Page 406 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

396 FLUID THERAPY

compliance of the tissue. With small increases in intersti- This finding may account for some of the clinical signs
tial fluid volume, a large increase in hydrostatic pressure observed, including depression, vomiting, cardiac
occurs, resisting additional filtration across the capillary. arrhythmias, coagulopathy, and oliguria (see later
However, with large increases in intravascular hydrostatic discussion).
pressure and further capillary filtration, the interstitial vol-
ume will increase. When the colloid osmotic gradient fails Pulmonary Edema
and the lymphatic system is overloaded, the tissue safety Pulmonary interstitial fluid dynamics differ from those of
factors are overcome with little resistance to further fluid other tissues. Pulmonary capillary pressure is lower
loss. When the interstitial pressure reaches 0 mm Hg, the (approximately 7 mm Hg); interstitial fluid pressure in
compliance of the tissue is increased, facilitating larger the lung is more negative ( 8to 5 mm Hg) than that
volumes of fluid to accumulate in the tissues with little of peripheral subcutaneous tissue; and the pulmonary
change in interstitial hydrostatic pressure. This effect is capillaries are relatively permeable to protein molecules,
called stress relaxation. The proteoglycan meshwork rendering the COP of the pulmonary interstitial fluid
within the interstitium is disrupted as the increased vol- approximately 14 mm Hg. These differences favor fluid
ume of fluid pushes the brush pile of proteoglycans apart, movement from the alveoli into the interstitium and
allowing the fluid to flow freely through the tissues. 27 lymphatics. 25 Pulmonary edema occurs in the same man-
When this occurs, pitting edema is detected by pressing ner as does edema elsewhere in the body. Therefore the
on an area of skin and noting pitting for several seconds conditions discussed above can result in pulmonary
until the fluid flows back into the area. Edema also may edema after an excessive volume of crystalloid, and poten-
occur in patients with moderate to severe capillary leak tially colloid, is administered. As pulmonary interstitial
after administration of moderate volumes of crystalloid pressure increases into the positive pressure range and
solutions. Assessing interstitial tissue edema is an essential the lymphatics are unable to remove this fluid, it leaks into
component of monitoring during fluid administration. the alveolar space. In the absence of capillary leak
Three body regions that are useful to evaluate are the hock disorders, when the pulmonary capillary pressure exceeds
because nonedematous animals, regardless of the amount 25 mm Hg (approximately 18 mm Hg above normal) in
of body fat, have well-defined lateral saphenous veins, normal dogs, fluid accumulates in the lungs. Experiments
Achilles tendons, and bony prominences; the mandibles performed on dogs showed that pulmonary capillary
and intermandibular space because these areas also are pressure must increase to a value at least equal to the
well defined in most animals; and the movement of the COP of the plasma inside the capillaries before clinically
skin and subcutaneous tissues over the torso. With the relevant pulmonary edema occurs. 25 The COP of normal
development of interstitial edema, these anatomic regions dogs is 19.95 2.1 (range, 15.3 to 26.3) mm Hg, and
become less defined, and a “jelly-like” appearance of the measuring COP in addition to evaluating physical
skin develops. If these findings are generalized, findings helps guide fluid management. This information
overhydration resulting from excessive fluid administra- applies to normal dogs and not dogs with capillary leak
tion or capillary leak can be assumed. These regions conditions or those that are hypoproteinemic. When
should always be examined for baseline assessment before COP is decreased, as in patients with hypoproteinemia,
fluid administration. Chemosis also may occur with edema formation may occur even at lower hydrostatic
overhydration, but this finding tends to occur later than pressures. In experimental models, edema begins to form
those previously mentioned. Bandages placed around at 11 mm Hg when COP is decreased. 29
the neck to secure a catheter into the jugular vein may Monitoring physical signs (see Table 16-2) can be
cause edema of the head and chemosis unassociated with effective in assessing potential fluid overload. The author
overhydration. Likewise, edema of a distal limb may occur has observed shivering, restlessness, nausea (as indicated
if it is the dependent limb or a bandage is placed above the by swallowing and licking the lips), and rarely vomiting in
hock or carpus. Although body weight did not change in some animals as a response to an excessive rate of infusion
the majority of animals treated for dehydration during a of a balanced electrolyte solution. These signs stopped
24- to 48-hour period, 41 body weight should be moni- within 1 or 2 minutes after discontinuing or reducing
tored because an increase above that calculated to treat the fluid rate for a short period, and the observed behav-
dehydration may indicate fluid overload if confirmed by ior was reproduced when a high rate of fluid administra-
other physical findings. Fluid losses into third spaces will tion was reestablished. These animals did not have
increase body weight without improvement in overall identifiable cardiac disease. While monitoring CVP in
fluid repletion. one animal with oliguric renal failure that demonstrated
When edema is noted in subcutaneous tissues, it is this behavior, CVP rapidly increased from 1 cm H 2 Oto
likely that a similar degree of edema also exists in body 11 cm H 2 O. The fluids were discontinued, but when
organs. It has been my observation at necropsy that restarted, the same behavior occurred when CVP reached
edema of the brain, gastrointestinal tract, heart, 10 to 11 cm H 2 O again. The fluid rate in this instance was
liver, and kidney coexists with subcutaneous edema. reestablished at a more reasonable level for this dog. The