Page 556 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

CHAPTER • 22

Managing Fluid and Electrolyte

Disorders in Renal Failure

Cathy Langston

The kidneys are responsible for maintaining homeostasis FLUID TREATMENT
in the body, and kidney failure may lead to derangements
of fluid, electrolyte, and acid-base balance. It is the Normal fluid losses consist of insensible and sensible
clinician’s goal in treatment to reverse these losses. Insensible losses are those that are not consciously
derangements and to prevent on-going damage. perceived, such as water lost via respiration, normal stool,
Kidney disease is classically compartmentalized into or sweating. Sweating is of negligible volume in dogs and
acute andchronicdisease,whichisaconvenientway toview cats. There is variation in respiratory losses in dogs, which
what are very frequently notably different manifestations of may lose considerable amounts of fluid by excessive
kidney disease. Both acute and chronic kidney disease may panting, but 22 mL/kg/day is the average. The main
vary from mild to severe. Many patients with acute kidney sensible fluid loss in the normal patient is urine output.
injury require hospitalization for optimal management. Additional sensible losses include the volume lost from
Patients with chronic kidney disease may present in a vomiting, diarrhea, body cavity drainage, burns, etc. In
decompensatedstateandneedhospitalization,ortheirfluid healthy animals, these losses are replaced by drinking
and electrolyte management may occur on an outpatient and the fluid contained in food. In sick animals, who
basis.Despitemanydifferencesinthisdiversefieldofkidney may not be voluntarily consuming food or water, or
disease, many of the principles of fluid and electrolyte man- who may be restricted from consumption due to
agement are the same despite the cause. vomiting, fluid therapy is necessary to replace these losses.
Intrinsic renal failure occurs when damage to the renal With renal disease, urine volume is frequently abnormally
parenchyma occurs. The damage may be reversible or high or low, or inappropriate for the situation, and fluid
irreversible, and includes damage to the glomerulus, therapy is tailored for the individual patient to maintain
tubules, interstitium, or renal vasculature. Hemody- fluid balance.
namic-mediated azotemia occurs when blood flow to FLUID THERAPY FOR HOSPITALIZED
the kidney is diminished, as may occur with hypovolemia, PATIENTS
hypotension, or increased renal vascular resistance.
Hemodynamic azotemia is rapidly reversible once the Although oliguria or anuria are the classic manifestation
underlying disorder has been controlled. Postrenal azote- of acute kidney injury (AKI), AKI may present with poly-
mia occurs when there is an obstruction to urine flow, uria, which frequently portends a less severe renal
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from the level of the renal pelvis to the urethra, or when injury. AKI may also be a subtle increase in creatinine
urine leaks into surrounding tissue and is reabsorbed (i.e., (>50% of baseline) or urine volume inappropriate for the
ruptured bladder, ureter, or urethra). Postrenal azotemia volume of fluid administered. In this early stage of injury,
can also be rapidly reversed by diverting the urine either attempts to lessen further renal damage are warranted.
by a urinary catheter or peritoneal catheter (in cases of an Patients with chronic kidney disease (CKD) may present
intraabdominal rupture). With both hemodynamic and in a decompensated uremic crisis, which may represent an
postrenal causes of azotemia, long-standing problems acute kidney injury superimposed on chronic disease.
may progress to intrinsic renal failure. Although signifi- Many drugs have been evaluated for their benefit in
cant renal disease can be present without azotemia, fluid treatingAKI,andsomearehelpfulincertainsettings.How-
therapy is generally not necessary in those situations. In ever, the most effective therapy of AKI is careful manage-
fact, fluid therapy may not be necessary in compensated ment of fluid balance, which involves thoughtful
chronic renal failure with mild to moderate azotemia. assessmentofhydration,afluidtreatmentplanpersonalized

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