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Microdialysis
Microdialysis is based on
diffusion across a semi-permeable membrane due to a concentration
gradient. Perfusate, pH and osmotically balanced with plasma, exchanges
across a semi-permeable membrane. The dialysate contains molecular
contents originating in the interstitium which have entered by passive
diffusion. Dual functions are served: medication may be administered to
the interstitium in perfusate and interstitial contents may be sampled
in dialysate. Relative recovery, the fraction of the biochemical found
in the dialysate, is independent of the concentration of the analyte,
but depends on flow rate, composition of the perfusate, surface area,
physicochemical properties of the microdialysis catheter, and the nature
of the sampled site. Given these restrictions and similar application of
similar catheters to similar sites at similar pump rates, it is
reasonable to assume that differences measured in dialysate reflect
differences in biochemical concentrations within the interstitium
between patients and controls. Also, it is reasonable to assume similar
rates of delivery of perfused drugs. We use microdialysis catheters
inserted into the dermal space of the lateral aspect of the calf on
although most commonly the non-glabrous skin of the forearm is used. .
Catheters contain 10 mm long, 20 kDa membranes with good recovery up to
at least 5000 Daltons. A 25 gauge needle is inserted into the dermal
space and exits the skin approximately 15 mm distant. The microdialysis
catheter is inserted into the lumen of the needle, and withdrawn so that
the dialysis membrane is 10 mm from the point of insertion. Once the
membrane is placed, the needle is withdrawn. Multiple catheters are
typically used but must be spaced at least 6 cm apart throughout all
procedures and are taped in place. This is to avoid spurious effects of
the axon reflex. Perfusion rates are usually at 2μl/min in studies.
Thus, rather small quantities of perfusate are delivered and dialysate
are sampled. Chemicals are dissolved in Ringer solution. Cutaneous flow
is monitored by LDF. Before measurements are performed, hyperemia
related to trauma will need to abate as indicated by the return of laser
Doppler flow rates to approximate baseline levels. This takes
approximately at least 60 minutes and may be much longer depending on
the subject. The flow measured is affected by the optical properties of
the skin and other factors. It is usually reported in arbitrary
perfusion units. Most often investigators express results as a percent
of maximum conductance of the skin frequently obtained by using high
dose sodium nitroprusside at the end of experiments. [ Up ]
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Intradermal Microdialysis and Laser-Doppler Flowmetry
