Skin blood flow: a comparison of transcutaneous oximetry and laser Doppler flowmetry

In normal limbs skin blood flow (SBF) falls on standing, but in ischaemic limbs there is a characteristic blush on dependancy. We studied the effect of changing posture in a range of ischaemic limbs, using two non-invasive techniques--laser Doppler (LD) and transcutaneous oximetry (TcPO2). Warmed TcPO2 and LD probes were placed over the first metatarsal cleft and SBF recorded with the limb horizontal, dependant and elevated. Ankle to brachial pressure ratios (A:B) were also measured. Twelve patients with lower limb ischaemia, mean A:B ratio 0.5 +/- 0.21 (+/- S.D.) were compared with 21 asymptomatic controls, mean A:B ratio 1.2 +/- 0.15. In the controls, neither TcPO2 nor LD values changed significantly with limb position. Both flux and oxygen tension were significantly lower in ischaemic limbs in all positions; on lowering the ischaemic limb, flux unexpectedly rose to 23.5 +/- 17.7 V from a mean of 14 + 9.0 V (horizontal). Both TcPO2 and LD correlated well with A:B ratio in two positions: horizontal r (TcPO2) = 0.73, r(LD) = 0.57; elevated r(TcPO2) = 0.78, r(LD) = 0.68. Overall there was a highly significant correlation between LD and TcPO2 (r = 0.6, P less than 0.001, Student's test), but this was strongest in the elevated position (r = 0.87, P less than 0.001). LD and TcPO2 relate to A:B ratio and to each other, particularly in the stressed limb; in the well perfused limb, the poorer correlation suggests skin perfusion is not linearly related to tissue oxygen tension.