Comment regarding “The role of wearable home blood pressure monitoring in detecting out-of-office control status” HY Pan and CK Lee et al. Hypertens Res 2024.

Blood pressure (BP) is not constant, and its fluctuations vary over both short and long terms due to various factors [1]. The BP fluctuations during periods of wakefulness in daily life may be due to a variety of lifestyle-related factors including physical activity and psychological stress [2]. These fluctuations cannot be detected by a snapshot measurement of office BP at clinic visits. BP variability during daily life is reported to be a cardiovascular risk factor itself, as it is associated with cardiovascular outcomes [1]. It is therefore clinically relevant to identify elevated BP in daily life in order to prevent future cardiovascular disease. Although BP variability in daily life can be monitored using ambulatory blood pressure (ABP) monitoring (ABPM) and home blood pressure (HBP) monitoring (HBPM), the diagnostic agreement between ABPM and HBPM is not always consistent. The diagnostic inconsistency may be explained in part by the differences in the timing and conditions of these two methods [3]. ABPM is performed with short-term (15- to 60-min) intervals and is usually performed over only a 24-h period. HBPM can be conducted for a longer period and is usually applied using specific timing (e.g., in the morning and evening).

As an ABPM monitor is worn during daily activity (i.e., a wearable device), and although it can measure an individual’s BP variability influenced by physical activity and psychological stress in daily life, it is usually performed for only one day. Notably, the amplitude and frequency of lifestyle-related BP variability varies from day to day, and a single day’s monitoring is not sufficient to assess an individual’s cardiovascular risk. A recently developed wristwatch-type wearable oscillometric blood pressure (WBP) monitoring (WBPM) device has features of both ABPM and HBPM [4]; it is a wearable device and can initiate BP measurements at any time and at any location, even during daily activity. Such a wristwatch-type WBPM device can be used to measure a person’s BP during daily activities in the same manner as ABPM does, and it can also be used for long-term monitoring, similar to HBPM (Fig. 1) [3, 5, 6].

Fig. 1
figure 1

Comparison of three out-of-office blood pressure monitoring methods: ambulatory blood pressure monitoring (ABPM), home blood pressure monitoring (HBPM), and wearable oscillometric blood pressure monitoring (WBPM). BPV blood pressure variability, N/A not applicable, ME morning–evening

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Pan et al. investigated their study participants’ BP control status defined by both daytime BP automatically measured by ABPM (via an oscillometric upper-arm-type device) and WBP self-measured at prespecified timepoints by WBPM (via a HeartGuide [Omron Healthcare] wristwatch-type wearable oscillometric BP monitoring device) [7]. The participants’ WBP was measured at six prespecified timepoints daily for one week of each month over three consecutive months, and the participants’ daytime ABP was measured at 30-min intervals over a 24-hr period. Among the study participants who had office BP < 140/90 mmHg and were receiving antihypertensive therapy, 6.5% had uncontrolled daytime ABP (≥130/80 mmHg) and controlled WBP (<130/80 mmHg).

We have demonstrated the diagnostic inconsistency between ABPM and HBPM (seated home BP measured in the morning and evening) recorded with the use of a single oscillometric upper-arm all-in-one device [8]. In our treated hypertensive patients with office systolic blood pressure (SBP) < 130 mmHg (a lower threshold for the office BP control status compared to Pan et al. study), the proportion of patients with home SBP < 130 mmHg and daytime ABP > 130 mmHg was 16.6%. Although our study evaluated the patients’ BP control status with a focus on SBP and used a strict office BP threshold, the diagnostic inconsistency was higher in our study compared to the Pan et al. report. These results indicate that a WBPM device can monitor BP variability during daily activity, beyond seated home BP.

Pan et al. also investigated the reproducibility of WBP over the 3-month period [7], and their results demonstrated that the reproducibility of average WBP for one week in each of the three months was good, especially for WBP measured before breakfast and after dinner. Good reproducibility can be interpreted as a reliable index, but poor reproducibility has important clinical implications. Poor reproducibility of increases in BP (i.e., BP increases that are not observed every time) do not mean that these increases are not a risk for cardiovascular disease. Another investigation indicated that an increase in the standard deviation (SD) of HBP self-measured in the morning was an independent risk for future cardiovascular events [9]. In addition to a reproducible BP increase, an exaggerated morning BP surge with poor reproducibility and an increased SD of morning BP may increase cardiovascular risks [10].

In our investigation using the same WBPM device as that applied in the Pan et al. study, the average of the participants’ morning home WBP and the peak of their morning home WBP values were significantly associated with left ventricular mass index [11]. Given these results, repeated measurements of morning BP and the assessment of morning BP variability are clinically relevant, and a WBPM device is useful for the long-term daily measurement of BP (Fig. 1).

Although HBPM has well-recognized guideline recommendations concerning the measurement method and timing [12,13,14], there are no similar recommendations for WBPM (Fig. 1). The study by Pan et al. monitored WBP at six prespecified timepoints based on the participants’ daily behaviors (i.e., after waking up, before lunch, after lunch, before dinner, after dinner, and before bedtime) [7], and in our study we monitored WBP after the participants’ wakeup time, before bedtime, and at five other timepoints (every 2 hr from 10:00 a.m. to 6:00 p.m.) [11]. Both studies collected data at intervals of a few hours during the daytime period. Although the ability to initiate BP measurements anywhere and anytime in daily life is an advantage of oscillometric wearable devices, there is no evidence regarding whether WBPM should be performed at specific timepoints by specific methods, or whether these parameters can be left up to the user. To obtain accurate data and clinical assessments with the use of WBPM, it may be necessary to first establish measurement guidance or recommendations.

Smartwatches and smart wristbands have become popular in recent years, and some of these devices have an accompanying BP calculation function. They differ from cuff oscillometric devices in that they estimate BP values using techniques such as photoplethysmography. Since smartwatches and smart wristbands do not require cuff compression, these devices are referred to as cuffless BP monitoring devices, and their estimated BP values are currently not recommended for clinical use [15]. A WBPM device can be worn 24 h a day every day in daily life, but with such a device the wearer’s BP is self-measured and the wearer is required to be in a specific posture as shown in Fig. 1. Ideally, BP should be measured continuously and automatically since an individual’s BP may increase without his/her being aware of it, e.g., due to BP elevation caused by psychological stress [16].

As noted above, although WBPM is a novel device that addresses the pitfalls of ABPM and HBPM, its measurement methods and evidence-based guidelines have not yet been established. Further research and technological development are necessary for WBPM to become the third option for out-of-office BP measurement methods.