Toward Cuffless Blood Pressure Monitoring

Liam Davenport

January 18, 2018

A simple, inexpensive device that uses a light sensor to measure blood pressure (BP) via the index finger could offer a flexible and stress-free replacement for cuff-based BP measurement, results of a joint academic and industry study suggest.[1]

The research showed that the cuffless device (CLB), which uses photoplethysmography (PTG) to measure pulse-wave changes in the finger, had excellent repeatability, matched cuff-based devices in performance, and met industry-guideline standards.

Also, a substudy of participants wearing the device overnight showed that it was associated with significant improvements in sleep quality over a cuff-based device.

"Although our device has yet to overcome the requirement for pre-calibration using cuff-based blood pressure measurement, our study shows the high precision and great advantage of CLB as a paradigm shift in blood pressure monitoring in the digital health era," Dr Naoki Watanabe (Nagoya University Graduate School of Medicine, Nagoya, Japan) and colleagues write in the report, published December 25, 2017, in JACC: Basic to Translational Science.  

For this study, the researchers recruited 172 individuals (66.9% male) and placed a PTG sensor on their right or left index finger, which comprised a light-emitting diode on one side and a photodetector on the other.

PTG data from five contiguous pulse-wave intervals were digitized and averaged by using an algorithm developed from a database of pulse-wave parameters obtained under various conditions from 887 individuals. The findings were then validated against the Institute of Electrical and Electronics Engineers (IEEE) guideline for a wearable or cuffless BP device.

To provide a reference, BP was also measured simultaneously via cuff-based measurement, using both auscultatory and electronic sphygmomanometers on the left arm.

The team found that the correlation coefficients for systolic BP (SBP) between CLB and cuff-based measurement was highly significant, at a mean absolute difference of 6.2 mm Hg, indicating that the CLB met the IEEE standards.

The results also showed that repeatability of the measurements was good, defined as an intraclass correlation coefficient (ICC) greater than 0.8, or excellent (an ICC greater than  0.9) for almost all measures.

For example, overall SBP was associated with an ICC of 0.918 (95% CI, 0.907–0.927) and static SBP was linked to an ICC of 0.950 (95% CI, 0.940–0.959), while BP rise was linked to an ICC of 0.920 (95% CI, 0.889–0.942).

Using Bland-Altman plot analysis, the researchers also found that CLB was sufficient to replace cuff-based BP measurement, at a mean difference of –0.4 mm Hg for SBP.

Simultaneously recording BP via CLB and a cuff-based device during coronary angiography after administration of nitroglycerin in 29 participants showed that even during falling BP, CLB was highly correlated with the changes, at R=0.86 (P<0.0001) for SBP and R=0.78 (P<0.0001) for diastolic BP (DBP).

Finally, the sleep substudy conducted in 35 participants indicated no differences in mean SBP or DBP between the two measurement techniques. However, more than 70% of participants felt uncomfortable when wearing the cuff-based device, while this rate was significantly lower when they wore the CLB (P<0.001).

Electrocardiography data obtained during sleep revealed participants had significantly lower heart rates and lower components of heart rate variability in the first hour after patients went to bed when wearing the CLG than while they wore the cuff-based device.

Discussing the limitations of the novel device, the team writes: "To convert the PTG signal into BP, calibration using CB [cuff-based measurement] is unavoidable."

"We are not yet free from the cuff, and our device is therefore termed 'cuff-less', not 'cuff-free'. However, our CLB has the advantage of using a single sensor for BP recording, unlike previous devices that require multiple sensors."

Results "Promising"

In an accompanying editorial,[2] Drs Florian Rader and Ronald G Victor (Cedars-Sinai Heart Institute, Los Angeles, CA) say that the correlation between the PTG device and standard BP measurement "appears to be robust, and at least the summary statistics are promising."

While the device appears more "wearable" and flexible than traditional techniques, the reviewers note that all measurements were conducted in a completely still body position and that changes in body position, exercise, and other deviations may require repeat calibration of the device.

Furthermore, the mean age of the patients was 47 years and only 30% of patients were hypertensive, prompting Rader and Victor to suggest that "more work is to be done in a strictly hypertensive population."

They also point out that the reference device for ambulatory BP measurement is not listed and that, because several of the coauthors are employees of the manufacturer, independent validation has thus not yet been provided.

Approached for comment, Drs Saarraaken Kulenthiran, Sebastian Ewen, and Michael Böhm (Universität des Saarlandes, Homburg, Germany) told theheart.org | Medscape Cardiology in a statement that for ambulatory BP monitoring to be useful in the treatment of hypertension, "optimized and simplified devices" are required.

They described these results as "promising" and said the CLB could improve the feasibility of ambulatory BP measurement and that use of a single finger sensor "could minimize the adulterant effect triggered by the cuff and thereby represent more precisely real-time blood pressure."

While Kulenthiran and colleagues note that PTG is a "non-invasive, inexpensive, and simple diagnostic technique," they say that its limitations should be clarified further, including whether finger position and motion artifacts caused by poor contact or temperature variations affect measurement.

They also point out that irregular ventricular beats affect detection accuracy with PTG but that the study excluded patients with arrhythmias. "An accompanying Holter study would have provided clues to this question."

Further, they note that the autonomic nervous system plays a crucial role in BP regulation and influences peripheral blood flow.

"It will be important to investigate how the variable sympathetic activity, with accompanying peripheral vasoconstriction, affects blood pressure monitoring via PTG," they say.

Kulenthiran and colleagues also call for studies comparing CLB with other devices that use pulse transit time and wave analysis in combination with electrocardiography, which "would be helpful for detecting the most accurate techniques."

The research was supported in part by a grant from the Innovative Research Center for Preventive Medical Engineering, Nagoya University. Watanabe has disclosed no relevant financial relationships. Murohara reports lecture fees and research grants from Astellas, AstraZeneca, Bayer, Boehringer Ingelheim, Daiichi-Sankyo, Denso Corporation, Kowa, MSD, Pfizer, Takeda, and Tanabe-Mitsubishi. Bando reports lecture fees and research grants from AstraZeneca, Boehringer Ingelheim, MSD, Taisho-Toyama, Eli-Lilly, Astellas, Daiichi-Sankyo, and Tanabe-Mitsubishi. Ishii reports lecture fees from Astellas, AstraZeneca, and Daiichi-Sankyo. Kawachi, Yamakita, Honda, and Futatsuyama are employees of Denso. All other authors disclosed no relevant financial relationships.

Kulenthiran, Ewen, Böhm, and Radar report no relevant financial disclosures. Victor reports research funding from ReCor Medical.

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