Strong Ion Difference

Strong Ion Difference

17/1/11

A-Z

http://www.acid-base.com/strongion.php

- Stewart method

- 1980’s

- pH is not only determined by the [H+] and [HCO3-] -> other ions in solution influence pH

- there are dependent and independent variables determining pH

Dependent

- H+

- OH-

- HCO3-

- CO32-

- HA (weak acid)

- A- (weak ions)

Independent

- PCO2

-ATOT (total weak non-volatile acids)

- SID (net Strong Ion Difference)

The influence of the independent variables can be predicted through 6 simultaneous equations:

1. [H+] x [OH-] = K 'w (water dissociation equilibrium)

2. [H+] x [A-] = KA x [HA] (weak acid)

3. [HA] + [A-] = [ATOT] (conservation of mass for "A")

4. [H+] x [HCO3-] = KC x PCO2 (bicarbonate ion formation equilibrium)

5. [H+] x [CO32-] = K3 x [HCO3-] (carbonate ion formation equilibrium)

6. [SID] + [H+] - [HCO3-] - [A-] -[CO32-] - [OH-] = 0 (electrical neutrality)

STRONG ION DIFFERENCE

- strong = those ion that dissociate totally at the pH of interest in a particular solution.

- in blood (pH 7.4): strong cations = Na+, K+, Ca2+, Mg2+ and strong anions = Cl- and SO42-

- SID = the difference between the concentrations of strong cations and strong anions.

SID = (Na+ + K+ + Ca2+ + Mg2+) – (Cl- - other strong anions)

Abbreviated SID = (Na + K+) – (Cl-)

- the number of positive and negative ions in a solution must be equal (SID = 0)

- increased SID (>0) -> alkalosis (increase in unmeasured anions)

- decreased SID (<0) -> acidosis

- with normal protein levels SID is about 40mEq/L -> alkaline (any departure is roughly equivalent to the standard base-excess, although because SID doesn’t allow for Hb there is often a discrepancy)

- the SID can be changed by two methods:

(1) Concentration change

- dehydration -> concentrates the alkalinity -> increases SID

- overhydration -> dilutes the alkaline state -> dilutional acidosis -> decreased SID

(2) Strong Ion changes

- low Na+ -> decreased SID -> acidosis

- high Na+ -> increased SID -> alkalosis

- increased Cl- -> decreased SID -> acidosis (NAGMA)

- increased in organic acids (lactate, formate, ketoacids) -> acidosis (RAGMA))

ATOT

= total plasma concentration of inorganic phosphate, serum proteins and albumin (weak non-volatile acids)

ATOT = [PiTOT] + PrTOT] + albumin

- hypoproteinaemia -> base excess

PCO2

- at a molecular level it is the concentration of CO2, not the partial pressure which governs its effect on other molecules and ions. However, in practice our warm blood means that CO2 is scarcely soluble and measured PCO2 can be used measure effect.

PROS

- acknowledgement of the importance of other factors controlling pH.

- diminishes the importance of the HCO3- ion which is just a dependent variable

CRITISIMS

- complex

- calculation of small differences between large numbers of variables -> decreases accuracy

- SID only reflect plasma (where as SBE reflects the whole body and Hb’s influence)

- lack of clinical correlation to validate benefit

- standard base excess accuracy has been well validated and accepted in clinical correlation.

Jeremy Fernando (2011)