M-Health: The Development of Cuff-less and Wearable BloodPressure Meters for Use in Body Sensor Networks

Carmen C.Y. Poon, Yee Man Wong and Yuan-Ting Zhang*Joint Research Centre for Biomedical Engineering, Department of Electronic Engineering,

The Chinese University of Hong Kong, Shatin, Hong Kongytzhang@ee. cuhk. edu. hk

Abstract-To meet the needs of the increasingly agingpopulation, development of wearable medical devices and bodysensor networks technologies for applications in telemedicineand mobile health are imperative. In this direction, we havedeveloped a cuff-less and noninvasive pulse transit time(PTT)-based approach to overcome the shortcomings ofconventional invasive or cuff-based blood pressure (BP)measurement techniques for long-term and continuousmeasurement of BP and its variability. The technique is usefulfor designing wearable BP meters, which, if connected withvarious types of wearable devices by a body sensor network,could form a front-end infrastructure for long-term personalhealth monitoring. In this paper, we evaluated the PTT-basedapproach with data collected from two groups of 85 subjects.The results suggested that the PTT-based approach is apromising technique for the noninvasive and cuff-lessmeasurement of BP.

Keywords-Arterial blood pressure, wearable medical devices,body sensor networks, telemedicine, mobile health

I. INTRODUCTIONThe increasingly aging population is a global phenomenon.

Since these people are often patients suffering from chronicdiseases, but not necessarily requiring hospitalization,wearable medical devices and body sensor networks (BSN)technologies are urgently required so that their healthconditions can be monitored via telemedicine and mobilehealth (m-Health) during normal daily activities.

Ideally, wearable devices and BSN should be designed tobe autonomous, imperceptible and power conservative.Solving these challenges requires incorporating newprinciples of physiologic measurements with effectivebio-model-based signal processing techniques. Therefore,we have developed novel technologies that enable the designof wearable devices for telemedicine and m-Health,especially for the noninvasive monitoring of vital signs suchas blood oxygen saturation (SpO2), heart rate and itsvariability, and, in particular, arterial blood pressure (BP) bya cuff-less pulse transit time (PTT)-based approach. Fig. 1shows a prototype of the BP watch (provided by JetflyTechnology Ltd.) using this PTT-based technology. This

*This work was supported in part by the Hong Kong Innovation andTechnology Fund. We are grateful to Standard Telecommunication Ltd.,Jetfly Technology Ltd., Golden Meditech Company Ltd., Bird InternationalLtd. and Bright Steps Corporation for their supports to the ITF projects.

Fig. 1. A prototype of the cuff-less BP watch produced by JetflyTechnology Ltd. using the PTT-based technology developed at JCBME.

type of light-weight, wearable devices are some of the idealnodes ofBSN, whereas the conventional medical devices (e.g.the cuff-based BP meters) are too bulky to be carried aroundby users and thus, are undesirable for use in BSN.

II. BLOOD PRESSURE MEASUREMENTSThe development of a cuff-less and noninvasive BP

measurement technique was motivated by the fact thathypertension remained inadequately managed worldwide [1].An accurate and user-friendly device that allowsself-measuring of BP on a regular and frequent basis willhave additive values to BP management.

While the importance of controlling the absolute BP hasbeen emphasized for many years, recent studies suggestedthat the BP variability (BPV) also carries importantinformation for determining the relative risk of morbidity andmortality of cardiovascular diseases [2]. Nevertheless, inm-Health applications, BPV measurement is difficult to beaccomplished by the three major types of state-of-the-art BPmeasurement techniques/devices: 1) the invasive arterial-lineis clearly not suitable for home use; 2) the cuff-basedauscultative/oscillometric devices can only provide asnapshot ofthe BP and cannot provide short-term BPV; and 3)the cuff-based continuous measurement devices are built onthe tonometric/volume-clamp principles that require to keepthe vascular wall in an unloaded state and thus are unsuitableto be used continuously in BSN for extended periods of time.

Therefore, new cuff-less and noninvasive measurementtechniques are needed for long-term and continuousmonitoring ofBP and BPV in BSN for m-Health.

1-4244-0278-6/06/$20.00 ©2006 IEEE

III. EXPERIMENTS AND RESULTSWe developed a pulse transit time (PTT)-based technique

for the noninvasive and cuff-less measurement of arterial BP[3] and evaluated the technique on 85 subjects (aged 57±29yrs., 36 males) using a protocol similar to that suggested byAAMI [4]. Each subject underwent a calibration procedure atthe beginning ofthe study and 9-12 measurements were takenin the subsequent weeks. Overall, the PTT-based approachestimated systolic BP (SBP) and diastolic BP (DBP) within0.6±9.8mmHg and 0.9±5.6mmHg of the referencesrespectively [3]. After removing measurements where thetwo references differ by more than 1OmmHg [4], thedifference between estimated and reference SBP reduced to0.6±9.2mmHg. Within 30 min. after individual calibration,the difference is 0.8±5.4mmHg, as shown in Fig. 2(a).

The PTT-based approach were modified and implementedonto a novel stethoscope developed at our center. Aprototype of the device was tested on another group of 85subjects (aged 40±14 yrs., 41 males). The study protocol issimilar to that described in [3], except that in this study,subjects were asked to hold the stethoscope head on theirchest wall for measuring BP using the PTT-based approach.Within 30 min. after individual calibration, 3 sets ofmeasurements were made. Fig. 2(b) plots the referenceagainst the estimated SBP, where the difference wascalculated to be 3.2±8.7mmHg.

IV. DISCUSSION AND CONCLUSIONDespite the PTT-based approach has yet to fully satisfy the

AAMI requirements, the results were supportive to this newapproach of measuring BP. The approach has negligible biasand reasonable standard deviation of differences for theestimation of SBP. It is also worth noting that although thedetails for the estimation of DBP was not shown above, thedifference between the estimated and reference DBP aremuch less than 5±8mmHg (as required by AAMI) in all cases.A key challenge remains to be solved for this approach is

how to better calibrate the technique for different individualsso that a more accurate measure of BP can be obtained.Shaltis et al. [5] proposed calibrating plethysmography byhydrostatic pressure for cuff-less BP measurement and foundthe accuracy to be consistent within 20 min. after calibration.

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Fig. 2. Scatter plots of the reference vs. estimated SBP for measurementstaken within 30 min. after individual calibration. Data were collected by thein-house circuitry described in [3] (a) and novel stethoscope (b), each on 85subiects.

The different results found in the two studies also suggestthat measurement posture and circuitry modification couldsignificantly alter the measured parameters, which ultimatelyaffected the accuracy and robustness of the approach.

In order to implement the technique into wearable devicesfor m-Health, power consumption of it must be reduced. Apossible solution would be implementing the circuitry intoASIC chips [6]. Fig. 3 shows an ASIC chip designed in ourlab for capturing photoplethysmographic signals.

Development of technologies that enables the design ofwearable medical devices such as those described in thispaper is necessary but it falls short to facilitate m-Health andtelemedicine. As more and more wearable devices andbiosensors will be used by individual persons, it is alsoessential to connect these devices with a BSN such thatinformation and resources can be shared between them. Inthis respect, we are investigating specific issues such asmodes to secure communications between nodes ofBSN [7].When all these technologies are in shape, better homecaresystem through m-Health and telemedicine can be realized.

Fig. 3. An ASIC chip to capture photoplethysmographic signals for thePTT-based BP measurement approach.

ACKNOWLEDGMENT

The authors gratefully acknowledge the contribution oftheir colleagues at JCBME.

REFERENCES[1] WHO and ISH Writing Group, "2003 World Health Organization

International Society of Hypertension statement on management ofhypertension," JHypertens, vol. 21, 2003, pp. 1983-1992.

[2] M. Kikuya, "Prognostic Significance of Blood Pressure and Heart RateVariabilities," Hypertension, vol. 36, 2000, pp. 901-906.

[3] C.C.Y. Poon and Y.T. Zhang, "Cuff-less and noninvasivemeasurements of arterial blood pressure by pulse transit time," in 27thIEEE Int. Conf ofEng. in Med. and Biol. Soc., Shanghai, PRC, 2005.

[4] Association for the Advancement of Medical Instrumentation."American National Standard for Manual, Electronic, or AutomatedSphygmomanometers", SP10-2002, AAMI, 2003.

[5] P. Shaltis, A. Reisner and H. Asada, "Calibration of thePhotoplethysmogram to Arterial Blood Pressure: Capabilities andLimitations for Continuous Pressure Monitoring," in 27th IEEE Int.Conf ofEng. in Med. & Biol. Soc., Shanghai, PRC, 2005.

[6] A. Wong, K.P. Pun, Y.T. Zhang and K. Hung, "A near-infrared heartrate measurement IC with very low cutoff frequency using currentsteering technique," IEEE Trans. on Circuits & Systems I, vol. 52, no.12, 2005, pp. 2642-2646.

[7] C.C.Y. Poon, S.D. Bao and Y.T. Zhang, "A Novel Biometrics Methodto Secure Wireless Body Area Sensor Networks for Telemedicine andM-Health," IEEE Communication Magazine, Apr. Special Issue, 2006.