Measurement of compartments
Ref: KB2: p5-6
Characteristics of tracer
- Nontoxic
- Not metabolised or excreted
- Distribution is
* Rapid
* Uniform
* Confined to the compartment measured
- Easily measured
- Does not alter fluid distribution
ECF measurement
Types of tracers used in ECF measurement
- Ionic (e.g. 82Br, 35SO4, chloride isotopes)
- 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