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Acid–base chemistry of plasma: consolidation of the traditional and modern approaches from a mathematical and clinical perspective

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Abstract

Objective

Debate still exists as to whether the Stewart (modern) or traditional model of acid–base chemistry is best in assessing the acid–base status of critically ill patients. Recent studies have compared various parameters from the modern and traditional approaches, assessing the clinical usefulness of parameters such as base excess, anion gap, corrected anion gap, strong ion difference and strong ion gap. To compare the clinical usefulness of these parameters, and hence the different approaches, requires a clear understanding of their meaning; a task only possible through understanding the mathematical basis of the approaches. The objective of this paper is to provide this understanding, limiting the mathematics to a necessary minimum.

Method

The first part of this paper compares the mathematics of these approaches, with the second part illustrating the clinical usefulness of the approaches using a patient example.

Results

This analysis illustrates the almost interchangeable nature of the equations and that the same clinical conclusions can be drawn regardless of the approach adopted.

Conclusions

Although different in their concepts, the traditional and modern approaches based on mathematical models can be seen as complementary giving, in principle, the same information about the acid–base status of plasma.

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Abbreviations

A− :

Weak base form of non-bicarbonate buffers, amount of hydrogen binding sites in mmol l−1

AG:

Anion gap, in mmol l−1

AGcorr :

Corrected AG, in mmol l−1

Atot :

Non-bicarbonate buffer concentratition, both in their acid and base form. Non-bicarbonate buffers are treated as a single monovalent weak acid in accordance with the Stewart’s model. Because of monovalency, units of meq  l−1 and mmol l−1 can be treated interchangingly in most cases. We use mmol  l−1

A −pHn :

Charge of plasma buffer base at pH = 7.4; directly proportional to Atot. It depends linearly on the concentration of albumins, globulins and phosphates

β:

Buffer Capacity, amount of univalent strong base needed to increase pH by 1, in mmoll−1. Only plasma buffer capacity is discussed in this article, i.e. β = βpl here

BB:

Buffer base, in mmol l−1

BE:

Base excess in mmol l−1, amount of univalent strong base added to plasma (blood) of pH = 7.4

ΔA −I :

Change in charge of non-bicarbonate buffer base due to buffering, at normal albumin (plasma protein and phosphate) concentration

ΔA −II :

Change in charge of non-bicarbonate buffer base due to buffering, at any albumin (plasma protein and phosphate) concentration

ΔA −pHn :

Change in charge of non-bicarbonate buffer base due to change in albumin (plasma protein and phosphate) concentration at normal pH = 7.4

HA:

Weak acid form of non-bicarbonate buffers, in mmol l−1

KA :

Lumped dissociation constant of non-bicarbonate buffers, in mmol l−1

pCO2 :

Partial pressure of carbon dioxide in arterial blood, in kPa (mmHg)

SID:

Strong ion difference, in mmol l−1

SIDA :

Apparent SID, sum of strong cations minus sum of strong anions, in mmol l−1

SIDE :

Effective SID, sum of bicarbonate and non-bicarbonate buffer bases, in mmol l−1

SIG:

Strong ion gap, SIDA minus SIDE, in mmol l−1

X− :

Unmeasured anions, generally equal to SIG, in mmol l−1

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Authors and Affiliations

  1. Institute of Pathological Physiology, First Faculty of Medicine, Charles University in Prague, Prague, Czech Republic

    S. Matousek MD

  2. Department of Anaesthetics, Pain Medicine and Intensive Care, Chelsea and Westminster Hospital, London, UK

    J. Handy MD

  3. Department of Academic Anaesthetics, Pain Medicine and Intensive Care, Imperial College London, London, UK

    J. Handy MD

  4. Center for Model-Based Medical Decision Support, Institute for Health Science and Technology, Aalborg University, Fredrik Bajers vej 7E, 9220, Aalborg, Denmark

    S. E. Rees PhD

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  1. S. Matousek MD
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Correspondence to S. E. Rees PhD.

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Matousek S, Handy J, Rees SE. Acid–base chemistry of plasma: consolidation of the traditional and modern approaches from a mathematical and clinical perspective.

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Matousek, S., Handy, J. & Rees, S.E. Acid–base chemistry of plasma: consolidation of the traditional and modern approaches from a mathematical and clinical perspective. J Clin Monit Comput 25, 57–70 (2011). https://doi.org/10.1007/s10877-010-9250-4

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