Biophysical methods for microcirculatory studies in man

Abstract

The most frequently used methods are:

Perfusion can be measured by means of radioactive, diffusible (and inert) indicators such as β-emitters (85-Kr), single photon emitters (133-Xe) and positron emitters (11-C in CO₂, 13-N, 15-O in CO₂): clearance method.

The mean transit time can be determined by means of radioactive, nondiffusible indicators such as 123 (μ)-J, 131 (β⁻, μ)-J, 99m(μ)-Tc or 113 m(μ)-Jn specially after labeling human serum albumin (HSA) microaggregates diameter 0,01–0,02 µm) with the indicators.

The velocity of the blood stream at a particular point in selected small vessels can be determined by (i) intravital microscopy (nailfold capillaries) or (ii) very high frequency ultrasonic Doppler method.

Supply conditions can be investigated by means of (i) oxygen microelectrodes, specially in skeletal muscle, and (ii) administration of radioactive indicators, preferentially of the positron emitters 15-O and 11-C which can be linked to glucose.

Metabolism (consumption) can be determined by application of the same positron emitters as in d.

There is no ideal method which complies with all requirements. Thus, the limiting factors of each method have to be taken into account for each special application. Despite of evident advantages, radioactive indicators show also some restrictive disadvantages:

poor spatial resolution (dimension: ml) which only in special cases can be improved by application of mathematical procedures (e.g. peeling);

for diffusible indicators: the dependence on the blood-tissue partition coefficient, particularly on the local hematocrit.

for non-diffusible indicators: the recirculation problem and, dependent on the form of application, high radiation exposure;

for positron emitters: the short physical half-time (several minutes) which necessitates the neighborhood of a cyclotron;

limited accuracy (and even reproducibility) due to:

• the form of administration (inhalation, intravenous or intraarterial injection, infusion) which affects the input function;

• the error of measurement statistics, temporal resolution errors, noise scatter etc.;

• discontinuous measurement: usually the measured parameter must be constant during the measurement period.

If these restrictive aspects are carefully taken into account, the application of radioactive indicators permits detailed