Page 24 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

14 APPLIED PHYSIOLOGY

to 27 mEq/L in cats (see Chapter 9). The average anion gap vary with the formula used to calculate osmolality.
gap in dogs is 18 to 19 mEq/L 11 and in cats is approxi- Numerous formulas have been derived to calculate serum
mately 24 mEq/L. 32 The higher average anion gap in osmolality (see earlier section on exchange of water
cats suggests a higher net charge on proteins in this spe- between ICF and ECF spaces).
cies. Younger animals may have a lower anion gap. Little One of the most commonly used formulas to estimate
information on variations in anion gap in pediatric small osmolality is:
animal patients is available. In 3-day-old puppies, how-
ever, anion gap values were reported to be approximately þ þ glucose BUN
16 mEq/L in one study, 37 suggesting that anion gaps in 2Na þ Kð Þ þ 18 þ 2:8
neonatal puppies are within the reference ranges for
adults. Some laboratories report a calculated osmolality based
The primary usefulness of the anion gap is to detect an on these various formulas because it is easy to program
increase in UAs as an aid in the diagnosis of metabolic aci- the analyzer to perform the calculation. These are
dosis. Clinically relevant changes in the anion gap usually estimates of the actual osmolality, which must be
are from changes in UAs, and most of these changes are measured using an osmometer. Serum osmolality most
caused by increases in UAs associated with organic acids. frequently is measured by freezing-point depression.
For example, the ketoacidosis that occurs in some dia- Measured osmolality is higher than calculated osmolality
betic patients causes an increase in UAs, resulting in an because Osm m measures all osmotically active solutes,
increase in the anion gap. Similarly, the increased UAs whereas the formulas used for Osm c do not account for
that occur with ethylene glycol intoxication result in an all osmotically active solutes in serum. The difference
increased anion gap. The derivation and clinical applica- (gap) between the measured (actual) and calculated
tion of the principle of the anion gap are discussed further (estimated) osmolality is called the osmolal gap.
in Chapters 9 and 10. Calculation of the osmolal gap is most helpful when
unsuspected osmoles are present in ECF, thus increasing
THE OSMOLAL GAP the osmolal gap as a result of an increase in the measured
but not the calculated osmolality (e.g., ethylene glycol
The osmolal gap is defined as the difference between the poisoning), and when assessing the significance of the
þ
measured and the calculated serum osmolalities: serum Na concentration (see Chapter 3). During the
acute stage (6 to 12 hours after exposure) of ethylene gly-
col toxicity, the osmolal gap is increased. This increased
Osmolal gap ¼ Osm m Osm c
osmolal gap could be helpful in the diagnosis of ethylene
Reference values for osmolal gaps in dogs are given in glycol toxicity if a measured osmolality is requested.
Table 1-5. Data for osmolal gaps in cats are not reported Hyponatremia with a normal osmolal gap suggests
in the literature. Attempts to derive osmolal gaps from dilutional hyponatremia (e.g., overhydration). This rules
published data on measured serum osmolalities and elec- out the presence of abnormal osmotically active particles
trolyte concentrations in cats have yielded confusing that could cause a shift of water from ICF to ECF, thus
results (see footnote to Table 1-5). Values for the osmolal decreasing the serum sodium concentration. The osmolal
gap is discussed further in Chapter 3.

TABLE 1-5 Reference Ranges for HOMEOSTASIS: ZERO
Osmolal Gap BALANCE
Species Osmolal Gap (mOsm/kg) Reference In the healthy adult animal at rest in a thermoneutral
environment, the daily intake of water, nutrients, and
Dog 10 6 Grauer 15 minerals is balanced by daily excretion of these substances
Dog 10.1 5.9 Hauptman 20 or their metabolic by-products. Thus, in this homeostatic
Dog 0-10 Shull 53 state, the animal does not experience a net gain or loss of
Dog 5 6 Burkitt 7 water, nutrients, or minerals and is said to be in zero bal-
ance. In a sedentary dog or cat in a thermoneutral
Serum osmolality values in normal cats were reported to be approximately
8
308 5 mOsm/kg (Chew et al ). When mean values for serum Na environment, obligatory daily losses of water occur
(155 mEq/L), K (4 mEq/L), glucose (120 mg/dL), and blood urea (Fig. 1-7). Input is equal to output in zero balance,
nitrogen (BUN; 24 mg/dL) are substituted into the equation and the volume of water added to body fluids by food
2(Na þ K) þ glucose/18 þ BUN/2.8, a value of 333 mOsm/kg is and water consumption and by metabolism is equal to
obtained for cats. Calculated plasma osmolality values greater than the volume of water lost in urine, feces, and saliva (i.e.,
measured values have generally been attributed to laboratory error.
Why calculated plasma osmolality exceeds measured plasma osmolality sensible water loss) and evaporation from cutaneous
31
using mean values from normal cats is unclear. and respiratory epithelia (i.e., insensible water loss).