Page 561 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

Managing Fluid and Electrolyte Disorders in Renal Failure 549

dose can be administered every 6 to 8 hours. A continu- and body weight should be assessed at least twice daily,
ous rate infusion gives a more sustained diuresis with a and the fluid plan adjusted accordingly. Blood pressure
lower cumulative dose compared with bolus infusion. 14 should also be monitored. Urine output and other fluid
In people, the time to maximal effect with a CRI without loss should be monitored and correlated to other findings
a loading dose is 3 hours, and 1 hour with a loading dose. of volume status.
The dose in people is usually 1 to 9 mg/hr (about 0.01 to Determining urine volume can be performed by a vari-
0.15 mg/kg/hr) with some reports using doses as high as ety of methods, including placement of an indwelling uri-
0.75 mg/kg/hr. 34 In normal dogs, 0.66 mg/kg/hr nary catheter with a closed collection system, collection
resulted in diuresis, and doses of 0.25 to 1 mg/kg/hr of naturally voided urine, metabolic cage, or weighing
have been used in dogs and cats with naturally occurring cage bedding/litter pans (1 mL of urine ¼ 1 g). An
renal failure. 1,2,13 Because electrolyte and fluid balance indwelling catheter is usually the most precise method,
disorders can develop rapidly if a brisk diuresis ensues, fre- although technical issues such as urine leakage around
quent monitoring is necessary. the catheter or inadvertent disconnection may cause an
Dopamine has been shown to make some human artifactually low measurement. The risk of an iatrogenic
oliguric patients nonoliguric, but it does not increase urinary tract infection from the catheter can be decreased
the GFR or improve the outcome in people. 20,23,40,50 by careful attention to catheter and patient hygiene,
Because of lack of efficacy and side effects associated with including cleaning the external portions of the catheter
dopamine, it is no longer recommended for treatment of with antiseptic multiple times daily and changing the
oliguric renal failure, except for pressor control. 13,56 collection bag and tubing daily. 58
Selective dopamine agonists may have better efficacy Complete collection of voided urine may be difficult in
and fewer adverse effects compared to dopamine. There many patients because of a lack of patient cooperation, or
are two dopaminergic receptors, DA-1 and DA-2. urinary incontinence in obtunded or recumbent patients.
Fenoldopam is a selective DA-1 receptor agonist, and An accurate scale is necessary to measure small volumes of
as such, it selectively increases cortical and medullary urine in cats and small dogs, but weighing cage bedding
blood flow, sodium excretion, and urine output while or litter pans before and after use may provide an ade-
maintaining the GFR in people. It does not have DA- quate assessment of urine volume noninvasively in some
2or a or b adrenergic activity, so it does not cause vaso- patients. Fluid losses from vomiting and diarrhea are usu-
constriction, tachycardia, or arrhythmias as seen with ally estimated, and other losses such as body cavity drain-
dopamine. 20,45 Although no studies have shown a age (ascites, pleural effusion) or nasogastric tube
benefit, studies with this drug in people are encouraging suctioning can be measured.
and larger clinical trials are needed. 45 Although some
studies in dogs show an improvement in the GFR with Discontinuing Fluid Therapy
fenoldopam, the GFR may decrease in the first few hours With AKI, once a diuresis has been established, polyuria
after administration. 24,39,57 can be quite profound. Monitoring urine production to
Calcium channel antagonists have been used to prevent inadequate fluid administration is necessary in
decrease damage after renal transplantation. 35 Calcium this setting as well as with oliguria or anuria. Weaning
channel antagonists presumptively reverse renal vasocon- these patients off of IV fluids is a crucial step. When
striction by causing predominantly preglomerular the azotemia has resolved or has reached a plateau, the
vasodilatation, inhibit vasoconstriction induced by fluid dose can be decreased by 25% per day. If the urine
tubuloglomerular feedback mechanisms, and cause natri- output diminishes by a corresponding degree and the
uresis independent of the GFR. 35 Although the results of azotemia does not return, continue tapering over 2 to
one study using diltiazem in addition to standard care in 3 days. If the urine output does not diminish, the kidneys
dogs with AKI from leptospirosis were not statistically sig- are unable to regulate fluid balance and further reduction
nificant, there was a trend toward increased urine output in the fluid administered will lead to dehydration.
35
andmorecompleteresolutionofazotemia. Whether this Attempts to taper the fluid rate can be made again after
will prove to be a beneficial therapy requires further study. several days, but generally at an even slower rate (10%
Atrial natriuretic peptide (ANP) increases tubular to 20% per day). It can take weeks for the kidneys to
excretion of salt and water, and stimulates afferent arteri- regain the ability to control fluid volume in rare cases.
olar dilation and efferent arteriolar constriction, increas- With CKD, once the prerenal component of the azo-
ing the GFR. Although ANP reduced the severity of temia has resolved, the serum creatinine concentration
experimental ARF from ischemic but not nephrotoxic will usually decrease by at least 1 mg/dL/day (monitored
causes, it has not been effective in clinical trials thus far. 20 generally every 48 hr). When the creatinine concentration
reaches a baseline value (i.e., no longer decreasing despite
Monitoring Fluid Therapy IV fluid therapy), fluids should be tapered in preparation
Monitoring fluid status is an ongoing process that must for patient discharge. Aggressive diuresis should be
be repeated throughout the day. Physical examination tapered gradually over about 2 to 3 days.