Overview
This lecture reviews fluid management strategies in ICU patients, focusing on body fluid compartments, types of IV fluids (crystalloids and colloids), and the clinical considerations for their use.
Body Fluid Compartments
- The human body is 50-60% water, divided between extracellular fluid (ECF) and intracellular fluid (ICF).
- ECF is one-third of body fluids, split into plasma (20%) and interstitial fluid (80%).
- Water moves passively between compartments due to osmotic and hydrostatic pressure gradients.
- Electrolytes (e.g., sodium, potassium) move freely within ECF but require active transport to enter ICF.
- Plasma proteins like albumin maintain the osmotic (oncotic) pressure within the vasculature.
Types of IV Fluids
- Two main categories: crystalloids (contain water, electrolytes) and colloids (contain large molecules like proteins).
- Crystalloids are simple, cost-effective, and have low risk of immune response.
- Colloids stay in the vascular space longer, have higher oncotic pressure, but are more expensive and can cause immune reactions.
Crystalloids: Categories and Common Fluids
- Categorized by osmolality: isotonic (250–375), hypertonic (>375), hypotonic (<250).
- Isotonic fluids (e.g., normal saline, lactated ringers, plasmalyte) stay in ECF; differ in balance and specific electrolytes.
- Normal saline: slightly hypertonic, high chloride, unbalanced; can cause hyperchloremic metabolic acidosis.
- Lactated ringers: slightly hypotonic, more balanced, contains lactate converted to bicarb.
- Plasmalyte: balanced, closely matches plasma, uses acetate and gluconate as buffers.
- Hypertonic fluids (e.g., 3% saline): draw water out of cells; used in cerebral edema and severe hyponatremia.
- Hypotonic fluids (e.g., 0.45% saline): move water into cells; avoid in head injury due to risk of cerebral edema.
Dextrose-Containing Solutions
- Dextrose solutions provide calories but are not nutritional substitutes.
- Their tonicity changes as dextrose is metabolized (e.g., D5W becomes hypotonic).
- D5NS, D5LR, and D10W start hypertonic; end as isotonic or hypotonic.
- Use cautiously in patients with head injury due to risk of cerebral edema.
Colloids
- Colloids (e.g., albumin, hydroxyethyl starches) act as plasma expanders by increasing oncotic pressure.
- Albumin is available in 5% or 25% solutions; adjunctive use in hypovolemia and sepsis.
- Colloids can be harmful in conditions with capillary leak (e.g., sepsis, ARDS), causing edema.
Key Terms & Definitions
- ECF (Extracellular Fluid) — Fluid outside cells, includes plasma and interstitial fluid.
- ICF (Intracellular Fluid) — Fluid within cells.
- Isotonic — Fluid with osmolality similar to plasma, does not cause fluid shifts between ECF and ICF.
- Hypertonic — Fluid with higher osmolality; pulls water out of cells.
- Hypotonic — Fluid with lower osmolality; moves water into cells.
- Crystalloid — IV fluid with water and small solutes/electrolytes.
- Colloid — IV fluid with large molecules/proteins that stay in vasculature.
- Oncotic Pressure — Osmotic pressure exerted by proteins to retain fluid in blood vessels.
Action Items / Next Steps
- Review the next lesson in the fluid management series.
- Test your knowledge with the free quiz at icuadvantage.com.
- Memorize key fluid types, their uses, and contraindications for exam preparation.