5-step-approach-to-blood-gas-interpretation

1. What is the pH?

  • Identifies the primary acid base disturbance
  • Acidaemia exists if pH <7.35
  • Alkalaemia exists if pH > 7.45
  •  If pH is between 7.35 and 7.45 then there is either:
    • No acid base disorder
    • A compensated disorder (can never overcompensate)
    • A mixed disorder

 2. Is the primary process respiratory, metabolic or both?

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

3. Is there 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 x {(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 x {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 x {(40 – Measured pCO2)/10} (±2)
      • 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!!

 4. Are there other clues to diagnosis?

  •  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) – (pCO2 x 1.25)
      • At sea level, breathing room air (713 x 0.21) = 150
      • Therefore PAO2 = 150 – (pCO2 x 1.25)
    • A-a gradient = PAO2 – PaO2
      • Where PAO2 equals the calculated alveolar pO2 and PaO2 equals the measured arterial pO2
      • Normal = Age/4 + 4

5. Formulate and confirm the acid – base diagnosis.

  • Consider evidence from history, examination and investigations to formulate a complete acid – base diagnosis.
  • In some cases, further biochemical testing may be useful (eg drug levels, urine ketones) for confirmation of the diagnosis.

 

Normal ABG Values

A Systematic Approach to Blood Gas Interpretation
Tagged on:

Leave a Reply

Your email address will not be published. Required fields are marked *