4. Surgical and traumatic loss
Fluid loss from the vascular compartment is either absolute (e.g. hemorrhage) or relative (e.g. vasodilatation). Although conceptually thought to be a myth, third space losses can also be either absolute or relative.24 Loss of intravascular fluid to a third space can be either anatomical or nonanatomical.25 Anatomical third space loss represents the accumulation of fluid within the interstitium, whereas nonanatomical third space
losses include fluid accumulation in traumatized tissue, the bowel, and the peritoneal cavity. Nonanatomical third space losses
contain protein concentrations similar to that of plasma and are lost from further extracellular exchange. Therefore, they
represent an absolute loss of protein-containing fluid.
Blood loss during anesthesia should be estimated or quantified during surgical procedures, considered in relation to the animal's
hemoglobin concentration, and replaced as soon as possible.26 Timely fluid resuscitation reduces later volume requirements. Fluid loss into surgically traumatized tissues can be highly
variable and is independent of the animal's weight. Fluid accumulation at bowel anastomotic surgical sites is minimal and
has been experimentally determined to depend on the rate of fluid (crystalloid) administration and ranges from 5 to 10 ml.27
Blood loss can be replaced with adequate amounts of crystalloid, colloid, or blood (see sidebar "A guide to optimal perioperative fluid therapy"). The amount of crystalloid required to effectively replace blood loss is difficult to determine and usually exceeds the
traditional 3:1 replacement ratio derived from estimates for the distribution of water between the intravascular and interstitial
fluid compartments.28-30 Experimental data in isoflurane-anesthetized dogs and clinical studies in people suggest that crystalloid fluid replacement
ratios ranging from 4 to 5 ml or greater for each milliliter of lost blood (4:1 or 5:1) are more efficacious than a 3:1 ratio,
depending on the amount of blood loss and whether concurrent hypotension exists.
Administering large volumes of crystalloid (> 30 ml/kg) during the first hour of anesthesia to replace blood loss is more
likely to produce hemodilution, fluid accumulation at the surgical site, and interstitial edema.23,29,30 Hemodilution decreases hemoglobin and protein concentrations and can produce coagulation defects.33-35 It is important to monitor hemoglobin concentrations and coagulation and treat dilutional coagulopathy when it occurs (e.g. with fresh plasma: 5 to 15 ml/kg). Hemoglobin (should be > 7 g/dl) and total protein (should be > 4 g/dl) concentrations should
be measured before and during surgery to minimize oxygen delivery deficits and interstitial edema. Either a colloid or blood
(recommended if the hemoglobin concentration is < 5 to 7 g/dl) is a more rational and effective choice for replacement of
Reducing or terminating anesthesia, when possible, and co-administering fluids with vasopressors or inotropes are the best
methods for managing hypotension caused by anesthetic drugs.
Hypotension in dogs and cats is one of the most frequent complications of general anesthesia and is related to the animal's
health status.20,36-41 The development of hypotension during anesthesia in a normal healthy dog or cat is more a testament to the animal's physical
status and the anesthetist's competency than to blood loss or the vasodilatory and negative inotropic effects of anesthetic
drugs. But keep in mind that all anesthetic drugs can induce hypotension, particularly when administered to dehydrated, sick,
depressed, or debilitated animals.
Absolute hypovolemia (e.g. blood loss) requires fluid replacement therapy with a crystalloid, colloid, or blood, depending on the animal's hemoglobin
concentration (Table 2). The question is how to treat hypotension due to relative hypovolemia during anesthesia. The answer is determined by its
principal cause (e.g. vasodiliatation, poor cardiac function).
Anesthetic drugs. Relative hypovolemia (vasodilatation) is most likely due to the vasodilating effects of anesthetic drugs on venous vascular
tone and suppression of compensatory homeostatic mechanisms.3 Isoflurane, sevoflurane, and propofol inhibit sympathetic compensatory homeostatic mechanisms and directly interfere with
smooth muscle contraction (e.g. splenic contraction), thereby decreasing venous vascular tone and increasing venous capacitance.42,43 These effects combine to decrease venous return, cardiac filling pressures, cardiac output, and arterial blood pressure
and are as important, if not more important, than their arterial vasodilatation effects.3
Conventional rates of fluid administration, whether crystalloid or colloid, may be ineffective for restoring normal arterial
blood pressure unless anesthetic depth is reduced.5,33 An experimental study in isoflurane-anesthetized dogs demonstrated that arterial blood pressure improved only when anesthetic
depth was decreased.5 Notably, the fluid administration rate in this study was six times greater (60 ml/kg/hr) than the currently suggested conventional
rate (10 ml/kg/hr).5 This rate of crystalloid administration is likely to produce hemodilution and hypervolemia unless administered for a very
short time. Clearly, more practical and effective recommendations are required for the intraoperative treatment of hypotension
in anesthetized animals.
Toward this end, we have administered 3 to 5 ml/kg of tetrastarch over one to three minutes to isoflurane-anesthetized dogs
and produced an immediate and sustained increase in arterial blood pressure. The magnitude of the beneficial effect was directly
related to the depth of anesthesia (i.e. deeper level then less effect). Similar but shorter duration effects were produced when lactated Ringer's solution was administered.
The relatively short duration of fluid administration decreases the potential for fluid overload (e.g. for a 10-kg dog, 30 ml of crystalloid or colloid over one to three minutes). If arterial blood pressure does not increase,
then the administration of a catecholamine (e.g. vasoconstrictor, inotrope) should be considered (Table 2).