Question 1

Outline the abnormalities in the following arterial blood gas (25% of Marks). Explain the Stewart approach to acid-base interpretation (75% of Marks).

Parameter Patient Value Adult Normal Range
FiO2 0.6  
pH 6.94* 7.35 – 7.45
pO2 85.0 mmHg (11.3 kPa)       
pCO2 43.0 mmHg (5.7 kPa) 35.0 – 45.0 (4.6 – 6.0)
SpO2 98%  
Bicarbonate       9.0 mmol/L* 22.0 – 26.0
Base Excess -15 mmol/L* -2.0 – +2.0
Lactate 4.0 mmol/L* 0.5 – 1.6
Sodium 141 mmol/L 135 – 145
Potassium 5.0 mmol/L 3.5 – 5.0
Chloride 92 mmol/L* 95 – 105
Glucose 3.8 mmol/L 3.5 – 6.0
Urea 18.0 mmol/L* 3.0 – 8.0
Creatinine 145 μmol/L* 45 – 90

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College Answer

High performing answers correctly outlined the ABG findings including consideration of electrolyte abnormalities, A-a gradient, acid-base disturbance (including anion gap and strong ion difference) and whether compensation was appropriate. The best explanations of the Stewart approach described its physicochemical basis, discussed the independent variables (strong ions, total weak acids, and pCO2) in detail, and described their effect on the dependent variables and how they result in acid-base derangements. 

The ABG provided depicted an incorrect base excess with an omission of (-) symbol. Candidates were marked accordingly depending on their response to this and all candidates were compensated equally for the confusion that this may have caused.


As data interpretation questions seem to be appearing less and less frequently in the Second Part Exam, they seem to be migrating into the First Part, and bringing Stewart with them. The last time Stewart's physicochemical approach to acid-base interpretation appeared was in Question 24 from the first paper of 2014, where Second Part candidates were expected to briefly outline it. 

This ABG interpretation question is difficult to recreate, as the original ABG was omitted from the paper. We can be confident that some kind of metabolic acidosis was apparent, considering the erratum issued by the examiners, but beyond that, everything else is speculation. Moreover the examiners clearly just threw some numbers at the candidates without any clinical context, which is not usually advisable. For the purposes of revision, the ABG in this version of the question was borrowed from Question 11.3 from the second paper of 2021, in the Second Part Exam, except the SBE was printed correctly.

The interpretation:

  1. The A-a gradient is markedly raised:
    PAO2 = (0.6 × 713) - (43× 1.25) = 374
    Thus, A-a = (374 - 85) = 289 mmHg.
  2. There is severe acidaemia
  3. The PaCO2 is relatively normal, which is unexpected considering the pH.
  4. The SBE is -15, suggesting a metabolic acidosis. The bicarbonate is only 9.
  5. There is no attempt to compensate; the expected CO2 is (40 - 15) = 25, or (1.5 × 9) + 8 = 21.5 mmHg. 
    Thus, there is a respiratory acidosis as well.
  6. The anion gap is raised:
    141-(92+9) = 40. 
  7. The delta ratio is:
    (40-12)/(24-9) = 1.89, i.e. a pure high anion gap metabolic acidosis

Stewart approach:

  • The acid-base system is an interaction of several variables
  • There are independent variables, which can be altered from outside the system.
  • There are dependent variables which are altered by changes in the independent variables.
  • pH and HCO3- are dependent variables.
  • The independent variables are:
    • SID - the strong ion difference
    • ATOT - the total weak acid concentration
    • PaCO2
  • Thus, changes in any of the independent variables can cause a change in pH and HCO3-, i.e. acidosis and alkalosis.
  • All the independent variables must be known to calculate the dependent variables

Thus, acid-base disorders can be classified as:

  • Respiratory: increased or decreased PaCO2
  • SID changes:
    • due to excess or deficit of water
    • due to excess or deficit of strong ions
  • ATOT changes: excess or deficit of inorganic phosphate or albumin


Morgan, T. J. "What exactly is the strong ion gap, and does anybody care?" Critical Care and Resuscitation (2004) 6: 155-166.

Sirker, A. A., et al. "Acid− base physiology: the ‘traditional’and the ‘modern’approaches."  Anaesthesia 57.4 (2002): 348-356.

Story, D. A., S. Poustie, and R. Bellomo. "Quantitative physical chemistry analysis of acid− base disorders in critically ill patients." Anaesthesia 56.6 (2001): 530-533.