Page 426 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

416 FLUID THERAPY

added to the fluids and administered through the same also have a direct effect on the kidney, the end result of
line must be compatible (see Table 17-1). If an animal will which is marked diuresis (urine output increases three fold
be receiving several drugs, it may be necessary to create to tenfold). The mechanism for this effect appears to be
additional access sites to prevent incompatible drugs from related to antagonism of vasopressin, and this dehydrating
being administered through the same line. Consideration effect may be even more relevant in a patient that is avidly
also must be given to the site of access. In cats undergoing conserving water. The opioids have a variety of actions.
declawing of the front paws, it is advisable to place the The m-agonists (e.g., morphine, oxymorphone, meperi-
catheter in the hind leg so that it does not interfere with dine) have an antidiuretic effect, whereas the k-agonists
the surgery. When the caudal vena cava is to be occluded (e.g., butorphanol, pentazocine, nalbuphine) tend to pro-
during surgery, it is important to have the catheter in the mote diuresis. The antidiuresis associated with the m-
forelimb or neck so that fluids reach the remaining circu- agonists may be the result of stimulation of vasopressin
lation during the occlusion. In an emergency in an release. Release of vasopressin may be stimulated in the
anesthetized animal with no venous access, the most visi- awake patient, but there is a reduction in the release of
ble vessel usually is the sublingual vein. This vein can be vasopressin in anesthetized patients receiving large doses
catheterized rapidly if necessary. of potent opioids (causing a reduced stress response). 49
The dissociative drugs (e.g., ketamine, tiletamine) tend
THERMODYNAMIC to decrease urine output despite increases in cardiac out-
CONSIDERATIONS put and blood pressure. 60 These drugs also tend to
decrease baroreceptor responses, and this may be impor-
Infusion of fluids with temperatures less than normal tant in the anesthetized patient with relative hypovolemia
body temperature requires that the animal warms the that undergoes changes in body position.
fluid, and this effect cools the animal. If we assume that Drugs that are used for the induction and maintenance
the specific heat of water (and most of the crystalloid of anesthesia all tend to decrease urine output, mainly
solutions used in fluid therapy) is 1 kcal/kg/ C, it would through their hemodynamic effects. 93 Thiopental has

cost the animal 18 kcal to increase the temperature of 1 L been shown to alter renal sodium resorption, leading to
of fluid from 20 Cto38 C. If the specific heat of the increased sodium and water losses in dogs, but in human

body is 0.83 kcal/kg/ C, 1 L of fluid at 20 C would patients there is either no change or a decrease in urine

cool a 21.7-kg dog by 1 C. 106 Stated in another way, a output. 65,93 Thiopental also decreases hematocrit (which

fluid infusion rate of 10 mL/kg/hr at 20 C would cost may be important in an anemic patient), but it has little

the patient 0.18 kcal/hr and would tend to cool the body effect on plasma volume. 185 Propofol causes hypotension
by approximately 0.2 C/hr. These losses are relatively if given rapidly, and it may cause some reduction in the

minor in comparison with the body heat lost via radiation glomerular filtration rate and urine flow. 139 In normal
but may become more important when massive fluid sheep, there was minimal effect on renal function, but
volumes are required or the infused fluid is much colder there was a significant detrimental effect during sepsis. 18
(e.g., stored blood products). Etomidate preserves circulation better than most other
drugs administered intravenously for induction, but it
EFFECTS OF ANESTHESIA may alter renal function by virtue of the base in which
it is constituted. Etomidate usually is supplied in propyl-
Some drugs may alter sympathetic activity and thus affect ene glycol, which can induce renal failure if enough is
blood volume and the distribution and excretion of body given. This would be unlikely with an induction dose of
fluids. Acepromazine is a potent a 1 -antagonist, and even the drug, but continuous infusion might be associated
low doses of the drug (0.001 mg/kg) induce this effect. with nephrotoxicity from the propylene glycol or the
In the healthy patient, this effect is associated with minor hemolysis that is likely to occur. Severe renal insufficiency
decreases in arterial blood pressure and hematocrit. 45 was reported in dogs after an infusion of etomidate. 124 All
In an animal with increased sympathetic tone, however, of the inhalants are associated with a decrease in renal
the administration of acepromazine may result in pro- function, but this effect can be prevented to some extent
found hypotension. Acepromazine also is a dopamine by preloading the animals with fluids. There is some con-
antagonist and may inhibit the effect of dopamine to cern that sevoflurane can react with soda lime or
increase renal blood flow. Such an effect has been baralyme, releasing a polyvinyl compound (compound
demonstrated with chlorpromazine, 24 but dopamine A) that is nephrotoxic, but this has not yet been seen
maintained its vasodilatory effect in the presence of to be a clinically important issue.
acepromazine, suggesting that this did not hold true for Positive-pressure ventilation has been associated with
acepromazine. 123 The a 2 -agonists have profound effects changes in renal function. A reduction in urine output
on the circulation and on renal function. In dogs and cats, occurs with the institution of positive-pressure ventila-
administration of these drugs, even at low doses, causes a tion, with CPAP or PEEP. 98 The techniques of PEEP
substantial decrease in cardiac output (40% to 60%). They and CPAP increase CVP, mean pulmonary artery