Page 335 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice
P. 335
326 ACID-BASE DISORDERS
HCO 3 , and A . The requirement for electroneutrality whereas [UA strong ] is the sum of all unmeasured strong
dictates that at all times the SID equals the sum of bicar- anions (e.g., ketoanions, lactate, sulfate). The calculated
bonate buffer ion activity (HCO 3 ) and nonvolatile value of the SIG will change depending on the strong ions
buffer ion activity (A ) such that SID þ ¼ HCO 3 – measured. The most important strong cations in plasma
þ
A . This approach obviously assumes that all ionized based on their concentration are Na and K , whereas
þ
entities in plasma can be classified as a strong ion, a vola- the most prevalent strong anion is Cl . Thus SIG can
tile buffer ion, or a nonvolatile buffer ion (A ). This be defined in its simplest form when only these three
assumption forms the basis for the simplified strong ion strong ions are measured as:
equation. A complete description of the mathematical
background of the simplified strong ion model, as well SIG ¼½Na þ ½K ½Cl ¼½UC strong ½UA strong
þ
þ
þ
as its limitations, can be found elsewhere. 5,7
The simplified strong ion approach is a quantitative,
where [UC strong ] is the sum of all strong cations other
þ
mechanistic acid-base model. Unlike Stewart’s strong ion
þ
than [Na ] and [K ], and [UA strong ] is the sum of all
þ
equation, the simplified strong ion equation uses hydrogen
strong anions other than [Cl ]. Electroneutrality must
ion activity (pH) instead of concentration, provides a prac-
be maintained in plasma, and the excess of positive
tical experimental method for determining species-specific
charges from the SIG is balanced by the negative charges
values for K a and A tot (CO 2 tonometry of plasma), and
of HCO 3 and the nonvolatile buffers [A ]. Thus
simplifies to the Henderson-Hasselbalch equation when
electroneutrality can be expressed as:
applied to aqueous nonprotein solutions (where A tot ¼
0mEq/L and SID ¼ [HCO 3 ]). 5,7 The simplified strong
þ
½
½
½
½
SIG þ Na þ K½ þ Cl HCO 3 A ¼ 0
ion model also explains many of the anomalies of the
Henderson-Hasselbalch equation. It explains why the
apparent value for pK 1 in plasma is dependent on pH, pro- because
0
tein concentration, and sodium concentration and also
þ þ Cl HCO 3 ,
½
provides a mechanistic explanation for the temperature AG ¼ Na þ K½ ½ ½
5
dependence of plasma pH. The simplified strong ion SIG þ AG A ½ ¼ 0
model shares two of the disadvantages of Stewarts strong
ion model: (1) difficulty in accurately determining SID, or
and (2) mathematical complexity when compared with
the traditional Henderson-Hasselbalch equation. It is SIG ¼ A ½ AG
unlikely that the simplified strong ion approach will replace
the traditional Henderson-Hasselbalch approach clinically Based on the relationship above, the SIG has been
and in descriptive experimental studies because two (pH simplified (SIG simplified ) so as to allow an estimation based
and PCO 2 )of the three (pH, PCO 2 , and [HCO 3 ]) on [A tot ] (the sum of [A ] and its weak acid pair [HA])
6
unknowns in the Henderson-Hasselbalch equation can and AG. Albumin is used to estimate [A tot ] in the
be measured accurately and easily in plasma, whereas only SIG simplified because albumin is the most important buffer
two (pH and PCO 2 ) of the four unknowns in the simplified in plasma. At a normal plasma pH of 7.4, SIG simplified can
strong ion approach (pH, PCO 2 ,SID, and A tot )can be be calculated from the albumin concentration in g/dL
measured easily and accurately. However, in mechanistic in dogs as: 12
experimentalstudies,thesimplifiedstrongionmodelispre-
ferred because it conveys on a fundamental level the SIG simplified ¼ alb 4:9Þ AG
ð
½
mechanisms underlying acid-base disturbances. 5,7
In cats, at a normal plasma pH of 7.35, SIG simplified can be
STRONG ION GAP calculated from the albumin concentration in g/dL as: 37
The SIG concept is a modification of the simplified strong
ion equation that overcomes one of the limitations of this SIG simplified ¼ alb 7:4Þ AG
ð
½
model, namely, algebraic complexity. SIG is the difference
in charge between all unmeasured strong anions and all An increase in unmeasured strong anions is suspected
6
unmeasured strong cations. Because there are more whenever SIG simplified is less than 5 mEq/L. In patients
strong cations than strong anions, normal SIG is positive: with hyperphosphatemia, however, AG should be
corrected for the presence of hyperphosphatemia
þ
SIG ¼½UC strong ½UA strong {AG phosphate-adjusted ¼ AG þ (2.52 – 0.58 [Phosphate])}
before calculating SIG simplified . The SIG simplified offers a
where [UC strong ] is the sum of all unmeasured strong more accurate approach to identifying unmeasured
þ
cations (e.g., ionized calcium, ionized magnesium), strong ions in plasma than does the AG. The critical
