Summary of Acid-Base Measurement - Strong Ion
Difference (Stewart’s Theory)
Traditional Method:
Bicarbonate Approach: Use of the Henderson-Hasselbalch equation
Uses Arterial [CO2] and [HCO3-]
(Le Châtelier’sprinciple): The Equilibrium Law
Principle of Mass Balance
Anion Gap:
Henderson-Hasselbalch is still an accurate method for determining Acid-Base status, and you can use
the albumin correction from the Stewart Strong Ion Method to determine an accurate anion gap.
Traditional bicarbonate methods were used well before the times when albumin and lactic acid levels
were easily able to be drawn.
Stewart’s Method:
Author: Peter Stewart published an article in 1978 and a book in 1981 for a revised model in determining
acid-base disorder. Since his original model, the acid-base measurement methods have been
highly contested and re-organized or simplified with other’s theories proving similar efficacy in
determining acid-base disorders
Considered the Principle of Electroneutrality
Main Conclusions:
Dependent Variables: HCO , CO , and OH
3
3
2-
--
Independent Variables: [strong ion difference], [A ], and P
TOT
CO2
Carbon dioxide and its spontaneous relationship with hydrogen and bicarbonate from the
dissociation of water (unlimited supply)
P
CO2
= Partial pressure of CO
Increases in P
Dereases in P
2
CO2
cause an acidosis
CO2
cause an alkalosis
A = [HA] + [A ]
-
TOT
Where [HA] is the total amount of weak acid albumin, phosphate (trace), and sulfate
(trace)
Where [A ] is the dissociate ions of the weak acids
-
A
TOT
= albumin (simplified method)
Strong Ion Difference = [Na+] + [K+] + [Ca2+] + [Mg2+] − [Cl− ]
Simplified Strong Ion Difference = [Na+] - [Cl-]
2
Strong Ion Gap:
Standard Base Excess/Deficit from a blood gas machine
Sodium Chloride Effect = [Na+] - [Cl-] - 38
Albumin Effect =0.25[4.2 - albumin]
SIG = (Base Deficit) + (SID – 38) + 2.5 (4.2 ‐ Albumin (g/dL)) – Lactate
Comparison of the two Methods:
Furthermore, both experimental and clinical observations can be explained with the use of either model.
Yet the physicochemical model is useful in revealing individual processes in the development of an acid–
base disturbance because it associates the abnormality with specific electrolyte disturbances.
References:
1. Seifter JL. Integration of acid-base and electrolyte disorders. N Engl J Med. 2014;371(19):182131.
2. Hagberg CA, Gabel JC, Connis RT. Difficult Airway Society 2015 guidelines for the management
of unanticipated difficult intubation in adults: not just another algorithm. Br J Anaesth. 2015;