3 + 20 (±5)
    • In general, pCO2 cannot rise above 55 mmHg

•  Over-compensation never occurs!!

 Are there any other clues?

•  Metabolic acid-base disorders:

  • Anion gap = Na – (Cl + HCO3)
    • Normal 12 ± 4
  • In hypoalbuminaemia: add 2.5 per 10 g/L decrease in albumin below 40 g/L
    • Corrected AG = AG + (40 – albumin)/4
  • Expected potassium:
    • Acidaemia: each 0.1 pH fall below 7.4 expect K⁺ to rise by 0.5 mmol (above 5 mmol/L)
      • Expected K = 5 + (5 × (7.4 – pH))
    • Alkalaemia: each 0.1 pH rise below 7.4 expect K⁺ to fall by 0.5 mmol (below 5 mmol/L)
      • Expected K = 5 – (5 × (pH – 7.4))
  • In hyperglycaemia:
    • Corrected Na = Na + ((glucose – 5) / 3)
  • Delta Gap = (Anion gap – 12) ÷ (24 – HCO3) – Should be a 1:1 relationship between increase in anion gap above 12 and decrease in bicarbonate below 24
    • Delta gap < 0.4 suggests a NAGMA
    • Delta gap 0.4 – 0.8 suggests a coexisting HAGMA and a NAGMA
    • Delta gap 0.8 – 2.0 is usual for a HAGMA
    • Delta gap > 2 suggests a coexisting metabolic alkalosis, OR a pre-existing compensated respiratory acidosis
  • Calculated Osmolality = (2 x Na) + urea + glucose + ethanol – Convert all units to mmol/L
    • Ethanol (mmol/L) = %ethanol x 218
  • Osmolar gap = Measured osmolality – calculated osmolality
    • Normal osmolar gap < 10
  • Lactate, Glucose, Electrolyte disturbances
    • May be associated with predictable acid-base disorders
      

• Respiratory acid-base disorders:

  • Expected PAO2 = (713 x FiO2) – 5(pCO2)/4
    • At sea level, breathing room air (713 x 0.21) = 150
    • Therefore PAO2 = 150 – 5(pCO2)/4
  • A-a gradient = PAO2 – PaO2
    • Where PAO2 equals the calculated alveolar pO2 and PaO2 equals the measured arterial pO2
    • Normal = Age/4 + 4

What’s the differential diagnosis?

• Consider evidence from history, examination and investigations to formulate a complete acid – base diagnosis.
• In some cases, further biochemical testing may be useful (e.g. drug levels, urine ketones) for confirmation of the diagnosis.
• Check out these useful blood gas mnemonics…