Pedal Acceleration Time (PAT) as a Measure of Foot Perfusion

The use of ankle brachial pressure index (ABPI) is commonplace in the screening, diagnosis and management of peripheral vascular disease (PAD). This technique measures and compares the occlusion pressure of the brachial arteries for comparison with the occlusion pressure of the tibial arteries at the level of the ankle. The index is then calculated by dividing the highest of the tibial occlusion pressures by the highest brachial occlusion pressure. In normal subjects the ABPI should fall between 09-1.3 (1) . A pressure less than 0.9 suggest a compromise to the peripheral circulation (PAD), with the blood pressure at the ankle level being lower than in the arm. ABPI results can be erroneous in the presence of medial artery calcification (MAC) where the ankle arteries are more difficult to compress and hence give falsely elevated readings. In some patients it can be impossible to occlude the arterial flow as the vessel is so rigid (1). Conditions that increase the prevalence of MAC are diabetes mellitus, chronic kidney disease as well as advancing age which also increased the stiffness of the vessel walls having a similar effect as MAC (2).

When used in isolation as a PAD screening, ABPI can give a normal reading due to undiagnosed pathological vessel stiffness with studies reporting up to 14-27% of patients returning normal ABPI results at the same time as low TBPI. (3) Despite the potential pitfall of ABPI, it is still a widely recognized as its diagnostic limits have been validated in several large scale studies. (3) However, due to the level of arterial assessment, ABPI does not actually assess circulation into the foot itself (4). The toe vessels are much less susceptible to vessel stiffness (3) , and hence deemed are more reliable in such cases when the ABPI would return a falsely elevated result.

When used alongside detailed duplex imaging of the peripheral arterial tree to give image guided documentation of vessel calcification, TBPI is a useful indicator of perfusion into the foot when the ABPI is known not to be indicated. Therefore TBPI measurement has become the established method for assessing arterial perfusion to the foot, particularly in patients with risk factors for MAC.

When investigating perfusion into the critically ischaemic limb, ABPI is not indicated and therefore TBPI is the preferred method. However this becomes problematic when there has been previous amputation of the forefoot/toes, the patient is in significant pain and/or there is tissue loss which impedes accurate placement of the equipment on the affected limb. Furthermore, in a significant number of these patients it is not possible to obtain an adequate waveform in the toe due to poor arterial perfusion.

To overcome this predicament a novel technique has been developed which involves direct ultrasound imaging of the pedal vasculature and use the systolic rise time calculated from the returned waveform analysis as a surrogate for ABPI (4) This systolic rise time, known as pedal acceleration time (PAT) in foot vessels, has been has been reported to correlate well with ABPI in patients with compressible vessels and therefore is felt to be a predictor of wound healing following direct and indirect revascularization. (4) There are, however, a significant group of patients that attend the vascular department that have risk factors for MAC which make them unsuitable for ABPI. For this group of patients and those with coexisting with foot wounds/previous amputation, there is no ratified method for investigation perfusion pressure.

The aim of this pilot study is to gather information from all patients who present with significant PAD to investigate a possible link between PAT, ABPI and TBPI and if PAT is a reliable measurement in patients who fall outside of the group normally excluded.

This would allow objective perfusion information and therefore guide treatment and management plans.