Page 21 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

Applied Physiology of Body Fluids in Dogs and Cats 11

One of the most commonly used formulas for calculat- Therefore:
ing the osmolality of serum is: 48
2½Na þŠ þ 2½K þŠ
½glucoseŠ ½BUNŠ þ e e
½
þ þ 2 plasma Na Š
ð
ð
½
ECF osmolality mOsm=kgÞ ¼ 2Na Š þ K ŠÞ þ þ
½
18 2:8 TBW
and
In all formulas, Na and K are measured in millimoles
þ
þ
perliterormilliequivalentsperliter.Inthisformula,thecon-
2½Na þŠ þ 2½K þŠ e
e
tribution of Cl and HCO 3 is estimated by multiplying the plasma Na Š
þ
½
major cations by 2, assuming the serum must remain elec- TBW
trically neutral. The concentrations of glucose and blood This relationship is represented graphically in Figure
urea nitrogen (BUN) are divided by 18 and 2.8, respec- 14,49
1-4. Examination of Figure 1-4 shows that when
tively, to convert milligram per deciliter to millimoles
þ
total exchangeable Na increases, serum sodium concen-
per liter (The molecular weight of glucose is 180 and the 49
tration also increases, and these changes are usually
molecular weight of urea is 28, and there are 10 dL/L).
associated with body fluid hypertonicity. A decrease in
Several other formulas have been suggested for estima-
total exchangeable Na þ or K þ is associated with
tion (calculated osmolality) of the true serum osmolality
hyponatremia, a decrease in plasma osmolality, and hypo-
(measured osmolality). These formulas vary based on
tonicity. The effect of a decrease in total exchangeable K þ
which major solutes are included and whether constants
on serum [Na ] is not intuitively obvious but is clinically
þ
are added to estimate the effects of other solutes. Includ- 49
important. A decrease in serum [K ] results in a shift of
þ
þ
ing K is a more accurate estimate of measured osmolal-
þ
þ
K out of cells. To maintain electroneutrality, Na shifts
þ
ity. Remember, in all formulas, Na and K are measured
þ
into cells, thus causing hyponatremia.
in millimoles per liter or milliequivalents per liter. If
Serum (and therefore ECF) osmolality in dogs is
glucose and BUN are measured in milligrams per decili-
approximately 300 mOsm/kg, and fluids with effective
ter, the conversion factor is included in the formula. If
osmolalities greater than 300 mOsm/kg are hypertonic
glucose and BUN are measured in millimoles per liter,
to plasma, whereas those with effective osmolalities less
delete the conversion factor (see later discussion). Alter-
than 300 mOsm/kg are hypotonic to plasma. Those with
nate formulas are listed in the second edition of this book.
effective osmolalities of 300 mOsm/kg are isotonic to
Not all potentially osmotic substances are osmotically plasma. In health, addition or loss of fluid or solute to
active in body fluids. Cell membranes are permeable to or from the body results in alterations in body fluid space
urea and K ; therefore, these solutes are ineffective volumes and tonicity. These alterations elicit homeostatic
þ
osmoles. Effective osmolality (tonicity) is calculated as: 48
shifts of fluid between compartments so that fluid spaces
return to isotonicity (see Chapter 3).
½glucoseŠ In most disease states, fluid and solutes initially are lost
þ
Effective ECF osmolality ¼ 2 Na þ
18 from the ECF. Three basic types of fluid and solute loss
may occur: solute in excess of water (loss of hypertonic
In healthy dogs and cats, the contribution of glucose fluids), isotonic loss (loss of isotonic fluids), or water in
to the effective osmolality of the ECF is small (about 4 excessofsolute(lossofhypotonicfluids)(Table1-4). Sol-
28
to 6 mOsm/kg) based on blood glucose concentrations ute and water losses theoretically may occur in any propor-
þ
of 70 to 110 mg/dL. Therefore, 2 [Na ] is a good tion along the continuum between solute loss with no
approximation of the ECF effective osmolality. waterloss (e.g., peritonealdialysiswith a salt-poor solution)
All body fluid spaces are isotonic with one another. and water loss with no solute loss (e.g., water deprivation).
Thus, the effective osmolality of the ICF also may be When solute is lost in excess of water (hypertonic fluid
estimated by doubling the ECF Na þ concentration, loss), the osmolality of the ECF decreases relative to that
[Na ],eventhoughtheNa concentrationinICFissmall. of the ICF. This could be seen in oozing of serum from
þ
þ
Because all body fluid spaces are isotonic, the tonicity of the skin of burn patients, which occurs much more
total body water also may be approximated by doubling commonly in human medicine than in veterinary
þ
the plasma [Na ]. The tonicity of total body water also medicine. Water passes from the ECF through the cell
maybeexpressedastheratioofthesumofallexchangeable membrane to the ICF, thus diluting the ICF solute until
cations and all exchangeable anions to the volume of total the effective osmolalities of ECF and ICF are again equal.
body water. Exchangeable ions (denoted by the subscript The osmolalities of both ICF and ECF decrease. This
letter “e”) are able to move throughout the fluid compart- homeostatic fluid shift decreases ECF volume. When
ment. The total number of milliosmoles of exchangeable hypertonic fluid is lost from the ECF and volume deple-
cations and anions may be estimated from the expression: tion occurs, homeostatic water shifts further compromise
the ECF volume and effective circulating blood volume,
þ þ
2½Na Š þ 2½K Š thus compounding fluid losses.
e e