Page 327 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

318 ACID-BASE DISORDERS

TABLE 13-1 Diagnostic Approaches to Acid-Base Disturbances*
Diagnostic Type of Abnormal
Approach Parameters Measured Disorder Parameter Acidosis Alkalosis

Routine Total CO 2 Metabolic Abnormal total #total CO 2 "total CO 2
screening CO 2
Henderson- pH, PCO 2 ; calculate BE ECF { Respiratory Abnormal PCO 2 "PCO 2 #PCO 2
Hasselbalch Metabolic Abnormal #BE ECF "BE ECF
(preferred) or (preferred) or
BE ECF

#HCO 3 "HCO 3
Simplified pH, PCO 2 ,Na ,K ,Cl , lactate, Respiratory Abnormal PCO 2 "PCO 2 #PCO 2
þ

þ
{
strong ion albumin and inorganic phosphate Metabolic Abnormal #SID þ "SID þ
(SID) SID *
Metabolic Abnormal A tot " [phosphate] # [albumin]
(A tot )
Adapted from Constable PD. Clinical assessment of acid-base status: comparison of the Henderson-Hasselbalch and strong ion approaches. Vet Clin Pathol
2000;29:115–128.
*BE ECF indicates extracellular base excess; SID, strong ion difference; A tot , total plasma concentration of nonvolatile weak buffers.
{ þ þ þ
The anion gap is calculated to determine whether unmeasured anions are present (requires measurement of three other parameters: Na ,K ,Cl ).
{
The strong ion gap is calculated to determine if unmeasured strong ions are present.

DISORDERS OF PCO
Strong cations cations - anions Strong anions 2
Na Cl IncreasesinPCO 2 areassociatedwith respiratoryacidosis,
K Lactate whereas decreases in PCO 2 lead to respiratory alkalosis.
Ca 2 Ketoanions There are no differences between the Henderson-
Mg 2 SO 4 2 Hasselbalch approach and the strong ion approach in
relation to PCO 2 . See Chapter 11 for further discussion
SID
of respiratory acid-base disorders.
Acidosis PCO 2 Alkalosis
DISORDERS OF [A TOT ]
Atot Albumin, globulins, and inorganic phosphate are nonvol-
Albumin atile weak acids and collectively are the major
Globulin contributors to [A tot ]. Consequently, changes in their
Phosphate concentrations will directly change pH, and this
Figure 13-3 Mechanisms leading to alkalosis and acidosis. represents a major philosophical difference between the
strong ion and Henderson-Hasselbalch approaches.
There are two general mechanisms by which A tot can
Because clinically important acid-base derangements change: (1) an increase in plasma albumin, phosphate,
result from changes in PCO 2 , SID, or A tot , the strong or globulin concentrations; and (2) a decrease in plasma
ion approach distinguishes six primary acid-base albumin, phosphate, or globulin concentrations.
disturbances (respiratory, strong ion, or nonvolatile Changes in plasma albumin, phosphate, or globulin
buffer ion acidosis and alkalosis; Figure 13-3) instead concentrations can occur in response to a change in the
of the traditional four primary acid-base disturbances distribution volume for the three factors (denominator
(respiratory or metabolic acidosis and alkalosis) effect; clinically equivalent to changes in plasma free water
characterized by the Henderson-Hasselbalch equa- for albumin and globulin, and changes in extracellular
tion. 5,7,8,13 Acidemia (decrease in plasma pH) results fluid volume for phosphate). Changes in plasma albumin,
from an increase in PCO 2 and nonvolatile buffer phosphate, or globulin concentrations and therefore the
concentrations (albumin, globulin, phosphate) or from value for A tot can also occur due to increases or decreases
a decrease in SID. Alkalemia (increase in plasma pH) in the total number of moles in plasma or extracellular
results from a decrease in PCO 2 and nonvolatile buffer fluidwithnochangeinthedistributionvolume(numerator
concentration or from an increase in SID. effect).