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Background

Fluid therapy is the administration of fluids to a patient as a treatment or preventative measure (can be administered via intravenous, intraperitoneal, intraosseous, subcutaneous and oral routes)
Fluid compartments in the body vary according to age and gender
Fluid enters and leaves the body through different routes that yield a certain amount (divided into sensible losses that can be seen, felt, and measured and insensible losses)

Fluid Compartments According to Age and Gender
Category	Details
Children	80% of the body is made up of fluid
Adult Males	60% of the body is made up of fluid
Adult Females	50% of the body is made up of fluid
Elderly	45% of the body is made up of fluid

Sensible and Insensible Fluid Gain and Loss
Route	Average Gain (mL)	Average Loss (mL)
Sensible
Oral fluids	800 - 1500
Solid food	500 - 700
Urine		800 - 1500
Intestinal		0 - 250
Insensible
Water oxidation	250
Respiratory and skin		600

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The extracellular space

The extracellular space is subdivided into three spaces:
1. Intravascular space (inside blood vessels) – 20% of the extracellular fluid
2. Interstitial space – the functional tissue space between and around cells – 80% of the extracellular fluid
3. The “third space” – the “third” extracellular space (the “third space” refers to areas of the body that do not normally contain fluid and where fluid collection is not functional or desirable. This includes areas such as the peritoneal cavity, pleural cavity, pericardial cavity and joints)
Third-spacing refers to fluid shifting into this non-functional third space (edema, ascites, effusions)
When fluid moves into a non-functional space, this may come at the expense of the intravascular space, resulting in hypotension and reduced perfusion of tissues

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Crystalloid solutions
- Aqueous solutions with mineral salts and other water-soluble molecules which are able to enter through the compartments of the body (the most commonly used fluids in a hospital setting)
Colloid solutions
- Contain large insoluble molecules and preserve a high colloid osmotic pressure in the blood. Therefore, they should theoretically preferentially increase the intravascular volume
- One example of a colloid is human albumin solution, which may be used in patients with decompensated liver disease. Albumin is an important component of plasma and increases the plasma volume. It increases the oncotic pressure of the plasma, drawing in and retaining fluid. In decompensated liver disease, the patient’s liver is not producing adequate albumin, leading to reduced oncotic pressure in the intravascular space, resulting in reduced circulating blood volume. Human albumin solution may be used to help correct this, although the effects are only temporary

Types of Crystalloid Solutions	Notes
Isotonic crystalloids (e.g., lactated Ringer’s solution, 0.9% NaCl)	Distribute uniformly through the extracellular fluid compartments so that after 1 hour, only 25% of the total volume infused remains in the intravascular space
Hypertonic saline solutions	-
Hypotonic solutions (D5W, 0.45% NaCl)	Distribute throughout the total body water compartment, expanding the intravascular compartment by 10% of the volume infused

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Osmolality
- The concentration of dissolved particles per unit mass of solution (mOsm/kg); preferred term to describe the osmotic pressure of biological systems
Osmolarity
- The concentration of solutes per unit volume of solvent (mOsm/L); often used interchangeably with osmolality in clinical practice
Tonicity
- The capacity of an extracellular fluid to create an osmotic gradient that will cause water to move into or out of the intracellular compartment; cannot be measured and has no units
- The osmolarity and tonicity of a solution are not the same thing. Administering solutions with inappropriate tonicity can lead to life-threatening fluid and electrolyte imbalances

Content and Composition of Commonly Used IV Fluid Solutions
Solution	Cl	Na	K	Ca	Lactate
Normal saline	154 mEq	154 mEq	-	-	-
½ saline	77 mEq	77 mEq	-	-	-
¼ saline	39 mEq	39 mEq	-	-	-
3% saline	512 mEq	512 mEq	-	-	-
Ringer lactate	109 mEq	130 mEq	4 mEq	3 mEq	28 mEq

Osmolarity versus Tonicity
Solution	Osmolarity	Tonicity
Isoosmolar	Equivalent to the intracellular compartment	Can be isotonic or hypotonic
Hyperosmolar	Higher than the intracellular compartment	Can be hypertonic, isotonic, or hypotonic
Hypoosmolar	Lower than the intracellular compartment	Can only be hypotonic

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Rapid introduction of large volumes of fluid through multiple large-bore (16G or wider) peripheral venous catheters is preferred to resuscitate patients with hypovolemic shock

Route of Parenteral Fluid Therapy
Route	Details
Peripheral IV Access	Most commonly used for administering fluids and/or medication
Intraosseous Access	In difficult/collapsed peripheral veins, IO access is preferred over central venous access for resuscitation
Central Venous Access	Longer length than most peripheral IV and IO access catheters