Applications

Postural Vasoconstriction

On changing posture from a supine position to standing, the microvasculature of the lower limbs vaso-constricts; this is the veno-arteriolar reflex (VAR) and the associated change in blood flow can be assessed by laser Doppler (Rayman et al 1986, Shore et al, 1994, in diabetes). The VAR is a mechanism that protects the delicate microvasculature of the lower limbs from damage due to excess hydrostatic blood pressure. The impairment of the VAR is seen in diabetes and more recently in primary biliary cirrhosis (Stevens et al, 2009).

Equipment

The moorVMS-LDF with a VP1T or VPIT/7 probe can be used in conjunction with moorVMS-PC software.

The VAR can also be assessed by imaging if measures are taken to enable the scan head to move by the 50cm change in height and any angle adjustments during change of foot position; the moorLDLS2 or the moorFLPI on an MS3 mobile stand or a moorLDI2 on an MS2 mobile stand would be appropriate choices.

Methods

For posture assessed VAR, patient reclined on a special couch that was hinged at the hip to allow passive lowering of either leg. The laser Doppler probe was attached to the pulp of the great toe and baseline measurements made after 30minutes acclimatisation in a room at 22 – 23°C. The leg was then lowered so that the toe was 50cm below the mid-axilliary line and LD flux was assessed between 3 and 4minutes after lowering (after the blood flow had stabilised).

Analysis

Multiple LD flux measurements are averaged;

The VAR index is Flux average in the dependent position / Flux average in the supine position x 100%

Observed values for VAR: normal healthy = 18%; diabetic without neuropathy = 29%; diabetic with neuropathy = 54%.

Related Topics

Oldfield and Brown (2006) have demonstrated that venous congestion, induced by partial occlusion at the thigh, does not evoke a VAR: baseline data were gathered over a period of 3 min, during which resting calf blood flow was measured four times. Venous distension was then induced by rapid thigh cuff inflation to 50 mm Hg in a single step, which was maintained for 5 min. Blood flow rapidly reduced, as a consequence of deep vein filling and a reduction in perfusion pressure. Thereafter vascular resistance gradually reduced rather than increased as would be expected from an active veno-arteriolar constrictor response.

LDI has been used to assess the improvement in plantar blood flow in patients following infrainguinal revascularization for critical lower limb ischemia (Saucy et al, 2006).

References

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