INDIVIDUALZING FLUID THERAPY
The dangers of hypovolemia and hypotension
Intraoperative hypovolemia (loss of > 10% of the blood volume) results in tissue hypoperfusion, which leads to metabolic acidosis
and increased morbidity and mortality.12,13
Absolute hypovolemia refers to the actual loss of volume from the extracellular space, such as from blood loss. Furthermore, the stress of major
surgery and anesthesia can alter fluid distribution to the body's fluid compartments and promote salt (sodium, chloride) and
water retention, which may take five to 10 days to return to normal.
Relative hypovolemia (blood volume is normal but insufficient because of widespread vasodilation) is caused by diseases (e.g. sepsis, trauma) or drugs (e.g. anesthetics) that produce vasodilation and results in dilatation of the intravascular space and a decrease in the effective
circulating blood volume.
Both absolute and relative hypovolemia cause hypotension. Absolute hypovolemia requires fluid replacement, while relative
hypovolemia requires careful consideration of its cause (anesthesia, trauma, sepsis) and the side effects of intravenous fluid
administration. Note that fluids do not correct vasodilatation.
Five factors to consider
The question is, how should current knowledge be used to provide the most medically rational perioperative fluid therapy possible?
The following are five considerations to help optimize intravenous fluid delivery. (Also see sidebar " A guide to optimal perioperative fluid therapy".) 29-32
A guide to optimal perioperative fluid therapy
1. Preoperative fluid loss and fluid loading
Basal fluid water requirements depend on metabolic rate and rarely exceed 1 to 2 ml/kg/hr at room temperature (70 F) in dogs
and cats.14 Any attempt to restore estimates of fluid deficit due to dehydration with a crystalloid immediately preoperatively (within
one to four hours of the surgery) or intraoperatively almost always leads to tissue edema. Simple dehydration (i.e. loss of water alone) results in proportional reductions of both interstitial fluid and plasma volume. The dehydrated interstitium
absorbs the crystalloid solutions that are infused and decreases their effectiveness to produce plasma volume expansion.
Furthermore, there is no evidence to support preoperative fluid loading with a crystalloid to prevent hypotension during anesthesia
(Table 2).15 Studies in both hydrated and dehydrated human surgical candidates suggest that it is an ineffective and unfounded practice
whether or not patients have been fasted.11,14-18 Fluid loading immediately before anesthesia and surgery in an attempt to replace fluid losses due to mild dehydration should
Table 2: Treatment of hypotension
Arterial blood pressure and tissue perfusion (mucous membrane color, capillary refill time) should be normalized and stabilized
in animals that are moderately to severely dehydrated, if time permits, before the animals are anesthetized. And only 75%
to 80% of the dehydration deficit should be replaced during the 24 hours before anesthesia, to avoid fluid overload. The fluid
deficit due to dehydration can gradually be replaced over 24 to 48 hours after surgery.19
2. Fasting loss
Mature dogs and cats can be fasted for six to eight hours; however, for older and younger animals, two to three hours of fasting
before surgery is more appropriate.20,,21 Except for medical or behavioral reasons, access to water should be allowed until the preanesthetic medication is administered.
An additional 1 to 2 ml/kg of crystalloid can be added to the base fluid administration rate for each hour the animal does
not have access to water before surgery and can be administered during the first hour of surgery to replace water losses if
water has been withheld.14
3. Insensible loss
Insensible fluid loss in people and animals is generally less than 1 to 2 ml/kg/hr during anesthesia.9,14 Evaporative losses from surgically traumatized tissues are more difficult to assess but have been experimentally determined
to range from 2 to 30 ml/hr in nonexteriorized and exteriorized viscera, respectively.22 Surgically manipulated tissues do not accumulate marked amounts of fluid unless they are severely inflamed or traumatized.
These insensible fluid losses, taken together with basal and fasting losses, suggest that initial rates for intravenous fluid
replacement to otherwise normal healthy dogs and cats rarely need to exceed 5 ml/kg/hr (basal + fasting + insensible). In
those circumstances in which greater rates of fluid administration are required (> 10 to 30 ml/kg/hr), the fluid should be
administered during the first hour and decreased to the basal rate while accounting for blood loss thereafter.23