Perioperative Fluid Balance

Perioperative Fluid Balance

Intraoperative fluid management is basically composed of providing normal maintenance fluid requirements, correcting any preexisting (preanesthetic) deficits, replacing ongoing losses (surgical trauma) and adjusting the type and quantity of fluids (Table 6.5).

Maintenance Fluids

Regardless of patients underlying state of health and the surgical disease or condition, there is a constant loss of water and electrolytes from the body. These losses come about through several organ systems, including the skin, lungs, kidneys, and gastro intestinal tract. Maintenance fluids are providing to counter balance these ongoing sensible and insensible losses. One can estimate the maintenance fluid requirements of patients of all ages using the "4-2-1'" rule (Table 6.4)

Table 6.4 Maintenance Fluid Requirements

According to this guideline the maintenance hourly fluid requirements for a 70 kg man would be: (4X10 = 40) + (2X10 = 20) + (1X50 = 50) = 110 ml/hour.

The maintenance fluid requirement of a patient is not static; it may change with certain conditions. Change in body temperature affect fluid requirements. Febrile patients will increase caloric expenditure and fluid requirements by approximately10% to 12% above normal for each degree centigrade rise in temperature above normal. Hypothermia results in a decrease in fluid requirements of the same magnitude for each centigrade drop in temperature below normal.

In addition to water replacement, electrolytes losses may also need to be replaced. In adults, 75 mEq of sodium should be adequate to replace daily renal sodium losses, and 40 to 50 mEq potassium should be adequate to replace normal renal & fecal potassium losses. Calcium & magnesium rarely need to be given to patients who are on short term maintenance fluid therapy. Replacement of calories (glucose) in the form of dextrose is ideal and prevents ketosis for debilitated and premature infants; these patients may have only minimal fat or glycogen stores thus glucose has been routinely used in children to prevent hypoglycemia. Post operatively there are several factors (increased release of ADH, increased endogenous water production) that may necessitate a reduction in the rate of maintenance fluids.

Pre anesthetic Fluid Deficit

Whenever possible, volume problems should be corrected prior to anesthesia. The goal of preoperative fluid therapy is to correct all concentration and composition disturbances and, most importantly, to attain normal or slightly expanded functional ECF volume.

It should be replaced with maintenance type solution. The fluid deficit incurred during fasting is estimated: first the hourly maintenance fluid rate must be calculated & this hourly rate is then multiplied by the time of restricted intake. 50% of this deficit volume is replaced in the first hour of parenteral fluid therapy & the rest 50% in the next two hours.

Common clinical situation that may be associated with a substantial decrease in the circulating blood volume includes: hemorrhage (gastro intestinal bleeding, ruptured spleen, ruptured ectopic pregnancy, fractured hip, retroperitoneal bleeding, leaking aneurysm, etc), protracted vomiting, diarrhea, bowel obstruction, chronic hypertension, sepsis, preoperative bowel preparation (enemas & purgatives), prolonged nasogastric suction, etc.

The first step in treating these preoperative deficits is to determine the extent of the problem (look at the management of hypovolemia). The fluid used to replace pure volume losses should be nearly isotonic with respect to plasma and should also contain salt. Preoperative fluid deficits should undergo total correction before the administration of an anesthetic. However, an urgent need for surgery may preclude replacement of the entire deficit. Relatively small volume deficits (i.e., less than 20% of the blood volume) can often be replaced with an isotonic or balanced salt solution administered over a period of 15 minutes or less. If the patient has a large deficit or there is little time before surgery, 25%-50% of replacement can be given over 1 hour with remainder infused in a detrimental fashion over several hours. Large deficits can also be replaced by a continuous infusion given over 8 to 24 hours, when time allows. Urinary catheter help to monitor urine output and hydration status along with blood pressure and heart rate monitor.

Should be corrected before anesthesia is induced that will prevent possible adverse reaction. Many conditions can result in electrolyte imbalance. Chronic gastric fluid loss may lead to metabolic alkalosis; since gastric secretions are rich in hydrochloric acid. Chronic diarrhea, on the other hand, may result in metabolic acidosis. With burn injury or ascites, the fluid that is lost is rich in protein and contains electrolytes in concentrations similar to plasma concentrations.

Intra Operative Fluid Losses (Fluid Loss Associated With Surgery)

During surgery the deficits develop rather quickly and should be replaced in an immediate and ongoing fashion.

The volume of fluid sequestrated from functional compartment to non functional compartment (it does not transport nutrients or metabolites and is unable to return to the vascular space in the event of sudden hemorrhage.) which is proportional to the amount of surgical trauma. The exact quantity of sequestrated fluid is difficult to ascertain, and replacement of these third space losses is - at best - an approximation. Estimate for initiating replacement of third spacing: minimal trauma, 2 to 4 ml/kg/hr (e.g., superficial); moderate trauma (lower abdominal surgery), 4 to 6ml/kg/hr; extensive trauma (upper and lower abdominal surgery), 6 to 8 ml/kg/hr.

Insensible Losses which increase with fever, an elevated ambient temperature, perspiration and high flow of non humidified gasses may loss hypotonic fluids. These insensible losses may vary substantially and are difficult to quantify depending on the conditions, 1 to 4 ml/kg/hr of hypotonic fluid may appropriately replete these deficits.

This is a relatively frequent occurrence, but it remains difficult to quantify. Depending the amount of blood lost, the preoperative blood lost, the preoperative hemoglobin concentration, and the patient's tolerance to a reduced red cell mass (i.e., tissue oxygen delivery). Crystalloid solutions alone may be administered to maintain or normalize the blood volume. A volume of 3 to 4 ml of isotonic or balanced salt solution should be infused for each 1 ml of blood loss. The replacement fluid should be given concurrently with the blood loss at a rate that exactly balances on the going loss. Often hemorrhagic losses of less than 2 to 3 units are replaced with the only normal saline or lactated Ringer's solution, whereas larger losses of blood may require the use of colloid solution &/or transfusion.

Estimating Blood Volume: Patients undergoing a surgical procedure that may result in significant blood loss should have an estimated blood volume (EBV) calculated. The table below (Table 6.6) is the approximate blood volume based on age and weight.

Table 6.6 Normal Blood Volume in Different Age

To estimate the maximal allowable blood loss (the amount of blood loss which can be replaced using crystalloid solution) the anesthesia provider should have an idea, based on the patient's physical condition, at what level of hematocrit blood replacement should occur. A general rule is that at a blood loss greater than 25-30% of the patient's estimated blood volume a transfusion may become necessary. It may be less than this if the patient has significant disease processes such as cardiovascular and pulmonary disease. To calculate the maximal allowable blood loss, first estimate the patient's blood volume. Example, a 60 kg adult would have an estimated blood volume of 70 ml per kg. 60 multiplied by 70 equal 4,200 ml of estimated blood. Next, the preoperative hematocrit should be known. In this example it was 36%. Since the patient is healthy, the anesthesia provider has decided to allow the hematocrit to decline to a level of 25%. The formula for calculating the maximal estimated blood loss is as follows:

Fluid losses continue in to the postoperative period. Sequestration or third spacing of ECF continues after surgery. These edema fluid remains sequestrated for 1-3 days & then mobilized in to the functional ECF compartment. Although maintenance fluid should be continued post operatively until the patient tolerate an adequate degree of oral intake, infusion rate of post operative maintenance fluid require a transient reduction (24-72 hours) reduction of up to 30% to prevent hypervolemic, and hyponatremia.