Fluid balance concepts in medicine: Principles and practice

Area of Science:

• Physiology
• Nephrology
• Critical Care Medicine

Background:

• Body fluid balance is critical for health and disease, involving tonicity, sodium balance, and extracellular volume.
• Tonicity disturbances affect cell volume, impacting brain function.
• Effective arterial blood volume (EABV) ensures organ perfusion but is complex to assess.

Purpose of the Study:

• To elucidate the three key concepts of body fluid balance regulation.
• To highlight the challenges in measuring and managing extracellular volume in severe illness.
• To propose extracellular volume in severe illness as a distinct concept.

Main Methods:

• Conceptual analysis of body fluid regulation.
• Review of factors influencing effective arterial blood volume (EABV).
• Discussion of challenges in extracellular volume assessment in critical illness.

Main Results:

• Body fluid balance is conceptualized into three distinct areas: tonicity, sodium balance/extracellular volume, and extracellular volume in severe illness.
• Tonicity is determined by solutes distributed across body fluid compartments.
• EABV regulation is multifactorial, and its assessment in severe illness is complex and error-prone.

Conclusions:

• Understanding body fluid balance requires considering tonicity, sodium balance, and the unique challenges of extracellular volume in severe illness.
• Accurate assessment and determination of optimal extracellular volume in critical illness remain significant challenges.
• Further research is essential to define optimal extracellular volume targets for various severe illnesses.