Peripheral arterial disease: diagnostic challenges and how photoplethysmography may help

ABPI methodology

The first problem we face is that a significant proportion of patients in general practice who are arteriopaths are asymptomatic and ABPI has been routinely adopted to confirm and quantify the diagnosis of peripheral vascular disease in those with IC. Unfortunately the exact methodology is not standardised, beginning with a choice of three arteries in the ankle each with potentially differing pressures and some of which are commonly congenitally absent. Similarly the clinician has a choice of two arms for the brachial pressure and there is no consensus on how the central reference pressure should be assessed. Indeed, arteriosclerosis is a systemic disease. Failure to lie supine can introduce a hydrostatic pressure difference between arm and leg. A number of environmental factors such as ambient temperature can also affect peripheral vasomotor tone. The occlusive cuff method itself has been called into question with a likely underestimate of true pressure drop varying with site of lesion and which may be exacerbated with baseline hypotension or hypertension confounding ABPI readings. Finally, there is no consensus on normal ranges for ABPI ratios, and hence referral thresholds, evidenced by its absence in the most recent National Institute for Health and Care Excellence guidance.³

Limitations of ABPI

As well as methodological limitations and despite the claimed 95% sensitivity and 99% specificity, there are a number of problematic assumptions. Many of the studies of ABPI have used control subjects that are clinically asymptomatic volunteers rather than angiographically proven normals. A rich collateral vessel network may be responsible for a proportion of asymptomatic PAD patients and ABPI would fail to unmask this cohort. Heavy arterial calcification resulting in incompressible arteries in diabetic, renal, and older patients renders the ABPI non-diagnostic. This is also problematic in rarer conditions such as systemic sclerosis and rheumatic diseases. There is a need for a non-invasive alternative to ABPI for assessing peripheral vascular disease in general practice.

Methodology and challenges

PPG typically uses near-infrared (or red) light to measure the changes in tissue blood volume associated with each heartbeat.⁵ In its simplest form a light source is directed towards the tissue microvascular bed, for example to the ear lobe, finger, or toe pad, and an adjacent photodetector measures the small changes in light absorption to form a PPG pulse waveform (Figure 1). Although the exact nature of PPG is not fully understood its signatures in health and vascular disease have been described and reviewed by Allen.⁵

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Figure 1.

Photoplethysmography.

The waveforms from PPG technology are typically pulsatile and can be compared between sites such as the toes and fingers. In PAD a damping effect is produced with a reduction in amplitude and changes in waveform shape and timing compared to a healthy vascular bed.

A simpler variant of PPG, pulse oximetry, has already been successfully deployed in general practice. The next stage in the development of this technology is in its routine use for PAD patients in general practice.