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).

BOX 14-3 Patient Conditions at Increased Risk for Aspiration

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.

Lungs

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).

Gastrointestinal tract

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.

Skin

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.

Kidneys

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.

Surgical fluid loss

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.

Distribution of body fluids

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).

Edema

A delicate balance of pressures keeps fluids passing between compartments (Fig. 14-1). A dynamic equilibrium exists between the plasma and the interstitial fluid because proteins are too large to cross the capillary barrier, which creates a colloid osmotic pressure between the two components. The hydrostatic pressures of the blood and the interstitial fluid tend to oppose each other, which is called the effective filtration pressure. Similarly, the colloid osmotic (or oncotic) pressure is the opposition between the blood and the interstitial fluid. The final common pathway is that these pressures result in a pulling in opposite directions when in appropriate physiologic equilibrium that does not allow fluid to accumulate into the interstitial spaces. Edema then results when either of the two pressures are in dysfunction.

FIG. 14-1 Components of the capillary pressure gradient. Filtration reflects the difference between the combined forces that push fluid out of the capillary (capillary pressure and interstitial fluid colloid osmotic pressure) and those that attempt to hold fluid in the capillary (plasma colloid osmotic pressure and interstitial fluid pressure).

(From Copstead LE, Banasik JL: Pathophysiology, ed 3, St. Louis, 2005, Saunders.)

Electrolytes

Electrolytes are any substance in solution that contains free ions that make the substance electrically conductive (e.g., elements, chemicals, minerals). These ions carry an electric charge. The cations are positively charged and include sodium, potassium, calcium, and magnesium. The anions are negatively charged ions and include chloride, bicarbonate, phosphate, sulfate, and ions of inorganic acids such as lactate. Protein also carries a negative charge at physiologic pH. Each of the fluid compartments of the body contains electrolytes. The concentration and specific composition of electrolytes in each compartment vary, and the number of cations in each compartment balances the number of anions to maintain electric neutrality.

The major ions found in the ECF are sodium and chloride. Potassium and phosphate are predominately intracellular ions. The predominance of sodium outside the cell and potassium inside the cell is the result of a cell membrane pump that exchanges sodium and potassium ions. This active transport mechanism requires energy from adenosine triphosphate. Although electrolytes constitute only a small fraction of the body weight, they are essential for facilitation of normal body function. They maintain electroneutrality and chemical conditions in the body fluids, equilibrium between ECF and ICF, and regulation of neuromuscular activity. Monitoring of electrolyte concentrations is usually analyzed before and after surgery, many times requiring blood being drawn in the PACU.

Sodium

Sodium is the major cation in the extracellular fluid. Blood plasma sodium averages 135 to 145 mEq/L and usually does not vary (±5 mEq/L). Variations greater than this can affect many physiologic activities; therefore mechanisms for regulation of sodium concentration are of prime importance in maintenance of balance. Basically, the body regulates sodium with conservation mechanisms when the sodium is low. If body stores of sodium are high, the body excretes sodium via sweat, feces, and, in large part, the kidneys.

The body fluids are maintained in an isotonic state with regulation of the concentration of sodium and its most abundant anion, chloride. Concentration of sodium and chloride in the fluids is maintained primarily with loss or retention of water. Loss of salt is accompanied by loss of water and retention of salt by retention of water. Water moves into areas where salt is in higher concentration, which is why patients are often placed on a low-salt diet in an effort to reduce fluid overload on the heart and other major organs. However, patients who receive magnesium sulfate can also have impaired fluid excretion.

Of particular interest to the perioperative nurse are patients who have undergone urologic surgery and have been or are currently receiving irrigation fluids in the bladder. These patients are at risk of developing hyponatremia. The most common surgical procedure associated with this complication is a transurethral resection of the prostate. Irrigation fluids typically consist of sorbitol and mannitol or glycine in distilled water. The irrigants are isotonic. The amount of irrigation solution absorbed through the venous sinuses in the bladder averages 10 to 30 mL per minute of resection time. For this reason, the resection time should ideally be limited to 1 hour or less. The absorption of the irrigating fluid results in the fluid entering the vascular system; this can lead to volume overload and ultimately to dilutional hyponatremia.

The resulting lowered serum sodium concentrations can cause serious cardiac and neurologic consequences. Concentrations of sodium at 140 mEq/L are usually associated with the development of cardiac dysrhythmias that can lead to cardiac arrest. Progressive neurologic symptoms include restlessness, confusion, nausea, vomiting, coma, and convulsions.

Hypernatremia is most often caused by a loss of body fluids resulting in excess sodium. Elective surgery should be postponed until sodium levels greater than 150 mEq/L are corrected. Correction of water deficit should take place over 48 hours with hypotonic solutions. Rapid correction can result in seizures, cerebral edema, and coma.³

Potassium

Potassium is the most important intracellular cation. Measurement of intracellular potassium is difficult; therefore only extracellular potassium is measured. The normal values are between 3.5 and 5.5 mEq/L. Potassium affects the excitability of nerve and muscle tissue, and is important in the maintenance of cardiac rhythm, deposition of glycogen in liver cells, and transmission and conduction of nerve impulses. It also contributes to cellular energy production. Overall, abnormal potassium concentrations can have serious effects on the contractility of the heart, resulting in dysrhythmias and potential cardiac arrest.

Potassium depletion can be accompanied by changes in plasma potassium concentration. True depletion develops only with a net loss of potassium, whereas a decrease in plasma potassium, hypokalemia, can occur with a shift of potassium from the ECF to the ICF. Decreased intake can cause a mild deficit because the mechanisms for potassium conservation are not as efficient as those for sodium. Severe depletion results from abnormal losses rather than decreased intake. Most common causes of severe potassium loss are usually associated with diuretics (see Chapter 13), vomiting, acute blood loss, gastrointestinal surgery, and nasogastric suctioning. Cardiac arrhythmias, polyuria, confusion, and weakness of skeletal muscle are commonly observed in mild hypokalemia. Hallucinations, diminished reflexes, ST segment depression, widened QRS, flattened T waves, and cardiac arrest result from severe depletion. Oral replacement with potassium chloride is in the range of 60 to 80 mEq/day. Peripheral intravenous (IV) potassium should not exceed 8 mEq/h so as not to irritate veins. Central IV potassium can be infused at 10 to 20 mEq/h. The administration of 0.5 mEq/kg of potassium chloride usually raises the serum potassium concentration by 0.6 mEq/L. If the patient is receiving catecholamine drugs, the increase is approximately 0.1 mEq/L; if the patient is receiving beta-adrenergic antagonists, the serum concentration increases by approximately 0.9 mEq/L. It is important to note that correction of hypomagnesemia may be needed to avoid the increased loss of potassium by the kidneys.

Hyperkalemia is often associated with situations in which cells are injured or destroyed. Examples include chronic and acute renal failure, crush injuries, burn victims, and newborns who receive relatively large transfusions. The administration of succinylcholine, a depolarizing skeletal muscle relaxant, can produce also produce hyperkalemia (discussed in Chapter 23). Accidental lethal doses of supplemental IV potassium have been administered to patients with rapid intravenous infusion in the PACU. Cases have been recorded in which death occurred within 5 minutes of the rapid injection of just 25 mEq of potassium; therefore under no circumstance should potassium chloride be given via IV push.³