Fluid and Electrolytes - Tonicity

Tonicity and IV Fluids

  • Tonicity refers to the osmolality of fluid surrounding cells and how it affects them, especially when administering IV fluids.

Types of Tonicity

Isotonic Fluid

  • "Iso-" means equal; isotonic fluid has the same osmolality as the fluid inside the cell.
  • Same concentration of solutes or particles inside and outside the cell, leading to minimal water movement.
  • Cell size remains relatively constant.
  • Examples:
    • 0. 9% Sodium Chloride
    • Lactated Ringers (contains various electrolytes)
  • Used to restore fluid volume lost due to:
    • Vomiting
    • Diarrhea
    • Insufficient fluid intake
    • Massive inflammatory response
    • Sepsis

Hypotonic Fluid

  • "Hypo-" means low; hypotonic fluid has lower osmolality, meaning fewer solutes or particles, than inside the cell.
  • Water moves into the cell, causing it to swell and potentially burst.
  • Example: 0.45% Sodium Chloride (half normal saline).
  • Used for cellular dehydration.
  • Not ideal for emergency situations; better for patients with adequate circulating volume but dehydrated cells.

Hypertonic Fluid

  • "Hyper-" means more; hypertonic fluid has higher osmolality, meaning more solutes or particles, than inside the cell.
  • Fluid moves out of the cell, causing it to shrink and potentially die.
  • Examples:
    • 3% Sodium Chloride (much higher salt concentration than 0.9% NaCl)
    • Mannitol
  • Used when patient is in fluid overload, pulls fluid from tissues back into bloodstream.

IV Fluids and Fluid Compartments

  • IV fluids are administered into the intravascular space (ECF - extracellular fluid).
  • Fluid type depends on the patient's specific problem.

Fluid Volume Imbalances

Fluid Excess (Hypervolemia)

  • Too much fluid in the bloodstream.
  • Can cause edema (excess fluid in interstitial and intracellular spaces).

Causes of Edema

  • Increased hydrostatic pressure:
    • Extra fluid increases pressure in capillary beds, pushing fluid out.
    • Can result from high blood pressure or kidney disease.
  • Lower oncotic pressure:
    • Decreased solute concentration (e.g., low albumin) reduces pull of fluid back into bloodstream.
    • Can result from malnutrition or low protein intake.
  • Obstruction of the lymph system or increased capillary permeability

Types of Edema

  • Generalized edema: all over the body.
  • Dependent edema: occurs in the lowest body parts due to gravity.
    • Swollen ankles/feet (standing).
    • Sacral edema (bedridden).
  • Decreased venous return contributes to dependent edema due to:
    • Reduced muscle tone
    • Valve dysfunction
  • Pitting edema: indentation remains after pressing on the skin.
  • Third spacing: fluid accumulation in areas where it shouldn't be.
    • Ascites: fluid in the peritoneal cavity (abdominal area).
    • Pleural effusion: fluid around the lungs.
    • Pulmonary edema: fluid in the lungs due to left-sided heart failure.

Effects of Edema

  • Decreased function and impaired joint movement.
  • Pain due to nerve stimulation.
  • Decreased circulation: fluid compresses vessels, reducing nutrient and oxygen supply.
    • Can lead to ischemia, tissue necrosis, and ulcerations.
    • Edematous skin is at high risk for tissue breakdown.

Signs of Fluid Volume Excess/Hypervolemia

  • Edema
  • Full, bounding pulses
  • Elevated blood pressure
  • Weight gain (approximately 2 pounds is about 1 liter of fluid)
  • Pulmonary edema (crackles, cough)
  • Decreased hematocrit (percentage of blood cells in volume decreases)
  • Lethargy
  • Functional impairment

Devices to improve Venous Return

  • Sequential compression devices (SCDs) simulate muscle contractions to improve venous return.