Page 416 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

406 FLUID THERAPY

volume or more will probably require blood component accurate iterative formula has been published that uses
therapy and possibly additional crystalloid or colloid sup- a more sophisticated calculation technique. 121
port. Occasionally, fluid therapy is not sufficient, and sur- Although these techniques may be beneficial under
gical management is required to stop bleeding (e.g., special circumstances, they have been evaluated in human
penetrating trauma with major blood vessel rupture). medicine and have been found to be very expensive and to
In these instances, it is crucial to have one or more provide little benefit to the patient. 21,37,73,100,163 Never-
large-gauge venous catheters in place in an attempt to theless, the American Society of Anesthesiologists (ASA)
keep pace with the loss. In cats and dogs weighing less published guidelines for the use of packed red cells that
than 5 kg, it usually is feasible to place an 18-gauge cath- include the following statements: 143
eter in the jugular vein. In many dogs of 5 to 15 kg, it is 1. When appropriate, preoperative autologous blood
feasible to place a 16-gauge catheter in a cephalic vein, donation, intraoperative and postoperative blood
whereas in dogs more than 15 kg, a 14-gauge catheter recovery, acute normovolemic hemodilution, and
normally may be placed. After the catheter has been measures to decrease blood loss (deliberate hypoten-
placed, the animal should be anesthetized using a tech- sion and pharmacologic agents) may be beneficial.
nique that induces minimal disturbances in volume status 2. The indications for transfusion of autologous red
and cardiovascular function. Some investigators advocate blood cells (RBCs) may be more liberal than for allo-
withholding fluids from trauma patients with major vessel geneic RBCs because of the lower (but still significant)
rupture before surgical intervention. In one study of risks associated with the former.
human patients, a marginal benefit was demonstrated
using this approach. 14 Others have advocated resuscita- HYPERVOLEMIA
tion to lower than normal blood pressures to minimize Hypervolemia is likely to be either iatrogenic or the result
the chance of dislodging a fragile clot or increasing the rate of oliguric renal failure or heart failure. In the former sit-
of hemorrhage. 55 It is likely to be a realistic approach only uation, it may simply be sufficient to monitor the patient
when blood loss is rapid and surgery can be performed carefully until its fluid volume status has normalized.
immediately. 157 In patients with major blood loss but no In the case of oliguric renal failure, it is difficult to reduce
central vessel rupture, it is more appropriate to replace the blood volume without dialysis. The primary risk of
the volume deficit before anesthetizing the animal. hypervolemia is related to hypertension and an increase
If the patient is expected to lose a large volume of blood in myocardial work, which could lead to failure in a heart
duringananticipatedelectivesurgery,theanimalcandonate with marginal reserve. Hypervolemia also may lead to
blood in advance so as to have autologous blood available. pulmonary edema, in which case the circulating volume
The animal can donate 1 unit of blood and then return 3 should be reduced by administration of a diuretic (if renal
weeks later, at which time the first unit of blood can be function is normal) or by phlebotomy if necessary.
returnedtotheanimaland2unitsofblooddrawn.Thispro-
cedurecanberepeatedtocollectseveralunitsfromthesame CHANGES IN CONTENT
animal. This approach usually is not possible because of the
leadtimeneededtocompletethesemultiplecollections,but Occasionally, changes in vascular volume do not affect the
a single donation technique has been reported for cats composition of blood, but in many cases changes in com-
undergoing partial craniectomy. 64 Another alternative in position also occur and require attention.
an animal with relatively normal hematocrit and total pro-
teinconcentration istouse acute normovolemichemodilu- ANEMIA
tion, collect blood immediately before surgery, and replace The major concern with anemic patients is the supply of
itwiththreetimesthevolumeofcrystalloidor the samevol- oxygen to the tissues after the animal has been
ume of colloid. The expectation is that the animal will lose anesthetized with drugs that may impair cardiovascular
less protein and red cell volume during the surgery because function. In the chronically anemic animal, some com-
of hemodilution, and the collected blood will be available pensation already has occurred to facilitate delivery of
for transfusion when it is needed after surgery. The formula oxygen to the tissues. This compensation usually occurs
forcalculatingthehemodilutionwasoriginallydescribedby as a result of an increase in cardiac output and a change
20
Bourke and Smith : in the affinity of hemoglobin for oxygen. When the ani-
mal is anesthetized, especially using drugs such as
Exchangeable blood volume ¼ a 2 agonists or inhalants, cardiac output is decreased,

Hb o which reduces the delivery of oxygen to the tissues.
Actual blood volume ln
Hb t Administration of 100% oxygen increases the amount
of oxygen in solution (0.3 mL per 100 mL of blood
where Hb 0 is the original hemoglobin concentration, and per 100 mm Hg pressure), but this effect provides little
Hb t is the target value. This formula tends to overesti- compensation for the decline in cardiac output.
mate the exchangeable blood volume, and a more Figure 17-1 illustrates the relationship between