IV Fluids – Study Notes

IV Fluids – Study Notes

Fluid Compartments

• Total body water: ~50–60% of weight

• Extracellular fluid (ECF): 1/3 total

  • Plasma (intravascular) = 20% of ECF

  • Interstitial = 80% of ECF

• Intracellular fluid (ICF): 2/3 total

• Movement:

  • Water moves passively between compartments (osmosis, hydrostatic pressure).

  • Electrolytes move via channels/pumps (active transport).

  • Plasma proteins (esp. albumin) help hold water in vasculature via oncotic pressure.

  • Types of IV Fluids

1. Isotonic (osmolality ~250–375; ~same as plasma 285–295)

• Effect: Stays in ECF; does not shift into ICF. Expands intravascular volume.

• Examples:

  • 0.9% NS (slightly hyper, unbalanced: ↑Cl → hyperchloremic metabolic acidosis, ↓GFR if high volumes)

  • LR (slightly hypo, balanced: contains Na, K, Ca, Cl, lactate→bicarb; avoid in ↑ICP or liver failure)

  • Plasma-Lyte (balanced, closest to plasma; uses acetate/gluconate as buffers; less risk acidosis)

• Uses: Hypovolemia, shock, resuscitation.

2. Hypertonic (osmolality >375)

• Effect: Pulls water from ICF → ECF. Cells shrink. Expands intravascular space.

• Examples:

  • 3% NS (osm 1030) → treats cerebral edema, severe hyponatremia. Risks: hypernatremia, overload.

  • D5NS, D5LR, D5½NS (start hyper → become iso/hypo as dextrose metabolizes).

  • D10W (osm 505 → metabolizes → free water, hypotonic).

• Uses: Cerebral edema, hyponatremia, ICP control.

3. Hypotonic (osmolality <250)

• Effect: Water shifts from ECF → ICF. Cells swell.

• Examples:

  • 0.45% NS (½ NS): Na 77, Cl 77; unbalanced. Risks: ↑ICP, hemolysis, intravascular depletion.

  • D5W: Starts isotonic (253), becomes free water (hypo) once dextrose is metabolized. Avoid in head injury.

• Uses: Hypernatremia, free water replacement.

4. Colloids (large molecules, ↑oncotic pressure)

• Stay in vasculature; pull water in.

• Examples:

  • Albumin 5% / 25% → volume expander, sepsis adjunct, renal protection.

  • Hetastarch / Dextran → less common, risks in sepsis/liver disease.

• Caution: Capillary leak (sepsis, ARDS) → proteins leak → worsen edema.

Summary Table – Fluid Shifts

Fluid Type	Osmolality	Shift	Effect
Isotonic	~250–375	Stays in ECF	Expands volume, no cell change
Hypertonic	>375	ICF → ECF	Shrinks cells, ↑ vascular volume
Hypotonic	<250	ECF → ICF	Swells cells, ↓ vascular volume
Colloid	Variable (oncotic)	Pulls interstitial → vascular	Plasma expander

Key dangers:

• Hypotonic → cerebral edema/ICP ↑

• Hypertonic → fluid overload, hyperNa

• NS large volume → hyperchloremic metabolic acidosis, ↓GFR

• Colloids in sepsis/ARDS → worsen edema