Page 560 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

548 FLUID THERAPY

blood flow at perfusion pressures between 80 to 180 mm Mannitol should not be given to patients that are
Hg, but renal perfusion may be more linear in damaged dehydrated because it will further exacerbate intracellular
kidneys. 10,12 The mean arterial pressure should be dehydration. Conversely, it is also contraindicated if
maintained above 60 to 80 mm Hg, or the systolic pres- overhydration is present, and may worsen pulmonary
sure above 80 to 100 mm Hg when measured by Doppler edema.
technology. Apparent anuria due to obstruction of the Hypertonic dextrose can be used as an osmotic
urinary tract or leakage into the peritoneal, retroperito- diuretic, if mannitol is not available. A total daily dose
neal, or subcutaneous tissues should be excluded before of 22 to 66 mL/kg of a 20% dextrose solution should
determining that a lack of urine is due to intrinsic renal cause hyperglycemia and glucosuria. 47
damage. Loop diuretics such as furosemide can increase urine
Various values have been used to define oliguria, flow without increasing the GFR. 14,20,37,40,61 Despite
including less than 0.25 mL/kg/hr, less than 0.5 mL/ the increase in urine output, loop diuretics do not
kg/hr, and less than 1 to 2 mL/kg/hr. 13 In a hydrated, improve outcome, suggesting that those who respond
well-perfused patient, less than 1.0 mL/kg/hr can be have less severe renal failure, resulting in a better outcome
considered absolute oliguria, and urine production for a recovery independent of drug therapy. 14,20,40,55,61
between 1 and 2 mL/kg/hr in a patient on fluid therapy For example, in one human study, patients that could
is considered relative oliguria. 10,13 Anuria is defined as be converted from oliguric to nonoliguric renal failure
essentially no urine production. 13 Urine volume above had better APACHE scores (a disease severity scoring sys-
2 mL/kg/hr is generally considered polyuria. tem used for people in ICU settings) and higher creati-
If pathologic oliguria or anuria persists despite nine clearance before treatment, suggesting that they
55
correcting hemodynamic parameters, most clinicians had less severe renal injury. Due to the perception that
attempt to convert oliguria to nonoliguria using there is a low complication rate associated with the loop
diuretics. There is no evidence that diuretics improve diuretics, they are often used despite lack of proven

the outcome of AKI, and some surmise that the ability benefit. Loop diuretics inhibit the Na -K -2Cl pump
þ
þ
to respond to diuretics is a marker of less severe renal in the luminal cell membrane of the loop of Henle,
injury associated with a better prognosis. In people, an decreasing transcellular sodium transport. Basal Na ,
þ
increase in urine output with diuretic use delays referral K -ATPase activity becomes unnecessary and the medul-
þ
for dialysis, perhaps inappropriately. 38 However, in veter- lary oxygen consumption decreases, which is
inary medicine where dialysis is not as readily available to hypothesized to protect the kidney from further
control fluid status, an increase in urine output from injury. 25,55 The amount of structural damage to the thick
diuretic use may allow an increase in the volume of other ascending limb of the loop of Henle is subsequently
medications or nutrition, and may be justified even with- decreased in isolated perfused kidneys. 25 Loop diuretics
out improvement in renal function. also have renal vasodilatory effects. 45 Despite the theoret-
Mannitol is an osmotic diuretic that causes extracellu- ical reasons to use loop diuretics, one retrospective study
lar volume expansion, which can improve GFR and in people showed an increased risk of death or failure of
inhibit sodium reabsorption in the kidney by inhibiting renal recovery in the furosemide treatment group. Poten-
renin. Mannitol also increases tubular flow, which may tial reasons for this finding include a detrimental effect of
relieve intratubular obstruction from casts and debris. the drug, delay in recognizing the severity of renal failure
Mannitol decreases vascular resistance and cellular with subsequent delay in starting dialysis, or preferential
swelling; increases renal blood flow, the GFR, and solute use of loop diuretics in patients with a more severe course
excretion; protects from vascular congestion and RBC of disease. 14,38 Loop diuretics may make fluid manage-
aggregation; scavenges free radicals; induces intrarenal ment easier in people, without changing the outcome. 55
prostaglandin production and vasodilatation; and induces In animals, loop diuretics may play a larger role in man-
5,10,13,20
atrial natriuretic peptide release Mannitol may agement because dialysis is not universally available.
blunt the influx of calcium into mitochondria in suble- Established indications for use of furosemide in veteri-
thally injured renal cells, thus decreasing the risk of sub- nary medicine include treatment of overhydration or
lethal injury progressing to lethal damage. Despite the hyperkalemia. 13 Furosemide should not be given to
theoretical advantages, no randomized studies have patients with aminoglycoside-induced ARF. 10
shown a better clinical response with the use of mannitol An increase in urine output should be apparent 20 to
and volume expansion than with volume expansion alone 60 minutes after an intravenous dose of furosemide of
in people or healthy cats. 20,37 2 to 6 mg/kg. Ototoxicity has been reported at high
Mannitol is administered as a slow intravenous bolus of doses in people, and doses of 10 to 50 mg/kg may cause
0.25 to 1.0 g/kg. If urine production increases, mannitol adverse effects in animals (apathy and anorexia in cats;
may be administered as a constant rate infusion (CRI) of 1 hypotension, apathy, and staggering in dogs). 10 If there
to 2 mg/kg/min IV or 0.25 to 0.5 g/kg q 4 to 6 hr. 13 is no response to high doses of furosemide, therapy
Doses in excess of 2 to 4 g/kg/day may cause ARF. should be discontinued. If a response does occur, this