This systematic literature search revealed that there are few publications regarding nocturnal home BP measurements currently available. Furthermore, seven of the 11 selected articles came from Japan, the world's largest producing country of blood pressure monitoring devices . Self-measured home BP has been widely accepted in Japan, and Japanese Guidelines emphasize the need of having home blood pressure measured in the morning and in the evening . However, there may be a risk of bias, as results may differ according to population.
In 2001, the first data on a novel validated oscillometric upper-arm cuff home BP monitor (Omron HEM-747IC-N; Omron Healthcare Co., Ltd, Kyoto, Japan) equipped with a timer that triggered automated BP measurements during sleep were published by a Japanese research group . After instruction by nurses on the home BP methodology, 49 patients set up the devices themselves before sleep. The devices automatically measured BP at the programmed clock time, 0200 h, and stored the readings on their memory chips. The next morning, after awakening, patients removed the device and documented in a diary whether the automated nocturnal BP measurement affected their quality of sleep. The average SBP of the patients increased in proportion to the degree of sleep disturbance according to a graded three-level sleep disturbance score, that is, ‘no sleep disturbance and no awareness of measurement,’ ‘mild sleep disturbance with an awareness of measurement,’ and ‘serious sleep disturbance’ (from 116.1 ± 10.9 to 121.7 ± 15.5 mmHg; P < 0.001) . The same research group later examined the reproducibility of nocturnal home BP  and reported that the correlation coefficients of the nocturnal measurements between two sessions using this method with an average interval of 5.9 days were modest (r = 0.67/0.55 for SBP/DBP) and standard deviations (SDs) of the BP differences were large (≥13.6/≥9.3 mmHg), despite taking into account the degree of sleep disturbance; it may be partly because the BP was measured only once, at 0200 h, per session. Subsequently, Ushio et al.  reported that another home device (Omron HEM-5041) provided estimates of the nocturnal BP in Japanese healthy volunteers similar to those measured by nocturnal ambulatory BP monitoring .
The J-HOP study group simultaneously conducted the Japan Morning Surge-Target Organ Protection (J-TOP) trial in which the nocturnal home BP was measured under the same protocol as the J-HOP study . On the basis of this first nocturnal home BP-based clinical trial on antihypertensive medication received either at bedtime or upon awakening, a significant reduction in the nocturnal home BP of the 161 participants was observed during 6-month (from 6.4 to 8.8 mmHg and from 3.1 to 5.3 mmHg for SBP and DBP, respectively) . This reduction did not differ between home and ambulatory BP measurements within a subgroup of 50 patients, regardless of the time of drug administration (systolic/diastolic: P = 0.22/0.34) . However, the individual patient measurement plots demonstrated wide differences between home and ambulatory BP values both in the systolic (mean ± 2 SD; −35.5 to +29.8 mmHg) and diastolic (−25.3 to +22.1 mmHg) measurements . The nocturnal home BP technology has also been applied to drug therapy studies in hypertension [55,56]. Using the same schedule of three fixed measurements per night, the reduction of the nocturnal BP during 8 weeks of combination therapy with irbesartan with amlodipine was found to be significantly greater than that in patients who received irbesartan with trichlormethiazide (14.4/7.3 versus 10.5/5.6 mmHg, respectively; P ≤ 0.0056) .
The Microlife WatchBPN device (Microlife Corp., Widnau, Switzerland) also enables automated monitoring of nocturnal home BP . A research group in Greece performed a pilot study using the Microlife WatchBPN device in 39 patients referred to a sleep clinic and showed nocturnal home BP to be correlated with indices of obstructive sleep apnea severity . The same investigators used the WatchBPN home monitor and the Microlife WatchBP O3 ambulatory monitor in 81 patients with hypertension and showed similar night-time BP values with the two methods, as well as substantial agreement between them in detecting nondippers . It is notable that both WatchBP O3 and WatchBPN use the same oscillometric BP measurement algorithm, which minimizes the systematic error caused by using a different device for each method (home and ambulatory), and therefore the net BP difference between the two methods could be demonstrated in the study. The same group further reported that in 131 untreated patients with hypertension, nocturnal home BP was a significant determinant of left ventricular mass index, carotid intima–media thickness, and urinary albumin excretion (P ≤ 0.01), and the determination coefficient R 2 of each multivariable model was 0.26, 0.25, and 0.28, respectively . These findings were corroborated by a Finnish study using the same ambulatory (Microlife WatchBP O3) and home (WatchBP Home) devices for nocturnal BP evaluation in a 248 subject cohort, in which multivariable-adjusted models showed the nocturnal home BP to be significantly associated with pulse wave velocity, intima–media thickness, and left ventricular mass index (P ≤ 0.003) except the diastolic value with intima–media thickness (P = 0.13) .
Individual values of average ambulatory BP have shown a moderate correlation between two monitoring sessions (r = 0.56 and 0.82 for systolic, and r = 0.51 and 0.79 for diastolic), as reported by Keren et al.  and Ash et al. , respectively. The reproducibility of average 24-h, daytime, and night-time ambulatory BP is better than that of average hourly values and much better than that of office BP readings [42,61]. Of note, the reproducibility of clinic BP in the outpatient setting has been reported to be increased by averaging repeated BP measurements as they occur in day-by-day BP monitoring . Among untreated patients with hypertension, the test–retest correlation coefficients of awake and sleep ambulatory monitoring of the SBP/DBP were 0.74/0.80 and 0.81/0.79, respectively . The SD of the differences were 10.0/6.6 mmHg for awake, and 9.2/7.0 mmHg for sleep ambulatory BP . Physicians should thus be cautious about clinical decision-making based on short ambulatory BP measurement periods. Reproducibility of self-measured home BP was comparably high between two sets of 2-day measurements (days 2–3 versus 4–5; correlation coefficients, 0.91/0.86 in SBP/DBP; SD of differences, 6.9/4.7 mmHg) , and even between two sets of measurements taken at a 1-year interval (correlation coefficients, 0.84/0.83; SD of differences, 7.7/5.5 mmHg) .
Although no direct comparison between the reproducibility of nocturnal ambulatory and home BP has been published, it should be emphasized that, at variance from the good reproducibility of the average of a consistent number of nocturnal BP monitoring , the reproducibility of a few night-time BP readings is imperfect regardless of whether it is determined by ambulatory  or home measurements [45,47]. Eguchi et al.  reported similarly low reproducibility between two pairs of two sleeping BP readings, regardless of whether the patients received antihypertensive drugs (SD of the difference: 13.0/7.9 mmHg for SBP/DBP before the initiation of treatment, and 15.0/8.3 mmHg on-treatment) in comparison with measurements recorded while the patient was awake (12.1/6.6 and 14.6/8.5 mmHg). The reproducibility of the circadian BP pattern and the amplitude of the nocturnal fall of BP were also relatively poor [40–42] (w17–w21, http://links.lww.com/HJH/B36); however, during long-term observation, the nocturnal BP dip was essentially stable over time when expressed as a continuous variable . A repeated nondipping pattern at two different 24-h ambulatory monitoring sessions has been determined to better reflect cardiac abnormalities than a single monitoring session , whereas day-to-day nocturnal dipping patterns can be influenced by sleep quality . However, in the clinical setting, repeated monitoring of the nocturnal BP in diverse populations is rarely performed by 24-h ambulatory recording. Thus, in conclusion, the reproducibility of BP readings, regardless of whether self-measured at home, recorded during ambulatory BP monitoring, or obtained in the office during consultation, deserves attention by clinicians as it has demonstrated important associations with cardiovascular outcomes.
The clinical usefulness of BP measurement relies on the commitment and preference of the people in need of these measurements. Overall acceptance and preference by patients with hypertension were reported to be better for home BP measurement compared with ambulatory BP monitoring [69,70]. These findings were independent of potential characteristics of patients, including previous experience with BP measurement . In addition to nocturnal BP measurement, typical reasons for preferring 7-day home measurement over 24-h ambulatory monitoring included the ability to instantly see BP values, a sense by participants that they were more ‘in control,’ and less embarrassment or awkwardness because ambulatory BP measurements sometimes occur in public spaces . Conversely, one of the perceived benefits of ambulatory BP monitoring was the shorter duration of the monitoring period (only 1-day) . Interestingly, on the day of ambulatory recording, patients were sedentary for an average of 27 min longer during waking hours than they were on nonrecording days (P = 0.002) . Moreover, nocturnal ambulatory BP monitoring has been associated with a progressive increase in perceived sleep deprivation in comparison with days without ambulatory BP monitoring (P < 0.01) . Both reports [33,71] support the general concern that ambulatory BP monitoring may not always capture the ‘real’ BP of an individual. Despite the many advantages of ambulatory BP monitoring, excessively frequent cuff inflations cause disruption of sleep, resulting in reduced adherence of patients to repeat the measurements . Recent studies suggest that home BP measurement, particularly nocturnal measurement, is at least as well accepted as ambulatory monitoring, with a trend towards less-severe sleep disturbance [43,48,58,72]. However, the preferences of patients with regard to the number of home BP measurements per night is still not known because different schedules of nocturnal home BP measurements and different questionnaires for assessing patient preferences have been used [48,58,72].
Evidence is accumulating regarding the possibility of cost savings with out-of-office BP compared with conventional clinic BP measurement ; however, no cost-effectiveness analysis on nocturnal home BP has yet been reported at this emerging stage of the method. Ambulatory BP monitors are generally much more expensive than currently available home BP-measuring devices, most of which cost less than 100 Euros. Although clinicians can evaluate hundreds of patients by using the same ambulatory BP monitor, an individual low-cost home BP device enables us to repeatedly measure BP in the daily life of each person. Moreover, the widespread clinical application of home devices is now being further favoured by currently developing technologies for remote telemonitoring and telemedicine [55,56,74]. For this very reason, nocturnal home BP has the potential to avoid the limited application of nocturnal BP measurement by ambulatory monitoring. Nevertheless, we cannot ignore the initial cost of introducing home devices to each patient.
Wrist-cuff oscillometric BP measurement devices have been marketed and widely used in clinical practice, particularly for self-measurement of BP during daytime. Such devices may be acceptable in limited situations, for example, day-to-day monitoring during travel, patients with arm circumferences that are too large or where the upper arm length is too short to apply a large cuff , or – as described later – to measure nocturnal BP during sleep. The measurement-induced reactive rise of home BP values by a wrist-cuff device was reported to be smaller than that by an upper-arm cuff device in hypertensive patients  as the wrist-cuff device may cause less discomfort and muscle compression . Nevertheless, there are some problematic aspects related to use of the wrist-cuff devices. One problem is the hydrostatic height-related pressure difference between wrist and heart levels in the lying posture during sleep; a 10-cm hydrostatic pressure difference between the heart level and cuff position results in a 7-mmHg difference in BP levels [78,79]. Even when patients remain in the lateral recumbent position during sleep, the difference in the position between the level of the heart and the mid-arm level is less than that at the wrist level. Another limitation of wrist devices is the different accessibility of radial and ulnar arteries by the device in different people, because of individual anatomical characteristics of the wrist [3,78]. Although wrist position sensors help patients to self-measure their wrist BP more accurately while they are awake , there is no guarantee that even the most advanced wrist-cuff oscillometric device will measure the nocturnal BP accurately during sleep – at least to the level of precision seen with validated upper arm-cuff devices.
The major advantages of nocturnal home BP measurement include that it allows the participants’ quality of sleep to be self-reported  and that it enables us to easily obtain a periodic recording of nocturnal BP. Although more research is needed on the quality of sleep during nocturnal BP recordings, a single nightly measurement repeated over a relatively long period (e.g. 5 days , 7 days , or much longer) can yield highly reliable sleep BP measurement values as documented by sleep quality questionnaire administered on the night of the BP recording. Just as the daily recording of morning/evening home BPs can reveal the time of the maximal effect (stabilization time) of antihypertensive treatment , the periodic measurement of the nocturnal home BP may have additional clinical value as it may allow to assess the long-term BP-lowering effect at night of antihypertensive drug therapy [11,80] and seasonal variations in nocturnal home BP levels [81,82].
Measuring nocturnal home BP for even just one night, which is the usual approach when using ambulatory BP monitoring, can capture basic information on nocturnal BP level associated with comparably accurate information on quality-of-sleep self-reported over the nocturnal measurement period . This approach does not take advantage of the strengths of multiple nocturnal home measurements, however. The number of nocturnal home BP measurement varied among previous studies (Table 1), and further research is needed to specify optimal nocturnal home BP measurement schedules according to the purpose for which they are performed, including the preference of patients for the frequency of the measurement. This issue must remain open until robust evidence on nocturnal home BP becomes available.
The 2017 ACC/AHA Hypertension Guidelines in the United States estimated that a nocturnal ambulatory BP level of at least 110 mmHg/at least 65 mmHg corresponded to the new recommended definition of stage 1 hypertension (≥130/≥80 mmHg by conventional clinic BP measurement) although such an estimate is not outcome-based . Thus, the suggestion to consider people with nocturnal home BP of at least 110 mmHg/at least 65 mmHg as having nocturnal home hypertension under this revised office BP threshold  would need to be further evaluated in the light of additional evidence derived from future studies. Taking the recent publications on nocturnal ambulatory BP monitoring [2,20,21,85] into consideration, the diagnostic threshold of nocturnal home hypertension as at least 120 mmHg/at least 70 mmHg remains reasonable for most patients.
Given that the recommended threshold for daytime ambulatory BP is the same as for home BP and that nocturnal home BP levels appear to be similar to nocturnal ambulatory BP, home BP measurement might be a useful and practical alternative to ambulatory BP monitoring in detecting participants with nondipping patterns. Furthermore, three studies reported the agreement between home and ambulatory BP in detecting nondippers to range from 74 to 79% [44,58,59], which is similar to the test–retest reproducibility of repeated ambulatory BP monitoring in detecting nondippers [40,67]. The reported similarity between the home-ambulatory BP agreement [44,58,59] and the agreement between repeated ambulatory BP monitoring in diagnosing a nondipping BP pattern [40,67] also implies that the ambulatory and home BP might be regarded as interchangeable approaches in the detection of nondippers.
Despite the major role that nocturnal ambulatory BP has gained in predicting cardiovascular risk, an important problem remains that the measurement of BP affects sleep quality. The effect on sleep quality is mainly attributed to the disturbance of sleep by cuff inflation. As shown by studies, which combined polysomnography or electroencephalography with nocturnal BP monitoring, cuff inflation was associated with increased arousal and wakefulness . Davies et al.  reported that the mean duration of arousal by ambulatory measurement during sleep was 8–16 s. The BP recording procedure caused a tiny but significant decrease in the slow-wave sleep period and an increase in nocturnal awakening . Brazilian investigators reported that 35.1% of patients had an abnormal sleep quality on the day of 24-h ambulatory BP monitoring . Furthermore, 24-h ambulatory monitoring sometimes causes adverse effects such as pain, skin irritation including upper-arm ecchymoses, and disruption of work . A questionnaire-based sleep quality assessment revealed that 61% of patients undergoing nocturnal ambulatory BP monitoring reported a minor disturbance of sleep, 14% had poor sleep, and 2% did not sleep at all . According to French cardiologists, severe difficulty (defined as ≥3 instances of disturbance by the device during 24-h measurement) occurred in 32% of their patients in relation to cuff pressure, whereas 14% reported that it was cumbersome . As a consequence, 45% of the patients reported experiencing a sleeping delay, 36% reported arousal, and 14% reported lower sleep quality. Moreover, a remarkable 95% of patients who did not sleep alone reported that it disturbed their spouse's sleep as well . More than half (58%) of the participants recalled cuff inflation during the nocturnal ambulatory monitoring .
Investigators have reported an inverse association between BP levels and sleep quality. A lower BP was significantly associated with deeper sleep [34,35]. Although the sleep stage was not changed on electroencephalography, arousal stimuli produced a significant increase in BP (8.6 mmHg in SBP) . A recent report by Oume et al.  also supported this association based on the actigraphic sleep parameters of 1127 residents aged at least 60. It is, therefore, a reasonable assumption that cuff inflation induces an increase in BP. Heude et al.  reported that upper arm-cuff inflation itself raises the nocturnal BP, even when a carbon dioxide cartridge is used to inflate the cuff silently. In recent studies by Sheshadri et al., the mean changes in the invasive arterial SBP/DBP level among 97 patients during each of six contralateral arm-cuff inflations were 6.7 ± 5.9/2.6 ± 4.0 mmHg . SBP increases of 0–10 mmHg were observed in 73.4% of cuff inflations, which was independent of the baseline BP levels . Interestingly, despite the fact that two-thirds of BP measurements caused arousal evaluated by simultaneous electroencephalography, neither the average SBP (1.2 ± 6.4 mmHg) or DBP (0.5 ± 4.1 mmHg) BP levels differed to a statistically significant extent between the uninterrupted sleep and the arousal period . The addition of noninvasive automatic or semiautomatic BP monitoring did not cause any alterations in the day and night intra-arterial BP or heart rate profiles in a few studies by the Milan group [93–95]. Studies on Brazilian patients  and Danish diabetes patients  supported the absence of an association between arm-cuff inflation and BP levels, too. However, it is worth noting that bedside monitoring with a catheter represents a different condition from the usual ambulatory setting, although the studies carried out in Milan were based on 24-h intra-arterial ambulatory BP monitoring. There might be some individual variability in the degree of BP rise triggered by arm-cuff inflations, and that arm-cuff inflation causes a transient reactive increase in BP in some individuals and that the averaging of the nocturnal BP values could mask the elevation that occurs at the precise moment of BP during arousal or cuff inflation.
There are technologic advances in BP device pumps to limit noise during cuff inflation. For example, an innovative, upper arm-cuff inflated by an electric motor drive unit that produces low noise cuff inflation of 36.7–41.5 dB has recently been developed (communication from Research and Development section, Omron Healthcare Co., Ltd). This is a noise considered to slightly more than a whisper (≈35 dB) or the background noise in a public library (≈40 dB). This device is nearly as quiet as the ABPM-630 (Nippon Colin, Komaki; currently Fukuda Colin, Tokyo, Japan) [16,91,97] which provided silent inflation; but because of the frequent exchange of a carbon dioxide gas cartridge required for this device, it became impractical and has been discontinued from clinical commercial use. It is also noteworthy that wrist-cuff devices usually generate less noise because of its small bladder size; for example, the wrist-cuff oscillometric HEM6310F-N (Omron Healthcare, Co., Ltd) produces a noise of only 8.1–18.0 dB (communication, Research and Development section, Omron Healthcare Co., Ltd). This quiet measurement condition further favors an accurate measurement of BP during the cuff inflation phase  as it does not cause the usual loud noise by upper arm-cuff devices that compromises an accurate oscillometric signal capturing during cuff inflation. A wrist-cuff device may represent a favorable solution for nocturnal home BP measurement with minimal disruption of sleep, an issue which needs to undergo adequate investigation, together with other abovementioned crucial features characterizing wrist-cuff devices for nocturnal BP measurement.
In the clinical setting – as opposed to the research setting – there is room for debate on how to define sleep because of the difficulties in obtaining accurate documentation. The use of the standard narrow-fixed clock intervals with ambulatory BP monitoring, that is, defining the time periods 0900–2100 h as daytime and 0100–0600 h as night-time, has allowed to define the prognostic significance of a rising pattern in nocturnal BP (hazard ratio, 1.57; 95% confidence intervals, 1.08–2.27 in comparison to participants with a dipping pattern) . However, diary record-based classifications provided the best predictive power for stroke incidence (hazard ratio 2.31; 95% confidence intervals, 1.47–3.62) . The recall of subjective sleep disturbances during nocturnal home BP measurement [45,47] would be a feasible option to investigate interference with sleep quality, although this approach does not guarantee whether an individual actually remained asleep during the measurement. Given the evidence that the quantity and quality of sleep predict cardiovascular outcomes [100,101], sleep per se has been indicated as area of interest, together with nocturnal BP, for research in cardiovascular prevention. Hence, there is a need for a convenient and widely available method of determining the state of sleep during BP measurement. Actigraphy has been used for decades as a tool to screen the state of sleep and to identify people who might benefit from a more precise diagnostic approach through polysomnography , that is applicable to either patients  and general population .
Specific event-triggered BP measurement, such as at the peak of hypoxemia in patients with obstructive sleep apnea patients [31,103,104] or at the time of the lowest heart rate that may coincide with low levels of sympathetic nervous system activity , would be approaches worth considering for research, aimed at improving the clinical relevance of nocturnal home BP measurement. Such measurements can capture specific episode-related sleep BP surges or declines and may reduce the number of BP readings required during sleep [106,107]. Moreover, a night-time home BP telemonitoring system has become available by recently marketed devices, such as the Omron HEM-7252G-HP, in which a mobile communication facility is embedded [55,56,74]. Continuous research and development into new home BP devices will enhance the usefulness of night-time home BP measurement in the near future .
In the last two decades, technological advancement in home BP devices have allowed the evaluation of BP also during night-time sleep [7,43]. Preliminary evidence shows that nocturnal home BP measurement is feasible and provides levels of nocturnal BP and ability to detect nondippers similarly as ambulatory BP monitoring does. Thus, nocturnal home BP measurement might be a practical and reliable alternative to ambulatory BP monitoring, which has been indicated as the gold standard for out-of-office BP measurement but has been limited in its dissemination by practical and economic concerns . Subheadings of the present article represent possible items of a research agenda for future studies on nocturnal home BP (Table 3), among which associations of nocturnal home BP with cardiovascular outcomes are most important. These research data are needed to guide the implementation of nocturnal home BP measurement in the management of patients with hypertension in clinical practice.
We gratefully acknowledge Toshikazu Shiga, Jim Li, Mitsuo Kuwabara, and Noboru Shinomiya (Omron Healthcare Co., Ltd) for their valuable contribution to establish and maintain the International Expert Group of Nocturnal Home Blood Pressure.
Sources of funding: Omron Healthcare Co., Ltd supported the travel and lodging expenses for the expert panel meetings of the International Expert Group of Nocturnal Home Blood Pressure which were held in Milan and Kyoto on 20 June 2017 and on 17 May 2018, respectively; the company had no role in the procedures for consensus on the contents and statements of the manuscript.
Y.I., T.O. and K.K. received research grants from Omron Healthcare. KA., T.O., K.K., and M.A.W. are consultants for Omron Healthcare. K.K., G.S.S., G.P., and Y.I. conducted validation studies for various manufacturers and advised manufacturers on device and software development. The other authors declare no conflicts of interest in association with the present study.
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