Clinical validation of smartphone-based activity tracking in peripheral artery disease patients

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Abstract

Peripheral artery disease (PAD) is a vascular disease that leads to reduced blood flow to the limbs, often causing claudication symptoms that impair patients’ ability to walk. The distance walked during a 6-min walk test (6MWT) correlates well with patient claudication symptoms, so we developed the VascTrac iPhone app as a platform for monitoring PAD using a digital 6MWT. In this study, we evaluate the accuracy of the built-in iPhone distance and step-counting algorithms during 6MWTs. One hundred and fourteen (114) participants with PAD performed a supervised 6MWT using the VascTrac app while simultaneously wearing an ActiGraph GT9X Activity Monitor. Steps and distance-walked during the 6MWT were manually measured and used to assess the bias in the iPhone CMPedometer algorithms. The iPhone CMPedometer step algorithm underestimated steps with a bias of −7.2% ± 13.8% (mean ± SD) and had a mean percent difference with the Actigraph (Actigraph-iPhone) of 5.7% ± 20.5%. The iPhone CMPedometer distance algorithm overestimated distance with a bias of 43% ± 42% due to overestimation in stride length. Our correction factor improved distance estimation to 8% ± 32%. The Ankle-Brachial Index (ABI) correlated poorly with steps ( R = 0.365) and distance ( R = 0.413). Thus, in PAD patients, the iPhone’s built-in distance algorithm is unable to accurately measure distance, suggesting that custom algorithms are necessary for using iPhones as a platform for monitoring distance walked in PAD patients. Although the iPhone accurately measured steps, more research is necessary to establish step counting as a clinically meaningful metric for PAD.

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Most cited references 27

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Peripheral arterial disease detection, awareness, and treatment in primary care.

Alan Hirsch (2001)

Peripheral arterial disease (PAD) is a manifestation of systemic atherosclerosis that is common and is associated with an increased risk of death and ischemic events, yet may be underdiagnosed in primary care practice. To assess the feasibility of detecting PAD in primary care clinics, patient and physician awareness of PAD, and intensity of risk factor treatment and use of antiplatelet therapies in primary care clinics. The PAD Awareness, Risk, and Treatment: New Resources for Survival (PARTNERS) program, a multicenter, cross-sectional study conducted at 27 sites in 25 cities and 350 primary care practices throughout the United States in June-October 1999. A total of 6979 patients aged 70 years or older or aged 50 through 69 years with history of cigarette smoking or diabetes were evaluated by history and by measurement of the ankle-brachial index (ABI). PAD was considered present if the ABI was 0.90 or less, if it was documented in the medical record, or if there was a history of limb revascularization. Cardiovascular disease (CVD) was defined as a history of atherosclerotic coronary, cerebral, or abdominal aortic aneurysmal disease. Frequency of detection of PAD; physician and patient awareness of PAD diagnosis; treatment intensity in PAD patients compared with treatment of other forms of CVD and with patients without clinical evidence of atherosclerosis. PAD was detected in 1865 patients (29%); 825 of these (44%) had PAD only, without evidence of CVD. Overall, 13% had PAD only, 16% had PAD and CVD, 24% had CVD only, and 47% had neither PAD nor CVD (the reference group). There were 457 patients (55%) with newly diagnosed PAD only and 366 (35%) with PAD and CVD who were newly diagnosed during the survey. Eighty-three percent of patients with prior PAD were aware of their diagnosis, but only 49% of physicians were aware of this diagnosis. Among patients with PAD, classic claudication was distinctly uncommon (11%). Patients with PAD had similar atherosclerosis risk factor profiles compared with those who had CVD. Smoking behavior was more frequently treated in patients with new (53%) and prior PAD (51%) only than in those with CVD only (35%; P <.001). Hypertension was treated less frequently in new (84%) and prior PAD (88%) only vs CVD only (95%; P <.001) and hyperlipidemia was treated less frequently in new (44%) and prior PAD (56%) only vs CVD only (73%, P<.001). Antiplatelet medications were prescribed less often in patients with new (33%) and prior PAD (54%) only vs CVD only (71%, P<.001). Treatment intensity for diabetes and use of hormone replacement therapy in women were similar across all groups. Prevalence of PAD in primary care practices is high, yet physician awareness of the PAD diagnosis is relatively low. A simple ABI measurement identified a large number of patients with previously unrecognized PAD. Atherosclerosis risk factors were very prevalent in PAD patients, but these patients received less intensive treatment for lipid disorders and hypertension and were prescribed antiplatelet therapy less frequently than were patients with CVD. These results demonstrate that underdiagnosis of PAD in primary care practice may be a barrier to effective secondary prevention of the high ischemic cardiovascular risk associated with PAD.

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The 6-minute walk: a new measure of exercise capacity in patients with chronic heart failure.

G H Guyatt, M J Sullivan, P Thompson … (1985)

Cycle and treadmill exercise tests are unsuitable for elderly, frail and severely limited patients with heart failure and may not reflect capacity to undertake day-to-day activities. Walking tests have proved useful as measures of outcome for patients with chronic lung disease. To investigate the potential value of the 6-minute walk as an objective measure of exercise capacity in patients with chronic heart failure, the test was administered six times over 12 weeks to 18 patients with chronic heart failure and 25 with chronic lung disease. The subjects also underwent cycle ergometer testing, and their functional status was evaluated by means of conventional measures. The walking test proved highly acceptable to the patients, and stable, reproducible results were achieved after the first two walks. The results correlated with the conventional measures of functional status and exercise capacity. The authors conclude that the 6-minute walk is a useful measure of functional exercise capacity and a suitable measure of outcome for clinical trials in patients with chronic heart failure.

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Functional decline in peripheral arterial disease: associations with the ankle brachial index and leg symptoms.

Luigi Ferrucci, J M Guralnik, Christopher Pearce … (2004)

Among individuals with lower-extremity peripheral arterial disease (PAD), specific leg symptoms and the ankle brachial index (ABI) are cross-sectionally related to the degree of functional impairment. However, relations between these clinical characteristics and objectively measured functional decline are unknown. To define whether PAD, ABI, and specific leg symptoms predict functional decline at 2-year follow-up. Prospective cohort study among 676 consecutively identified individuals (aged > or =55 years) with and without PAD (n = 417 and n = 259, respectively), with baseline functional assessments occurring between October 1, 1998, and January 31, 2000, and follow-up assessments scheduled 1 and 2 years thereafter. PAD was defined as ABI less than 0.90, and participants with PAD were categorized at baseline into 1 of 5 mutually exclusive symptom groups. Mean annual changes in 6-minute walk performance and in usual-paced and fast-paced 4-m walking velocity, adjusted for age, sex, race, prior-year functioning, comorbid diseases, body mass index, pack-years of cigarette smoking, and patterns of missing data. Lower baseline ABI values were associated with greater mean (95% confidence interval) annual decline in 6-minute walk performance (-73.0 [-142 to -4.2] ft for ABI <0.50 vs -58.8 [-83.5 to -34.0] ft for ABI 0.50 to <0.90 vs -12.6 [-40.3 to 15.1] ft for ABI 0.90-1.50, P =.02). Compared with participants without PAD, PAD participants with leg pain on exertion and rest at baseline had greater mean annual decline in 6-minute walk performance (-111 [-173 to -50.0] ft vs -8.67 [-36.9 to 19.5] ft, P =.004), usual-pace 4-meter walking velocity (-0.06 [-0.09 to -0.02] m/sec vs -0.01 (-0.03 to 0.003] m/sec, P =.02), and fastest-pace 4-meter walking velocity (-0.07 [-0.11 to -0.03] m/sec vs -0.02 [-0.04 to -0.006] m/sec, P =.046). Compared with participants without PAD, asymptomatic PAD was associated with greater mean annual decline in 6-minute walk performance (-76.8 (-135 to -18.6] ft vs -8.67 (-36.9 to 19.5] ft, P =.04) and an increased odds ratio for becoming unable to walk for 6 minutes continuously (3.63; 95% confidence interval, 1.58-8.36; P =.002). Baseline ABI and the nature of leg symptoms predict the degree of functional decline at 2-year follow-up. Previously reported lack of worsening in claudication symptoms over time in patients with PAD may be more related to declining functional performance to than lack of disease progression.

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Author and article information

Contributors

Oliver Aalami: +1-(650)-725-5227 , aalami@stanford.edu

Journal

Journal ID (nlm-ta): NPJ Digit Med

Journal ID (iso-abbrev): NPJ Digit Med

Title: NPJ Digital Medicine

Publisher: Nature Publishing Group UK (London )

ISSN (Electronic): 2398-6352

Publication date (Electronic): 11 December 2018

Publication date PMC-release: 11 December 2018

Publication date Collection: 2018

Volume: 1

Electronic Location Identifier: 66

Affiliations

[1 ]ISNI 0000000419368956, GRID grid.168010.e, Division of Vascular & Endovascular Surgery, Department of Surgery, , Stanford University, ; Stanford, CA 94305 USA

[2 ]ISNI 0000 0004 0419 2556, GRID grid.280747.e, Division of Vascular Surgery, , Veterans Affairs Palo Alto Health Care System, ; Palo Alto, CA 94304 USA

[3 ]ISNI 0000000419368956, GRID grid.168010.e, Precision Health and Integrated Diagnostics Center at Stanford, , Stanford University, ; Stanford, CA 94305 USA

Author information

Kara Rothenberg http://orcid.org/0000-0002-5123-5876

Article

Publisher ID: 73

DOI: 10.1038/s41746-018-0073-x

PMC ID: 6550212

PubMed ID: 31304343

SO-VID: 4e031375-2dfa-4f3f-bfda-31e3d1f51cc5

License:

Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/.

History

Date received : 2 July 2018

Date accepted : 14 November 2018

Funding

Funded by: Precision Health and Integrated Diagnostics Center at Stanford. Clinical Translational Research Spectrum Stanford Predictives and Diagnostics Center