Page 557 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

Managing Fluid and Electrolyte Disorders in Renal Failure 545

for the specific patient, repeated and frequent reassessment changes in trends for PCV and total solids may reflect
of fluidand electrolytebalance,withappropriatechangesin changes in volume, in the absence of bleeding or blood
the treatment planinresponse tothe rapidlychangingclini- transfusion. Because each parameter is impacted by
cal situation of the kidney failure patient. aspects beyond hydration status, these factors must be
viewed in aggregate.
Assessing Hydration
Route of Fluid Administration
The key feature to an appropriate fluid plan is accurate
determination of hydration status. Blood volume can be In most hospitalized patients, the intravenous route is the
measured using indicator dilution techniques, radioactive most appropriate route of administration. In some
tracers, bioimpedance spectroscopy, or other methods. 54 situations, such as extremely small patients, including
Unfortunately, readily available accurate measurement neonates or very young puppies or kittens, IV catheteri-
of blood volume is not feasible in general practice settings. zation may be difficult. Intraosseous fluid administration
Despite a lack of precise objective data, there are many can be used in that setting. In dehydrated patients, fluids
ways to estimate hydration. A deficit of the extravascular administered into the peritoneal cavity will be readily
fluid compartment (interstitial and intracellular) causes absorbed, but this method is not reliable for promoting
dehydration. A severe deficit may decrease the intravascu- diuresis or in oliguric patients. Fluid administered subcu-
lar compartment, leading to poor perfusion. Dehydration taneously may not be absorbed rapidly or completely, and
of less than approximately 5% is difficult to detect it is not possible to administer a large volume by this
clinically. A 5% to 6% deficit leads to sticky mucous route, making subcutaneous fluid inappropriate for the
membranes. Six to eight percent dehydration causes hospital setting. It may play a role in outpatient therapy
dry mucous membranes and decreased skin elasticity. (see later discussion).
By 8% to 10% dehydration, the eyes may be sunken,
and over 12% dehydration, corneas are dry, mentation Type of Fluid
is dull, and perfusion is impaired. 28 Overhydration may A balanced polyionic solution (i.e., lactated Ringer’s solu-
manifest as wet mucous membranes, increased skin elas- tion [LRS], Plasmalyte 148, Normosol-R) is an appropri-
ticity (heavy or gelatinous), shivering, nausea, vomiting, ate choice for the initial volume resuscitation fluid and
restlessness, serous nasal discharge, chemosis, tachypnea, replacement of the dehydration deficits. Physiologic
cough, dyspnea, pulmonary crackles and edema, pleural (0.9%) NaCl contains no potassium and is a suitable initial
effusion, ascites, diarrhea, or subcutaneous edema (espe- choice for the hyperkalemic patient.
cially hock joints and intermandibular space). 13,36 After rehydration, maintenance fluids with a lower
Difficulties exist in interpreting these physical findings. sodium concentration are more appropriate (i.e., 0.45%
Uremic patients frequently have xerostomia, causing dry NaCl with 2.5% dextrose, one half strength LRS with
mucous membranes independent of hydration status. 2.5% dextrose).
Hypoalbuminemia or vasculitis may cause interstitial fluid Dextrose 5% in water (D 5 W) is rarely appropriate as a
accumulation despite an intravascular volume deficit. sole fluid choice, but may be combined with LRS or 0.9%
Emaciation or advanced age decrease elasticity of the skin. saline to make one half or three fourths strength sodium
Central venous pressure (CVP) measurement through solutions (25 mL LRS þ 25 mL D 5 W ¼ 50 mL 1=2
a centrally placed intravenous catheter may provide infor- strength LRS þ 2.5% dextrose).
mation about intravascular filling. A volume depleted ani- Colloidal solutions (i.e., hydroxyethyl starch, 6% dex-
mal will have a CVP less than 0 cm H 2 0. A CVP over tran) may be appropriate if hypoalbuminemia is present.
10 cm H 2 0 is consistent with volume overload or right- Hypoalbuminemia may be present with protein-losing
sided congestive heart failure. 62 However, pleural effu- nephropathy, diseases associated with vasculitis, or severe
22
sion falsely elevates the CVP. An accurate body weight gastrointestinal losses or bleeding. The recommended
recorded before an illness is an invaluable aid to assessing dose is 20 mL/kg/day, and may be used to replace the
hydration. Body weight should be measured two to four insensible portion when using the “ins-and-outs”
times a day on the same scale to monitor fluid balance. method (see later discussion). Higher doses may be
A sick animal may lose up to 0.5% to 1% of body weight associated with coagulopathy. Despite initial concerns
per day due to anorexia; changes in excess of this amount that colloidal solutions may cause acute kidney injury
are due to changes in fluid status. 10 An increase in blood (specifically, osmotic injury), there is no evidence that
pressure may indicate a gain of fluid; conversely, colloids are directly nephrotoxic. 46 An alternative to syn-
a decrease in blood pressure may indicate a net loss of thetic colloids is human albumin, but this product carries
fluid. Because of the high percentage of patients with a risk of anaphylaxis. 11,21 Canine and feline albumin have
hypertension (80% of dogs with severe acute uremia recently become commercially available and can be used
and 20% to 30% of dogs and cats with CKD), the trend for colloidal support.
rather than the absolute value is of more utility in Treatment of the patient with an acute uremic
assessing changes in hydration status. 13,26,59 Similarly, crisis from a protein-losing nephropathy with severe