In recent years, out of office BP measurements have become an important part of blood pressure management and control. The 2020 International Society of Hypertension Global Practice Guidelines for Hypertension clearly states that 24-hour ambulatory blood pressure monitoring (ABPM) or home blood pressure monitoring (HBPM) can be combined to assist in the diagnosis of hypertension when conditions permit. These two out-of-office BP measurements have been shown to provide better prognostic information on target organ damage and cardiovascular risk than office BP measurements [1]. However, both of them have certain limitations in blood pressure monitoring. First, the number of measurements that can be recorded with these two approaches is limited, which provides insufficient basis for diagnosis in terms of long-term monitoring; second, 24-hour ambulatory blood pressure monitoring usually needs to be completed in the hospital that would usually cost a lot compared to home blood pressure monitoring; rather, In addition, both of them have shortcomings in terms of mobile convenience while they cannot measure blood pressure anytime or anywhere.
With the rapid development of science and technology and the increasing awareness of long-term blood pressure monitoring and health management, wearable blood pressure monitoring devices have emerged in the digital age. Wearable blood pressure monitoring devices can meet the long-term monitoring needs of people to measure blood pressure in any occasions and is superior to traditional devices in terms of patient comfort. Potential cardiovascular risk can be effectively identified by increasing the number of blood pressure measurements over time and in multiple situations. The massive data collected by wearable devices can also help to do time series analysis and predict hypertension through artificial intelligence [2].
Because of these technologies, wearable blood pressure monitoring devices have attracted the attention from academia, industry and even ordinary consumers. There has been growing public concern about its accuracy of blood pressure measurements and monitoring.
Wearable Blood Pressure Monitoring Devices, Blood Pressure Variability and Cardiovascular Risk
The synergistic resonance theory suggests that all forms of BPV (beat-by-beat, diurnal, day-by-day, seasonal and yearly) have the potential to create dynamic surges in BP, which could coincide with peak BP and different external triggers (eg, temperature, mental stress, sleep apnea, exercise) to precipitate cardiovascular events, especially in patients with arterial stiffness who are less able to absorb BP surges in the peripheral arteries [3]. The combination of these various factors means that everyone has their own unique 24-hour blood pressure changes. In addition, the variability of blood pressure in hypertensive patients also varies by season, possibly because the temperature in winter is lower than that in summer.
Many studies have shown that blood pressure variability is closely related to cardiovascular events. Generally speaking, blood pressure tends to be high in the morning and lower at night. Excessive morning blood pressure surge is associated with an increased risk of stroke, cerebral hemorrhage, and various forms of target organ damage. Rather, no reduction in nocturnal blood pressure has also been shown to increase the risk of target organ damage and cardiovascular events. These all imply the importance of accurate detection of blood pressure fluctuations to control cardiovascular risk [4]. Wearable blood pressure monitoring devices can continuously monitor blood pressure and evaluate all environmental factors, which can well perform the role of blood pressure monitoring and management.
Wearable Blood Pressure Monitoring Devices
Compared with office blood pressure detection equipment, the reliability and accuracy of wearable devices have obviously not been fully verified. But growing evidence is beginning to support wearable medical devices from experienced manufacturers. These devices use a variety of different methods and techniques to monitor blood pressure, some of which are even certified.
Oscillometric Measurement at the Wrist
The oscillometric technique is widely used in current office, home and ambulatory BP measurements devices. The oscillometric method is also called the oscillation method. The basic principle behind this approach is that when the pressure oscillations in a cuff sphygmomanometer are recorded during gradual depressurization, the point of maximum oscillation corresponds to the average intra-arterial pressure. This approach has great advantages as no transducer placement on the brachial artery is required and it is less sensitive to external noise (except for low frequency mechanical vibrations). Compared to traditional upper arm cuff measurement devices, the wrist-based device uses the same principles, but offers a significant improvement in comfort.
In a recent head-to-head study, it was for the first time that an oscillometric BP monitoring wrist device was compared to a 24h ambulatory BP measuring device [5]. After continuous in-office and out-of-office measurements of 50 patients, the mean difference between the wearable device and ABPM were 0.8 ± 12.8 mmHg in the office (P=0.564) and 3.2±17.0 mmHg outside the office (P< 0.001). The proportion of differences that were within ±10 mmHg was 58.7% in the office and 47.2% outside the office. Through a mixed-effects model analysis, the study found that the difference between the out-of-office blood pressure values measured by the two devices was not statistically significant, which means that the difference in blood pressure detection between the two devices is acceptable, which also indicates the accuracy of wrist oscillometric measurement of blood pressure.
Figure 1 Comparison of blood pressure measurement results between a manufacturer's wrist oscillometric blood pressure watch and ambulatory blood pressure monitoring equipment
Figure 1 above shows an example of blood pressure levels obtained using the wearable device and ambulatory blood pressure detection device in the above study. We can clearly see that when the time line is extended to one day, and after multiple measurements during the day, the blood pressure readings measured by the two devices almost display the same fluctuation pattern. Both patterns measured by the two devices can well reflect the blood pressure fluctuations throughout the day. BP Doctor Pro and BP Doctor Med are two blood pressure smartwatches that are designed and manufactured by YHE. They use the oscillometric method to measure blood pressure by patented air pump pressurization and micro airbags, which ensures the accuracy of blood pressure measurement and monitoring.
BP Measurement Using Oscillometric Finger Cuffs
(METHOD OF PENAZ)
Arterial pulse in the finger is detected by a photoplethysmographic sensor, located underneath a finger cuff. The output of the plethysmograph is used to guide a pump, which quickly moderates the cuff pressure so that the artery is kept partially open. The pressure oscillations of the cuff are Recorded and it is proved that they resemble the intra-arterial pressure wave fluctuations in most individual. Compared with brachial artery pressure, this method can accurately estimate changes in systolic and diastolic blood pressure. Two devices currently on the market, the Finometer and Portapres, can record pressure fluctuations and have been certified in several studies comparing finger and intra-arterial blood pressure monitoring.
PHOTOPLETHYSMOGRAPHY
Photoplethysmography (PPG) is a non-invasive technique that appeared in 1930s, through which changes in blood flow are detected in selected parts of the body during the cardiac cycle. Clinical applications of this technique are found in the assessment of BP, heart rate and blood oxygen saturation, but also in the detection of peripheral venous diseases. Pulse transit time (PTT), indicating the time it takes for a pulse wave to travel along the length of the arterial tree, is an essential part of measuring BP with this method. The pulse pressure wave form, occurs when blood is ejected from the left ventricle and the impulse transmitted to the arterial wall moves at greater velocity than the blood itself. In addition to measuring blood pressure, PTT is an indicator of arterial stiffness. PTT can be calculated from signals from electrocardiogram (ECG) and PPG.
ECG data is used as the basis for calculating time, while PPG provides a visual assessment of the volumetric changes of blood in the tissues during the cardiac cycle. Optical PPG and ECG sensors have been used in wearable devices for measuring heart rate. The PTT measurement involves calculating the time between the R wave on the ECG and a reference point in the pulse pressure wave measured using PPG, providing data on blood pressure levels. However, PTT-based estimates of blood pressure values may be inaccurate because blood pressure regulation is a multifactorial process. Studies have shown that measuring PTT alone is not sufficient in terms of measuring reliability. The integration of PPT, heart rate, and a recent conventional blood pressure measurement in the equation can improve the reliability of blood pressure assessment [6].
However, PPG blood pressure measurement has implicit limitations. Firstly, the stability of the PPG signal is affected by motion, so its use is limited to non-mobile devices only; secondly, it requires frequent calibration of the equipment. But it takes time to find a doctor to carry out the calibration.
Integrating PPG in Smartphone Apps
Studies have attempted to explore the effectiveness of measuring blood pressure through a smartphone camera or integrated into a portable detector connected to a smartphone. In a single study, 205 individuals were enrolled and data from PPG signals were collected, using the heart rate sensor of a smartphone. An algorithm was developed based on patient demographics, which improved the recording accuracy of systolic and diastolic blood pressure by 11.5% and 18%, respectively. Based on PPG signals and this integrated algorithm, there were measurements achieved with accuracies of 7 mmHg mean absolute effor for SBP, and 5 mmHg for DBP.
The Future of Wearable Blood Pressure Monitoring Devices
Existing researches suggest that digital management of hypertension and wearable blood pressure detection technology are the direction of the future. The goal of these approaches is to reduce or even eliminate the incidence of cardiovascular events in hypertensive patients. The latest guidelines from the American College of Cardiology and the American Heart Association highlight the importance of health information technology solutions in the diagnosis and management of hypertension. The integration of individual time-series data with environmental factors is increasingly recognized as an important factor in blood pressure measurement. In this context, wearable technology provides convenience for better long-term monitoring of hypertension, and also provides a better diagnostic basis for remote medicine. Several studies have also shown that wearable devices produced by some companies specializing in high blood pressure detection technology are effective and reliable [8]. What’s more, wrist-worn wearable devices perform well in out-of-office blood pressure monitoring, and even become an important alternative approach when traditional office blood pressure monitoring method cannot monitor blood pressure at all time and places.
Despite the promising prospects of wearable blood pressure monitoring devices and the achievements in practice, most existing wearable blood pressure monitoring devices rely on specific body positions, relatively strict measurement standards and necessary frequent calibrations. Because of those limitations, wearable blood pressure monitoring devices are still confined in clinical practice. More researches, more testings, more data accumulation, better algorithm optimization, and better continuous calibration may be the key to the future of wearable blood pressure monitoring technology and devices.
List of reference:
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【6】 Wang R, Jia W, Mao ZH, Sclabassi RJ, Sun M. Cuff-free blood pressure estimation using pulse transit time and heart rate. Int Conf Signal Process Proc. 2014;2014:115–8.
【7】 Dey J, Gaurav A, Tiwari VN. InstaBP: cuff-less blood pressure monitoring on smartphone using single PPG sensor. Annu Int Conf IEEE Eng Med Biol Soc 2018;2018:5002–5
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