creation date: 2026-05-30 01:55
tags: Pharmacology
Intravascular Fluids
Background
Fluid Distribution
Under normal physiological conditions, renal function regulates volume, osmolality, and electrolyte balance. Normal minimum daily water intake (from all sources) is approximately 1600 mL with an equivalent output.
The body consists of 50-60% water, of which is split into extracellular fluid (1/3) and intracellular fluid (2/3). The ECF is split into 25% plasma and 75% interstitial fluid.
Fluid moves passively between the plasma, interstitial space, and intracellular space through passive, osmotic and hydrostatic gradients. Owing to the active transport control of electrolytes and plasma proteins, concentrations and thus fluid distribution can be controlled.
- Primary intracellular: K+
- Primary extracellular: Na+
Normal plasma has a osmolality of 280-295 mOsmol/L.
Fluid Dosing
When dosing fluids, make sure to create stops in order to allow for reassessment of volume status.
Note that addition of albumin is not indicated except in hepatic disease.
Bolus
Adults
Bolus is typically 500 mL or 1000 mL of NS or LR. May be more in some situations.
Maintenance
Adults
Adults dosing can be calculated as:
- (Weight in kg) + 40 mL/hr
This is derived from the 4-2-1 rule.
Pediatrics
Pediatric dosing uses the 4-2-1 rule:
- First 10 kg: 4 mL/kg/hr
- Next 10 kg: 2 mL/kg/hr
- Over 20 kg: 1 mL/kg/hr
Example: 45 kg patient maintenance infusion rate:
- (10 kg * 4 mL/kg/hr) + (10 kg * 2 mL/kg/hr) + (25 kg * 1 mL/kg/hr) = 85 mL/hr
Note that with very young infants, total fluid intake is used instead because formula/breast milk will contribute to fluid intake.
Fluid Types
Two primary types are the crystalloids and colloids.
Crystalloids are made up of water and electrolytes and other water soluble molecules. This makes it simple, cost-effective, and free of immune responses. These fill both the ICF and ECF.
Crystalloid types are based on osmolality:
- Isotonic: osmolality 250-375/L, similar to plasma; won’t shift from ECF to ICF
- Hypertonic: osmolality >375; pulls fluids from ICF to ECF
- Hypotonic: osmolality <250; moves fluids from ECF to ICF
Crystalloids can also be classified as balanced or unbalanced based on the electrolyte balance of normal plasma.
Colloids consist of blood products and other fluids with large insoluble molecules. They have a higher osmotic pressure, are more expensive, and risk immune responses. These will go into the ICF.
Fluids
Common Fluids
| Fluid | Osmolality | Na+ | Cl- | K+ | Ca2+ | Mg2+ | HCO3- | Lactate |
|---|---|---|---|---|---|---|---|---|
| Plasma | 280-290 | 135-145 | 96-106 | 3.5-5.2 | 4.4-5.2 | 1.8-24 | 22-26 | 1 |
| NS (0.9%) | 308 | 154 | 154 | - | - | - | - | - |
| LR | 273 | 130 | 109 | 4 | 3 | - | - | 28 |
| PlasmaLyte | 295 | 140 | 98 | 5 | 4 | Acetate 27, gluconate 23 | ||
| 3% NS | 1030 | |||||||
| 0.45% NS | 154 | 77 | 77 | - | - | - | - | - |
Isotonic
Normal Saline (0.9% NaCl)
Unbalanced fluid
Issues:
- Higher chlorine than plasma can lead to hyperchloremic metabolic acidosis
- Negative effects of GFR (risk of AKI) due to inflammation/edema from hyperchloremia
Lactated Ringers (LR)
Balanced fluid
- Lactate metabolized to bicarbonate by the liver
Issues:
- Slight hypotonic can cause ICF shifts (cerebral edema/increased ICP)
- Hepatic hypoperfusion can reduce lactate metabolism and loses out on lactate benefits
PlasmaLyte
Balanced fluid
- Acetate and gluconate for bicarbonate production instead of lactate which removes the reliance for liver conversion
- Lower risk of hypercholemic metabolic acidosis
Hypertonic
3% Normal Saline
- Draws fluid from ICF to ECF
- Treats cerebral edema and severe, symptomatic hyponatremia
- Can result in hypernatremia and hyperchloremia, and intravascular fluid overload
Hypotonic
1/2 Normal Saline (0.45% NS)
Unbalanced fluid
- Shifts fluids from ECF to ICF
- Do not use with head injuries (increase cerebral edema and ICP)
- Treats hypernatremia and replaces free water deficit
Issues:
- Rapid administration can lead to lysis of RBCs and depletion of intravascular volume (and thus cardiovascular collapse)
Dextrose Containing Fluids
Osmolality depends; as dextrose is metabolized the osmolality decreases. Dextrose is always given instead of water as water alone will result in hemolysis.
Calories:
- 5% is 50 g/L = 170 cal/L
- 10% is 100 g/L = 340 cal/L
Dextrose is often used for pediatric populations (maintenance) as children have a significant metabolic load during illness. Note, dextrose generally isn’t used for pediatric boluses as it can cause rebound hypoglycemia due to insulin spike.
Isotonic
5% Dextrose-Water (D5W)
Starts as isotonic (osmolality 253/L); as dextrose is metabolizes, becomes free water (hypotonic)
- Same issues as 1/2 NS
Indications
- Pediatric fluid of choice, often with 20 mmol KCl (ensure kidney function)
Hypertonic
5% Dextrose-1/2 Normal Saline (D5 1/2NS)
Starts as hypertonic (osmolality 406/L); becomes 1/2 NS (hypotonic)
5% Dextrose-Normal Saline (D5NS)
Starts as hypertonic (osmolality 560/L); becomes 1NS (isotonic)
This is the most commonly used fluid for pediatric populations.
5% Dextrose-Lactated Ringers (D5LR)
Starts as hypertonic (osmolality 527/L); becomes LR (isotonic)
10% Dextrose-Water (D10W)
Starts as hypertonic (osmolality 505/L); becomes free water (hypotonic)
Colloids
Referred to as volume or plasma expanders.
- High oncotic pressure that stays in the vasculature (does not pass into the interstitial space) results in fluid being retained within the vasculature for longer
Issues:
- Potential harm with capillary leak disorders (eg. sepsis, ARDS); worsens oncotic gradient pulled water from vasculature to interstitial space when these proteins leak out
Albumin
Derived from plasma; some sites consider this a blood product.
- 5% - osmolality 309/L
- 25% - osmolality 312/L
Uses:
- Preserve renal function in critically ill patients
- Adjuncts to other fluids for hypovolemia (eg. crystalloids with albumin)
Hydroxyethyl Starches (Hespan) and Dextran
Less common but there are potential complications – functions like albumin