The 'third space'--fact or fiction?

Area of Science:

• Anesthesiology
• Critical Care Medicine
• Surgical Fluid Management

Background:

• The 'third space' concept historically guided perioperative fluid regimens, assuming significant fluid sequestration.
• Traditional approaches involved generous fluid replacement to compensate for this perceived loss and maintain blood volume.
• This often resulted in a markedly positive fluid balance during major surgery.

Purpose of the Study:

• To re-evaluate the validity of the 'third space' concept in perioperative fluid management.
• To investigate the mechanisms of fluid shifts during major surgery.
• To propose an optimized fluid regimen based on current understanding.

Main Methods:

• Review of existing literature and physiological principles regarding fluid compartments.
• Analysis of factors contributing to fluid shifts, including inflammation and iatrogenic hypervolemia.
• Evaluation of the role of the endothelial glycocalyx in vascular barrier integrity.

Main Results:

• The 'third space' is likely a fictional construct, with minimal fluid loss to such a compartment.
• Insensible perspiration and preoperative deficits are often negligible.
• Excess fluid primarily accumulates in the interstitial space due to endothelial glycocalyx damage.

Conclusions:

• Eradicating the 'third space' notion is crucial for optimizing perioperative fluid management.
• Fluid regimens should focus on avoiding interstitial fluid accumulation rather than replacing a fictional 'third space' loss.
• Optimized fluid strategies benefit patients undergoing major surgery by preventing detrimental fluid shifts.