Stewart's physicochemical approach to acid-base analysis

Stewart's approach is a mathematically coherent explanation of acid-base balance, which allows one to make precise predictions regarding the interactions of the numerous interacting variables which affect pH. Laws governing the behaviour of aqueous solutions can be used to create equations which describe these interactions. It is a concept (or rather, a model? An ethos, a worldview?) that straddles the First and Second Part exams, having appeared in both. Originally, in their model answer for Question 24 from the first paper of 2014, the college examiners commented that they only expected "an overall understanding of the principles involved". Most trainees possess this understanding; however, the pass rate was only 5.7%. The main problem was therefore a failure to articulate this knowledge into a coherent answer.  Years later, the First Part exam candidates did not do much better in answering Question 1 from the first paper of 2023 (21% passed). This chapter ambitiously compacts a concept requiring a 504-page book to explain into a 10-minute SAQ answer or a sixty-second viva.

In its briefest form:

  • 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

Advantages of the Stewart method:

  • Quantitative mathematical explanation of acid-base disorders
  • A more scientific approach - applies the concepts of physical chemistry to traditional acid-base concepts
  • Accessible, logical framework for the design of resuscitation fluids
  • Attributes an appropriately low importance to HCO3

Disadvantages of the Steward method:

  • Complex
  • Substantially different to the well-validated classical approach; thus, produces confusion
  • Numerous variables, each measured with a small error, are combined- thus amplifying the measurement error
  • Fails to incorporate the buffering contribution of haemoglobin
  • No evidence that this approach has any influence on mortality


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.