Intravenous (IV) Fluids

Intravenous (IV) fluids are sterile solutions that are administered directly into a vein through an IV catheter. They are one of the most common and versatile interventions as they can be used to restore and maintain fluid and electrolyte balance, provide nutrients, and deliver medications directly into the bloodstream. 

There is a wide variety of IV fluids available, each with its own indications and possible side effects. Understanding the composition of each IV fluid can help medical professionals choose the best treatment for every clinical scenario.  

There are four main indications for IV fluid therapy, including fluid resuscitation, maintenance fluid therapy, correction of electrolyte imbalances, and delivery of IV medications. 

Fluid resuscitation involves administering IV fluids to rapidly restore circulating blood volume in cases of acute blood loss, severe dehydration, or shock. In critically ill individuals, IV treatment is administered in the form of rapid boluses of 500-1000 ml of fluid and repeated as needed depending on clinical response. In hemodynamically stable individuals, fluids are administered less aggressively due to risk of fluid overload.  

After stabilization, IV fluids can be used to provide daily fluid and electrolyte requirements, especially if oral intake is not possible or cannot compensate for the individual’s requirements alone. In those with ongoing fluid losses (e.g., burns, gastrointestinal tract losses from vomiting or diarrhea, fistulas) maintenance therapy should consider the amount of lost fluid and try to mimic its composition. Generally, daily baseline requirements are 20-25 ml/kg of water; 1 mmol/kg/day of potassium, sodium and chloride; and around 50-100 g/day of glucose to prevent starvation ketosis. Other sources of fluid intake (e.g., IV medication, enteral nutrition, blood transfusions) should be subtracted from the required daily fluid volume to avoid fluid creep, which is the gradual and often unnoticed accumulation of fluid in the body.  

Another indication of IV therapy is the correction of electrolyte imbalances, such as hyper- or hyponatremia, hypokalemia, or hypocalcemia. 

Finally, IV fluids like normal saline or dextrose solutions are often used as a vehicle to deliver IV medications.  

Intravenous fluids work by causing water to shift across the body’s fluid compartments. Water makes up around 60% of our total body weight, roughly 42 liters in a 70 kg individual. Around 40% of this water is distributed in the intracellular compartment (i.e., within cells) and the remaining 20% in the extracellular compartment (i.e., outside of cells). Extracellular fluid can be further subdivided into interstitial fluid, which is found surrounding cells, and plasma, which is the aqueous portion of blood. 

Each compartment has a specific osmolarity determined by the concentration of ions and other solutes. Free water moves across compartments to maintain a steady state so that osmolarity remains the same. If one compartment has a few more solutes than the other, water will flow in that direction to lower the concentration until it reaches a new state of balance. 

Once administered, IV fluids redistribute throughout the body causing the movement of water and ions from one compartment to another. Isotonic fluids have a similar osmolarity to plasma, so they do not cause significant shifts between compartments. Hypotonic fluids have a lower osmolarity than plasma, causing water to move into the cells, whereas hypertonic fluids have a higher osmolarity than plasma, resulting in the movement of water out of the cell.