Chapter 39 Fluid, Electrolyte, & Acid-Base Balance PDF

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Notes from Fundamentals of Nursing Yoost/Crawford...

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! Normal Structure and Function of Fluids, Electrolytes, Acids, and Bases • The equilibrium between the acidity and alkalinity of body fluids is acid-base balance, which is regulated by the pulmonary and renal compensatory systems and by chemical (buffering) systems. • Composition of body fluids

‣ Total body water (TBW) is the percentage of body weight that consists of water. ! ‣ Almost 60% of a healthy adult's weight is water.!

‣ Vitreous and aqueous fluid in the eye and bile and other digestive fluids are included in this category. !

‣ Solutes are chemical substances that dissolve in a liquid (the solvent). ! • Solutes that dissolve easily are called crystalloids. ! • Substances such as proteins that do not dissolve easily are called colloids. ! • Solutes consist of electrolytes and nonelectrolytes, such as oxygen, carbon dioxide, and glucose.!

‣ The # of electrolytes that carry a positive charge and the # that carry a negative charge should be equivalent.! ‣ The term milliequivalent denotes the ability of cations to bond with anions to form molecules. ! • Electrolytes are measured in milliequivalents per liter of water (mEq/L). • Movement of body fluids

‣ Osmotic pressure is the force created when two solutions of different concentrations are separated by a selectively permeable membrane. The cell membrane allows water to move across but not the solutes.! • Water moves from an area of low concentration to area of high concentration. ! • The higher the difference in concentration of solutes, the greater is the osmotic pressure.! ‣ Osmolality refers to the number of osmols (unit of osmotic pressure) per kilogram of solvent, which in this case is water. ! ‣ Osmolarity is the number of osmols per liter of solvent. In discussions of body fluids, the differences between the two values are very small. ! • Another term used to describe osmolarity is tonicity of the fluid. ! • Tonicity refers to the level of osmotic pressure of a solution. ! • An administered solution that has the same osmolarity as blood plasma is an isotonic solution. (NS, D5W, LR)! • A hypertonic solution pulls water from the cell to the extracellular fluid compartment, causing cellular shrinkage. (D5LR)! ‣ Osmotic pressure in the intravascular space is controlled by the concentration of plasma proteins, specifically albumin. This is known as colloid osmotic pressure, or oncotic pressure. Because these plasma proteins are too large to pass through the capillary membrane, they hold the fluid in the intravascular space.

move together from an area of higher pressure to one of lower pressure. ! ‣ Hydrostatic pressure is the force of the fluid pressing against the blood vessel; it is controlled by the force of myocardial contraction, rate of contraction, and blood flow.! ‣ Filtration across a selectively permeable membrane occurs when the hydrostatic pressure is greater than the oncotic pressure.! ‣ As oncotic pressure rises on the venous side, fluid and waste products are pulled back into the intravascular space. • Regulation of body fluids

‣ A small amount of water is generated through various metabolic processes, including the breakdown of carbohydrates.

‣ Water is lost from the body primarily through the kidneys and excreted as urine. Water loss also occurs through insensible loss, which is moisture lost through respiration and perspiration; a very small amount is lost through feces.!

secretion of antidiuretic hormone, and the thirst mechanism. ! ‣ The renin-angiotensin system regulates blood pressure and fluid balance through vasoconstriction and excretion or reabsorption of sodium. ! ‣ Antidiuretic hormone (ADH), which is secreted by the pituitary gland, maintains serum osmolality by controlling the amount of water excreted in the urine. Homeostasis is monitored by the kidneys through changes in blood pressure.!

‣ Renin converts angiotensinogen into angiotensin I. ! ‣ Angiotensin-converting enzyme (ACE), found in the lungs and kidneys, then changes angiotensin I to angiotensin II.! ‣ Angiotensin II is a hormone that causes several things to occur:! • Vasoconstriction (narrowing of blood vessels) throughout the body increases arterial blood pressure.! • Water from the nephron is reabsorbed back into the intravascular space due to changes in hydrostatic pressure in the nephron.! • Stimulation of the adrenal cortex releases the hormone aldosterone. Aldosterone acts on the distal convoluted tubule of the kidney to increase the amount of water and sodium reabsorbed back into the bloodstream.! • ADH is secreted.! • The thirst mechanism is stimulated.!

‣ When osmolarity increases (becomes more concentrated, as in dehydration), the osmoreceptors stimulate the posterior pituitary to secrete ADH. ! ‣ ADH works on the collecting ducts of the kidneys to reabsorb more water, and less urine is produced as a result.! ‣ The osmoreceptors, in response to increased osmolarity, stimulate the cerebral cortex to produce a conscious awareness of thirst. ! ‣ Atrial natriuretic peptide (ANP) is secreted by cells in the atrium of the heart in response to an increase in blood pressure. ! • When released, ANP causes an increase in the glomerular filtration rate (GFR), which leads to increased excretion of water in urine. ! • It inhibits secretion of renin by blocking the renin-angiotensin system, inhibits secretion of ADH, and inhibits reabsorption of sodium chloride and water into the bloodstream.! • Movement of electrolytes

‣ Diffusion is the movement of solutes across a selectively permeable membrane from areas of higher concentration to areas of lower concentration until equilibrium is reached. The rate of diffusion is influenced by the following factors:! • Temperature: The higher the temperature, the faster the particles diffuse.! • Molecular weight: Lighter weight molecules (i.e., gases and electrolytes) diffuse more quickly than heavier molecules.! • Steepness of the concentration gradient: The more uneven the concentrations on either side of the membrane, the faster solutes move across.! • Membrane permeability: There are conditions that cause the membrane to become more permeable to larger molecules, which otherwise would not be able cross.! ‣ Facilitated diffusion occurs when a solute is unable to pass through a membrane and requires a carrier.! ‣ Active transport is the transport of a solute from areas of lower to higher concentration; it is the opposite of diffusion.! • The process is considered active because it requires energy. • Regulation of electrolytes

plasma). • Acid-base balance

scale of 0 to 14, 7 is neutral. A pH less than 7 is acidic, and a pH greater than 7 is basic.!

such as ketone bodies, phosphoric acid, carbonic acid, and lactic acid.! ‣ There is an inverse relationship between pH and the hydrogen ion. ! • The greater the number of hydrogen ions, the lower is the pH value. ! • The lower the pH number, the stronger is the acid.!

• Hydroxide ions can combine with hydrogen ions to form water, thereby neutralizing the acidic solution. ! • Bicarbonate is a weak base found primarily in the plasma; it plays a crucial role in acid-base balance.!

primary mechanisms for maintaining acid-base balance; respiratory control of carbon dioxide; and renal control of bicarbonate. ‣ Buffering systems • A buffering system consists of a weak acid and weak base. It is designed to adjust to small changes in pH. ! • This is a reversible chemical reaction. !

greater degree in the renal tubules than in the cells. It converts alkaline sodium phosphate (NaHPO4) to the acid solution phosphate (Na2H2PO4) with the addition of a hydrogen ion.! • When there are excess hydrogen ions in the extravascular space, potassium moves out of the cell as the hydrogen ions move into the cell. As a result, acid-base imbalances can affect serum potassium levels and potassium imbalances can influence acid-base balance.! • Several plasma proteins and the hemoglobin in red blood cells have the ability to bind or release hydrogen ions. This helps maintain balance in the intracellular fluid and blood plasma. ‣ Respiratory regulation • The lungs control the amount of carbonic acid available by retaining or exhaling carbon dioxide. !

• The kidneys neutralize more acid or base than the chemical buffers or the lungs. They do so by excreting or retaining hydrogen ions and excreting or forming bicarbonate ions. ! • It is the only system that removes excess hydrogen ions from the body. !

‣ It can take up to 3 days for normal pH levels to be achieved by the renal mechanism. • Blood groups

and Rh groups of antigens

‣ Individuals with type O blood are classified as universal donors because their blood cells contain no antigens.! ‣ People with AB blood type are considered universal recipients because their blood cells contain both A and B antigens, and they can receive type A, B, or O blood.! ‣ The Rhesus (Rh) factor was discovered in 1940 and is the second most important antigen to test for before blood transfusions.! • After the person has been exposed, subsequent contact with Rh+ blood can lead to life-threatening destruction of red blood cells, known as hemolytic reactions. ‣ Blood testing • To be eligible to donate blood, a person must be at least 17 years of age, weigh at least 110 pounds, and have not donated blood within the past 56 days.! • The donated blood is then tested for ABO and Rh type, screened for antibodies, and tested for infectious disease markers such as hepatitis and HIV.! • Autologous blood donation uses the patient's own blood donated 2 to 3 weeks before elective surgery. ‣ Blood components (see chart) —>

Altered Structure and Functions of Fluids and Electrolytes • Fluid imbalances

fluid. Two types of fluid deficits can occur: isotonic and hypertonic.! • Isotonic fluid deficit, or hypovolemia, occurs when water and sodium are lost at the same rate. In this situation, circulating volume decreases, but serum osmolarity remains unchanged. ! • Hypertonic fluid volume deficit, or dehydration, can be serious if not recognized and treated. As fluid loss continues, the circulating fluid volume decreases and serum osmolarity increases.!

‣ ‣ ‣ ‣

A 2% loss is mild dehydration.! A 5% loss is moderate dehydration.! An 8% loss is severe dehydration.! A 15% loss is life-threatening, usually fatal.

‣ Severity of the fluid volume excess is assessed by the corresponding increase in total body weight:! • A 2% gain is mild excess.! • A 5% gain is moderate excess. ! • An 8% gain is severe excess.! ‣ Edema is the abnormal accumulation of fluid in the interstitial spaces, typically in the extremities such as fingers, ankles, and feet, and also may be present in the face and/or abdomen. ! • Edema, also known as third spacing, develops when fluid moves into a tissue at a faster rate than it can be reabsorbed into the intravascular space.! • There are four primary causes of edema formation: increase in hydrostatic pressure due to fluid overload, decreased production of circulating plasma proteins, obstruction of lymphatic drainage, and increased capillary permeability due to tissue damage.! ‣ In isotonic fluid volume excess, there is an equal increase in fluid and sodium retention. ! • The result is an increase in circulating blood volume while serum osmolarity remains unchanged. ! ‣ Hypotonic fluid volume excess occurs when water is ingested at a rate greater than sodium.! • Swelling leads to pulmonary congestion and cerebral edema. There are situations in which inappropriate secretion of ADH (i.e., syndrome of inappropriate antidiuretic hormone secretion [SIADH]) occurs.

simultaneously. ! • When the serum albumin level is very low, the hydrostatic pressure is greater than the oncotic pressure, and fluid seeps into the interstitial spaces.! • As a result, many of the signs of fluid volume excess are present, such as weight gain, pulmonary congestion, and edema.!

as hypotension, weak and thready pulse, and tachycardia. !

• Electrolyte imbalances Calcium

Phosphate

• Hyponatremia and hypernatremia

relation to body water.!

and sodium, but the sodium loss is greater. !

hyponatremia. In this situation, water intake exceeds sodium intake, which leads to an overall decrease in serum sodium levels.!

with sodium or a greater intake of sodium.! ! ! • Hypokalemia and hyperkalemia

‣ transfusions and medications! ‣ impaired renal excretion! ‣ cellular movement! • Because potassium levels are lowered by renal excretion, homeostasis requires normal renal functioning. • Hypocalcemia and hypercalcemia

‣ Decreased serum calcium levels occur when calcium cannot be absorbed from the small intestine. !

serum calcium. ! ‣ An increase in secretion of the parathyroid hormone, as occurs in hyperparathyroidism, leads to increased calcium released from the bones. • Hypomagnesemia and hypermagnesemia

‣ Decreased serum levels result from decreased intake, decreased absorption, or increased loss through the kidneys.!

‣ It occurs in patients on total parenteral nutrition (TPN) containing too much magnesium or with the prolonged use of IV magnesium such as magnesium sulfate. ! ‣ Renal failure can lead to decreased excretion of magnesium. ! ‣ Severe dehydration can lead to hemoconcentration of magnesium. ! ‣ Adrenal insufficiency and leukemia also have caused increased serum levels of magnesium. • Hypochloremia and hyperchloremia

‣ Fluid volume excess and certain medications such as corticosteroids and/or diuretics may cause hypochloremia. !

‣ Excessive infusion of normal saline intravenously also may cause hyperchloremia. • Hypophosphatemia and hyperphosphatemia

gastrointestinal tract, or increased excretion of phosphorus by the kidneys.!

‣ As the GFR decreases, the ability of the kidneys to excrete excess phosphate ions diminishes. ! ‣ Increased levels of phosphate ions occur as a result of decreased parathyroid hormone secretion seen in hypoparathyroidism. • Acid-base imbalances

abnormal ventilation, perfusion, or diffusion.! • This leads to hypercapnia— increased carbon dioxide levels—in the blood. ! • Renal compensation of respiratory acidosis focuses on excreting excess metabolic acids and conserving bicarbonate. However, renal compensation is fairly slow, taking 12 to 24 hours to adjust to the abnormal pH levels. As a result of compensation, pH levels move toward the normal range, and bicarbonate levels are abnormally high.

exhalation of carbon dioxide. ! • The pH is increased to greater than 7.45, and the PaCO2 is decreased to less than 35 mm Hg. ! • Acute hypoxia leads to an increased respiratory rate by overstimulating the respiratory center of the brain, which leads to hypocapnia (i.e., decreased carbon dioxide levels in the blood). ! • Renal compensation of this imbalance focuses on preserving metabolic acids, thereby decreasing the amount of available bicarbonate. !

pH less than 7.35 and a bicarbonate level less than 21 mEq/L. ! • It occurs when there is a loss of bicarbonate ions or an increase in acids produced as by- products of a metabolic process (i.e., ketones).! • Lactic acidosis, a result of anaerobic respiration, can quickly lead to a metabolic acidotic state. !

kidneys lose the ability to secrete hydrogen ions or conserve bicarbonate ions. ! • In metabolic acidosis, the lungs compensate by increasing the rate and depth of respirations in an attempt to remove excess carbon dioxide, thereby lowering the amount of carbonic acid in the blood. This type of deep, rapid breathing is known as Kussmaul respirations

bicarbonate levels greater than 28 mEq/L. ! • This occurs as a result of excessive intake of bicarbonate-containing medications (i.e., antacids), loss of gastric acids through vomiting or nasogastric suctioning, and frequent blood transfusions (due to the citrate preservative used in the storage of the red blood cells).

Assessment • Health history

‣ Aloe: Acts as a cathartic/laxative and can lead to hypokalemia. Aloe also can cause hypoglycemia when taken with oral diabetic medications.! ‣ Celery and Dandelion: Each acts as a diuretic in the body! ‣ Ginseng: Contraindicated in renal failure! ‣ Licorice: Pseudoaldosteronism leading to sodium retention and hypokalemia • Vital signs

mechanism for metabolic imbalances. !

‣ With fluid volume deficit, the blood pressure decreases. ! ‣ It is important to assess for orthostatic hypotension, a decrease of more than 20 mm Hg in systolic pressure or 10 mm Hg in diastolic pressure when moving from one position to another, such as sitting to standing. ! ‣ The pulse pressure, which is the difference between the systolic and diastolic blood pressures, is approximately 40 mm Hg. A pulse pressure less than 24 mm Hg can indicate severe fluid volume deficit.!

• Intake and output

enemas.!

‣ Containers with calibrations, such as urinals and urine hats, can be used to collect and measure the fluids.!

• Edema

based on the depth of the indentation:! ! ! ! !

much as 30% before even 1+ edema is apparent. • Skin turgor and mucous membranes

• Diagnostic tests

essential information on a patient's acid-base balance and oxygenation status.! • The first step in interpreting ABGs is to examine the oxygenation status by examining the PaO2 and O2 saturation values. !

decreases:! ‣ PaO2 of 60 to 80 mm Hg: Mild hypoxemia! ‣ PaO2 of 40 to 60 mm Hg: Moderate hypoxemia ! ‣ PaO2 less than 40 mm Hg: Severe hypoxemia! • The second step is to examine the pH and determine whether the value falls within the normal range. ! • The third step is to examine the PaCO2 and HCO3 values to determine whether the underlying disorder is metabolic or respiratory.!

! ! ! ! ! ! • Factors affecting fluid, electrolyte, and acid-base balances

obese.!

fluids from nasogastric tubes or wounds and postoperative vomiting.!

Implementation and Evaluation • Monitoring fluid balance

(i.e., intake greater than output) can be a desired outcome in patients with fluid volume deficits or can indicate fluid volume excess.!

• Maintaining fluid and electrolyte balance

amount of fluid the patient takes in. ! ‣ Normally, the order indicates the amount of fluid to be consumed in a 24-hour period (e.g., 1000 mL). It is up to the nurse to divide that total into smaller amounts throughout the day. ! • The normal recommendation is to use 50% of the amount during the day, when the patient is most active and consumes two meals. ! • The amount is then further divided into fluid with meals and fluid available between meals and with medication administration. ! ‣ If the patient has IV fluids running, the fluids need to be subtracted from the ordered fluid restriction; the remaining oral fluid is then divided throughout the 24-hour period.! • Explain the purpose of the restriction. Make sure the patient understands that ice chips, Jell-O, and ice cream are considered fluids and are included.! • Offer fluids frequently in small amounts to prevent dry mouth. Using a smaller 4- or 6-oz glass offers the impression of more fluid. Give the patient a choice in fluid preferences.! • Instruct the patient to avoid overly sweet or salty foods. Both stimulate the thirst sensation.! • Offer small amounts of ice chips. Remember that 100 mL of ice chips is approximately 50 mL of water.! • Encourage the patient to assist in maintaining the intake and output record. This helps the patient feel more in control of the situation.

restrictions are classified as mild, moderate, or severe:! • Mild restriction is 3000 to 4000 mg/day; it is indicated as “no added salt.”! • Moderate restriction is 2000 mg/day; it is indicated as the need to ...