14 Fluid and electrolytes
Anions: Ions that carry a negative charge and migrate to the anode (terminal) in an electric field.
Autologous: Originating within the same person, such as an autotransfusion.
Cations: Ions that carry a positive charge and migrate to the cathode (terminal) in an electric field.
Chvostek Sign: An abnormal spasm of the facial muscles elicited by light taps on the facial nerve that indicates hypocalcemia.
Colloids: Compounds such as red blood cells, albumin, or dextran that, because of size, are retained within a specific fluid compartment and increase the oncotic pressure of that compartment.
Cryoprecipitate: A preparation rich in factor VIII needed to restore normal coagulation in hemophilia. The preparation is collected from fresh human plasma that has been frozen and thawed.
Crystalloids: Balanced electrolyte solutions that are in isotonic solutions of water or dextrose and can move between the intravascular and interstitial compartments.
Edema: Accumulation of fluid in the interstitial spaces.
Hemolysis: A disruption of the integrity of the red cell membrane that causes release of cell contents to include hemoglobin.
Hemostasis: The arrest of bleeding by the interaction of the platelet with the blood vessel wall and the formation of the platelet plug.
Hypercalcemia: Increased plasma concentration of calcium (>5.6 mEq/L).
Hyperkalemia: Greater than 6 mEq/L blood concentration of potassium.
Hypermagnesemia: An increase in the plasma concentration of magnesium (>2.6 mEq/L).
Hypernatremia: An increase in sodium in the plasma of more than 145 mEq/L.
Hypertonic Solutions (Hyperosmotic): Solutions that have an osmolality greater than that of plasma.
Hypocalcemia: Reduced plasma concentration of calcium (<4.4 mEq/L).
Hypokalemia: Less than 3 mEq/L blood concentration of potassium.
Hypomagnesemia: A decrease in the plasma concentration of magnesium (<1.6 mEq/L).
Hyponatremia: A decrease of sodium in the plasma of less than 135 mEq/L.
Hypotonic Solutions (Hypoosmotic): Solutions that have an osmolality less than that of plasma.
Isotonic Solutions: Solutions that have the same osmolality as plasma.
Milliequivalent (mEq): Replaced with the SI units millimole (mmol); mEq/L has been replaced by mmol/L.
Osmolality: A physical property of a solution, one that is dependent on the number of dissolved particles in the solution.
Tetany: A condition characterized by cramps, muscle twitching, sharp flexion of the wrist and ankle joints, and convulsions.
Third Space: Losses of fluid and electrolytes from the extracellular fluid to a nonfunctional space, an acute sequestered space that accompanies surgery.
Trousseau Sign: A test for latent tetany in which carpal spasm is induced with inflation of a sphygmomanometer cuff on the upper arm to a pressure that exceeds systolic blood pressure for 3 minutes.
Body fluid balance
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.
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).
Surgical patient considerations
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).
BOX 14-3 Patient Conditions at Increased Risk for Aspiration
• Increased intracranial pressure
• Decreased level of consciousness
• Gastrointestinal obstruction or dysfunction
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).
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
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 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).
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