Fluid and electrolytes

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Milliequivalent (mEq):  Replaced with the SI units millimole (mmol); mEq/L has been replaced by mmol/L.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.

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.

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.

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).

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).

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).

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).

Only gold members can continue reading. Log In or Register to continue

Stay updated, free articles. Join our Telegram channel

Nov 6, 2016 | Posted by in NURSING | Comments Off on Fluid and electrolytes

Full access? Get Clinical Tree

Get Clinical Tree app for offline access