Approach to Volume Disorders, Part 1

Volume disorders are tough. They haven't come easily to me, but I hope once I start practicing the experience will help me along. I will do a 3 part series on the approach to volume disorders.

Fluid Compartments (Normal)
Assessing Volume Status
Fluid Replacement Therapy

Hopefully they will help you in your basic understanding.

Fluid Compartments:
  • Men and women are different. Men: TBW* = 60% of body weight, Women: TBW = 50%
  • % of TBW decreases with age and increases with obesity (Why? Fat contains little water)
  • How is water distributed?
    • Intracellular (ICF) = 2/3rd of TBW (the largest proportion of TBW = skeletal muscle mass)
    • Extracellular (ECF) = 1/3 of TBW
      • Interstitial fluid = 1/3 of ECF
      • Plasma = 2/3 of ECF
  • Water exchange:
    • Intake (normal) = 1500mL PO fluids, 500mL in solids/oxidation PER DAY
    • Output (normal) = 800-1500mL in urine daily is normal
      • 600-900mL per day is from insensible losses (variable bases on fever, trachs, hyperventilation, etc)
      • 250mL lost in stool
      • MIN OUTPUT per day = 500-600mL assuming normal kidney function
  • Fluid shifts are based on hydrostatic and oncotic pressures (pull out your physiology book for a refresher in this)
For the fluid compartments think 60-40-20!
TBW is 60% of body wt (50% for women)
ICF is 40% of body wt
ECF is 20% of body wt

What are 3 reasons for oliguria?
1. low blood flow to kidneys
2. kideny problem
3. post-renal obstruction (need a Foley cath!)

*TBW = total body water

Source: Step up to Medicine (Agabegi and Agabegi)

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