Lewis: Medical-Surgical Nursing in Canada, 4th Edition Chapter 19: Fluid, Electrolyte, and Acid–Base Imbalances Key Points • Body fluids and electrolytes play an important role in homeostasis (the state of equilibrium in the internal environment of the body, naturally maintained by adaptive responses that promote healthy survival). • Many diseases and their treatments have the ability to affect fluid and electrolyte balance. • Water is the primary component of the body, accounting for approximately 60% of the body weight in the adult. • The two major fluid compartments in the body are intracellular fluid (ICF), or inside the cells, and extracellular fluid (ECF), or outside the cells. ECF is composed of interstitial fluid, plasma, and transcellular fluids. • The measurement of electrolytes is important to the nurse in evaluating electrolyte balance, as well as in determining the composition of electrolyte preparations. • Many different processes are involved in the movement of electrolytes and water between the ICF and ECF. Some of these include simple diffusion, facilitated diffusion, and active transport. Two forces drive water movement: hydrostatic pressure and osmotic pressure. • Osmosis is the movement of water between two compartments separated by a membrane permeable to water but not to a solute. Water moves through the membrane from an area of low solute concentration to an area of high solute concentration. Osmotic pressure is the amount of pressure required to stop the osmotic flow of water. Measuring osmolality is important because it indicates the water balance of the body. • In the metabolically active cell, there is a constant exchange of substances between compartments, but no net gain or loss of water occurs. • The tonicity of the fluid surrounding cells affects them. Fluids with the same osmolality as the cell are isotonic. Solutions in which the solutes are less concentrated than the cells are hypotonic; those with solutes more concentrated than cells or an increased osmolality are hypertonic. • Oncotic pressure (colloidal osmotic pressure) is osmotic pressure exerted by colloids in solution. The major colloid in the vascular system contributing to the total osmotic pressure is protein. Copyright © 2019 Elsevier Canada, a division of Reed Elsevier Canada, Ltd. Key Points 19-2 • The amount and direction of movement between the interstitium and the capillary are determined by the interaction of (1) capillary hydrostatic pressure, (2) plasma oncotic pressure, (3) interstitial hydrostatic pressure, and (4) interstitial oncotic pressure. • If capillary or interstitial pressures are altered, fluid may abnormally shift from one compartment to another, resulting in edema or dehydration. • Fluid is drawn into the plasma space whenever there is an increase in the plasma osmotic or oncotic pressure. This could happen with administration of colloids, dextran, mannitol, or hypertonic solutions. • Fluid spacing is a term sometimes used to describe the distribution of body water. First spacing describes the normal distribution of fluid in the intracellular fluid (ICF) and extracellular fluid (ECF) compartments. Second spacing refers to an abnormal accumulation of interstitial fluid (i.e., edema). Third spacing occurs when fluid accumulates in a portion of the body from which it is not easily exchanged with the rest of the ECF. • Water balance is maintained by a balance of water intake and excretion. • An intact thirst mechanism is critical for fluid balance. The patient who cannot recognize or act on the sensation of thirst is at risk for fluid deficit and hyperosmolality. • An increase in plasma osmolality or a decrease in circulating blood volume will stimulate antidiuretic hormone (ADH) secretion. Reduction in the release or action of ADH produces diabetes insipidus. • Aldosterone is a naturally occurring mineralocorticoid with potent sodium-retaining and potassium-excreting capability. • The primary organs for regulating fluid and electrolyte balance are the kidneys. The kidneys regulate water balance through adjustments in urine volume. Similarly, urinary excretion of most electrolytes is adjusted so that a balance is maintained between overall intake and output. • With severely impaired renal function, the kidneys cannot maintain fluid and electrolyte balance. This condition results in edema, potassium and phosphorus retention, acidosis, and other electrolyte imbalances. • Insensible water loss, which is invisible vaporization from the lungs and skin, assists in regulating body temperature. Copyright © 2019 Elsevier Canada, a division of Reed Elsevier Canada, Ltd. Key Points 19-3 • Older adults experience normal physiological changes that increase susceptibility to fluid and electrolyte imbalances. Structural changes to the kidney and a decrease in the renal blood flow lead to a decrease in the glomerular filtration rate, decreased creatinine clearance, the loss of the ability to concentrate urine and conserve water, and narrowed limits for the excretion of water, sodium, potassium, and hydrogen ions. • Fluid and electrolyte imbalances are commonly classified as deficits or excesses. • ECF volume deficit (hypovolemia) and ECF volume excess (hypervolemia) are commonly occurring clinical conditions. Fluid volume deficit can occur with abnormal loss of body fluids (e.g., diarrhea, fistula drainage, hemorrhage, polyuria), inadequate intake, or a plasma-to-interstitial fluid shift. • The use of 24–hour intake and output records gives valuable information regarding fluid and electrolyte problems. • Monitoring the patient for cardiovascular and neurological changes is necessary to prevent or detect complications from fluid and electrolyte imbalances. • Accurate daily weights provide the easiest measurement of volume status. Weight changes must be obtained under standardized conditions. • Edema is assessed by pressing with a thumb or forefinger over the edematous area. • The rates of infusion of IV fluid solutions should be carefully monitored. • The goal of treatment in fluid and electrolyte imbalances is to treat the underlying cause. Sodium • Sodium is the major ECF cation and plays a major role in maintaining the concentration and volume of the ECF. • An elevated serum sodium (hypernatremia) may occur with water loss or sodium gain. • Hyponatremia o Hyponatremia is a low serum sodium level. o Common causes include water excess from inappropriate use of sodium-free or hypotonic IV fluids. o Symptoms of hyponatremia are related to cellular swelling and are first manifested in the central nervous system (CNS). Potassium Copyright © 2019 Elsevier Canada, a division of Reed Elsevier Canada, Ltd. Key Points 19-4 • Potassium is the major ICF cation and the major factor in the resting membrane potential of nerve and muscle cells. Changes in potassium balance cause a number of clinical problems. • Factors that cause potassium to move from the ICF to the ECF include acidosis, trauma to cells (as in massive soft tissue damage or in tumour lysis), and exercise. • Hyperkalemia o Hyperkalemia is an elevated serum potassium level. o The most common cause is renal failure. Hyperkalemia is also common with massive cell destruction (e.g., burn or crush injury, tumour lysis); rapid transfusion of aged blood; and catabolic state (e.g., severe infections). o Manifestations of hyperkalemia include cramping leg pain, followed by weakness or paralysis of skeletal muscles. o All patients with clinically significant hyperkalemia should be monitored electrocardiographically to detect dysrhythmias and to monitor the effects of therapy. Cardiac depolarization is impaired, leading to flattening of the P wave and widening of the QRS wave. Repolarization occurs more rapidly, resulting in shortening of the Q–T interval and causing the T wave to be narrower and more peaked. Ventricular fibrillation or cardiac standstill may occur. o The patient experiencing dangerous cardiac dysrhythmias should receive IV calcium gluconate immediately while the potassium is being eliminated and forced into cells. • Hypokalemia o Hypokalemia is a low serum potassium level. o The most common causes are from abnormal losses via either the kidneys or the gastro-intestinal tract. Abnormal losses occur when the patient is diuresing, particularly in the patient with an elevated aldosterone level. o In the patient with hypokalemia, cardiac changes include impaired repolarization, resulting in a flattening of the T wave and eventually in emergence of a U wave. The incidence of potentially lethal ventricular dysrhythmias is increased in hypokalemia. o Patients taking digoxin experience increased digoxin toxicity if their serum potassium level is low. Skeletal muscle weakness and paralysis may occur with hypokalemia. Severe hypokalemia can cause weakness or paralysis of respiratory muscles, leading to shallow respirations and respiratory arrest. o Hypokalemia is treated by giving potassium chloride supplements and increasing dietary intake of potassium. Calcium • Hypercalcemia o Hypercalcemia is an elevated serum calcium level. o About two-thirds of cases are caused by hyperparathyroidism and one-third are caused by malignancy, especially from breast cancer, lung cancer, and multiple myeloma. Copyright © 2019 Elsevier Canada, a division of Reed Elsevier Canada, Ltd. Key Points 19-5 o Manifestations of hypercalcemia include decreased memory, confusion, disorientation, fatigue, muscle weakness, constipation, cardiac dysrhythmias, and renal calculi. o Treatment of hypercalcemia is promotion of excretion of calcium in urine by administration of a loop diuretic and hydration of the patient with isotonic saline infusions. • Hypocalcemia o Hypocalcemia is a low serum calcium level. o Is caused by a decrease in the production of parathyroid hormone. o Hypocalcemia is characterized by increased nerve excitability and sustained muscle contraction (tetany). o A patient who has had neck surgery including thyroidectomy is observed carefully for signs of hypocalcemia, due to the proximity of the surgery to the parathyroid glands. Phosphate • Phosphorus is a primary anion in the ICF and is essential to the function of muscle, red blood cells, and the nervous system. • Hyperphosphatemia is an elevated serum phosphorus level. o The major condition that can lead to hyperphosphatemia is acute or chronic renal failure. • Hypophosphatemia (low serum phosphate) is seen in the patient who is malnourished or has a malabsorption syndrome. Magnesium • Hypomagnesemia (low serum magnesium level) produces neuromuscular and CNS hyperirritability. A major cause of magnesium deficiency is prolonged fasting or starvation. Chronic alcoholism commonly causes hypomagnesemia as a result of insufficient food intake. • Hypermagnesemia (high serum magnesium level) depresses neuromuscular and CNS functions. It usually occurs only with an increase in magnesium intake accompanied by renal insufficiency or failure. Protein • Protein imbalances can lead to hypoproteinemia. • Causes related to intake are anorexia, malnutrition, starvation, fad dieting, and poorly balanced vegetarian diets. Clinical manifestations of protein deficit include edema (from decreased oncotic pressure), slow healing, anorexia, fatigue, anemia, and muscle loss that results from the breakdown of body tissue to meet the body’s need for protein. Management of protein deficit includes providing a high-carbohydrate, high-protein diet and dietary protein supplements. Acid-Base Imbalances Copyright © 2019 Elsevier Canada, a division of Reed Elsevier Canada, Ltd. Key Points 19-6 • Patients with diabetes mellitus, chronic obstructive pulmonary disease, and kidney disease frequently develop acid-base imbalances. Vomiting and diarrhea may cause loss of acids and bases in addition to fluids and electrolytes. • The nurse must always consider the possibility of acid-base imbalance in patients with serious illnesses. • The buffer system is the fastest acting system and the primary regulator of acid-base balance. • The lungs help maintain a normal pH by excreting CO2 and water, which are byproducts of cellular metabolism. • The three renal mechanisms of acid elimination are (1) secretion of small amounts of free hydrogen into the renal tubule, (2) combination of H+ with ammonia (NH3) to form ammonium (NH4−), and (3) excretion of weak acids. • Acid-base imbalances are classified as respiratory (affect carbonic acid concentrations) or metabolic (affect the base bicarbonate). o Respiratory acidosis (carbonic acid excess) occurs whenever there is hypoventilation. o Respiratory alkalosis (carbonic acid deficit) occurs whenever there is hyperventilation. o Metabolic acidosis (base bicarbonate deficit) occurs when an acid other than carbonic acid accumulates in the body or when bicarbonate is lost from body fluids. o Metabolic alkalosis (base bicarbonate excess) occurs when a loss of acid (prolonged vomiting or gastric suction) or a gain in bicarbonate occurs. • Arterial blood gas (ABG) values provide valuable information about a patient’s acidbase status, origin of the imbalance, the body’s ability to regulate pH, and the patient’s overall oxygen status. • In cases of acid-base imbalances, the treatment is directed toward correction of the underlying cause. • Fluid replacement therapy is used to correct fluid and electrolyte imbalances. o A hypotonic solution provides more water than electrolytes, diluting the ECF. o Administration of an isotonic solution expands only the ECF. There is no net loss or gain from the ICF. An isotonic solution is the ideal fluid replacement for a patient with an ECF volume deficit. o A hypertonic solution initially raises the osmolality by the ECF and expands it. o Plasma expanders stay in the vascular space and increase the osmotic pressure. • Central venous access devices (CVADs) are catheters that are placed in large blood vessels (e.g., subclavian vein, jugular vein) of people who require frequent access to Copyright © 2019 Elsevier Canada, a division of Reed Elsevier Canada, Ltd. Key Points 19-7 the vascular system. CVADs permit frequent, continuous, rapid, or intermittent administration of fluids and medications. o These include centrally inserted catheters, peripherally inserted central catheters, and implanted infusion ports. o Removal of CVADs should be done according to institution policy and the nurse’s scope of practice. Copyright © 2019 Elsevier Canada, a division of Reed Elsevier Canada, Ltd.
