14 Fluid and electrolytes
The goal of fluid management in the perioperative period is to maintain adequate intravascular fluid volume, left ventricular filling pressure, cardiac output, systemic blood pressure, and oxygen delivery to tissues. The maintenance of appropriate concentrations of body fluid and electrolytes is essential to normal physiologic function of all body systems. An understanding of basic human physiology in this area along with a brief introduction to the various types and protocols of fluid management of the patient is presented in this chapter.
Water is the most abundant and essential component of the body. It represents approximately 50% to 60% of adult body weight and 75% to 77% of body weight in infants less than 1 month of age. By approximately 17 years of age, the adult composition is attained; and in a person weighing 154 lb (70 kg), the total body water is approximately 42 L. Because women have higher fat content in their bodies and because fat is essentially free of water, they have a lower water content than men do. Older adults and those with diabetes, hypertension, or obesity also have a lower proportion of water in their bodies.
Body water is the medium within which metabolic reactions take place to facilitate the ionization of electrolytes; it acts as a reagent in many chemical reactions; it transports nutrients to cells and removes waste products; and its high specific heat and heat of vaporization make it especially suitable as a temperature regulator. The total amount of body water remains relatively stable; intake usually slightly exceeds bodily needs and the excess is excreted. Removal or output of water from the body is normally through four types of excretion: through the lungs, gastrointestinal tract, skin, and kidney.
Water intake includes not only the water consumed in beverages but also the fluids obtained from the metabolism of solid foods. The water taken in via beverages and food is referred to as exogenous water. Although variance occurs on a day-to-day basis, overall the average adult in a moderate climate with a mixed diet consumes 2500 to 3000 mL daily. Approximately 1000 mL is obtained from beverages and 1500 mL from solid and semisolid foods.
The water formed during metabolism of ingested food is called endogenous water. Because metabolism varies with body temperature, the amount of exercise performed, and other factors, the amount of endogenous water available also varies on a daily basis. In a healthy adult who performs a moderate amount of exercise, an average of 300 to 350 mL of endogenous water is available daily. Intake is influenced by the thirst center located in the hypothalamus, which is stimulated by either a decrease in blood pressure or extracellular fluid, or an increase in serum osmolality. If the fluid volume inside the cells decreases, salivary secretion is reduced, thereby causing a dry mouth and the sensation of thirst. In normal circumstances, an individual then drinks and restores the fluid volume (Box 14-1).
The surgical patient experiences even greater fluid losses. Unless the patient is coming to the operating room for a surgical emergency, in most cases adults will be NPO for at least eight hours (Box 14-2). The goal of preoperative fluid therapy is to replace preexisting fluid deficits, normal intraoperative losses (maintenance requirements), and surgical wound losses (third spacing and blood loss).
BOX 14-2 Summary of Fasting Recommendations
From American Society of Anesthesiologists Committee on Standards and Practice Parameters: Practice guidelines for preoperative fasting and the use of pharmacologic agents to reduce the risk of pulmonary aspiration: application to healthy patients undergoing elective procedures, Anesthesiol 114:495-511, 2011.
NPO guidelines are enforced because of the risk of pulmonary aspiration. Over the past few years, fasting times have become more liberal after studies have shown that reduced fasting times lower residual gastric volumes. Furthermore, prolonged fasting can contribute to hypovolemia, hypoglycemia, and patient anxiety. Longer fasting times are generally enforced in patients who are at increased risk for aspiration (Box 14-3).
From Nagelhout J, Plaus K: Nurse anesthesia, ed 4, St. Louis, 2010, Saunders; Barash P, et al: Clinical anesthesia, ed 6, Philadelphia, 2009, Lippincott Williams & Wilkins.
The amount of fluid lost through the lungs varies with the humidity, temperature of inspired air, and the rate and depth of respiration. As a rule, 300 to 400 mL of water is lost daily. This loss of water via the respiratory tract is termed insensible loss of water, so named because one is not aware of this loss. The water content of inhaled gases decreases as the ambient temperature decreases. Consequently, the insensible water loss from the lungs is higher in cold environments. Therefore patients with respiratory dysfunction need a greater water intake to offset the increased insensible water loss when they are in cold environments. In addition, an increase in the respiration rate can increase the water loss to as much as 2000 mL, which is significant in patients with chronic obstructive pulmonary disease (see Chapter 48).
The amount of water lost via the feces averages 100 mL per day; however, with vomiting or diarrhea, this loss may be greatly increased. Up to 7000 mL can be lost with diarrhea and 6000 mL with vomiting. The implications to the perianesthesia nurse are great because active vomiting can significantly affect the volume status of the surgical patient. In such cases, fluids should be increased in rate, any ordered antiemetics should be given, and the anesthesia provider should be notified.
Water lost via the skin can be via insensible (not obvious) or sensible (obvious) loss. There is constant diffusion of moisture from deep body layers to the dry surface where the evaporation occurs. Insensible loss depends largely on environmental humidity. Sensible perspiration refers to loss of water with production of sweat. Sweating is an emergency mechanism for regulation of body temperature when the heat produced by metabolic processes is excessive. The amount of sweat therefore varies with exercise and with body temperature. In a moist atmosphere, sweat may be more visible than in a dry atmosphere, but the amount of water lost is the same. Despite its role as a protective mechanism, sweating can become a hazard when body water supplies are low because the body continues to lose sweat to maintain its temperature. In the healthy adult who performs moderate exercise in a comfortable environment, approximately 500 mL of water is lost in both sensible and insensible perspiration.
Water loss via the kidneys varies with the supply of body water. The kidneys are able to concentrate the urine, and the specific gravity may approach 1.040 (normal, 1.002 to 1.030); however, if the amount of excess water is great, such as might occur when a large quantity is administered intravenously, the kidneys excrete dilute urine, the specific gravity of which might approach 1.001. In normal conditions, the kidneys excrete approximately 1.5 L per day. In patients who are vomiting or have diarrhea, obviously less water is available, and the kidneys respond promptly by curtailing water loss via urine. The two hormones responsible for control of the volume of urine are antidiuretic hormone from the posterior pituitary and aldosterone from the adrenal cortex. Antidiuretic hormone, by increasing the permeability of the renal distal convoluted tubule and collecting ducts, increases the amount of water reabsorbed and thus decreases the urine volume. Aldosterone increases the renal reabsorption of sodium and of water secondarily. Both hormones are secreted in response to lowered blood volume and serve to control output to balance intake. However, a minimal loss of fluids is obligatory; therefore perioperative monitoring of fluid balance is critical.
Patients undergoing surgery require fluid replacement for that lost through their lungs, gastrointestinal tract, skin, and urine. They also need to have fluid replacement of their NPO deficit, intraoperative fluids lost through blood loss, third space fluid shifts, evaporation through incisions, and tissue manipulation. The amount of replacement is based on the type of fluid used for replacement (i.e., crystalloids, colloids, blood products) and calculations based on a patient’s age, sex, and weight.
The fluids in the body can be divided into two compartments along with a potential third compartment or space. The two compartments are normally divided relative to the location of the cell membrane: intracellular (inside the cell) and extracellular (outside the cell). The intracellular fluid (ICF) is estimated to be approximately 40% of the body weight, or approximately 28 L of fluid, and represents approximately two thirds of the total body water. ICF provides a medium for all intracellular activities. The other compartment, the extracellular fluid (ECF), is approximately 20% of the body weight and ranges from 12 to 14 L of fluid. The fluid compartment includes the blood plasma or intravascular fluid, the interstitial fluid (ISF) that bathes the cells, the lymph, the cerebrospinal fluid (CSF), and the transcellular fluids. The transcellular fluids include the synovial fluid, peritoneal fluid, digestive fluids, and fluids of the eye and ear. The lymph, CSF, and the transcellular fluids normally constitute approximately 1% of the body mass. Blood constitutes approximately 4% of the body weight, and the interstitial fluid constitutes 15.7%.1
There is a potential third compartment, which is commonly called the third space. It is a concept that is defined as a compartment that includes the interstitial spaces that are swollen by local responses to tissue trauma, inflammation, and hormonal influx from the stress of surgery. This third space can occur even when patients have undergone massive surgical procedures and the fluid loss, to include insensible loss, is appropriately replaced. This accumulation of fluid in the third space compartment usually occurs during and immediately after the surgical procedure and is difficult to clinically differentiate from actual blood loss. Clinically, the signs of hypovolemia reflect third space loss and actual fluid loss. The treatment includes infusion of fluids in the range of 3 to 10 mL/kg/h and is usually adequate along with establishment and treatment of the underlying cause (e.g., active bleeding). The third space loss usually resolves in several postoperative days, and the nurse on the unit that receives the patient after the postanesthesia care unit (PACU) should be alert for signs of possible fluid overload as the fluid returns after surgery to the ECF.
Fluid balance involves not only the total amount of body water but also the maintenance of a relatively constant distribution of that water in the different compartments. Circulation of fluid between compartments depends on the relative hydrostatic and osmotic pressures in each compartment. Hydrostatic pressure is the force that pushes fluid from one compartment to the other. If the hydrostatic pressure in the capillaries (blood pressure) exceeds the pressure in the interstitial space, fluid moves from the capillary into the interstitial space. Osmotic pressure is the “pull” of fluids into the compartment; it is a function of the number of dissolved molecules in the solution and is not influenced by weight or size of the molecule. Electrolytes are the major contributors to the osmotic pressure of the fluids.2
The major difference between the two major compartments that make up the extracellular fluid is the much higher protein content in the plasma than in the interstitial fluid. Because capillary membranes are not selectively permeable to small particles, ions and small molecules can exchange rapidly between the plasma and the ISF. However, proteins remain in the plasma because they are too large to cross the capillary barrier. As a result, the electrolyte composition differs slightly from the plasma and the interstitial fluid. The sodium concentration in plasma is slightly greater, whereas the chloride concentration is slightly less than in the interstitial fluid and the sum of the diffusible ions. Thus, the osmotic pressure in the plasma is greater than that of interstitial fluid. The osmotic pressure caused by plasma colloids is called the colloid osmotic pressure (COP) or oncotic pressure. Protein molecules are responsible for the COP or oncotic pressure. The proteins that exert a COP help to retain the plasma water in the intravascular compartment. Albumin is the major protein in the plasma that contributes to the COP.
The extracellular fluid is regulated carefully by the kidneys to facilitate the cells being bathed in fluid that contains appropriate concentrations of electrolytes to include sodium, potassium, and nutrients. A patient with major abdominal surgery usually excretes large amounts of potassium during the first 48 hours postoperatively and for several days thereafter. As a result, the potassium is usually administered intravenously in the immediate postoperative period. The body has significant stores of potassium; therefore hypokalemia might not be evident for a number of days postoperatively. Potassium levels are generally monitored closely postoperatively, and replacement is administered intravenously when needed. It is important to note that plasma potassium measurements do not exactly predict total body potassium, because potassium is primarily an intracellular ion. From a clinical chemistry point of view, the international standard unit is the millimole (mmol), commonly called the milliequivalent (mEq). The clinical implications for the perianesthesia nurse is that patients who undergo major surgery should routinely have potassium levels checked and evaluated before surgery for determining whether they are receiving any non–potassium-sparing diuretics (see Chapter 13).