The cell wall separates the intracellular compartment from the extracellular compartment. The capillary endothelium and the walls of arteries and veins divide the extracellular compartment into the intravascular and interstitial compartments. Water moves freely through cell and vessel walls and is distributed through all these compartments (Grocott, Mythen, and Gan, 2005).

Holliday and Segar (l957) established that regardless of age, to dissolve and eliminate metabolic wastes all healthy persons require approximately 100 milliliters (mL) of water per 100 calories metabolized. This means that a person who expends 1800 calories of energy requires approximately 1800 mL of water for metabolic purposes. The metabolic rate increases with fever; it rises approximately 12% for every 1° C (7% for every 1°F) increase in body temperature. Fever also increases the respiratory rate, resulting in additional loss of water vapor through the lungs (Porth, 2007). Water and solute depletion may occur because of either decreased intake (e.g., fasting before surgery, anorexia, or altered consciousness level) or increased losses (e.g., diarrhea, vomiting, or pyrexia). There are two main physiological mechanisms that assist in regulating body water: thirst and antidiuretic hormone (ADH). Thirst is primarily a regulator of water intake and ADH a regulator of water output. Both mechanisms respond to changes in extracellular osmolality and volume (Porth, 2007).

Osmosis is a process by which a solvent, usually water, moves through a semipermeable membrane from a solution of lower concentration to a solution of higher concentration. The osmotic pressure exerted by particles in a solution is determined by the number of particles per volume of fluid versus the mass or size of the particles (Phillips, 2005). The osmotic pressure at the cell membrane differs from that at the capillary membrane. At the capillary membrane, the pressure is referred to as oncotic pressure, while at the cell membrane the pressure is referred to as osmotic pressure (Hankins, 2006). Because proteins are the only dissolved substances in the plasma and interstitial fluid that do not diffuse readily through the capillary membrane, the colloid osmotic pressure is influenced by proteins. The concentration of proteins in plasma is two to three times greater than the concentration of proteins found in the interstitial fluid. Only those substances that do not pass through the semipermeable membrane exert osmotic pressure, and proteins are the only substances that do not readily penetrate the pores of the capillary membrane. Therefore proteins in the extracellular fluid spaces are responsible for the osmotic pressure at the capillary membrane.

In a healthy person, the net intracapillary pressures are more than the interstitial pressures, and this results in a pressure gradient that produces a slow continuous flow of fluid from capillary lumen to interstitium. This tissue space, or interstitial fluid, drains via the lymphatic system back into the systemic circulation. In disease, all these factors can be altered, often resulting in an increase in the loss of fluid from the circulation (Grocott et al, 2005). Starling (1896) formulated the Law of Capillaries, which states that equilibrium exists at the capillary membrane when the fluid leaving the circulation and the amount of fluid returning to circulation are exactly equal.

Capillary pressure tends to force fluid and dissolved substances through the capillary pore into the interstitial spaces. Colloid osmotic pressure tends to cause fluid to move via osmosis from the interstitial spaces into the blood, thus preventing a significant loss of fluid volume (Hankins, 2006). The volume of distribution of infused fluids is dictated by their solute content. In turn, the plasma volume expansion effect is directly related to the volume of distribution:

Infusion of isotonic crystalloid (e.g., 0.9% sodium chloride or lactated Ringer’s solution) will expand all the components of the extravascular volume, and 20% of the volume infused will remain in the intravascular space. Infusion of an “ideal colloid,” containing large molecules that do not escape from the circulation, will expand the intravascular volume by 100% of the volume infused (Grocott et al, 2005). To rationally prescribe fluid replacement, it is important to identify which compartment is depleted.

The osmotic activity of a solution may be expressed in terms of either its osmolarity or its osmolality. Osmolarity refers to the osmolar concentration in 1 liter (L) of solution (mOsm/L) and osmolality is the osmolar concentration in 1 kilogram (kg) of water (mOsm/kg of H ₂O). Osmolarity is usually used when referring to fluids outside the body and osmolality for describing fluids inside the body. Because 1 L of water weighs 1 kg, the terms osmolarity and osmolality are often used interchangeably (Porth, 2007). The term osmolality will be used in this chapter. Extracellular osmolality is primarily determined by the sodium level because it is the main solute found in extracellular fluid. A rough estimation of extracellular osmolality can be made by multiplying the plasma sodium concentration by 2 (Porth, 2007).

A change in water content causes cells to swell or shrink. The term tonicity refers to the tension or effect that the osmotic pressure of a solution, with impermeable solutes, exerts on cell size because of water movement across the cell membrane. Tonicity is determined solely by effective solutes such as glucose that cannot penetrate the cell membrane, thereby producing an osmotic force that pulls water into or out of the cell, causing it to change size.

Solutions to which body cells are exposed can be classified as isotonic, hypotonic, or hypertonic depending on whether they cause cells to swell or shrink. Cells placed in an isotonic solution, which has the same effective osmolality as intracellular fluids (280 to 295 mOsm/L), neither shrink nor swell. When cells are placed in hypotonic solution, which has a lower effective osmolality than intracellular fluids, they swell as water moves into the cell, and when they are placed in a hypertonic solution, which has a greater effective osmolality than intracellular fluid, they shrink as water is pushed out of the cell.

Water has the priority in maintenance therapy. The body needs water to replace insensible loss, which can occur as perspiration from the skin and moisture from respirations. The average adult loses 500 to 1000 mL of water over 24 hours through insensible loss. Water is also an important dilutor for waste products excreted by the kidneys. In addition, an individual’s fluid requirements are based on age, height, weight, and amount of body fat.

Maintenance therapy provides nutrients that meet the daily needs of a patient for water, vitamins, electrolytes, glucose, and protein, with water having priority. The typical patient profile for maintenance therapy is an individual who is allowed nothing by mouth (NPO) or whose oral intake is restricted for any reason. Remember that insensible loss is approximately 500 to 1000 mL every 24 hours. Maintenance therapy should be 1500 mL per square meter (m ²) of body surface area over 24 hours (Metheny, 2000). For example, a man weighing 85 kg (187 lb) has a body surface area of 2 m ²; 1500 times 2 equals 3000; therefore he needs 3000 mL of fluids for maintenance therapy. Balanced solutions for maintenance therapy include water, daily needs of sodium and potassium, and glucose.

Replacement therapy is necessary to take care of the fluid, electrolyte, or blood product deficits of patients in acute distress; this type of therapy is supplied over a 48-hour period. The following are examples of conditions of patients needing replacement infusion therapy (and their replacement requirements):

Replacement therapy has a twofold rationale. The first rationale is to restore fluid loss when the previous output exceeded intake. After kidney status has been considered, a hydrating solution (e.g., 5% dextrose in 0.2% sodium chloride) is administered. This restores an adequate output of urine and then allows electrolytes to be replaced. The other rationale for replacement therapy is to restore present fluid and electrolyte losses, such as loss of intestinal fluid through continuing diarrhea. A solution such as lactated Ringer’s injection can be used to replace this type of loss. Replacing continuing losses prevents acidosis and alkalosis.

When the maintenance of body requirements cannot be met, the authorized prescriber should institute replacement therapy. The authorized prescriber must determine the losses and calculate replacement over a 48-hour period. Assessment of kidney function is the first step in determining appropriate replacement therapy. Patients requiring replacement therapy, except those in shock, require potassium. Patients under stress from tissue injury, wound infection, or gastric or bowel surgery also require potassium. Adequate replacement is achieved using 20 mEq/L of potassium (Metheny, 2000). A key nursing assessment before beginning replacement therapy is validation of kidney function before administering potassium in replacement therapy.

Therapy for concurrent losses is achieved on an ongoing daily basis. Critical evaluation of concurrent losses of fluids and electrolytes is done at least every 24 hours. Accurate documentation of intake and output is extremely important in this type of fluid and electrolyte management. Restoration of homeostasis depends on the nursing assessment of intake of IV solutions, as well as on the documentation of all body fluid losses. The types of clinical patients who require 24-hour evaluation are those with draining fistulas, abscesses, nasogastric tubes, burns, and abdominal wounds. The fluid and electrolyte management of these patients cannot be completed in 48 hours. Therefore maintenance therapy and replacement therapy do not meet these patients’ needs. A day-by-day restoration of vital fluids and electrolytes is necessary. With these types of patients, you will see frequent changes in the types of solutions ordered, in the amounts of electrolytes ordered based on laboratory test results, and in the rate of infusion. Restoration of electrolyte imbalance is imperative for proper homeostatic management therapy. The type of restoration fluid ordered depends on the type of fluid that is being lost. For example, excessive loss of gastric fluid must be replaced by solutions that include sodium, potassium, and chloride.

Assessment of a patient for delivery of parenteral solutions includes collecting a nursing history, performing a focused physical assessment, monitoring pertinent laboratory tests, and evaluating intake and output. The purpose of this assessment is to identify patients at risk for or already experiencing alterations in fluid and electrolyte balance.

Nursing history related to fluid and electrolyte balance includes questions about the patient’s medical history, current health concerns, food and fluid intake, fluid elimination, medications, and lifestyle. Physical assessment correlates data with the nursing history and laboratory studies. Focused assessment would include, but not be limited to, the following:

In addition to the systems’ assessment listed, the nurse will need to track vital sign changes, monitor daily weights, and record intake and output. The intake and output record should be a standard nursing order on all patients receiving intravenous solutions. Accuracy is of utmost importance; therefore all liquids (intake or output) should be measured carefully and should be estimated when they cannot be measured. Monitoring for signs and symptoms of fluid volume overload includes assessment of lungs, symptoms of fluid volume excess, vital signs, and laboratory values. These are independent nursing actions, and they do not require an authorized prescriber’s order (Heitz and Horne, 2005).

The next important aspect of determining fluid needs is the review and interpretation of a patient’s laboratory findings. The two systems that have the most direct impact on fluid and electrolyte balance are the renal and cardiovascular systems. Therefore tests that reflect the proper functioning of the kidneys and heart require consistent and close scrutiny. Box 13-1 summarizes laboratory findings for monitoring effective parenteral fluid therapy.