Page 559 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

Managing Fluid and Electrolyte Disorders in Renal Failure 547

estimate of the loss should be included in the fluid admin- over 6 hours. Add this amount to the fluid volume
istration rate. In practical terms, after an initial fluid resus- required over the next 6 hours (6 hours of insensible
citation if needed for shock, the volume of fluid to losses þ previous 6-hour urine output). Divide the total
administer is calculated by adding average maintenance volume by 6 to get the hourly rate for the CRI.
fluids (66 mL/kg/day) plus replacement of dehydration An anuric patient should receive fluid administration
(over a selected time frame) plus ongoing losses to replace insensible loss only. If the patient is
(estimated volume of polyuria, vomiting). overhydrated, withhold the insensible loss.
Because uremic toxins are retained in renal failure, Overhydration in an anuric patient or inability to induce
administration of a volume of fluid exceeding “mainte- diuresis in an oliguric or anuric patient is an indication for
nance” can improve excretion of some uremic toxins in dialysis, which is the only other effective therapeutic
animals with the ability to increase urine output in option.
response to a fluid challenge. The volume is varied based Not withstanding the conventional wisdom that fluid
on clinical situation and clinician preferences, but gener- therapy is cornerstone of treatment of kidney failure, evi-
ally ranges from 2.5% to 6% of body weight per day, in dence of harm from volume overload is mounting. Rapid
addition to the maintenance fluid administration rate. restoration of renal perfusion may decrease renal damage,
In practical terms, twice the maintenance fluid rate is but there is no evidence that fluid therapy will reverse
equivalent to a maintenance rate plus a 6% “push” for established renal injury. 44,60 Patients with volume over-
diuresis (60 mL/kg/day ¼ 6% of body weight). An load (>10%) had decreased survival and impaired renal
increase in urine volume does not automatically mean recovery. 6–8 In fact, one study in adult humans found that
there is an increase in toxin or solute excretion. a 1 L positive fluid balance in 24 hours was associated
42
If the urine output varies substantially from normal, with a 20% increase in mortality.
either oliguria (<0.5 mL/kg/hr) or polyuria (>2 mL/ Critically ill patients frequently have increased capillary
kg/hr), a fluid plan based on these assumptions may be leakiness, leading to tissue edema as a consequence of
inadequate. Animals with kidney failure may have urine aggressive fluid therapy. 44 Tissue edema impairs oxygen
output in a “normal” range (0.5 to 2.0 mL/kg/hr), delivery and metabolite diffusion, distorts tissue architec-
but if their kidneys are unable to alter the urine volume ture, and impairs capillary blood flow and lymphatic
to excrete a fluid load, the patient has “relative oliguria.” drainage. 44 The adverse effects of tissue edema may be
The ins-and-outs method of fluid administration is more predominant in encapsulated organs, such as the
appropriate in these situations. It should only be used kidneys and liver, as the increased tissue volume increases
after rehydration is complete and is not appropriate if a interstitial pressure and decreases organ blood flow. Car-
patient is still dehydrated. diac dysfunction caused by increased preload and
There are three components of volume calculations in myocardial edema further impairs tissue oxygen delivery
the “ins and outs” method, consisting of (1) insensible and may impair renal recovery. 44,53 The lungs are perhaps
loss (fluid lost via respiration and normal stool), which the most sensitive to volume excess, and the development
is about 22 mL/kg/day in the average patient; (2) urine of pulmonary edema is a common life-threatening condi-
volume replacement calculated by actual measurement tion in oliguric dogs and cats on fluid therapy.
(see later discussion for measuring techniques); and (3) In light of these concerns, restricted fluid administra-
ongoing losses (i.e., vomiting, diarrhea, body cavity tion to avoid fluid retention, and early referral for dialysis
drainage), which are generally estimated. if azotemia cannot be controlled with the amount of fluid
To write treatment orders for “ins and outs” using two therapy the patient can tolerate, may prove to be a bene-
IV catheters, divide the daily insensible loss (22 mL/kg) ficial therapeutic strategy.
by 24 to determine the hourly dose of IV fluids to admin-
ister through one catheter. You can use this fluid dose to Converting Oliguria to Nonoliguria
deliver any drugs that need to be given by constant rate A decrease in urine production may be due to hemody-
infusion (CRI) (metoclopramide, furosemide, mannitol, namic, intrinsic renal, or postrenal causes. An appropriate
etc.), being cognizant of drug incompatibilities. Measure renal response to inadequate renal perfusion from
urine output to determine the rate of replacement fluid to hypovolemia or hypotension includes fluid retention with
administer over the next time period. For example, if you a concomitant decrease in urine volume. Before deter-
are measuring urine output every 6 hours, take that vol- mining whether oliguria is pathologic or physiologic,
ume and divide by 6 to give the hourly rate of fluid renal perfusion should be optimized by ensuring ade-
replacement to administer over the next 6 hours. Add quate hydration. A volume of fluid equal to 3% to 5%
to this an estimate of losses during that time period of body weight should be administered to patients that
(vomiting and diarrhea). For the starting fluid dose, appear normally hydrated because dehydration of less
select a volume based on your estimate of the patient’s than 5% cannot be detected clinically. In patients that
needs. If only one IV catheter is available, calculate the are volume overexpanded, this fluid administration is
amount of medication to be administered by CRI to give not necessary. Healthy kidneys can autoregulate renal