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:

• ⁵¹Cr (to measure RBC volume)
• ¹⁵¹I (to measure plasma volume)

Haematocrit

When using haematocrite, remember:

• Haematocrit is different in different parts
  --> Distribution is uneven
  --> Some error
• Haematocrit is higher in venous blood because of larger RBC volume
  --> Whole body Hct
  = 0.91 x Venous Hct

NB:

• Hct in muscle capillaries is only 20% due to axial streaming

Plasma volume measurement

Tracer:

• Evan's blue
• Radioiodine (¹⁵¹I)

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