4.2. Measurements
4.2.7. Measurement of compartments

Measurement of compartments

Ref: KB2: p5-6

Characteristics of tracer

  1. Nontoxic
  2. Not metabolised or excreted
  3. Distribution is
    * Rapid
    * Uniform
    * Confined to the compartment measured
  4. Easily measured
  5. Does not alter fluid distribution

ECF measurement

Types of tracers used in ECF measurement

  1. Ionic (e.g. 82Br, 35SO4, chloride isotopes)
  2. Crystalloids (e.g. inulin, mannitol)

Ionic tracers

Very small

Distribute throughout ECF, but some also enter cells (ICF)
--> ECF volume overestimated

Crystalloid tracers

Larger molecules

Do not diffuse equally throughout ECF, but stay out of cells
--> ECF volume underestimated

Blood volume measurement

1. Indirect method

Measuring haematocrit and plasma volume (see below)

--> Blood volume
= Plasma / (1 - Hct)

2. Direct

Use radio-chromium (51Cr) labelled RBC as tracers

3. Mixed

Using both:

Haematocrit

When using haematocrite, remember:

NB:

Plasma volume measurement

Tracer:

Both tracers bind to albumin and stay mostly intravascularly
--> Some are lost to ISF
--> Multiple samples taken
--> Plasma concentration at time zero extrapolated

Keywords:  Blood volume, measurement of ; Compartments, measurement of ; Extracellular fluid, measurement of ; Haematocrit ; Plasma volume, measurement of ; Tracer
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Created20050603
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