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 (TcPO 2). Warmed TcPO 2 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 TcPO 2 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 TcPO 2 and LD correlated well with A: B ratio in two positions: horizontal r (TcPO 2) = 0.73, r(LD) = 0.57; elevated r(TcPO 2)=0.78, r(LD)=0.68. Overall there was a highly significant correlation between LD and TcPO 2 (r=0.6, P