Although the auscultatory method with a mercury sphygmomanometer has been the gold standard for blood pressure (BP) measurement for more than a century, this traditional technique has increasingly been replaced by the oscillometric method because of environmental concerns 1–3. However, the accuracy of automated oscillometric BP monitors has always been called into question 4–6. The BP values might be estimated incorrectly by oscillations using different algorithms that are proprietary to different devices 3–6. Therefore, the oscillometric method may not be as reliable as the Korotkoff sounds auscultation method. With the development of transducers, sensors, and mobile phone central processing units, considerable progress has been made in the sound-processing capabilities over the last few years, including the advent of the smartphone auscultatory BP kit – Accutension XYZ-110.
The Accutension XYZ-110 is the first BP monitoring device based on automated Korotkoff sounds auscultation and iOS application (App) software for self/home and clinical office use. Our previous pilot study 7 showed that the BP measurement difference between the XYZ-110 device and the mercury sphygmomanometer was acceptable. The aim of the present study was to further validate the accuracy of the XYZ-110 BP device in adults according to the American National Standards Institute/Association for the Advancement of Medical Instrumentation/International Organization for Standardization (ANSI/AAMI/ISO) 81060-2:2013 standard guidelines 8.
Participants and methods
Study participants of both sexes older than 18 years of age were recruited from among the hospital staff and outpatients or hospitalized patients of Shanghai General Hospital (Shanghai, China). The number, sex, arm circumference, and BP reading distributions of the participants were in accordance with the ANSI/AAMI/ISO 81060-2:2013 standard requirements. Individuals with cardiac arrhythmias, bilateral upper limb arterial occlusion, and arm circumference beyond the cuff range were excluded. The present study was approved by the Ethics Committee of Shanghai General Hospital. Written informed consent was obtained from all the participants.
The Accutension XYZ-110 device
To the best of our knowledge, the Accutension XYZ-110 (Shanghai Zhihu Co. Ltd, Shanghai, China) is the first upper-arm auscultatory BP kit for self/home and clinical office BP measurement using the smartphone and the Accutension App software (Shanghai Zhihu Co. Ltd) to record, process, visualize, and analyze Korotkoff sounds during cuff deflation. The product consists of a manual cuff and a stethoscope. The cuff is compliant with the ANSI/AAMI/ISO 81060-2:2013 standard, suitable for arm circumference range from 22 to 36 cm, and with a pressure-sensing module. The power of the sensor is provided by built-in rechargeable batteries. The stethoscope is similar to a traditional BP device, but with a standard 3.5 mm headphone plug at the end position. The sounds from the tube of the stethoscope are fed into the smartphone and auscultated by the App software through a microphone plugged into the jack. The pressure from the cuff is synchronously transferred to the smartphone by a pressure sensor through Bluetooth connection, which is different from the mercury sphygmomanometer using the mercury column to sense and display the pressure (Fig. 1a). Each auscultation sound has a corresponding BP value with the Accutension App. The final BP values are then displayed on the smartphone screen. When uncertain measurements occur because of noise disturbance, the observers may obtain accurate BP readings ultimately by playing back the measurement recording and resetting the red lines on the screen (Fig. 1b).
The manufacturer provided two devices, one of which was selected randomly for the validation process.
The validation procedure
The general characteristics, medical histories, and the arm circumference of the participants were collected before validation. BP measurement was carried out in a quiet room without disturbing noises. The participants were comfortably seated with the legs uncrossed, the feet laid flat on the floor, and the forearm supported at the level of the heart. The investigators were experienced in BP measurement and trained repeatedly according to the recommendations of the British Hypertension Society 9.
The validation procedure used the ‘same arm simultaneous method’ by strictly following the ANSI/AAMI/ISO 81060-2:2013 standard. The same cuff was connected in parallel to the mercury sphygmomanometer (two observers) and the XYZ-110 device (one supervisor) through a Y-shaped tube. Two observers and the test device shared one stethoscope head by two Y-tube connectors. BP was measured simultaneously by utilizing the same inflation/deflation cycle (at a constant deflation rate of 2–3 mmHg/s) and three BP values were obtained independently. The two observers were blinded to each other’s readings as well as to the device readings. An initial manual BP value was taken and was not used in the calculation of the accuracy of the test device. Each participant simultaneously underwent three valid pairs of manual and device measurements. There was a 60 s pause between two successive measurements.
SPSS 22.0 for Windows (SPSS Inc., Chicago, Illinois, USA) and Microsoft Excel 2013 (Microsoft, Redmond, Washington, USA) were used for analysis. Differences in systolic blood pressure (SBP) and diastolic blood pressure (DBP) between the test device and the mercury sphygmomanometer were calculated by subtracting the mercury sphygmomanometer measurement from the Accutension measurement. Data were presented as mean±SD, and data analysis was carried out according to both criteria 1 and 2 as required in the ANSI/AAMI/ISO 81060-2:2013 standard. A Bland–Altman graph was also created to assess the agreement between the two devices, which showed the differences of between-device readings compared with the corresponding averages.
A total of 91 consecutive participants were screened and 85 participants were enrolled, of whom three participants were excluded because the required number of participants in the BP recruitment ranges was completed according to the ANSI/AAMI/ISO 81060-2:2013 standard, one was excluded because of arrhythmias, and two were excluded because of unavailable cuff size. The 85 participants included 37 (43.5%) men and 48 (56.5%) women with a mean age of 43.2±12.6 years (range: 20–72 years) and a mean arm circumference of 28.4±3.7 cm (range: 22–36 cm). The arm circumference was 22.0–25.5, 25.6–29.0, 29.1–32.5, and 32.6–36.0 cm in 20 (23.5%), 31 (36.5%), 17 (20.0%), and 17 (20.0%) of the 85 participants, respectively. All the values of sex, age, and arm circumference distribution fulfilled the protocol requirements.
The observer difference in SBP and DBP measurement was within −4−4 mmHg, with a mean value of 0.26±1.99 and −0.08±1.88 mmHg, respectively. The number of SBP reference readings ≥160, ≥140, and ≤100 mmHg was 11 (12.9%), 22 (25.9%), and 15 (17.6%), respectively. The number of DBP reference readings ≥100, ≥85, and ≤60 mmHg was 9 (10.6%), 35 (41.2%), and 10 (11.8%), respectively.
The mean device–observer difference in the 255 separate BP data pairs was 2.45±2.24 mmHg for SBP and 0.69±2.09 mmHg for DBP. The data were in agreement with the criterion 1 of the ANSI/AAMI/ISO 81060-2:2013 standard requirements (≤5±8 mmHg). In addition, the mean device–observer difference of the 85 participants was 2.45±1.47 mmHg for SBP and 0.69±1.36 mmHg for DBP, which also fulfilled the criterion 2 with SD less than or equal to 6.47 and less than or equal to 6.90 mmHg for SBP and DBP, respectively.
Bland–Altman plots of the data set are presented in Fig. 2a and b for SBP and DBP, respectively. These figures show no underestimation or overestimation of BP at either low or high BP level by the Accutension XYZ-110 device.
The results of the present study showed that the Accutension XYZ-110 BP kit fulfilled the validation criteria 1 and 2 of the ANSI/AAMI/ISO 81060-2:2013 standard requirements for both SBP and DBP 8.
The low Accutension–mercury differences in SBP and DBP showed acceptable agreement between the test device and the mercury sphygmomanometer. The Accutension XYZ-110 BP kit uses direct BP measurements similar to the traditional auscultatory method, rather than statistical algorithms. The accuracy of oscillometric devices is known to be affected significantly by arterial stiffness, heart rate, and cuff size 3–6. In addition, the validation procedure was performed using the ‘same arm simultaneous method’ instead of the sequential method, which may reduce the impact of BP fluctuations at every moment 10.
A major innovation of the XYZ-110 device is that it records not only the BP readings but also the raw data including the Korotkoff sounds, the corresponding cuff pressures, and the deflation rate. The raw data may be used to evaluate the BP measurement quality and redetermine the BP values by medical professionals. In addition, the XYZ-110 device has some features other than BP measurement. For example, the irregular Korotkoff sounds of the patients suspected of having paroxysmal atrial fibrillation can be captured during the routine BP measurement and kept for further analysis by email or office visit. The recorded BP data may be useful for hypertension management and the mobile health service.
De Greeff et al. 11,12 evaluated the accuracy of the Tensoval duo control device using Korotkoff sounds to determine BP. The mean Tensoval duo control I/II-mercury difference was −2.4±6.0/−1.8±6.5 mmHg for SBP and 0.9±6.3/1.9±5.1 mmHg for DBP. Thus, in comparison with those traditional automated auscultatory devices, the Accutension XYZ-110 has apparent advantages because of the distinctive design concept in that the Korotkoff sounds are recorded with high fidelity; the sound visualization shows the strength of the sounds; and the process of measurement may be played back using the Accutension App software. The sound visualization and BP marking lines can help identify the start and the end of the Korotkoff sounds. All the innovations of Accutension make the automated auscultatory BP measurement more accurate, consistent, and reliable.
There are some limitations in this study. The selected participants did not include children younger than 12 years, pregnant women, and individuals with cardiac arrhythmia or an arm circumference beyond the cuff size. The Accutension XYZ-110 should be subjected to further validation study in these special populations according to the ANSI/AAMI/ISO 81060-2:2013 standard requirements in future.
In summary, the Accutension XYZ-110 BP kit passed the ANSI/AAMI/ISO 81060-2:2013 standard requirements, and hence could be recommended for both clinical and self/home BP measurement in adults.
Shanghai Zhihu Co. Ltd provided funding for this validation study.
Conflicts of interest
There are no conflicts of interest.
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