Fluid therapy: for anesthesia
Synonym(s): anesthesia, perioperative
- Think about fluid therapy prior to anesthesia, during anesthesia, and after anesthesia.
- Fluid homeostasis is disturbed by anesthesia and surgery.
- Upsetting this balance will have more serious consequences for old, very young and diseased patients.
- Appropriate fluid therapy can improve the outcome following anesthesia and surgery.
The effects of anesthesia on fluid balance.
What is the significance of anesthesia to fluid balance?
- Almost all drugs used to anaesthetize patients have deleterious effects on the circulation. The use of fluid therapy will help to minimize these.
- Patients undergoing anesthesia are denied access to water. This is usually from the time the water bowl is removed at premedication, until the patient is recovered enough to resume eating and drinking. During this time the patient continues to have "normal" fluid losses (such as urine production and evaporation) and possibly "abnormal" losses (such as vomiting or diarrhea).
- During anesthesia and surgery, losses may be greater than normal due to increased evaporative losses from the respiratory tract (due to dry anesthetic gas mixtures) and visceral surfaces (exposed to the environment during abdominal and thoracic surgery). Some anesthetic drugs have a diuretic side effect.
- Despite our best efforts, surgery is "stressful" to patients. Stress hormones (such as cortisol and catecholamines) will upset fluid balance.
- The procedure the patient is being anaesthetized for may also result in abnormal fluid losses (typically blood).
What are the effects of commonly used anesthetic drugs?
- Acepromazine Acepromazine maleate , an alpha-1 adrenoceptor antagonist, causes vasodilation. In healthy patients this is associated with a mild decrease in arterial blood pressure and PCV. Acepromazine is contraindicated for hypovolemic patients.
- The alpha-2 adrenoceptor agonists (e.g. medetomidine Medetomidine , Domitor") cause a fall in cardiac output and are diuretics.
- Propofol Propofol causes hypotension (especially when given rapidly).
- All inhaled anesthetics are associated with a decrease in renal perfusion, but this effect can be reduced with fluid therapy.
- All anesthetic drugs have some effect (see individual drugs for further details).
Prior to anesthesia
The patient's fluid balance should be as close to normal as possible prior to administering any anesthetic drugs.
What should I consider prior to anesthesia?
- The circulating volume. Hypovolemia may be due to hemorrhage, dehydration, or changes in vascular tone. In all cases fluid should be given to replace the loss. Occasionally, surgical management is required to stop rapid blood loss before the fluid deficit can be corrected. In these cases a number of large gauge intravenous catheters will be required to "keep pace" with the ongoing loss until hemostasis is achieved. However, overly aggressive fluid therapy prior to this may result in a worse outcome as a result of excessive dilution of blood components.
- The oxygen carrying capacity of the blood. The primary concern with anemic patients is that oxygen delivery to the tissues will become insufficient when the patient is anesthetized with drugs that impair cardiovascular function.
- The blood pH. Acidosis or alkalosis will interfere with enzyme function.
- Plasma protein concentrations. Many anesthetic drugs have a high tendency to bind to proteins; when plasma proteins are low more of the drug may be free/unbound and available to exert an effect. Plasma proteins also help to retain water within the circulation; insufficient levels may allow water to "escape" contributing to pulmonary edema.
- Plasma electrolyte concentrations. Ideally, these should be within normal levels prior to surgery. Appropriate fluid therapy can help to correct any abnormalities. Commonly the derangements are iatrogenic following incorrect fluid therapy.
- Blood glucose levels.
How should I give the fluids?
Access to the circulation should be via the intravenous route Fluid therapy: overview (the intraosseous route may be more applicable in some patients) throughout the peri-operative period. Before surgery, always consider if adequate "lines" have been placed:
- The necessity to place an intravenous catheter in an emergency should be strenuously avoided.
- Prior to draping the patient, ensure a catheter will always be readily accessible throughout the procedure, even if this requires additional cannulation.
- Ensure the patient's catheter(s) are of sufficient diameter to allow rapid administration of fluids.
- Some intravenous solutions are incompatible with others. For example, Hartmann's solutions Lactated Ringers solution should not be administered through the same "line" as either blood products or sodium bicarbonate Sodium bicarbonate .
Should I warm the fluids?
- Giving fluids at room temperature will cool the patient; however, at 10mL/kg/hr it has a minimal effect compared to other losses.
- It is prudent to warm them to body temperature prior to administering fluids rapidly or giving massive volumes.
- Always take care to follow the manufacturer's recommended guidelines; in particular some specifically prohibit the use of microwave ovens to heat the fluids.
Monitoring fluid therapy
- Although intravenous fluid therapy is frequently used to support arterial blood pressure, adequate blood pressure does not imply an adequate circulating volume. Often hypovolemic patients will vasoconstrict to maintain a normal arterial blood pressure.
- The use of urine output to monitor renal perfusion may be complicated by the effects of many anesthetic drugs.
- Heart rate often increases with hypovolemia (e.g. blood loss during surgery); however, it is a non-specific sign and this physiological response may be obtunded by anesthetic drugs.
- Clinical signs, such as skin turgor, are only a crude guide to the patient's circulating volume. They are not useful during anesthesia.
- A clinically useful measure of circulating blood volume is the Central Venous Pressure (CVP) Central venous pressure . It can be measured with equipment available to the general practitioner.
What about the patient's Packed Cell Volume (PCV)?
- The use of PCVs Hematology: packed cell volume to assess any hemodilution/concetration may complicated by several factors:
- As we are administering fluids, we need to consider to what extent we are diluting the blood components, such as red blood cells and clotting factors. If fluids are added to the circulation faster than they are being lost, the concentration of cells will fall. This could be detected as a fall in the PCV.
- If the PCV falls too low, this will have a detrimental effect on how much oxygen is delivered to the tissues. Generally, it is thought that ensuring adequate circulating volume is of paramount importance until the PCV gets to be quite low (about 15%).
- After administering large volumes of crystalloids, the PCV may fall dramatically only to rise again as the fluid moves out of the circulation.
What fluid and how fast?
- Generally speaking, the losses during anesthesia are best matched by one of the isotonic (130-160 mEq/l) crystalloid solutions referred to as the "replacement solutions" Fluid therapy: overview .
- Hartmann's is the most common choice for peri-operative veterinary use in the UK.
- "Maintenance solutions", Fluid therapy: overview ,do notmatch the expected losses during surgery.
What is the "normal" rate of administration?
- Often arbitrarily set at 10mL/kg/hr throughout the period of anesthesia.
- This should only be considered as a starting point.
- Well tolerated by most patients as it does not result in excessive dilution of blood components and is helpful in maintaining a "good circulation".
- Sufficient to meet fluid losses when a patient undergoes abdominal surgery.
- 5mL/kg/hr may be more appropriate for situations where losses will be smaller (such as diagnostic procedures); however, even then 10mL/kg/hr does not result in overload in the vast majority of patients.
- If it was not possible to correct any pre-existing fluid deficits prior to induction 10mL/kg/hr may be insufficient.
What do I do about blood loss?
- Special consideration is needed for hemorrhage as this loss is borne by the circulation only Fluid therapy: for hemorrhage .
- If the patient is losing large amounts of blood additional crystalloids should be given (as well as the 10ml/kg/hr) to match this loss and maintain the circulating volume.
- Crystalloid fluid administered intravenously will rapidly leave the circulation. As a rule of thumb, only one third of the volume given will still be in the circulation one hour later.
- Estimate the blood volume lost, and gives three times the volume as replacement crystalloids.
- If the blood loss is extensive this could require the administration of very large volumes of crystalloids. If this is the case, artificial colloids Fluid therapy: overview may be a better choice of fluid. As colloids have a greater tendency to remain in the circulation than crystalloids, the volume given should equal the volume of blood lost.
- Due to the cost of colloids, and concern over potential side effects, they should only be used when crystalloids are insufficient.
Should I give a bolus of fluids to crystalloids to patients with tachycardia?
- There are several possible reasons for tachycardia, including insufficient analgesia. However, if the patient is suspected to be hypovolemic, it is prudent to give a bolus of 5 to 10ml/kg of replacement crystalloid over fifteen minutes to see if the heart rate slows. Greater care must be taken with certain patients.
When do I stop administering fluids?
- It is necessary consider the patient's fluid balance until it is fully recovered and able to regulate it for itself.
- Fluid therapy may be necessary until the animal is eating and drinking normally.
- Fluid losses will continue in the post-operative period but at a decreased rate; consequently, fluid administration should be reduced.
What can go wrong?
- Diluting blood constituents(in particular RBCs and clotting factors). If very large volumes of fluids are administered then the concentrations of RBCs and clotting factors may become dangerously low. These patients will require transfusion with appropriate blood products Blood transfusion .
- Hypervolemia. Excessive volume in the circulation can result in a dangerously high blood pressure, increased work for the heart and lead to pulmonary edema.
- Hypoproteinemia Hypoproteinemia . The concentration of plasma protein can become excessively low with very large volumes of crystalloids causing lots of water to move out of the circulation. This could result in pulmonary or cerebral edema.
- Iatrogenic derangements in blood pH and electrolyte concentrations.
- Special care needs to be taken with patients with the following conditions, as there will be a smaller margin for error:
- Heart failure. A failing heart has only a small reserve to cope with excessive fluid load; also, it will not compensate as well for a deficient fluid volume.
- Coagulation defects. Abnormally low concentrations of platelets or clotting factors may be exacerbated by excessive fluid therapy.
- Kidney disease. This will worsen if the adverse effects of anesthesia and surgery on fluid balance are not counteracted.
- Severe liver disease. These patients may have low plasma proteins and/or clotting factors.
- Endocrine disease. These patients are likely to have an abnormality, such as a derangement in plasma electrolytes or blood glucose levels, requiring correction prior to induction.