Management of Fluid and Electrolytes Case File

Management of Fluid and Electrolytes Case File

Lydia Conlay, MD, PhD, MBA, Julia Pollock, MD, Mary Ann Vann, MD, Sheela Pai, MD, Eugene C. Toy, MD

Case 8

A 54-year-old man is undergoing a laparotomy and colon resection for carcinoma. The anesthesiologist is attempting to calculate the fluid replacement.

➤ What are the components that must be considered when calculating the volume of fluid that should be replaced?

ANSWER TO CASE 8:

Management of Fluid and Electrolytes

Summary: A 54-year-old man is undergoing bowel surgery, for which the volume of fluid replacement is being calculated.

➤ Factors affecting volume of fluid to be replaced: Preoperative fluid deficits, insensitive fluid losses, intraoperative blood loss, and urine output

ANALYSIS

Objectives

1. Review the distribution of infused fluids within the various bodily compartments.

2. Acquaint the student with methods of calculating the fluid replacement in the intraoperative period.

3. Allow the student to become familiar with the advantages and disadvantages of crystalloids and colloids.

Considerations

Patients undergoing intestinal surgery are well known to require more fluid replacement than might otherwise be expected. This patient will no doubt have preoperative fluid deficits, because he has not been eating or drinking, and has also undergone a bowel prep. Intraoperatively, the patient will lose blood, and there is also likely to be a significant insensitive loss from the intestines exposed to air. Fluids—crystalloids or blood products—will be required to ensure the preservation of renal and cardiovascular function, maintaining urine output at normal levels.

APPROACH TO

The Management of Fluids and Electrolytes

Many factors make fluid management in the perioperative setting different from any other situation. Preoperative fasting, insensible fluid losses, blood loss from the surgical site, postoperative dietary restrictions, and nasogastric tube drainage require the use of replacement fluids. Such fluids include crystalloids, colloids, and blood products. Administration of fluids is guided by the nature of the bodily fluid loss, and the patient’s disease process.

Fluid Compartments

Total body water constitutes approximately 70% of lean body weight, although this percentage varies with the deposition of adipose tissue. Total body water is distributed between the extracellular and intracellular compartments.

Intracellular fluid is the fluid contained within cells and comprises approximately two-thirds (30 L) of total body water. (Please see Figure 8–1.)

Extracellular fluid is further compartmentalized into intravascular fluid and extravascular fluid, and comprises about one-third (15 L) of total body water.

Intravascular fluid consists of plasma (3 L), plus blood cells (2 L), which together constitutes circulating blood volume (5 L).

Extravascular fluid (12 L) is mostly found in tissues adjacent to the microvascular circulation, and is called the “functional compartment.” In contrast, the “nonfunctional” fluid compartment is ill defined, and devoid of fluid in the normal physiologic state. However, fluid can distribute into this “third” space postoperatively. Common examples of “third-spacing” include the bowel, peritoneal cavity, and traumatized tissues.

During normal circumstances, the lymphatic system removes the excess interstitial water and returns it to the intravascular compartment, maintaining the equilibrium and preventing intravascular depletion. In the perioperative setting, this mechanism may be depressed due to processes such as inflammation, rendering the body unable to redistribute the interstitial fluid and effectively causing edema.

Figure 8–1. Relationship and distribution of bodily fluid spaces.

Distribution of Infused Fluids

The prediction of plasma volume expansion in response to a fluid administration

(assumes that the body fluid spaces are static).

Plasma volume increment = volume infused × plasma volume/distribution volume. Rearranging this equation facilitates an estimation of the amount of fluid that needs to be administered.

Volume infused = desired plasma volume increment × distribution volume/plasma volume.

For example, to get 2 L increment in plasma volume:

2 × 45(TBW)/3 = 30 L D5W, or 2 × 15(ECF)/3 = 10 L crystalloid, or 2 × 3(PV)/3 = 2 L of colloid will be required.

The primary goal of any fluid management regimen is to maintain adequate tissue perfusion. Fluid shifts during the perioperative period, and the physiological responses to surgical stress have significant implications for perioperative fluid prescribing.

The homeostasis of total body water balance is maintained with intake and losses of 2.5 to 3 L/d. Intake from ingested fluid (1300 mL), solid food (800 mL) and metabolic waste (400 mL) is balanced by insensible losses of 0.5 mL/kg/h (850 mL) from skin and lungs, plus urine (1500 mL) and feces (100 mL). The insensible losses are typically replaced by oral intake, lacking in presurgical fasting patient. Therefore, the perioperative fluid replacement has to take this “(NPO) loss” into account.

Still, many patients arrive in the operating room volume-depleted from preoperative fasting and/or the use of bowel preps. A patient’s volume status may be assessed by the conventional clinical signs of oliguria, hypotension, tachycardia, and a positive “tilt” test (a reduction in systolic blood pressure > 20 mm Hg and increase in heart rate > 20 beats/min when a patient assumes an upright position). A positive “tilt” test signals at least a 20% deficit in circulating blood volume. Hypotension while in the supine position indicates a 30% deficit in blood volume. (Please see Table 8–1.)

Intraoperative losses are frequently underestimated, and excess losses, both surgical and third-space losses, often persist into early postoperative period. Intraoperative volume status is assessed by monitoring heart rate, blood pressure, the respiratory variation of pulse pressure, urine output (≥ 1/2 cc/kg/min), central venous pressure and if indicated, echocardiographic assessment of the heart.

In the perioperative setting, fluid replacement also takes into consideration the insensible fluid losses, as well as the perioperative stress response which increases the permeability of the capillary endothelium and causes movement of water into the extravascular compartment. The stress response also causes retention of sodium and water

Table 8–1 SIGNS OF PREOPERATIVE HYPOVOLEMIA

Hypotension Tachycardia Oliguria Supine hypotension Positive tilt test Increased BUN/Creatinine (>10:1) Increasing urinary osmolality (>450 mOsm/L) Decreased urinary sodium (<10 mEq/L) Increase in urine specific gravity (>1.010) Metabolic alkalosis (mild hypovolemia) Metabolic acidosis (severe hypovolemia) Increasing serial hematocrits

Fluid Replacement Therapy

There are many ways of calculating the fluid replacement in the intraoperative period all of which take into consideration the preoperative fluid deficits, intraoperative blood loss, and urine output.

Pre-existing Fluid Deficits

Patients undergoing surgery often have 6 to 8 hours of fasting and the deficit can be estimated by multiplying the maintenance rate by the duration of fast. Patient’s preprocedural volume status may vary due to factors such as vomiting, diarrhea, ileus, fever, burns, ascites, effusions, hemorrhage, bowel preparations, or diuretics.

Maintenance Requirements

Two simple formulas are often used to estimate maintenance fluid requirements:

WEIGHT (kg)	mL/kg/h	mL/kg/d
1-10 11-20 >20	4 2 1	100 50 20

For example, a 70-kg adult will require (10 × 4 ) + (10 × 2) + (50 × 1) = 110 mL/h of maintenance. To calculate the deficit, this amount is multiplied by the hours of fasting. The deficit is infused over 3 hours, half in the first hour and the rest over the next 2 hours.

The hourly fluid replacement will also include the maintenance and losses. Daily requirements for sodium and potassium are approximately 2 mmol/kg and 1 mmol/kg, respectively. Therefore a healthy 70-kg adult requires about 2500 mL of water containing 30 mEq/L of Na and 15 to 20 mEq/L of K. Intraoperatively, fluids containing sodium-free water Na <30 mEq/L are rarely used because of the need to replace isotonic losses and the risk of postoperative hyponatremia.

Surgical Volume Losses

3 cc crystalloid = 1 cc blood

1 cc colloid = 1 cc blood

The redistributive and evaporative losses are proportionate to the degree of tissue trauma on the surgical field. This can be estimated by the following values:

• 4 mL/kg/h for minimal trauma to the tissues (arthroscopy, hand surgery, etc.)

• 6 mL/kg/h for moderate tissue trauma without significant bowel exposure (cholecystectomy, hysterectomy, etc.)

• 8 mL/kg/h for severe tissue trauma (aortic aneurysm repair, Harrington rod spinal fusion, most bowel surgery)

When a significant amount of fluid is required for a more major surgery, fluids may be titrated to physiologically relevant biometric end points such as urine output, central venous pressure, wedge pressure, pulse pressure variation, venous oxygen saturation, or left ventricular end-diastolic volume measured dynamically in the intraoperative setting. However, calculations which permit a periodic estimation of appropriate fluid administration are an invaluable addition to the physiologic end points.

The Choice of Fluids: Crystalloid vs Colloids

Despite a decade’s long debate, systematic reviews show no difference in pulmonary edema, mortality, or length of hospital stay following preferential administration of crystalloids or colloids.

Crystalloids are solutions typically containing glucose or saline, and are useful for the replacement of insensible losses. They are nontoxic, nonallergenic, and inexpensive. However, their ability to remain within the intravascular space is limited, leading to a propensity to cause interstitial edema in large quantities due to dilution of plasma proteins and thus plasma oncotic pressure. The most commonly used crystalloids are normal saline and lactated Ringer solution. Rarely, large amounts of 0.9% or normal saline can lead to hyperchloremic metabolic acidosis, whereas large quantities of lactated Ringer may result in metabolic alkalosis due to increased production of bicarbonate from the metabolism of lactate.

Colloids are homogenous noncrystalline substances consisting of large molecules dissolved in a solute. They are more likely to remain in the intravascular space and therefore serve as effective volume expanders. Hypersensitivity reactions, including anaphylaxis, have been reported with all colloids.

Coagulation abnormalities are seen with the synthetic colloids. Dextrans produce a dose-related reduction in platelet aggregation and adhesiveness, whereas hydroxyethyl starch can lead to a reduction in factor VIII and von Willebrand factor, impairing platelet function and prolonging partial thromboplastin time. Coagulation and bleeding times are not usually prolonged after 1 L of colloid infusion.

The types of colloids in clinical use today include:

Albumin, purified from human plasma pasteurized at 60°C for 10 hours. Albumin is available as 5% or 25% solution and has a plasma half-life of 16 hours, and is by far the most expensive colloid available today.

Hydroxyethyl starch is composed of amylopectin linked with hydroxyethyl groups in a glucose moiety, resulting in a polymer similar to glycogen. Examples are 6% Hespan and Hextend. Their half-life is 17 days.

Dextran is a biosynthesized commercially from sucrose by the bacterium Leuconostoc mesenteroides. Dextran 40 and Dextran 70 differ based on molecular weight. Dextran 40 is thought to improve blood flow in microcirculation, presumably by decreasing blood viscosity. Dextran 1 (also known as Promit) may be administered before dextran 40 or 70 to prevent severe anaphylaxis.

CONCLUSIONS

Perioperative fluid management is a complicated and controversial topic, with an ongoing debate regarding the type and amount of intraoperative fluid to be administered. Currently, there is a trend toward limiting fluid administration in the intraoperative setting, especially crystalloids, due to their inability to stay in the intravascular space for long periods of time. Colloid administration is not without risk, as allergic reactions and coagulopathies are not uncommon. The bottom line for fluid administration: biometric end points guide fluid replacement with the goal of maintaining adequate tissue perfusion.

Comprehension Questions

8.1. A patient suffering a stab wound to the abdomen arrives in the operating room for an emergency exploratory laparotomy. His BP is 70/40, and his HR is 118. Which of the following fluid regimens are appropriate for his fluid resuscitation?

    A. Normal saline, at a ratio of 3 mL per estimated 1 mL of blood lost

    B. Blood at a ratio of 3 mL blood administered to 1 mL of blood lost

    C. Hetastarch (Hespan) in a ratio of 3 mL per estimated 1 mL of blood lost

    D. D5W in a ratio of 3 mL of D5W to 1 mL of blood lost

8.2. A 70-kg college freshman is undergoing an arthroscopy of the knee as an 8 AM case. Does he need fluids intraoperatively? If so, what type of fluids, and at what rate?

    A. Crystalloids, 10 mL/kg/h maintenance

    B. Crystalloids, 1 mL/kg/h maintenance

    C. Crystalloids, approximately 1,000 mL for NPO losses

    D. Crystalloids, approximately 1,600 mL for NPO losses

8.3. The surgery of the freshman mentioned is delayed until 3 PM in order to accommodate an emergency. Does he need fluids intraoperatively? If so, what type of fluids, and at what rate?

    A. Crystalloids, 10 mL/kg/h maintenance

    B. Crystalloids, 1 mL/kg/h maintenance

    C. Crystalloids, approximately 1,000 mL for NPO losses

    D. Crystalloids, approximately 1,600 mL for NPO losses

ANSWERS

8.1. A. Normal saline, at a ratio of 3 mL per estimated 1 mL of blood lost, or blood at a ratio of 1 mL blood administered to 1 mL of blood lost are appropriate resuscitation regiments. Since hetastarch is a colloid, it would be administered in a ratio of 1 mL of hetastarch per 1 mL of blood lost. D5W is not an appropriate resuscitative fluid for hemorrhage secondary to trauma.

8.2. C. If the patient has been NPO for 8 hours and weighs 70 kg, he should receive approximately 1000 mL of crystalloids to replace his NPO losses. Since an arthroscopy is a minimally invasive procedure, maintenance fluids should be administered at a rate of 4 mL/kg/h.

8.3. D. If the patient’s surgery is delayed and he has been NPO for some 15 hours, he should receive approximately 1,600 mL of crystalloids to replace his NPO losses. Since an arthroscopy is a minimally invasive procedure, maintenance fluids should also be administered at a rate of 4 mL/kg/h.

Clinical Pearls

➤ Factors affecting fluid management in the perioperative setting include preoperative fasting, insensible fluid losses, blood loss from the surgical site, postoperative dietary restrictions, and nasogastric tube drainage.

➤ There is little evidence to support the advantages or disadvantages of crystalloids over colloids, or vice versa.

➤ The bottom line for fluid administration: biometric end points guide fluid

replacement with the goal of maintaining adequate tissue perfusion.

References

Barash PG, Cullen BF, Stoelting RK. Clinical Anesthesia. 6th ed. Philadelphia, PA: Lippincott, Williams and Wilkins, 2009. 

Chappell D, Jacob M. A rational approach to perioperative fluid management. Anesthesiology. 2008;109: 723-740. 

Grocott MPW, Mythen MG, Gan TJ. Perioperative fluid management and clinical outcomes in adults. Review article. Anesthesia and Analgesia. 2005;100: 1093-1106. 

Miller RD. Anesthesia. 5th ed. Vol 1. 2000. 

Rassam SS, Counsell DJ. Perioperative electrolyte and fluid balance. Continuing Education. In Anesthesia Critical care and Pain. 2005;5: 157-160. 

Roberts I, Alderson P, Bunn F, et al. Colloids versus crystalloids for fluid resuscitation in critically ill patients. Cochrane Database Syst Rev. 2004;4:CD 000567.

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