Maintaining fluid balance during surgical or prolonged procedures

It is of vital importance to support the circulation by correcting any fluid imbalances, and hypovolaemia should always be considered a possible primary cause of cardiovascular failure.

Blood loss during surgery can be very gradual, and assessment of the volume lost is frequently highly inaccurate. One simple measure is to weigh the swabs used during surgery. This should always be done, as it is rapid, simple and can be applied in all species. Weigh a dry swab, then weigh any swab used to control haemorrhage or clean the surgical site. It is sufficiently accurate to assume that 1g is the equivalent of 1ml of blood. This will provide a reasonable estimate of blood loss, but additional blood will have been lost by seepage into surgical wounds, body cavities and surgical drapes.

Additional losses of plasma occur by exudation both into traumatised tissues and into the peritoneal cavity during prolonged abdominal surgery (approximately 100–200 ml/h in people). Further depletion of the extracellular fluid (ECF) occurs due to water loss by evaporation from the respiratory tract and from any surgical wounds and exposed viscera.

As a routine, fluid should be replaced at a rate of 10 ml/kg of body weight per hour using either Hartmann’s solution or 0.9% saline. It is common practice to warm fluids to body temperature before administration, but infusing fluids at 20 °C rather than 38 °C will have minimal effects on the animal’s body temperature compared to other sources of heat loss. The effect of infusing fluids at 40 °C would be greater, especially if administered rapidly, so it is recommended that fluids are warmed if practicable, but this should not delay or prevent their administration.

A healthy, unanaesthetised animal can withstand the rapid loss of 10% of its circulating volume. Once the loss exceeds 15–20% of circulating volume, signs of hypovolaemia and shock may develop. In an anaesthetized animal, many of the physiological mechanisms that act to maintain cardiovascular stability are depressed, and hence less severe losses can still have serious effects. If blood loss exceeds 20–25% of the circulating volume, replacement with whole blood may be necessary. Smaller losses can be replaced by the infusion of crystalloid solutions or plasma volume expanders. Blood volume is approximately 70 ml/kg of body mass, so a 30 g mouse has a total blood volume of about 2.1 ml. It is easy to appreciate that any blood loss in these small species can rapidly become significant. A key measure in preventing problems is therefore to ensure careful surgical technique, with rapid and effective control of any haemorrhage.

Blood for transfusion can be obtained from a donor animal of the same species and collected in acid citrate dextrose solution (1 part ACD to 3.5 parts blood, using ACD from a blood collection pack manufactured for storing human blood). It is preferable to use the blood within 4–6 hours as platelet function and red cell viability is likely to be well maintained during this period. More prolonged storage at 4 °C is possible, but the storage characteristics of blood from many animal species have not been properly evaluated. Although cross-matching will rarely be possible when dealing with laboratory animals, in the author’s experience the incidence of adverse reactions to an initial transfusion appears to be low. Selection of donors of the same breed or strain as the recipient may help reduce the likelihood of transfusion reactions. Use of blood from a single individual, rather than pooled from a number of donors, will also help to reduce the risk of an adverse reaction. When using an inbred strain of rodents, there are obviously no problems of this nature.

Blood should be replaced at a rate of 10% of the calculated blood volume every 30–60 minutes. If severe and rapid haemorrhage has occurred, the estimated volume of blood lost should be transfused as rapidly as possible. If whole blood is unavailable, either previously stored plasma or a plasma volume expander such as Haemaccel (Hoechst) or Hespan (Du Pont) should be administered. If these fluids are unavailable, or if blood loss has been less severe, then Hartmann’s solution or 0.9% saline should be administered, at the rate described for whole blood and at a volume of three to five times the estimated blood loss. Considerably greater volumes are needed because these crystalloids are distributed throughout the ECF, unlike blood, plasma and plasma volume expanders that remain in the circulatory system. Some controversy exists concerning the merits of crystalloids and plasma volume expanders for restoring the circulating volume following severe haemorrhage. Such controversy should not be a deterrent to the use of fluid therapy, and it should be remembered that it is almost always better to give than to withhold fluids.

In small animals in which intravenous therapy is difficult, 0.9% sodium chloride or Hartmann’s solution can be administered intraperitoneally to correct intra-operative fluid loss. It is often particularly convenient to replace intra-operative water losses and anticipated post-operative deficits by the administration of 0.18% sodium chloride with 4% dextrose by subcutaneous injection at a rate of 10–15 ml/kg. These routes of administration result in slow absorption and will be of no immediate value in treating cardiovascular failure, but will help maintain fluid balance in the post-operative period.