2.3 Clinical Nutrition, Fluid & Electrolyte Balance, and Parenteral Nutrition

Intravenous Fluid Therapy

Intravenous (IV) fluids are categorized based on their tonicity (osmolality) compared to blood plasma. Understanding tonicity is essential for selecting the correct fluid to achieve the desired clinical outcome without causing dangerous fluid shifts.

Fluid Classifications

1. Isotonic Fluids: Have an osmolality equal to plasma. They remain within the intravascular space, expanding blood volume without pulling water into or out of cells.
   • Examples: 0.9% Sodium Chloride (Normal Saline or NS), Lactated Ringer's (LR).
   • Indications: Fluid resuscitation, hypovolemia, hemorrhage, shock, and mild hyponatremia.
2. Hypotonic Fluids: Have a lower osmolality than plasma. They cause fluid to shift out of the intravascular space and into the intracellular space, hydrating the cells.
   • Examples: 0.45% Sodium Chloride (Half Normal Saline).
   • Indications: Intracellular dehydration (e.g., hypernatremia, diabetic ketoacidosis).
   • Contraindications: Do not use in patients with increased intracranial pressure (ICP) or severe burns, as fluid shifting into cells can exacerbate cerebral edema or hypovolemia.
3. Hypertonic Fluids: Have a higher osmolality than plasma. They act as a sponge, pulling fluid out of the intracellular and interstitial spaces and into the intravascular space.
   • Examples: 3% Sodium Chloride, Dextrose 10% in Water (D10W).
   • Indications: Severe symptomatic hyponatremia, cerebral edema.
   • Nursing Consideration: Must be administered very slowly, typically via a central line, and requires intensive care monitoring due to the risk of volume overload and pulmonary edema.

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Fluid and Electrolyte Imbalances

Electrolytes are critical for cellular function, nerve conduction, and cardiac rhythm regulation. Nurses must recognize the clinical manifestations of imbalances and the appropriate interventions.

Potassium (Normal: 3.5 - 5.0 mEq/L)

Potassium is the major intracellular cation. Small deviations can profoundly affect cardiac muscle.

• Hypokalemia (< 3.5 mEq/L):
  • Causes: Diuretics (loop and thiazide), vomiting, gastric suctioning.
  • Signs/Symptoms: Muscle weakness, leg cramps, decreased bowel sounds.
  • ECG Changes: ST depression, flat or inverted T waves, and prominent U waves.
  • Intervention: Oral or IV potassium replacement. Safety Alert: IV potassium must always be diluted and administered via an infusion pump. Never exceed 10-20 mEq/hr, and never administer IV push.
• Hyperkalemia (> 5.0 mEq/L):
  • Causes: Renal failure, potassium-sparing diuretics, severe tissue trauma (burns, crush injuries).
  • Signs/Symptoms: Muscle twitching followed by weakness, diarrhea.
  • ECG Changes: Tall, peaked T waves, widened QRS complex, prolonged PR interval.
  • Intervention: Administer calcium gluconate (to protect the heart), regular insulin with IV dextrose (to shift potassium back into cells), and sodium polystyrene sulfonate (Kayexalate) for gastrointestinal excretion.

Sodium (Normal: 135 - 145 mEq/L)

Sodium is the major extracellular cation and the primary determinant of plasma osmolality. Sodium imbalances are fundamentally water imbalances.

• Hyponatremia (< 135 mEq/L):
  • Signs/Symptoms: Lethargy, confusion, muscle twitching, seizures (if severe).
  • Intervention: Fluid restriction (if hypervolemic) or hypertonic saline (if severe and symptomatic).
• Hypernatremia (> 145 mEq/L):
  • Signs/Symptoms: Extreme thirst, dry mucous membranes, altered mental status.
  • Intervention: Gradual fluid replacement with hypotonic fluids (e.g., 0.45% NS). Rapid correction can cause cerebral edema.

Calcium (Normal: 8.5 - 10.5 mg/dL)

• Hypocalcemia (< 8.5 mg/dL): Characterized by neuromuscular irritability. Classic signs include Trousseau's sign (carpal spasm when inflating a blood pressure cuff) and Chvostek's sign (facial twitching when tapping the facial nerve).
• Hypercalcemia (> 10.5 mg/dL): Characterized by neuromuscular depression. Symptoms include lethargy, muscle weakness, constipation, and kidney stones.

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Clinical Nutrition

Nutritional support is vital for healing, immune function, and overall recovery. When oral intake is insufficient, specialized nutritional therapy is required.

Enteral Nutrition (Tube Feeding)

Enteral nutrition involves delivering nutrients directly into the gastrointestinal (GI) tract (e.g., via NG tube, PEG tube).

• Principle: "If the gut works, use it." Enteral feeding is always preferred over parenteral nutrition because it maintains the integrity of the intestinal mucosa, preventing the translocation of gut bacteria into the bloodstream (which can lead to sepsis), and is significantly cheaper with fewer severe complications.
• Nursing Considerations:
  • Aspiration Precautions: Elevate the head of the bed to 30-45 degrees during and for at least one hour after feedings.
  • Placement Verification: X-ray is the gold standard for initial placement verification. For ongoing assessment, check gastric aspirate pH (should be acidic, < 5.0).
  • Gastric Residual Volume (GRV): Check regularly (e.g., every 4 hours). High residuals may indicate delayed gastric emptying and an increased risk of aspiration.

Parenteral Nutrition

Parenteral nutrition delivers nutrients directly into the bloodstream, bypassing the GI tract entirely.

• Total Parenteral Nutrition (TPN): A highly concentrated, hypertonic solution containing glucose, amino acids, lipids, vitamins, and minerals. Because of its high osmolality, it must be administered through a central venous catheter (e.g., PICC line, subclavian line) to allow rapid dilution in a large volume of blood.
• Indications: Severe malabsorption, non-functioning GI tract, severe burns, or prolonged paralytic ileus.
• Major Complications:
  • Hyperglycemia: The high dextrose concentration often causes elevated blood glucose. Regular capillary blood glucose monitoring (e.g., every 6 hours) is mandatory, and insulin coverage may be required.
  • Infection/Sepsis: The high glucose content makes TPN an excellent medium for bacterial growth. Strict aseptic technique is required for dressing and tubing changes.
  • Fluid Overload: Due to the hypertonic nature pulling fluid into the vascular space.
• Critical Nursing Action: If the TPN bag is empty and a new bag is not immediately available, the nurse must hang a solution of 10% Dextrose in Water (D10W) at the same infusion rate to prevent profound rebound hypoglycemia.