Although ambulatory blood pressure (BP) monitoring provides a better guide to the management of hypertension, 1, 2, 3, 4 these devices are cumbersome and inconvenient, and therefore not widely used. HealthSTATS International (Singapore) developed a wrist-bound BP measurement device (BPro), which measures BP using arterial tonometry. 5 We aimed to validate the accuracy of this device in measuring BP according to the modified European Society of Hypertension (ESH) protocol 6 and Association for the Advancement of Medical Instrumentation (AAMI) standard. 7
The wrist-bound BP measurement system consists of a wrist-bound BP monitor that works by applanation tonometry, with a hemispheric plunger placed on the radial artery. Before use, it is calibrated by an arm-based oscillometric monitor. A proprietary strapping system for the plunger provides constant pressure on the artery without obstructing venous return and compression of surrounding structures, for example, median nerve. The position of the sensor, housed in the watch-head, is stabilized, allowing the forearm and wrist to move reasonably freely. Pulsation of the radial artery is sampled at a rate of 60–200 Hz to generate the arterial waveform. Several indices and coefficients are obtained to generate a template of the individual's waveform. During subsequent samplings, waveforms that match the template are collected and sorted in an array, and the mean BP is computed. Proprietary software using derived coefficients is used to calculate systolic BP (SBP) and diastolic BP (DBP) by detecting BP changes from the calibrated level through changes in the arterial waveform.
The study protocol was approved by the Institutional Review Board. Subjects with sustained arrhythmia were excluded. Only subjects ⩾30 years of age were used for the ESH analysis. A medical examination was performed prior to enrollment to determine suitability because the protocol requires a minimum number of men and women satisfying various BP and arm circumference measurements. Informed consent was obtained from all subjects.
The validation team, consisting of 12 registered cardiac nurses, was instructed in the use of the device during several practice sessions and received training in BP measurement according to the British Hypertension Society guidelines (www.abdn.ac.uk/, accessed on 16 September 2003). An Erkameter 3000 mercury sphygmomanometer (Erka, Bad Tölz, Germany) was used with a dual-headset Littmann Master Classic II stethoscope (3M Health Care, St Paul, MN, USA) as the reference. Cuff size was selected such that it encircled ⩾80% of the arm circumference.8 The onset of Korotkov phase V was used for DBP. Otherwise, phase IV was used.8 The subject was to relax for 10–15 min, after which paired systolic and diastolic readings were taken, to the nearest mm Hg, simultaneously by the device and by two independent observers who were blinded to each other's readings. The two observers' readings were accepted by the independent study supervisor if they deviated ⩽4 mm Hg. Measurement was repeated ⩽2 times until agreement was achieved. Otherwise, at the third attempt, the sequence was repeated. To avoid venous congestion and minimize variability, 30–60 s were allowed between measurements.
The protocol incorporates the requirements and recommendations of the ESH protocol and the AAMI standard.5, 6 Initially, the device was calibrated in the seated position and then tested in the seated, standing and supine positions, with the wrist held over the chest area with an oscillometric device (MC3000, HealthSTATS International). With the subject seated, nine alternate, sequential same-arm measurements with the oscillometric monitor and the sphygmomanometer were recorded in accordance with the ESH protocol,5 giving a total of four sets of oscillometric readings. The first two sets of readings were not used, while the mean of the SBP and DBP of the last three sets of oscillometric readings were used for calibration. Sphygmomanometer readings obtained during calibration were selected in accordance with the ESH protocol and also used to calculate the average BP level during calibration.
After calibration, nine alternate, sequential same-arm measurements with the device and the sphygmomanometer were recorded, with the subject seated. The average of the first set of sphygmomanometer readings was used to classify the entry BP of the subject into the low, medium or high range, separately, for SBP and DBP, and the second set was used to check the function of the device. Three pairs of device-sphygmomanometer readings were derived for both SBP and DBP from the last seven sets of readings. The same sequential measurements were made for the standing and supine positions.5 Subsequently, the validation continued with more subjects with a view to meeting the requirements of the AAMI standard,6 which requires ⩾85 subjects satisfying specific ranges of entry BP and arm circumferences.
Data management and analysis were performed independently by the Clinical Trials and Epidemiological Research Unit. Differences between the device and sphygmomanometer readings were analysed in accordance with the ESH protocol5 and AAMI standard.6 For the AAMI analysis, all original three pairs of SBP and DBP readings for each subject were used and limits of agreement were graphically represented using Bland–Altman plots.9
The subjects were mostly young men and ∼35% had hypertension. Distribution of the entry BP readings and arm circumferences met ESH and AAMI requirements. For ESH analysis, the 33 sets of SBP and DBP readings were derived from >33 subjects. The readings were selected consecutively on the basis of the age, gender and entry BP requirements. For the AAMI analysis, 89 subjects were selected from 113 available on the basis of the arm circumference and entry BP requirements.10, 11
The device met the accuracy criteria of the ESH protocol and the AAMI standard for both SBP and DBP. Furthermore, in two of three measurements carried out, the difference between device and sphygmomanometer readings of ⩽5 mm Hg was found in 23 and 29 subjects for SBP and DBP, respectively. There were only three and one subject for SBP and DBP, respectively, whose difference in reading was >5 mm Hg in all three measurements. For the AAMI analysis, the mean difference for all three positions fell within the AAMI limit of ±5 mm Hg, with the standard deviation of differences <8 mm Hg. However, the device overestimated SBP by ⩽1.3 mm Hg and underestimated DBP by ⩽4.6 mm Hg. The Bland–Altman plots for the AAMI analysis suggest that there was no clear relationship between the difference and the average BP (Figure 1). The average changes in BP from the average calibrated level ranged from −14.0 to +27.7 mm Hg (2.4±6.3) for SBP and –8.3 to +20.0 mm Hg (3.9±4.7) for DBP for all three positions combined (n=267). Agreement appeared to be better for the sitting and supine positions compared with the standing position, particularly for DBP (Supplementary data).
We found that the wrist-bound BP measurement device, when calibrated with the oscillometric monitor, was accurate in measuring BP under various stationary conditions. The device overestimated SBP by ⩽1.3 mm Hg and underestimated DBP by ⩽4.6 mm Hg. Understandably, this device may be used for ambulatory BP measurement. These differences are small and unlikely to impact on the clinical management, particularly when several readings are obtained over the 24-h period. Unlike arm-cuff-based devices, this wrist-bound device overcomes the limitations relating to ambulatory BP monitoring. It avoids use of the cumbersome cuff and noisy painful inflation. As readings are obtained without the subject's knowledge, the anticipatory BP response may be prevented. These advantages will make ambulatory BP monitoring more appealing to both patients and physicians. Outcomes of patients with hypertension and other vascular-associated diseases may be improved.
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KHM was the study director and principal investigator; he directed the entire study, including protocol development, training of staff, conduct of study, review of findings, and obtaining ethics approval. DN, SYT and HWG were the co-investigators. JT recruited subjects and coordinated the validation sessions. SFL supervised nurses during the validation sessions. ZM and GH managed the data and performed the statistical analysis in accordance with the protocol. WLP and NHC provided technical support. DN, SYT and KHM wrote the paper. All the authors reviewed and approved the final version. This study was funded by HealthSTATS, Singapore.
Conflict of interest
WLP is employed by HealthSTATS International. NHC is an executive director of the company. All the other authors have no financial or personal relationships with HealthSTATS International.
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Nair, D., Tan, S., Gan, H. et al. The use of ambulatory tonometric radial arterial wave capture to measure ambulatory blood pressure: the validation of a novel wrist-bound device in adults. J Hum Hypertens 22, 220–222 (2008) doi:10.1038/sj.jhh.1002306
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