Digital Ankle Brachial Index (ABI) as a Screening Tool in Detecting Peripheral Arterial Disease

Peripheral artery disease (PAD) affects 8-18 million in the US and is an economic burden, currently estimated to be greater than cancer and heart disease. Older age (> 65 years), smoking, diabetes and kidney diseases are some risk factors associated with PAD and are known to have increased morbidity and mortality. PAD progression directly results in claudication, impaired walking, and amputation. Amputation is not benign - impacts are physical and emotional. Recovery from amputation involves long-term rehabilitation. The American Heart Association (AHA) in 2016 guidelines suggests an algorithm for diagnosis and management of PAD. They recommend a collaborative multidisciplinary team approach between the primary care physician and specialists including; a diabetologists/ endocrinologist, smoking cessation expert, hypertension and lipid specialist, interventional radiologist, vascular surgeon, orthopedist, neurologist, wound healing expert, and/or others for 'the greatest chance of providing optimal care for the patient with PAD'.

Early detection is critical for mitigating PAD progression. Ankle-brachial index (ABI) testing is recommended by the US Preventative Services Task Force as an affordable and effective screening tool for evaluating PAD risk. ABI testing is especially useful in primary care settings allowing for earlier diagnosis and treatment. Several ABI methods are used; classically this test uses a device for measuring blood pressure with an inflatable cuff, and blood pressure measurements are taken at the upper arm and the ankle; healthcare providers have used a doppler probe to detect audible systolic pressure signals within the arteries - Doppler ABI. Barriers to implementation include; time required to conduct tests, training and required technical skills. Hence, Doppler-based ABI in primary care has been limited. Recently designed oscillometric (manual or digital with automatic electronic calculation of blood pressure) and photophlethysmographic devices are now available. However current guidelines do not endorse the use of these newer devices but recommend the hand-held doppler technique.

Digital ABI is noninvasive, automated ABI using pulsatile blood volume changes - plethysmography, irrespective of vessel compressibility. The QuantaFlo™ (Semler Scientific, Inc.) is an innovative, 510K FDA approved device that provides bilateral, non-invasive physiologic studies of upper and lower extremity arteries using volume plethysmography of the posterior tibial and anterior tibial/dorsalis pedis arterial distributions. QuantaFlo™ uses an optical sensor attached onto a digit in the hand and feet to assess extremity blood volume. It then displays a signal, directly related to blood volume, on a Volume Plethysmography Chart used to calculate PAD probability. This test is easy to use and portable, delivering fast, accurate results in ~5 minutes; an attractive option in primary care settings. While maintaining accuracy, results comparing digital methods to Doppler show concordance above 90%. The American Medical Association (AMA) concluded, "Evaluation of the digits in both legs with volume plethysmography is appropriate for this code provided that an ankle/brachial index (ABI) be performed." QuantaFlo™ meets both of these standards.

ABI ≤ 0.9 has been used as the criterion for diagnosing PAD based on ACC/AHA guidelines for the management of patients with PAD ; the cutoff value of ABPI ≤0.9 it has been reported that the sensitivity of diagnosing PAD was 83% to 85% and a specificity of 95% to 100% [5]. ABI has been validated against lower extremity contrast angiography obtaining high sensitivity (85%-95%) and specificity (90%-100%), for diagnosing PAD . Many studies have reported more than 95% of sensitivity and specificity of colour duplex ultrasound scan in detecting a significant stenosis of the arteries .

QuantaFlo™ (Semler Scientific, Inc.) is a novel, noninvasive, 510K FDA approved digital device that is used as a screening tool to measure ABI of patients at risk of PAD. This single center prospective clinical trial will evaluate the sensitivity and specificity of digital ABI in detecting PAD using color Doppler ultrasound and 'gold standard' angiography as reference. Specifically in patient undergoing dialysis and who cannot undergo ABI using pressure cuffs dialysis grafts/ fistulae in the arms, we will evaluate the value of digital ABI in detecting PAD.