Fluids and Electrolytes



Fluids and Electrolytes


Objectives


When you reach the end of this chapter, you will be able to do the following:



1 Review the function of fluid volume and compartments within the body as well as the role of each of the major electrolytes in maintaining homeostasis.


2 Identify the various electrolytes, and give normal serum values for each.


3 Briefly discuss the various fluid and electrolyte disorders that commonly occur in the body with attention to fluid volume and/or electrolyte deficits and excesses.


4 Identify the fluid and electrolyte solutions commonly used to correct states of deficiency or excess.


5 Discuss the mechanisms of action, indications, dosages, routes of administration, contraindications, cautions, adverse effects, toxicity, and drug interactions of the various fluid and electrolyte solutions.


6 Compare the various solutions used to expand and/or decrease a patient’s fluid volume and electrolytes with regard to how they work, why they are used, and specific antidotes available to counter any toxic effects.


7 Develop a nursing care plan that includes all phases of the nursing process for patients receiving fluid and electrolyte solutions.


Drug Profiles



Key Terms


Blood The fluid that circulates through the heart, arteries, capillaries, and veins, carrying nutriment and oxygen to the body cells. It consists of plasma, its liquid component, plus three major solid components: erythrocytes (red blood cells or RBCs), leukocytes (white blood cells or WBCs), and platelets. (p. 474)


Colloids Protein substances that increase the colloid oncotic pressure (p. 478)


Colloid oncotic pressure Another name for oncotic pressure. It is a form of osmotic pressure exerted by protein in blood plasma that tends to pull water into the circulatory system. (p. 475)


Crystalloids Substances in a solution that diffuse through a semipermeable membrane. (p. 476)


Dehydration Excessive loss of water from the body tissues. It is accompanied by an imbalance in the concentrations of essential electrolytes, particularly sodium, potassium, and chloride. (p. 475)


Edema The abnormal accumulation of fluid in interstitial spaces. (p. 475)


Extracellular fluid (ECF) That portion of the body fluid comprising the interstitial fluid and blood plasma. (p. 475)


Extravascular fluid (EVF) Fluid in the body that is outside the blood vessels. (p. 474)


Gradient A difference in the concentration of a substance on two sides of a permeable barrier. (p. 476)


Hydrostatic pressure (HP) The pressure exerted by a liquid. (p. 475)


Hyperkalemia An abnormally high potassium concentration in the blood, most often due to defective renal excretion but also caused by excessive dietary potassium or certain drugs, such as potassium-sparing diuretics or ACE inhibitors. (p. 480)


Hypernatremia An abnormally high sodium concentration in the blood; may be due to defective renal excretion but is more commonly caused by excessive dietary sodium or replacement therapy. (p. 482)


Hypokalemia A condition in which there is an inadequate amount of potassium, the major intracellular cation, in the bloodstream. (p. 480)


Hyponatremia A condition in which there is an inadequate amount of sodium, the major extracellular cation, in the bloodstream, caused either by inadequate excretion of water or by excessive water intake. (p. 482)


Interstitial fluid (ISF) The extracellular fluid that fills in the spaces between most of the cells of the body. (p. 474)


Intracellular fluid (ICF) The fluid located within cell membranes throughout most of the body. It contains dissolved solutes that are essential to maintaining electrolyte balance and healthy metabolism. (p. 474)


Intravascular fluid (IVF) The fluid inside blood vessels. (p. 474)


Isotonic Having the same concentration of a solute as another solution and hence exerting the same osmotic pressure as that solution, such as an isotonic saline solution that contains an amount of salt equal to that found in the intracellular and extracellular fluid. (p. 475)


Osmotic pressure The pressure produced by a solution necessary to prevent the osmotic passage of solvent into it when the solution and solvent are separated by a semipermeable membrane. (p. 475)


Plasma The watery, straw-colored fluid component of lymph and blood in which the leukocytes, erythrocytes, and platelets are suspended. (p. 474)


Serum The clear, cell-free portion of the blood from which fibrinogen has also been separated during the clotting process, as typically carried out with a laboratory sample. (p. 474)


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Anatomy, Physiology, and Pathophysiology Overview


Fluid and electrolyte management is one of the cornerstones of patient care. Most disease processes, tissue injuries, and surgical procedures greatly influence the physiologic status of fluids and electrolytes in the body. Understanding fluid and electrolyte management requires knowledge of the extent and composition of the various body fluid compartments.


Approximately 60% of the adult human body is water. This is referred to as the total body water (TBW), and it is distributed in the three main compartments in the following proportions: intracellular fluid (ICF), 67%; interstitial fluid (ISF), 25%; and plasma volume, 8%. This distribution is illustrated in Figure 29-1. The actual volume of fluid that would normally be in each compartment in an average 70-kg man with a TBW content of 60% is shown in Table 29-1.




The terms used to identify the various spaces within which the TBW is distributed can be quite confusing, and there are two basic approaches to distinguishing among the locations of the fluid. The TBW can be described as being in or out of the blood vessels (vasculature). If this point of reference is used, then the term intravascular fluid (IVF) is used to describe the fluid inside the blood vessels and the term extravascular fluid (EVF) is used to refer to the fluid outside the blood vessels. Examples of EVF include lymph and cerebrospinal fluid. As these concepts are learned, it is important to remember the difference between the prefixes intra- (inside), inter- (between), and extra- (outside). The term plasma is used to describe the fluid that flows through the blood vessels (intravascular fluid). Serum is a closely related term (see Key Terms). The interstitial fluid (ISF) is the fluid that is in the space between cells, tissues, and organs. When discussing blood vessels, the term extravascular volume is used; extravascular volume is made up of plasma and interstitial fluid (ISF). When discussing cells, the term extracellular volume is used; extracellular volume is composed of ISF and intracellular fluid (ICF). These terms are often confused and misused. Table 29-1 lists these definitions for further clarity and understanding.


Extracellular fluid (ECF) consists of both plasma and ISF. There is one big difference between the plasma and the ISF. Plasma has a protein concentration four times greater than that of the ISF, composed primarily of albumin. The reason for this higher intravascular concentration of protein is that these solutes (proteins) have a very large molecular weight, which makes them too large to pass through the walls of the blood vessels. Because of the difference in the concentration, fluid flows from the area of low concentration in the interstitial compartment to the area of high concentration inside the blood vessel, trying to create an isotonic environment on either side of the blood vessel wall. (Isotonic means an equal concentration of solutes across a membrane.) The protein in the blood vessels exerts a constant osmotic pressure that prevents the leakage of too much plasma through the capillaries into the tissues. Because proteins suspended in plasma are in a colloidal state, this particular pressure is called colloid oncotic pressure, and normally it is 24 mm Hg. The opposing pressure, that exerted by the interstitial fluid (ISF), is called hydrostatic pressure (HP), and normally it is 17 mm Hg—which is less than the colloid oncotic pressure. The phenomenon of colloid oncotic pressure is illustrated in Figure 29-2.



This regulation of the volume and composition of body water is essential for life, because body water is the medium in which all metabolic reactions occur. The body maintains the volume and composition remarkably constant by preserving the balance between intake and excretion. The amount of water gained each day is kept roughly equal to the amount of water lost. When the body cannot maintain this equilibrium, therapy with various agents is necessary. If the amount of water gained exceeds the amount of water lost, a water excess or overhydration occurs. Such fluid excesses often accumulate in interstitial spaces, such as in the pericardial sac, intrapleural space, peritoneal cavity, joint capsules, and lower extremities. This is referred to as edema. In contrast, if the quantity of water lost exceeds that gained, a water deficit, or dehydration, occurs. Death often occurs when 20% to 25% of TBW is lost.


Dehydration leads to a disturbance in the balance between the amount of fluid in the extracellular compartment and that in the intracellular compartment. Sodium is the principle extracellular electrolyte and plays a primary role in maintaining water concentration due to its highly osmotic chemistry. In the initial stages of dehydration, water is lost first from the extracellular compartments. The amount of further fluid losses, colloid oncotic pressure changes, or both determines the type of clinical dehydration that develops (Table 29-2). Clinical conditions that can result in dehydration and fluid loss, as well as the symptoms of dehydration and fluid loss, are presented in Table 29-3. When fluid that has been lost must be replaced, there are three categories of agents that can be used to accomplish this: crystalloids, colloids, and blood products. The clinical situation dictates which category of agents is most appropriate.




TABLE 29-3


CONDITIONS LEADING TO FLUID LOSS OR DEHYDRATION AND ASSOCIATED CORRESPONDING SYMPTOMS∗






















CONDITION ASSOCIATED SYMPTOMS
Bleeding Tachycardia and hypotension
Bowel obstruction Reduced perspiration and mucous secretions
Diarrhea Reduced urine output (oliguria)
Fever Dry skin and mucous membranes
Vomiting Reduced lacrimal (tears) and salivary secretions

∗There may be overlap involving more than one of the symptoms depending on the patient’s specific condition.


Acid-base balance is also important to normal bodily functions and is regulated by the respiratory system and the kidney. An acid is a substance that can donate or release hydrogen ions, such as carbonic acid or hydrochloric acid. A base is a substance that can accept hydrogen ions, such as bicarbonate. The pH is a measure of the degree of acidosis and alkalinity and is inversely related to hydrogen ion concentration. For example, when hydrogen ion concentration increases, the pH decreases and leads to acidity. As hydrogen ion concentration decreases, the pH increases, leading to more alkalinity. With the normal pH ranging from 7.35 to 7.45, acidosis occurs when there is an excess of hydrogen or carbon dioxide (CO2) and the pH falls below 7.35. Conversely, alkalosis occurs when there is a hydrogen or CO2 deficit and the pH rises above 7.45.


Regulation of the acid-base balance requires healthy functioning of the respiratory and renal systems. The respiratory system compensates for metabolic problems and pH imbalances by regulation of CO2. In acidosis, CO2 can be exhaled to try to normalize the lower pH; however, in alkalosis, carbon dioxide will be retained by the respiratory system to try and elevate the pH. The kidney also compensates by reabsorbing and generating bicarbonate and excreting hydrogen ions in acidosis to normalize the low pH. Conversely, the kidney can excrete bicarbonate and retain hydrogen ions to normalize the high pH seen with alkalosis. Certain drugs, such as the diuretic acetazolamide (see Chapter 28) and sodium bicarbonate (tablets or injection), can also be used to correct metabolic acid-base disturbances.


Pharmacology Overview


Crystalloids


Crystalloids are fluids given by intravenous (IV) injection that supply water and sodium to maintain the osmotic gradient between the extravascular and intravascular compartments. Their plasma volume–expanding capacity is related to their sodium concentration. The different crystalloids are listed in Table 29-4.



Mechanism of Action and Drug Effects


Crystalloid solutions contain fluids and electrolytes that are normally found in the body. They do not contain proteins (colloids), which are necessary to maintain the colloid oncotic pressure and prevent water from leaving the plasma compartment. In fact, the administration of large quantities of crystalloid solutions for fluid resuscitation decreases the colloid oncotic pressure, due to a dilutional effect. Crystalloids are distributed faster into the interstitial and intracellular compartments than colloids. This makes crystalloids better for treating dehydration than for expanding the plasma volume alone, such as in hypovolemic shock.


Indications


Crystalloid solutions are most commonly used as maintenance fluids. They are used to compensate for insensible fluid losses, to replace fluids, and to manage specific fluid and electrolyte disturbances. Crystalloids also promote urinary flow. They are much less expensive than colloids and blood products. In addition, there is no risk for viral transmission or anaphylaxis and no alteration in the coagulation profile associated with their use, unlike with blood products. The choice of whether to use a crystalloid or a colloid depends on the severity of the condition. The following are the common indications for either crystalloid or colloid replacement therapy:



Contraindications


Contraindications to the use of crystalloids include known drug allergy to a specific product and hypervolemia, and may include severe electrolyte disturbance, depending on the type of crystalloid used.


Adverse Effects


Crystalloids are a very safe and effective means of replacing needed fluid. They do, however, have some unwanted effects. Because they contain no large particles, such as proteins, they do not stay within the blood vessels and can leak out of the plasma into the tissues and cells. This may result in edema anywhere in the body. Peripheral edema and pulmonary edema are two common examples. Crystalloids also dilute the proteins that are in the plasma, which further reduces the colloid oncotic pressure. To be effective, large volumes (liters of fluid) are usually required. As a result, prolonged infusions may worsen acidosis or alkalosis, or adversely affect central nervous system function, due to fluid overload. Another disadvantage of crystalloids is that their effects are relatively short-lived.


Interactions


Interactions with crystalloid solutions are rare because they are very similar if not identical to normal physiologic substances. Certain electrolytes contained in lactated Ringer’s solution may be incompatible with other electrolytes, forming a chemical precipitate. Phenytoin precipitates if mixed with dextrose.


Dosages


For the dosage information on crystalloids, see Table 29-5.



Drug Profile


The most commonly used crystalloid solutions are normal saline (NS or 0.9% sodium chloride) and lactated Ringer’s solution. The available crystalloid solutions and their compositions are summarized in Table 29-4. Sodium chloride is also discussed briefly in the section on electrolytes and in the Nursing Process section under electrolytes.


sodium chloride


Sodium chloride (NaCl) is available in several concentrations, the most common being 0.9%. This is the physiologically normal concentration of sodium chloride, and it is referred to as normal saline (NS). Other concentrations are 0.45% (“half-normal”), 0.25% (“quarter-normal”), 3% (hypertonic saline), and 5% (hypertonic saline). These solutions have different indications and are used in different situations, depending on how urgently fluid volume restoration is needed and/or the extent of the sodium loss.


Sodium chloride is a physiologic electrolyte that is present throughout the body’s water. Thus, there are no hypersensitivity reactions to it. It is safe to administer it during any stage of pregnancy, but it is contraindicated in patients with hypernatremia and/or hyperchloremia. Hypertonic saline injections (3% and 5%) are contraindicated in the presence of increased, normal, or only slightly decreased serum electrolyte concentrations. Hypertonic saline is considered a high-risk drug because deaths have occurred when it is infused inappropriately. Correcting sodium too rapidly with hypertonic saline can lead to osmotic demyelination syndrome, which is potentially fatal. Conversely, infusing hypotonic saline (0.25% NaCl) is not recommended because it can cause hemolysis of the red blood cells. Adding potassium to hypotonic solutions makes them isotonic and safe to give. Sodium chloride is also available as a 650-mg tablet.


The dose of sodium chloride administered depends on the clinical situation. The volume of crystalloid or colloid needed to expand the plasma volume by 1 L (1000 mL) is given in Table 29-5, and this can be used as a general guide to dosing.



Colloids


Colloids are protein substances that increase the colloid oncotic pressure and move fluid from the interstitial compartment to the plasma compartment by pulling the fluid into the blood vessels. Normally, this task is performed by the three blood proteins: albumin, globulin, and fibrinogen. The total protein level must be in the range of 7.4 g/dL. If this level falls below 5.3 g/dL, fluid shifts out of blood vessels into the tissues. When this happens, colloid replacement therapy is required to reverse this process by increasing the colloid oncotic pressure. Colloid oncotic pressure decreases with age and also with hypotension and malnutrition. The commonly used colloids are listed in Table 29-6.



Mechanism of Action and Drug Effects


The mechanism of action of colloids is related to their ability to increase the colloid oncotic pressure. Because the colloids cannot pass into the extravascular space, there is a higher concentration of colloid solutes (solid particles) inside the blood vessels (intravascular space) than outside the blood vessels. Fluid thus moves from the extravascular space into the blood vessels in an attempt to make it isotonic. As such, colloids increase the blood volume, and they are sometimes called plasma expanders. They also make up part of the total plasma volume.


Colloids increase the colloid oncotic pressure and move fluid from outside the blood vessels to inside the blood vessels. They can maintain the colloid oncotic pressure for several hours. They are naturally occurring products and consist of proteins (albumin), carbohydrates (dextrans or starches), fats (lipid emulsion), and animal collagen (gelatin). Usually they contain a combination of both small and large particles. The small particles are eliminated quickly and promote diuresis and perfusion of the kidneys; the larger particles maintain the plasma volume. Albumin is the one exception in that it contains particles that are all the same size.


Indications


Colloids are used to treat a wide variety of conditions (see the list on p. 477). Clinically, colloids are superior to crystalloids because of their ability to maintain the plasma volume for a longer time. However, crystalloids are less expensive and are less likely to promote bleeding. Crystalloids are more likely to cause edema because of the larger volumes needed to achieve the desired clinical effect. Crystalloids are better than colloids for emergency short-term plasma volume expansion.


Contraindications


Contraindications to the use of colloids include known drug allergy to a specific product and hypervolemia, and may include severe electrolyte disturbance.


Adverse Effects


Colloids are relatively safe agents, although there are some disadvantages to their use. They have no oxygen-carrying ability and contain no clotting factors, unlike blood products. Because of this, they can alter the coagulation system through a dilutional effect, which results in impaired coagulation and possibly bleeding. They may also dilute the plasma protein concentration, which in turn may impair platelet function. Rarely, dextran therapy causes anaphylaxis or renal failure.


Interactions


No drug interactions occur with colloids.


Dosages


For dosage information on colloids, see Table 29-5.


Drug Profiles


The specific colloid used for replacement therapy varies from institution to institution. The three most commonly used are 5% albumin, dextran 40, and hetastarch. They all have a very rapid onset of action as well as a long duration of action. They are metabolized in the liver and excreted by the kidneys. Albumin is the one exception; it is metabolized by the reticuloendothelial system and excreted by the kidneys and the intestines. Hetastarch is a synthetic colloid with properties similar to those of albumin and dextran.


albumin


Albumin is a natural protein that is normally produced by the liver. It is responsible for generating approximately 70% of the colloid oncotic pressure. Human albumin is a sterile solution of serum albumin that is prepared from pooled blood, plasma, serum, or placentas obtained from healthy human donors. It is pasteurized (heated at 140° F [60° C] for 10 hours) to destroy any contaminants. Unfortunately, because it is derived from human donors, the supply is limited. Many institutions have specific indications for the use of albumin.


Albumin is contraindicated in patients with a known hypersensitivity to it and in those with heart failure, severe anemia, or renal insufficiency. Albumin is available only in parenteral form in concentrations of 5% and 25%. It is classified as a pregnancy category C drug. See Table 29-5 for dosing guidelines.


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May 9, 2017 | Posted by in NURSING | Comments Off on Fluids and Electrolytes

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