Page 325 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

CHAPTER • 13

Strong Ion Approach to Acid-Base

Disorders

Helio Autran de Morais and Peter D. Constable

“Assumptions can be dangerous, especially in science. They usually start as the most plausible or comfortable
interpretation of the available facts. But when their truth cannot be immediately tested and their flaws are not
obvious, assumptions often graduate to articles of faith, and new observations are forced to fit them. Eventually,
if the volume of troublesome information becomes unsustainable, the orthodoxy must collapse.”
John S. Mattick, Scientific American, October 2004

Determination of the mechanisms underlying acid-base PCO 2
disturbances has been an important clinical goal for more Carbon dioxide tension can be changed by alveolar ven-

than 100 years. Landmark advancements in the clinical tilation, which has a profound effect on [HCO 3 ] and
diagnosis and treatment of acid-base disturbances have pH. Approximately 50% of the daily variability of

included the Henderson-Hasselbalch equation (1916), [HCO 3 ] in normal dogs can be attributed to changes
the base excess (BE) concept (1960), calculation of in PCO 2 alone. The dependent nature of the relationship
the anion gap (AG) (1970s), 41 introduction of the between [HCO 3 ] and PCO 2 is best appreciated by

strong ion approach, 50 and the simplified strong ion rearrangement of the Henderson-Hasselbalch equation,
approach, 5 and development of the strong ion gap whereby:
(SIG) concept. 6,30,32
0
The two main goals of acid-base assessment are to ½ HCO 3 Š ¼ S PCO 2 10 ð pH pK a Þ

identify and quantify the magnitude of an acid-base dis-
turbance and to determine the mechanism for the acid- with pK a being the negative logarithm of the apparent
0
base disturbance by identifying changes in variables that dissociation constant for carbonic acid (H 2 CO 3 )in
independently alter acid-base balance. 5,14 Independent
variables influence a system from the outside and cannot
be affected by changes within the system or by changes in
other independent variables. In contrast, dependent pH
variables are influenced directly and predictably by
changes in the independent variables. Singer and
Hastings proposed in 1948 47 that plasma pH was deter- Respiratory Metabolic
mined by two independent factors, PCO 2 and net strong
ion charge, equivalent to the strong ion difference (SID, P SID A tot
or the difference in charge between fully dissociated CO 2

strong cations and anions in plasma). Stewart suggested Na K Albumin
in 1983 that a third variable, [A tot ] or the total plasma Ca 2 Mg 2 Globulin
concentration of nonvolatile weak buffers (e.g., albumin, Cl Phosphate
Lactate
globulins, and phosphate), also exerted an independent Ketoacids
effect on plasma pH. 50 One of Stewart’s major 2
SO 4
contributions to clinical acid-base physiology was his pro- Figure 13-1 Determinants of plasma pH at 37 C as assessed by

posal that plasma pH was determined by three indepen- the simplified strong ion model. Both [SID ] and [A tot ] provide
þ
dent factors: PCO 2 , SID, and [A tot ](Figure 13-1). An independent measures of the nonrespiratory (metabolic)
understanding of the three independent variables component of plasma pH. (From Stämpfli HR, Constable PD.
(PCO 2 , SID, A tot ) is required to apply the strong ion Experimental determination of net protein charge and A tot and K a of
approach to acid-base disorders in dogs and cats. nonvolatile buffers in human plasma. J Appl Physiol 2003;95:620–630.)

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