The routine administration of fluids during anesthesia is not common in veterinary practice. There are a number of reasons quoted for this, with the most common being (1) can't justify the added expense (2) appear unwarranted if the animal isn't ill (3) hadn't thought of it!
Even healthy animals show cardiovascular changes associated with anesthesia and surgery. Anesthesia alters vascular volume, depresses cardiac output and blood pressure and decreases compensatory responses to blood loss. In addition concurrently administered drugs (antibiotics, corticosteroids) can depress vascular tone and blood pressure, some surgical procedures depress venous return, cardiac output and blood pressure, and evaporation from exposed tissue decreases vascular volume. In the compromised patient there are additional problems of ongoing fluid losses (hemorrhage, sequestration etc) and incomplete therapy of hypovolemia, dehydration, electrolyte and acid-base abnormalities. For every patient undergoing anesthesia ask 'Does this patient need fluids?' If you decide not to use fluids make it a conscious decision rather than one of neglect.
Patients that generally don't need fluids: Healthy animals undergoing minor procedures lasting < 20 to 30 minutes.
All other patients should receive fluids
Healthy patient with no underlying pathology
A balanced electrolyte solution (BES) such as LRS at 1015 ml/kg/hr is administered throughout anesthesia. BES provide electrolytes in about the same concentration as that found in blood and for this reason can be given rapidly into the intravascular compartment.
Dehydration prior to anesthesia should be treated to ensure that patient stability is optimal. Well hydrated animals should continue to have access to water for up to about 2 hours before induction. Dehydration or hypovolemia should be corrected before anesthesia is induced. Hypovolemic animals SHOULD NOT be anesthetized. They are too unstable and irreversible tissue damage is likely with the added insult of cardiovascular depression induced by anesthetic agents and surgery.
Fall in blood pressure
Hypotension for a short period of time probably causes no deleterious effect but a profound fall in BP or hypotension lasting longer than 15-20 minutes can cause a number of adverse changes. Blood is shunted to vital organs (brain, heart and lung) with other organs (liver, GI tract and kidneys) receiving reduced blood flow. For example consider how many old cats develop renal insufficiency after anesthesia. GI hypoperfusion increases bacterial translocation through the GI mucosa and can to a systemic inflammatory reaction and shock.
If BP falls assess depth of anesthesia, decrease if necessary. Increase fluid infusion to 10-20 ml/kg over 10-15 minutes (dog), 5-10 ml/kg over 15-20 minutes (cat). Continue to assess BP and to give LRS until BP returns to normal. If a large volume of crystalloid is given check PCV and TP. Less than 20% of infused crystalloid solution remains in the intravascular space after 2 hours and about 11% in present after 3 hours in dogs so expect some change in PCV and TP as redistribution occurs. During anesthesia/surgery if PCV fall to < 20% or TP < 35 g/l consider providing blood or plasma etc.
The compromised patient
Fluid requirements during surgery depend on the underlying problem and the procedure. However, there is one common factor to all compromised patients. None should be anesthetized before the cardiovascular system has been stabilized. Compromised patients may only require a BES at 10-15 ml/kg/hr. However, colloid, plasma, blood or oxygen carrying solutions may be required during the procedure and should be on hand.
If bleeding occurs
Estimate volume of blood lost and provide 3 ml LRS for each ml of blood to compensate for the redistribution of LRS out of the intravascular space. Assess BP and depth of anesthesia. Consider whether a colloid or oxygen carrying solution may be needed.
When is blood or colloid necessary
Consider providing colloid before or during surgery when hypoproteinemia/ low protein is present because anesthesia further reduces TP e.g., ACP and thiopental potentially decrease PCV by 10%. Administration of crystalloid dilutes serum protein and may cause edema formation if TP falls below about 35 g/l.
Colloid Solutions
Colloids are aqueous solutions that contain large particles that don't normally pass into the intravascular space. They exert oncotic pressure that helps retain water in the intravascular space and so are primarily intravascular volume expanders. Colloids are removed from the intravascular space by glomerular filtration, movement into tissues, metabolization and loss through the GI tract. They restore microvasculature perfusion more effectively than crystalloids with lower volumes and less risk of edema. They may be given alone if only intravascular deficit is present or in combination with crystalloids to replace interstitial fluid. Crystalloid dose should be decreased by 40-60% to prevent a large increase in hydrostatic pressure and interstitial edema. Colloid solutions include albumin, plasma protein, dextrans, hydroxylethyl starch and gelatins.
Plasma proteins
Plasma has a long duration of action and may provide clotting factors if fresh or fresh frozen. Disadvantages include cost of collection and relative unavailability.
Dextran-70
Glucose polymer with weight of 70,000 daltons. Small molecules are removed immediately by renal filtration, larger molecules are metabolized into CO2 and water. About 30% of dextran-70 remains in the vascular space at 24 hours. Potential adverse effects include increased bleeding and rarely, anaphylactic reactions. The tendency to bleed is due to reduced platelet function, dilution of clotting factors, impairment of clot strength and decreased von Willebrand's factor. Bleeding occurs more readily in patients with pre-existing coagulopathies or platelet dysfunction. Recommended rate:4-5 ml/kg/h, maximum infusion:20 ml/kg/day.
Hydroxyethyl starch preparations
Branched starch molecules with a molecular weight of 250,000-400,000. Fifty % remains in the intravascular space at 24-48 hours. Small molecules are filtered by the kidneys and large molecules are metabolized to glucose. HES solutions are effective vascular expanders and 3 hr after infusion the effective intravascular volume is greater than the infused volume, an effect that may last for 18-24 hours. They may reverse the effects of free radicals on microvasculature permeability during reperfusion injury. Potential side effects include bleeding (see dextrans), anaphylaxis, renal dysfunction and vascular overload. Recommended dosage is 10-40 ml/kg (dogs), 5 ml/kg increments over 5-10 mins (cats).
Gelatins
Modified bovine collagen with a molecular weight of 30,000-35,000 daltons resulting in a short half-life (about 2 hours). Renal filtration is the main form of elimination but some is also excreted via the GI tract and metabolism. Compared to other colloids a large volume must be administered to achieve a therapeutic effect. Potential side effects include complement mediated anaphylaxis, hypotension, urticaria, pulmonary oedema, GI disturbances and dilutional coagulopathies. The recommended dosage is 10-20 ml/kg.
Fluids for the patient with Anemia
Anesthetic drugs and crystalloid reduce PCV mildly to severely depending upon agent and volume administered. RBCs should be administered to anemic patients before anesthesia to increase PCV to minimum 24-25% (dog) 16-20% (cat). Blood, packed RBCs, Hb solution are administered at about 2-10 ml/kg/hr +/- crystalloid or colloid depending upon procedure and underlying problem.
Fluid for the patients that bleeds during surgery
Expect hemorrhage in some surgeries e.g., amputation, tumor excision, nasal and sinus surgery, maxillectomy, mandibulectomy. Before surgery place 1, 2 or 3 catheters depending upon patient size. Monitor the patient with direct arterial blood pressure if possible, Doppler if necessary. Blood loss can rapidly result in decreased tissue oxygen delivery. If hemorrhage occurs estimate lost blood volume. Conscious healthy animals can lose about 10% blood volume without a problem. Sick and anesthetized patients cannot compensate to the same degree and treatment is required at an earlier stage.
After hemorrhage bolus with LRS to re-establish normovolaemia. This causes further hemodilution, but decreases blood viscosity and tissue oxygen delivery is not be adversely affected. An alternative to LRS is hypertonic saline at 2-4 ml/kg. If PCV acutely falls below 20% (dog) 16% (cat) tissue oxygen delivery will be compromised and indicates that blood or oxyglobin are needed. Aim for PCV of 25% (dog), 20% (cat). Cross match or typing, blood must be warm and filtered.
Blood Substitutes
Problems associated with administering blood include limited availability, disease spread and incompatibility. These can be avoided by using hemoglobin based oxygen carrying (HBOC) solutions (oxyglobin). Oxyglobin is an ultrapurified polymerized bovine hemoglobin solution that requires no cross-match. The recommended dose is 30 ml/kg but may overload the vascular volume. In cats HBOC is as effective as HES in restoring blood volume and as effective as blood in returning oxygen transport to normal.
Blood or packed red cells
If intra-op hemorrhage is possible have cross-matched blood available. In an emergency when cross-matching is not possible a transfusion reaction is unlikely in dogs provided the donor has not had a previous transfusion or litter and the recipient has not had a previous transfusion or immune mediated anemia. In cats an unmatched transfusion is not possible because of naturally occurring antibodies that can cause severe transfusion reactions.
Mitral insufficiency
Reduce infusion rate to 2.0-5.0 ml/kg/hr and change to low sodium fluid e.g., ½ strength LRS:D5W to prevent edema.
The pediatric patient
50:50 LRS:dextrose at 5 ml/kg/hr, 10 ml/kg/hr if an open body cavity. Hypoglycaemia is common because glycogen stores are low, 2.5% dextrose maintains normoglycaemia.
Summary
There is no single fluid protocol that fits all patients and so physiological state, surgical procedure, current medication and intercurrent disease must be taken into account when developing a fluid plan for anesthetized patients.