Pressure index

The pressure index is widely criticized because, when the arteries are calcified, it may be artificially raised. However, we feel that it is very relevant to the investigation of the diabetic foot as long as the potential difficulties of its interpretation are understood.

If the pressure index is 0.5 then it is truly low, and indicates severe ischaemia whether the arteries are calcified or not. Indeed, if the artery is calcified the true pressure index may be even lower and even more urgent action is required.

Difficulty comes at pressure indices of 0.5 and above, when one should always pay attention to the Doppler waveform, either in audible or visible form. The normal waveform is pulsatile with a positive forward flow in systole followed by a short reverse flow and a further forward flow in diastole, but in the presence of arterial narrowing the waveform shows a reduced forward flow and is described as 'damped' (Fig. 4.15a,b).

Dopplerwaveform From Normal Foot
Fig. 4.15 (a) Doppler waveform from normal foot showing normal triphasic pattern, (b) Doppler waveform from neuroischaemic foot showing damped pattern.

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