Page 427 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

Perioperative Management of Fluid Therapy 417

pressure, and PCWP. 120 The increase in CVP tends to of the ideal perfusion state can be defined and the type
increase renal vein pressure, which may alter interstitial and volume of fluid necessary to achieve this state then
pressure within the kidney. 153 The use of intermittent can be administered. 85,95 To some extent, this approach
positive-pressure ventilation (IPPV) and PEEP or CPAP has been used in veterinary medicine for many years, but
is associated with an increase in vasopressin secretion, but the primary goal has been to increase blood pressure
it is likely that increased renal interstitial pressure has a because it often is the only parameter measured. Inade-
more important effect because the decrease in urine out- quate perfusion of tissues occurs at low pressures but
put can be seen without changes in vasopressin. 152 the converse may not be true (i.e., adequate perfusion
Regional anesthetic techniques also may result in vol- may not necessarily occur at normal pressures). Normal
ume-responsive hypotension. This is particularly true arterial pressures can be achieved with very low cardiac
with epidural or intrathecal techniques. In people, the output if peripheral resistance is increased sufficiently
spinal cord ends at vertebral level L1/L2, and it is neces- because arterial blood pressure is the product of cardiac
sary to inject enough drug to extend high into the tho- output and systemic vascular resistance.
racic region to block enough spinal segments for
abdominal surgery. As a result, there is a significant block MONITORING CHANGES
of sympathetic outflow from the thoracic and lumbar spi- IN VOLUME
nal cord segments, which can result in hypotension. The
cord ends at vertebral level L6/L7 in dogs and at S1/S2 Methods for monitoring intravascular volume are not
in cats. Thus it is feasible to achieve an effective abdomi- available in routine practice. Most of the techniques that
nal block in dogs and cats without substantial loss of sym- have been used in the laboratory involve dye dilution and
pathetic tone. However, hypotension can occur with this require sophisticated measuring techniques and
technique, and the animal should be monitored calculations to determine intravascular volume. Even if
accordingly. such information was available, it is unlikely that absolute
Intraoperative blood loss is affected by blood pressure values for vascular volume would be of much use because
and body temperature. In some situations in which it is it is unlikely that a normal volume measurement for the
difficult to control blood loss, it may be possible to animal in question would be available before the proce-
reduce the loss by maintaining pressure at a lower than dure. However, trends over time may be helpful. A simple
normal value for the period of concern. It would be method for estimating a change in plasma volume is to
advantageous to be able to monitor lactate use the change in hematocrit with time:
concentrations to ensure that global perfusion was not
being adversely affected by this approach. Hypothermia Change in plasma volume ¼
has been shown to alter coagulation. The mechanism 79
ð½Baseline Hb New HbŠ 1Þ=ð1 Baseline Hct½L=LŠÞ
for this effect appears to be mainly related to platelet func-

tion until body temperature decreases to less than 33 C
when the effects on enzymes become manifest. 197 This calculation ideally would be based on hemoglobin
In dogs, it has been noted that platelet counts decrease and hematocrit values taken soon after the induction of

by up to 70% between 37 C and 32 C because of splenic anesthesia because substantial decreases in hematocrit

sequestration, but the defective release of thromboxane and hemoglobin can occur during anesthesia. Devices
A 2 , down-regulation of platelet glycoprotein Ib-IX, and that measure changes in blood volume are available on
up-regulation of platelet surface protein GMP-140 also sophisticated hemodialysis machines and provide a guide
alter platelet aggregation. 83,122 Several studies in humans to therapy in situations in which blood volumes can
have shown increased blood loss during procedures nor- change rapidly. In general, however, changes in blood
mally associated with hemorrhage, even with small volume must be inferred from clinical signs. An interest-
changes in body temperature (e.g., 30% greater loss with ing new approach to this problem is to use the response of
intraoperative temperature differences of <2 C). 160,196 the patient’s hemoglobin concentration to a fluid chal-

Other studies in humans have not been able to repeat lenge to differentiate dehydration from hypovolemia. 78
these findings, and there are no similar studies in dogs This method still is not very precise, but with refinement
and cats. 76,141 may prove useful in a clinical setting.
Loss of skin turgor is a helpful sign when present, but
MONITORING FLUID in many animals skin turgor changes little until volume
80
THERAPY depletion is severe ; skin turgor is not useful in monitor-
ing hypervolemia. Radiographic signs of hypovolemia
Determining the best fluid regimen and judging the ade- include microcardia and a decrease in the size of the cau-
quacy of therapy are dependent on monitoring the dal vena cava and pulmonary vessels. CRT is used to mon-
patient. In human medicine, the term “goal-directed itor the microcirculation and, if prolonged, implies poor
therapy” has been used to indicate that the parameters tissue perfusion. Poor tissue perfusion may be the result