14.1 Fluid and electrolyte homeostasis

Body fluid composition and distribution are crucial for maintaining homeostasis. Understanding how fluids and electrolytes are balanced in the body is essential for nurses. This knowledge helps in assessing patients and providing appropriate interventions for fluid imbalances.

Nurses must be able to recognize signs of fluid and electrolyte imbalances through physical exams and lab data. They play a key role in monitoring fluid intake and output, administering fluids and medications, and educating patients on maintaining proper balance. These skills are vital for preventing complications and promoting overall health.

Body Fluid Composition and Distribution

Composition of body fluids

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• Total body water (TBW) comprises 50-60% of total body weight varies based on factors such as age (higher in infants and children), sex (higher in males), and body composition (higher in lean individuals)
• Intracellular fluid (ICF) makes up approximately 2/3 of TBW contains high concentrations of potassium ($K^+$), magnesium ($Mg^{2+}$), and phosphate ($HPO_4^{2-}$)
• Extracellular fluid (ECF) accounts for about 1/3 of TBW
  • Interstitial fluid (ISF) constitutes the majority of ECF bathes the cells and facilitates nutrient and waste exchange
  • Plasma represents a smaller portion of ECF serves as the liquid component of blood
  • ECF is characterized by high levels of sodium ($Na^+$), chloride ($Cl^-$), and bicarbonate ($HCO_3^-$)

Fluid and Electrolyte Balance Mechanisms

Mechanisms of fluid balance

• Osmosis involves the movement of water across a semipermeable membrane from a region of low solute concentration to one with a higher solute concentration driven by differences in osmotic pressure determined by the concentration of solutes
• Diffusion refers to the movement of solutes from an area of high concentration to an area of low concentration down their concentration gradient
• Hormonal regulation plays a crucial role in maintaining fluid balance
  • Antidiuretic hormone (ADH) secreted by the posterior pituitary gland promotes water reabsorption in the collecting ducts of the kidneys reducing urine output and conserving water
  • Aldosterone released by the adrenal cortex stimulates sodium reabsorption and potassium excretion in the distal tubules and collecting ducts of the kidneys promoting water retention
  • Natriuretic peptides (ANP and BNP) secreted by the heart promote sodium excretion in the kidneys leading to increased urine output and reduced blood volume
• Thirst mechanism is stimulated by increased plasma osmolality and decreased blood volume triggering the sensation of thirst and promoting fluid intake to restore balance
• Renal regulation involves the kidneys adjusting urine concentration and electrolyte excretion based on the body's needs through processes such as glomerular filtration, tubular reabsorption, and secretion

Common electrolyte imbalances

• Hyponatremia is characterized by low serum sodium levels ($<$135 mEq/L)
  • Causes include excessive water intake, diuretic use, syndrome of inappropriate antidiuretic hormone secretion (SIADH), heart failure, and liver cirrhosis
  • Effects may include neurological symptoms (confusion, seizures, coma), muscle cramps, and fatigue
• Hypernatremia occurs when serum sodium levels are elevated ($>$145 mEq/L)
  • Causes encompass dehydration, excessive sodium intake, diabetes insipidus, and certain medications
  • Effects can include thirst, neurological symptoms (irritability, confusion, coma), and muscle twitching
• Hypokalemia refers to low serum potassium levels ($<$3.5 mEq/L)
  • Causes include diuretic use, gastrointestinal losses (vomiting, diarrhea), renal losses, and inadequate dietary intake
  • Effects may manifest as muscle weakness, fatigue, arrhythmias, constipation, and paralytic ileus
• Hyperkalemia is defined as high serum potassium levels ($>$5.5 mEq/L)
  • Causes include renal failure, angiotensin-converting enzyme (ACE) inhibitors, potassium-sparing diuretics, and tissue damage (rhabdomyolysis, burns)
  • Effects can include arrhythmias, muscle weakness, paresthesia, nausea, and diarrhea

Patient Assessment and Nursing Interventions

Assessment of fluid imbalances

• Physical examination findings indicative of fluid imbalances include
  • Altered skin turgor, dry mucous membranes, and prolonged capillary refill time suggesting dehydration
  • Changes in neurological status such as confusion, seizures, or coma
  • Abnormal cardiovascular parameters (blood pressure, heart rate, presence of arrhythmias)
  • Variations in muscle strength and tone
• Laboratory data used to evaluate fluid and electrolyte status include
  • Serum electrolyte levels ($Na^+$, $K^+$, $Cl^-$, $HCO_3^-$, $Ca^{2+}$, $Mg^{2+}$, $PO_4^{3-}$)
  • Serum osmolality to assess the concentration of solutes in the blood
  • Urine specific gravity and osmolality to evaluate the kidney's concentrating ability
  • Acid-base balance parameters (pH, $PaCO_2$, $HCO_3^-$) to identify metabolic or respiratory disturbances

Interventions for fluid balance

• Monitor fluid intake and output to maintain balance and detect discrepancies
• Administer prescribed fluids and electrolytes
  • Oral rehydration solutions for mild to moderate dehydration
  • Intravenous fluids tailored to the patient's needs (isotonic, hypotonic, or hypertonic solutions)
• Administer medications as prescribed (diuretics to manage fluid overload, electrolyte supplements to correct deficiencies)
• Educate patients on strategies to maintain proper fluid and electrolyte balance
  • Encourage adequate fluid intake based on individual needs and circumstances
  • Provide guidance on dietary sources of electrolytes (fruits, vegetables, dairy products)
• Monitor for and manage complications related to fluid and electrolyte imbalances
  • Adjust fluid and electrolyte therapy based on patient response and laboratory findings
  • Implement safety measures for patients with neurological symptoms (fall precautions, seizure management)
  • Collaborate with the healthcare team to address underlying causes of imbalances (treating infections, managing chronic conditions)