What is the approximate extracellular volume after equilibration in a 100kg man who receives a 3 liter infusion of sodium chloride (0.9%) solution?

Extracellular Volume After Normal Saline Infusion

After infusion of 3 liters of 0.9% sodium chloride solution into a 100 kg man with normal fluid homeostasis, his extracellular volume would be approximately 23 L.

Physiological Basis for This Calculation

To determine the final extracellular volume after infusion, we need to understand:

1. Normal extracellular fluid volume distribution:

   • In a healthy adult, extracellular fluid (ECF) comprises approximately 20% of body weight
   • For a 100 kg man, baseline ECF would be approximately 20 L

2. Distribution of infused normal saline (0.9% NaCl):

   • Normal saline is an isotonic crystalloid solution that primarily distributes in the extracellular space
   • When infused, it equilibrates across the extracellular compartment (intravascular and interstitial spaces)
   • Approximately 25% remains in the intravascular space while 75% moves to the interstitial space

3. Volume calculation after equilibration:

   • Baseline ECF: 20 L (20% of 100 kg)
   • Added volume: 3 L of 0.9% NaCl
   • Final ECF volume: 20 L + 3 L = 23 L

Clinical Implications

Understanding fluid distribution is critical in clinical practice for several reasons:

• Fluid resuscitation: When administering crystalloids like normal saline, clinicians must account for the distribution across the extracellular compartment rather than assuming all fluid remains intravascular 1

• Sodium balance: Sodium is the principal cation of the ECF that directly influences intravascular and interstitial volumes. When sodium is added to ECF (as with normal saline infusion), it creates an osmotic gradient that maintains fluid within the extracellular space 1

• Monitoring for fluid overload: Excessive crystalloid administration can lead to interstitial edema as the majority of infused fluid moves into the interstitial space

Potential Variations

Several factors could potentially alter this distribution:

• Pre-existing conditions: Patients with heart failure, renal dysfunction, or liver disease may have altered fluid distribution patterns
• Capillary permeability: Increased capillary permeability (as in sepsis or systemic inflammatory response) can alter the distribution of infused fluids
• Albumin levels: Hypoalbuminemia can affect oncotic pressure and fluid distribution between intravascular and interstitial spaces

The calculation assumes normal physiological conditions with intact homeostatic mechanisms and normal capillary permeability, which is specified in the question (normal fluid homeostasis prior to infusion).

In clinical practice, balanced crystalloid solutions might be preferred over 0.9% NaCl for large volume resuscitation to reduce the risk of hyperchloremic metabolic acidosis and adverse renal events 2.