Page 389 - Clinical Small Animal Internal Medicine

36 Fluid Therapy 357

composition. Both humans and dogs have been shown to osmoles, the majority of equilibration will occur by the
VetBooks.ir have a decrease in total body water with age. Body com- movement of water from the ICF to the ECF. This results
in a decrease of the ICF volume but a bolstering of the
position also plays a role and it is useful to base fluid
therapy recommendations on lean body mass as opposed
ria seen with renal disease. When isotonic fluid is lost,
to body weight. In the normally fleshed animal, the lean ECF. An example of hypotonic fluid loss would be polyu-
body mass is approximately 80% of total body weight. In there is no change in the tonicity between the ECF and
obese patients, fat (which has a lower water composition ICF. For this reason, there is no net fluid movement
than other tissues) is abundant, so lean body mass can be between fluid compartments and the volume lost results
estimated as 70% of body weight. By contrast, in the in an absolute decrease in the volume of the ECF. An
underconditioned animal, measured body weight is example of isotonic fluid loss would be acute hemor-
equivalent to the lean body weight. rhage. Finally, in the case of hypertonic fluid loss from
Total body water is further divided into body fluid the ECF, the ECF becomes hypotonic relative to the ICF
compartments. The largest portion of total body water is and water moves from the ECF into the ICF, which fur-
found within the cells, with the intracellular fluid (ICF) ther exacerbates the volume loss. It is important to rec-
compartment accounting for roughly two‐thirds of total ognize that an animal can have hypovolemia without
body water. The remaining one‐third is found in the dehydration but dehydration cannot occur without some
extracellular fluid (ECF) compartment. The ECF com- degree of hypovolemia.
partment is further divided into the interstitial space,
which contains roughly 75% of the water of the ECF, and
the vascular space, which contains the remaining 25% as Water Movement
plasma volume. While it is useful to describe total body
water based on its location, it is important to remember Movement of fluid between body fluid spaces is governed
that fluid balance is dynamic and movement between by Starling’s law.
fluid compartments is occurring constantly. Fluid fluxk f P c P i c i
Body fluids contain varying concentrations of solutes
in addition to water. The ECF and ICF have vastly differ- In this equation, k f is a filtration coefficient unique to
ent solute concentrations but the total numbers of each tissue bed that reflects the relative permeability of
cations and anions in all body fluids must be equal to the capillary wall within that tissue bed, P c refers to the
maintain electroneutrality. In the ECF, sodium is the hydrostatic pressure within the capillary and P i refers to
major cation and chloride and bicarbonate are the most the hydrostatic pressure within the interstitium.
common anions. By contrast, within the cell, the most Similarly, π c and π i refer to the oncotic pressure in the
common cations are potassium and magnesium and the capillary and interstitium respectively. Recall that
most common anions are phosphates and proteins. Body oncotic pressure differs from osmolality/osmolarity and
fluid compartments are dynamic and the relative volume refers to the force exerted by large, impermeant mole-
within the fluid compartments is dependent on the num- cules to hold water within either the capillary or intersti-
ber of osmotically active particles present within the tial space. Clinically, oncotic pressure is referred to as
space. The normal plasma osmolality (tonicity) is colloid osmotic or colloid oncotic pressure (COP). The
300 mOsm/kg in the dog and 310 mOsm/kg in the cat. balance between these opposing forces determines the
Approximately 95% of total serum osmolality in the direction and degree of net fluid movement.
normal animal results from the presence of sodium, When fluids are administered too aggressively, the
potassium, chloride, bicarbonate, urea, and glucose intravascular space is overexpanded. This results in an
while large molecules such as albumin have little effect increase in hydrostatic pressure within the capillaries
on osmolality. that exceeds the oncotic pressure in the vessels and
The body must balance daily fluid losses with intake. favors the movement of fluid into the interstitium, which
In addition to pathologic fluid losses, all animals have is composed of a meshwork of collagen and glycosami-
insensible fluid losses via the urinary tract, gastrointesti- noglycans as well as proteins and electrolytes in solution.
nal tract, and respiratory system. When fluid and solutes There are three mechanisms, known as tissue safety fac-
are lost from the body, the relative effect on the different tors, that prevent fluid accumulation within the inter-
body compartments is dependent on the tonicity of the stitium and their relative importance varies between
fluid lost. Lost fluid can be hypertonic to plasma, iso- tissue types. As fluid moves in, the hydrostatic pressure
tonic, or hypotonic. Usually fluid is lost from the ECF. rises rapidly due to the nondistensible nature of the
When hypotonic fluid is lost from the ECF, the ECF interstitium and opposes further fluid accumulation.
becomes hypertonic relative to the ICF. Because the Opposition to the movement of fluid into the interstit-
solutes (sodium, potassium, and chloride) are effective ium also occurs as interstitial oncotic pressure drops due