Page 326 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

Strong Ion Approach to Acid-Base Disorders 317

plasma, and S being the solubility of CO 2 in plasma. 180
Because plasma pH is vigorously defended by animals 160
to maintain a homeostatic environment, and because 140 Other SC HCO 3
values for pK a and S are fixed at constant temperature A SID
0

and ionic strength, the dependence of [HCO 3 ]on 120 AG Other SA
PCO 2 is clearly appreciated. 100
MeasurementofarterialPCO 2 (PaCO 2 )providestheclini- Ionic strength (mEq/L) 80 Strong cations Na
cian with direct information about the adequacy of alveolar 60 Cl
ventilation because PaCO 2 is inversely proportional to the Strong anions
alveolar ventilation. Increase in PaCO 2 or respiratory acido- 40
sis is caused by and synonymous with hypoventilation, 20
whereasadecreaseinPaCO 2 orrespiratoryalkalosisiscaused 0
by and synonymous with hyperventilation. Cations Anions
Figure 13-2 Gamblegram of normal plasma showing cations:
þ
þ
SID sodium (Na ), and other strong cations (SC ) in one column, and

Simple ions in plasma can be divided into two main types: anions: chloride (Cl ), other strong anions (SA ), net charge of
nonvolatile buffers (A ), and bicarbonate (HCO 3 ) in the

nonbuffer ions (strong ions or strong electrolytes) and second column.
buffer ions. Strong ions are fully dissociated at physio-
logic pH and therefore exert no buffering effect. How- (e.g., phosphate, imidazole [histidine] groups on plasma
ever, strong ions do exert an electrical effect because proteins). Also known as the non-HCO 3 buffer system,

the sum of completely dissociated cations does not equal they form a closed system containing a fixed quantity of
the sum of completely dissociated anions. Stewart termed buffer. The non-HCO 3 buffer system consists of a
this difference the SID. 50 Because strong ions do not par- diverse and heterogeneous group of plasma buffers that
ticipate in chemical reactions in plasma at physiologic pH, can be modeled as a single buffer pair (HA and A ).

they act as a collective positive unit of charge (SID). 5,50 An assumption in Stewart’s strong ion model is that
The quantitatively most important strong ions in plasma HA and A do not take part in plasma reactions that result

þ
þ
are Na ,K ,Ca ,Mg ,Cl , lactate,ß-hydroxybutyrate, in the net destruction or creation of HA or A . When HA

2þ
2þ

2
acetoacetate, and SO 4 . The influence of strong ions on dissociates, it ceases to be HA (therefore decreasing

pH and [HCO 3 ] can always be summarized in terms of plasma [HA]) and becomes A (therefore increasing
the SID. Changes in SID of a magnitude capable ofaltering plasma [A ]). The total amount of A, or A tot , is the

acid-base balance usually occur as a result of increasing sum of A in dissociated [A ] and undissociated [HA]

þ 2
concentrations of Na ,Cl ,SO 4 , or organic anions or forms, which remain constant according to the law of
decreasing concentrations of Na or Cl .An increasein conservation of mass. 5,50
þ

SID (by decreasing [Cl ]or increasing[Na ]) will cause The great advantage of Stewart’s strong ion approach

þ
a strong ion (metabolic) alkalosis, whereas a decrease in is that it provides a mechanistic view as to why pH is
þ 2
SID (by decreasing [Na ] or increasing [Cl ], [SO 4 ], changing and fully integrates electrolyte and acid-base
ororganicanions)willcauseastrongion(metabolic)acido- physiology. However, his approach is heavily mathemati-
sis. 5,50 A Gamblegram of normal plasma (Figure 13-2) cal and states that pH is a function of eight factors 50 (for
shows the relationship between strong cations, strong comparison, the Henderson-Hasselbalch equation is

anions, and buffer ions (HCO 3 and the nonvolatile weak mathematically simpler, stating that pH is a function of

acids, A ).A graphicrepresentation ofSID and the AG also four factors, whereas Constable’s simplified strong ion
is shown in Figure 13-2. equation states that pH is a function of six factors). With
5
that in mind, this chapter will focus on the concepts
[A tot ] behind the strong ion approach, emphasizing the
In contrast to strong ions, buffer ions are derived from relationship between weak and strong ions and acid-base
plasma weak acids and bases that are not fully dissociated balance and developing an understanding of why pH and
at physiologic pH. The conventional dissociation reaction [HCO 3 ] are changing. Frameworks adapting the strong

for a weak acid (HA) and its conjugated base (A ) pair is ion approach to clinical uses also will be reviewed. The
HA $ H þ A . At equilibrium, an apparent dissociation mathematical and physicochemical background of this
þ

constant (K a ) can be calculated. 5,50 For a weak acid to act approach is described in detail elsewhere.* A comparison
as an effective buffer, its pK a (defined as the negative log- of diagnostic approaches using routine screening (total
arithm of the weak acid dissociation constant K a ) lies CO 2 ), the Henderson-Hasselbalch approach, and the
within the range of pH 1.5. Because normal plasma simplified strong ion approach is shown in Table 13-1.
pH is approximately 7.4, substances with a pK a between
5.9 and 8.9 can act as buffers. The main nonvolatile
plasma buffers act as weak acids at physiologic pH *References 5, 7, 15, 48, 50, 55.