Chapter 20 Fluid Management
Overview of fluid and electrolyte physiology
1. What is the goal of perioperative fluid management?
2. What percentage of body weight is represented by water?
3. In what two compartments is total body water found?
4. What percentage of extracellular fluid volume is occupied by plasma volume? What are the other constituents of extracellular fluid?
5. How does plasma differ from other components of extracellular fluid?
6. What are the major sources of daily water loss? How does temperature affect daily water loss?
7. What body compartment has the highest concentration of potassium? What electrolytes are found in plasma?
8. Which source produces the greatest volume of gastrointestinal fluid? Which source of gastrointestinal fluid contains the highest concentration of bicarbonate?
Fluid replacement solutions
9. What are the daily water, potassium, and sodium requirements for the average adult?
10. How are crystalloids grouped? How do crystalloid solutions distribute?
11. What is the composition of a balanced salt solution?
12. What occurs when normal saline is used in large volumes?
13. What group of patients routinely receives normal saline during surgery? Why?
14. What is the sodium concentration of hypertonic saline? What are the potential benefits of hypertonic saline?
15. What is the advantage of using 5% dextrose water instead of pure water? What are the clinical indications for use of 5% dextrose water?
16. What are colloids? How should colloids be used to correct blood loss in clinical practice?
17. What are the differences between 5% albumin and 25% albumin?
18. What is dextran? What is the indication for its intravenous administration?
19. What are hydroxyethyl starches?
20. What are some potential adverse effects of using hydroxyethyl starches or dextran for volume replacement?
21. What are the arguments for crystalloids versus colloids for perioperative fluid replacement?
Perioperative fluid strategies
22. What is the 4-2-1 rule of perioperative fluid management?
23. What is compensatory intravenous volume expansion?
24. How is fluid deficit corrected?
25. How is blood loss replaced clinically?
26. What is “third-space loss of fluid”? What is used to replace it?
27. Why might the traditional fluid management strategy cause problems? How might one restrict the fluid administration?
Answers*
Overview of fluid and electrolyte physiology
1. Proper perioperative fluid management requires knowledge of the patients’ surgical procedure, their preexisting disease states, and the physiologic effects of the anesthetic plan. Balancing these three factors will allow the anesthesiologist to maintain the patient’s intravascular volume, cardiac preload, oxygen-carrying capacity, coagulation status, electrolyte balance, and acid-base homeostasis. (364)
2. Total body water represents approximately 60% of the body’s total weight in the average adult. The relative percentage of body water can vary depending on age, gender, and adiposity. The average 70 kg man contains approximately 600 mL/kg or 40 L of total body water. (364)
3. Total body water is found in the intracellular and extracellular compartments. The intracellular fluid volume averages 400 to 450 mL/kg and the extracellular fluid volume averages 150 to 200 mL/kg. (364)
4. The two main components of the extracellular compartment are blood volume, which averages 60 to 65 mL/kg, and the interstitial fluid volume, which averages 120 to 165 mL/kg. The other constituents of extracellular fluid include pleural fluid, peritoneal fluid, aqueous humor, sweat, urine, lymph, and cerebrospinal fluid. (364-365)
5. The difference between plasma and other components of extracellular fluid is the protein count. Plasma contains a much higher concentration of protein, which results in a much higher plasma oncotic pressure. This oncotic gradient between the plasma and interstitial fluid helps maintain intravascular volume. (364-365)
6. The major sources of daily water loss under normal activity and temperature are urine, sweat, feces, and insensible losses. The average 70 kg man loses approximately 2300 mL of water per day. The majority of this water loss is from urine (1400 mL/day) and insensible losses (700 mL/day). However, when body temperature is increased, daily water loss increases to 3300 mL/day, largely due to the increase in water loss from sweating (1400 mL/day). (Table 23-1, 365)
7. Potassium concentration is highest in intracellular fluid, which contains 150 mEq/L. Plasma contains 4 mEq/L of electrolytes, while extracellular fluid is responsible for 4.5 mEq/L. (Table 23-2, 365)
8. A normal adult produces about 6000 to 8000 mL of gastrointestinal fluid per day. The stomach and ileum can each generate up to 2000 mL/day of gastrointestinal fluid; however, the jejunum may produce the greatest volume of gastrointestinal fluid (4000 mL/day). The gastrointestinal fluid generated by the pancreas contains the highest concentration of bicarbonate (95 to 120 mEq/L). (Table 23-3, 365)
Fluid replacement solutions
9. The average adult needs 1.5 to 2.5 L of water, 50 to 100 mEq of sodium, and 40 to 80 mEq of potassium daily. (365)
10. Crystalloids are fluids that contain water and electrolytes. They are grouped as balanced, hypertonic, and hypotonic salt solutions. Crystalloid fluids distribute freely between the intravascular and interstitial compartments. Approximately one third of intravenously administered crystalloid remains intravascular. (365-366)
11. The most common balanced salt solutions used are lactated Ringer solution, Plasma-lyte, and Normosol. All balanced salt solutions have a composition similar to ECF. Their sodium concentrations are considered hypotonic and a buffer is present that takes the place of bicarbonate. (366)
12. Normal saline (0.9% NaCl) is isotonic, but contains more chloride than ECF. It contains no other electrolytes or buffer. Large scale fluid replacement with normal saline results in a nonanion gap metabolic acidosis. (366)
13. Normal saline is commonly used in patients with chronic renal failure. Their inability to excrete potassium makes normal saline a popular crystalloid choice. (366)
14. Hypertonic salt solutions contain 250 to 1200 mEq/L of sodium. The higher the sodium concentration, the less volume is needed for resuscitation because hypertonic salt solutions osmotically shift fluid from the intracellular space to the extracellular space. The reduced volume needed may reduce tissue edema. This may prove beneficial for those patients experiencing prolonged bowel surgery, burns, or brain injuries. (367)
15. Five percent dextrose water (D5W) is considered a free water solution because the dextrose is metabolized. It is considered iso-osmotic and does not cause hemolysis. Hemolysis results when pure water is infused intravascularly. D5W is commonly used to prevent hypoglycemia in diabetic patients taking insulin in the perioperative period. Also, it is used as a treatment for hypernatremia. (367)
16. Colloids are the fluids containing large molecules, which usually do not cross capillary membranes and remain in the intravascular space. Commonly used colloids are albumin, hydroxyethyl starch (hetastarch), and dextran. When used to correct the perioperative blood loss, colloids are generally administered in a volume equivalent to the volume of blood loss. Colloids are distributed entirely intravascularly, so the initial volume is equivalent to plasma volume. The half-life of albumin in circulation is 16 hours, but it can be as short as 2 to 3 hours. (367-368)
17. Albumin solutions are commercially available mostly in 5% and 25%. Five percent albumin is also called plasma protein fractions, which has an osmotic pressure around 20 mmHg (as plasma colloid osmotic pressure). Twenty-five percent albumin has five times the normal concentration and expands blood volume by five times after intravenous administration. Infectious agents are eliminated during the preparation process for all albumin solutions. (367)
18. Dextran is commercially available in Dextran 40 and Dextran 70, which indicate their mean molecule weight 40,000 Da and 70,000 Da, respectively. Dextran solutions are water-soluble glucose-polymers that are synthesized by certain bacteria and degraded enzymatically to glucose. Six percent Dextran 70 is administered for the same indications as 5% albumin (temporary volume expansion). However, Dextran 40 is administered for vascular surgery to prevent thrombosis and is rarely used to expand volume. (367)
19. Hydroxyethyl starches (HES) are synthetic colloid solutions and are characterized by their concentration, molar substitution, and molecular weight. Solutions with higher molar substitution and molecular weight hydroxyethyl starches usually have a more prolonged volume effect, but may experience more side effects. (367)
20. HES and dextran solutions are generally very safe. HES may produce coagulation disturbances and renal toxicity. HES interfere with von Willebrand factor, factor VIII, and platelet function. The effects on renal toxicity are controversial but appear more common in older, higher-molecular-weight solutions. Dextran can potentially cause anaphylactic or anaphylactoid reactions in about 1 in every 3300 administrations, increased bleeding time caused by decreased platelet adhesiveness (at doses of 20 mL/kg/24 hours), Rouleaux formation, and noncardiogenic pulmonary edema in rare cases. (367)
21. Crystalloid solutions are effective plasma volume expanders, which are cheaper than current colloid preparations, and do not contain the transmission risks associated with colloid fluids. Also, some studies indicate that albumin enters the pulmonary interstitial compartment freely and only increases the amount of albumin cleared by lymphatics. Historically, colloids have shown no advantage over crystalloid when used for expansion on intravascular volume. Proponents of colloids argue that continued crystalloid use dilutes plasma proteins and lowers plasma oncotic pressure. This increase in interstitial fluid supports edema formation. Lastly, colloid is administered 1:1 for every milliliter of blood loss. This may lead to a more rapid restoration of filling pressure and arterial blood pressures. (367-368)
Perioperative fluid strategies
22. The 4-2-1 rule provides a very close approximation of the water requirement. This rule states that the first 10 kg of body weight needs 4 mL/kg/hr and the second 10 kg of body weight needs 2 mL/kg/hr. After the first 20 kg of body weight, the water requirement is 1 mL/kg/hr. For example, a 70 kg man requires 110 mL/hr of water, or 2640 mL/day.
23. Most general and regional anesthetics cause venous and arteriolar dilation, thus increasing the total vascular capacity. Fluids must be infused to fill the expanded intravascular space. If not, the patient will potentially have decreased venous return and decreased preload. This may lead to lower cardiac output and lower organ perfusion pressure. Perioperatively, administration of intravenous fluids can maintain venous return, cardiac preload, stroke volume, and cardiac output. The administration of intravenous fluids to fill the increased vascular capacity due to anesthesia is called compensatory intravenous volume expansion (CVE). CVE with 5 to 7 mL/kg of balanced salt solution is advised before or simultaneously with the onset of anesthesia. (369)
24. The fluid deficit equals the maintenance fluid requirement times the hours since last intake (“NPO deficit”), plus unreplaced preoperative external and third-space losses. If hypovolemia is present, sufficient fluid should be infused to restore filling pressures, heart rate, and arterial pressure to preinduction baseline values. Fluid deficit is corrected by infusing three to four times the maintenance rate until the calculated deficit has been corrected. Balanced salt solutions are usually the most commonly used replacement of fluid deficits. (369)
25. Each milliliter of blood loss is usually replaced with 3 mL of balanced salt solution or 0.9% sodium chloride solution, 1 mL of colloid solution, or 0.5 mL of packed red blood cells (PRBC) plus colloid or crystalloid solutions. The general principle for the replacement of external losses (e.g., blood, ascites) is to maintain normal blood volume and normal composition of the extracellular fluid volume. Hemoglobin levels of 7.5 g/dL or higher are usually well tolerated in patients with reasonable cardiac function and without compromised regional circulations. A formula used to calculate the red blood cell (RBC) volume is based on a patient’s weight (kilograms), initial hematocrit, and desired hematocrit. The EBV is estimated blood volume, whose value is 55 mL/kg in an average adult woman and 70 mL/kg in an average adult man. (369)
26. Third-space loss of fluid is the fluid redistributed to spaces inside the patient’s body but functionally not participating in the intravascular blood circulation (e.g., ascites, pleural effusion, and gastrointestinal tract fluid accumulation). The composition of third-space losses is usually equivalent to extracellular fluids (in regard to electrolytes) but contains a lower concentration of proteins. A balanced salt solution is the most appropriate replacement for third-space fluid losses. (369)
27. Certain surgical procedures or patient disease states may lend themselves to restricting fluid administration. For instance, in patients undergoing pulmonary surgery, the risk of postpneumonectomy pulmonary edema is directly related to the amount of fluids administered. Patients undergoing liver resection may benefit from a low central venous pressure to prevent bleeding. Patients with a history of end-stage renal disease or congestive heart failure may also benefit from restricted fluid administration. Restrictive fluid management strategies include: replacing blood loss on a milliliter per milliliter basis with colloid, not replacing urine or third-space loss during surgery, no fluid loading prior to anesthesia, colloid bolus for treatment of hypovolemia, postoperative restriction of fluids, and administration of diuretics for weight gain. (370)
