Original Article

Journal of Human Hypertension (2010) 24, 207–212; doi:10.1038/jhh.2009.60; published online 16 July 2009

Are there really differences between home and daytime ambulatory blood pressure? Comparison using a novel dual-mode ambulatory and home monitor

G S Stergiou1, D Tzamouranis1, E G Nasothimiou1, N Karpettas1 and A Protogerou1

1Hypertension Center, Third University Department of Medicine, Sotiria General Hospital, Athens, Greece

Correspondence: Professor GS Stergiou, Hypertension Center, Third University Department of Medicine, Sotiria General Hospital, 152 Mesogion Avenue, Athens 11527, Greece. E-mail: gstergi@med.uoa.gr

Received 23 December 2008; Revised 3 June 2009; Accepted 19 June 2009; Published online 16 July 2009.

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Abstract

Several studies compared blood pressure (BP) at home (HBP) with ambulatory BP (ABP), but using different devices, which contribute to differences in measured BP. A novel dual-mode device allowing ABP and HBP monitoring (Microlife WatchBPO3) was validated according to the European Society of Hypertension International Protocol and used to compare the two methods. In the validation study, 33 subjects were assessed with simultaneous BP measurements taken by 2 observers (connected mercury sphygmomanometers) 4 times, sequentially with 3 measurements taken using the tested device. Absolute observer-device BP differences were classified within 5/10/15mmHg zones. Measurements with less than or equal to5mmHg difference were calculated per participant. In the validation study, the device produced 70/89/96 measurements within 5/10/15mmHg, respectively, for systolic BP and 67/95/99 for diastolic BP. Twenty-eight subjects had at least two of their systolic BP differences less than or equal to5mmHg and one subject had no difference less than or equal to5mmHg, whereas for diastolic BP, it was 22 and 1 subjects, respectively. Mean device-observers BP difference was −0.3±5.6/−2.4±4.8mmHg (systolic/diastolic). In the application study, the difference between daytime ABP and HBP was 0.5±7.9mmHg for systolic BP (mean±standard deviation, 95% confidence intervals (CI) −1.9, 2.9, P=NS) and 0.6±5.5 for diastolic BP (95% CI −1.1, 2.3, P=NS). In conclusion, the Microlife WatchBPO3 device for ABP and HBP monitoring fulfils the International Protocol validation criteria. Using this device, no clinically important difference between daytime ABP and HBP was detected. These data justify the use of the same diagnostic threshold for both methods.

Keywords:

ambulatory blood pressure, home blood pressure, self-measurement, comparison, validation, international protocol

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Introduction

For the accurate diagnosis and the optimal management of hypertension, current guidelines recommend the assessment of out-of-office blood pressure (BP) using either 24h ambulatory monitoring (ABP) or self-monitoring by patients at home (HBP).1, 2, 3 Studies have shown that both these methods allow the detection of the white coat and the masked hypertension phenomena and are more closely related to cardiovascular events than the conventional BP measurements at the office or clinic.1, 2, 3

Numerous studies have compared HBP with ABP measurements.4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14 In each of these studies, HBP and ABP measurements have been obtained using different devices, usually oscillometric, but of different manufacturers specifically designed for each method.15 Thus, the observed differences in BP levels between these two methods are attributed, at least in part, to the different devices used.

A novel dual-mode automated oscillometric device (Microlife WatchBPO3) that allows 24h ABP and also HBP monitoring has been developed. This device aims to provide a complete assessment of the ‘true’ BP of an individual over time at his/her usual environment by combining these two out-of-office monitoring techniques. Furthermore, this device provides a unique opportunity to assess the true difference between HBP and ABP, using the same device for both methods.

This paper presents the results of (i) a validation study of the Microlife WatchBPO3 device according to the European Society of Hypertension International Protocol 16 and (ii) a comparison of daytime ABP versus HBP in the same patients and using the same device.

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Materials and methods

Tested device

The Microlife WatchBPO3 (Microlife, Widnau, Switzerland) is a professional oscillometric device for ABP and also HBP measurement on the upper arm. ABP monitoring can be performed at 20–60min intervals for 24h and HBP monitoring for 7 days with duplicate morning and evening measurements per day as recommended by European and American guidelines.2, 3 Casual BP measurement might also be obtained at the user's discretion. The device has a switch to change measurement mode (ambulatory, home, casual). It measures BP at a range 30–280mmHg and pulse rate 40–200 beats per minute. Inflation is performed by an automatic electric pumping system and deflation by an automatic pressure release valve. It is powered by four 1.5V batteries and automated memory and PC link capacity. A PC software is available to report and summarize the recorded BP data (24h, awake and asleep ABP average according to the individual's sleeping times and HBP average of days 2–7). Three cuffs are available with the device for arm circumference 17–22, 22–32 and 32–42cm. The expected retail price in Europe is about 700 euros. Three devices were obtained from the manufacturer for the purpose of the study, with a written declaration that they were standard production models. One of them was randomly selected for the validation procedure.

Validation study

The study was conducted by a supervisor and three trained observers who rotated according to their availability. Before the study initiation, the observers were tested for agreement in BP measurement according to the British Hypertension Society protocol.17 Two standard mercury sphygmomanometers (Riester, diplomat-presameter, Rud. Riester GmbH Co. KG, Jungingen, Germany), the components of which have been checked before the study, and a teaching Littman stethoscope were used for simultaneous (Y tube) observer-taken reference BP measurements. The supervisor measured BP with the tested device and also checked the agreement of BP measurements taken by the two observers, who were blinded from each other's readings and those obtained by the device. Observer readings with a difference >4mmHg were repeated until closer agreement was reached. The cuffs of the tested device were used for measurements taken using the tested and the mercury device to fit the arm circumference of each individual. All measurements were taken on the left arm, which was supported at heart level. The protocol was approved by the hospital scientific committee.

According to the International Protocol,16 in phase 1 a total of 15 treated or untreated subjects are included, who fulfil the age, sex and entry BP range requirements (agegreater than or equal to30 years, greater than or equal to5 men and greater than or equal to5 women, 5 subjects with entry BP within each of the ranges 90–129, 130–160 and 161–180mmHg for systolic and 40–79, 80–100 and 101–130mmHg for diastolic BP). If analysis of these data is successful, additional subjects are recruited until 33 subjects fulfil the age, sex and entry BP range requirements for phase 2 (age greater than or equal to30 years, greater than or equal to10 men and greater than or equal to10 women, 11 subjects with entry BP within each of the above-mentioned BP ranges). Subjects with sustained arrhythmia or irregular pulse during the validation procedure were excluded. Signed informed consent was obtained from all subjects who participated in the study.

The validation study was conducted in an isolated room in which noise disturbance was avoided. Age, sex and arm circumference of each participant were recorded, together with the cuff size used and the date and time of the validation procedure. After 10–15min rest (sitting), BP was measured by the two observers (entry BP). This measurement was used to classify subjects into the low, medium and high range, separately for systolic and diastolic BP, as described above. Device detection measurement was followed by the supervisor to ensure that the device was able to measure BP of each individual. The two observers took readings BP1, BP3, BP5 and BP7 using the double-headed stethoscope and the mercury sphygmomanometers. The supervisor took readings BP2, BP4 and BP6 using the tested device. The validation analysis was based on measurements BP1-BP7.

Application study

Subjects aged >25 years attending an outpatients hypertension clinic, untreated or on stable antihypertensive drug treatment for at least 4 weeks were invited to participate in an application study using the tested device. Participants were trained in the conditions of ABP and HBP measurement and the use of the device and were instructed to perform 24h ABP monitoring and then to switch to the HBP monitoring mode and perform 7 days HBP measurements, or the reverse, according to each individual's preference. Before study, entry accuracy of the devices was tested against a mercury column in each participant (Y connector).

ABP was monitored on a routine workday at 30min intervals for 24h. Participants were instructed to follow their usual daily activities, but to remain still with the forearm extended during each BP reading. HBP was monitored before or after ambulatory monitoring with duplicate morning (0700–1000h) and evening (1800–2100h) measurements after 5min sitting at rest and with 1min between recordings. Further to the device memory, a form was supplied to the participants to report all their HBP values and also the time they went to bed and arose during ABP monitoring. Clinic BP was measured in two visits in the beginning and the end of the study by three physicians who fulfilled the British Hypertension Society Protocol criteria for agreement among observers in BP measurement.17 Triplicate BP measurements were performed during each visit after 5min sitting at rest and with at least 1min between recordings using the same device as for ABP/HBP monitoring.

Analysis

For the validation study analysis, each pair of observer measurements was averaged and was then subtracted from the device measurement. The absolute differences between BP2-BP1, BP2-BP3, BP4-BP3, BP4-BP5, BP6-BP5 and BP6-BP7 were calculated and paired according to the device reading. For each pair, the one with the smaller difference was used in the analysis. These BP differences were classified into three zones (within 5, 10 and 15mmHg), separately for systolic and diastolic BP, for 15 subjects in phase 1 and for all the 33 in phase 2.1. For each participant, the number of readings with a difference within 5mmHg was also calculated (phase 2.2).

For the application study, average HBP was calculated after excluding measurements of the first day, as recommended by current guidelines,2, 3 and average awake ABP using the sleeping times reported by each individual. Office BP measurements of two visits were also averaged for each individual. Student's paired t-tests with Bonferronis's correction for multiple comparisons were used to compare average office, home and awake ambulatory measurements of BP and pulse rate in the same subjects. Standard deviation (s.d.) of mean BP values was compared using F-tests. The variation among repeated BP measurements was quantified using the s.d. of measurements of each subject. The within-subjects s.d. was estimated by one-way analysis of variance for HBP and awake ABP.18 The BP variation is also reported as within-subjects coefficient of variation (within-subjects s.d./mean BP/100%). The range of BP values (highest–lowest BP reading) obtained by HBP and awake ABP monitoring was also compared. The Bland–Altman approach was used to investigate the degree of similarity between awake ABP and HBP. Statistical analysis was performed using the MINITAB INC Statistical Software (release 13.31) (Stage College, Pennsylvania, PA, USA).

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Results

Validation study

Forty-two subjects were recruited from an outpatients BP clinic and from patients and staff of a University Department of Medicine. One subject was excluded because of arrhythmia, 2 because their BP was out of range and 6 because their BP was within the ranges already completed. In 9 readings there was a difference between the observers’ measurements >4mmHg and were repeated to reach closer agreement.

The first 15 participants who fulfilled the protocol criteria regarding sex and entry BP were included in the analysis of phase 1 (8 men, mean age 53.4±12.3years [range 33–75], arm circumference 31.6±3.5cm [25.5–38], entry systolic BP 142.9±24.5mmHg [108–180] and diastolic 89.5±13.7mmHg [67–113]). Analysis of phase 2 was based on the first 33 participants who fulfilled the criteria regarding sex and entry BP (21 men, mean age 53.4±12.1 years [range 33–79], arm circumference 30.8±3.4cm [25–38], entry systolic BP 145.3±23.6mmHg [106–180] and diastolic 88.5±15.3mmHg [63–117]). In 14 subjects, the medium size cuff was used, in 19 the large cuff and the small cuff in none.

The use of the tested device was straightforward with no operational problems during the study. There was one failure of the device to record BP throughout the study, which was successful on repeated measurement. The results of the validation analysis are presented in Table 1. The BP differences between the tested device and the observer readings are presented in Figure 1. The tested device passed all the validation criteria of phases 1 and 2.1 of the European Society of Hypertension International Protocol (Table 1). The first criterion of phase 2.2 was marginally fulfilled for diastolic BP, whereas the second one was comfortably fulfilled for both systolic and diastolic BP.

Figure 1.
Figure 1 - Unfortunately we are unable to provide accessible alternative text for this. If you require assistance to access this image, please contact help@nature.com or the author

Scatterplots presenting differences in BP between the tested device and the observer readings (99 readings). Recruitment limits regarding entry BP ranges (low, medium and high) are indicated by the vertical lines (BP, blood pressure).

Full figure and legend (108K)


Application study

A total of 49 subjects were recruited and four were excluded because of incomplete HBP or ABP data. Finally, 45 subjects were included in the analysis. Mean age was 56±11.6 years, 29 were men (64%) and 32 (71%) were on antihypertensive drug treatment. Twenty participants (44%) performed HBP monitoring first and then self-started ABP monitoring, whereas the rest 25 (56%) had the ABP monitor fitted by the physician in the office and started HBP monitoring on the next day.

Average office BP was higher than average HBP or awake ABP (systolic and diastolic; Table 2). There was a tendency for pulse rate to be higher in the office than at home or during awake ABP monitoring, which did not reach statistical significance (Table 2). The mean difference between home and awake ambulatory measurements was 0.5±7.7mmHg for systolic BP (95% confidence intervals (CI) −1.8, 2.8, P=NS), 0.6±5.4mmHg for diastolic BP (95% CI −1.1, 2.2, P=NS) and –0.6±5.3 beats per minute for pulse rate (95% CI −2.2, 1.0, P=NS). The differences between HBP and awake ABP are presented in Figure 2. The home–awake ABP difference did not differ in subjects who performed HBP monitoring first and ABP monitoring second (0.10/0.91mmHg, systolic/diastolic) compared with those who did the reverse (1.26/0.31mmHg).

Figure 2.
Figure 2 - Unfortunately we are unable to provide accessible alternative text for this. If you require assistance to access this image, please contact help@nature.com or the author

Scatterplots presenting differences between home and awake ABP measurements obtained using the same device. Horizontal lines indicate mean differences between measurements and limits of agreement (±2s.d.) within which 95% of the differences are expected to lie.

Full figure and legend (74K)


Strong correlations were found among office BP, HBP and awake ABP measurements (P<0.001 for all r values). Although the correlation coefficients between systolic HBP and awake ABP (r=0.77/0.81, systolic/diastolic) were higher than those between office BP and awake ABP (r=0.59/0.80) or HBP (r=0.67/0.83), these differences did not reach statistical significance.

The first clinic BP measurement was by 3.3/2.1mmHg (systolic/diastolic) higher than the first HBP measurement, whereas the first ABP measurement was by 5.6/3.6mmHg higher than the first HBP measurement. None of these differences reached statistical significance, probably because of the small number of subjects analysed (25 subjects started ABP monitoring in the clinic). The first office BP measurement was strongly correlated with the first HBP (r=0.68/0.81) and the first awake ABP measurement (r=0.54/0.62; P<0.01 for all r values).

The s.d. of the mean HBP (10.6/7.8mmHg, systolic/diastolic) tended to be lower than that of the mean awake ABP (11.9/9.1mmHg); yet these differences did not reach statistical significance. The within-subjects s.d. was lower for HBP (systolic/diastolic, 9.3/5.4mmHg) compared with that of awake ABP (13.7/10.8mmHg) (P<0.05/<0.001). The within-subjects coefficient of variation was 7.0%/6.7% (systolic/diastolic) for HBP and 10.5%/13.4% for awake ABP. The average lowest and highest BP reading (systolic/diastolic) was 113.4–150.8/71.6–92.3mmHg for HBP and 103.2–165.7/59.9–109.3mmHg for awake ABP. The BP range for HBP measurements (37.4±12.9/20.8±8.1mmHg, systolic/diastolic) was significantly lower than that of awake ABP (62.5±16.6/49.4±18.9mmHg) (mean difference 25.1±20.5mmHg/28.6 ±19.7, P<0.001 for both).

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Discussion

This is the first study allowing the true difference between HBP and ABP measurements to be shown by using the same device for both methods. A formal validation study of a novel device that allows both HBP and ABP monitoring has also been performed. The main findings are that (i) there is no clinically important difference between average home and awake ABP and pulse rate as suggested by 95% CIs and (ii) the dual HBP and ABP monitor fulfils the validation requirements of the European Society of Hypertension International Protocol.

Multiple studies have compared ABP with HBP measurements obtained in the same patients. Some studies found no difference between HBP and daytime ABP,4, 5, 6, 7 whereas others showed the HBP values to be higher8 or lower,9, 10, 11 particularly in children and adolescents.12, 13, 14 Differences in BP levels between the two methods in these studies are attributed to intrinsic differences between the two methods, but also, at least in part, to the different devices used for each method (ambulatory and home monitoring). Interestingly, despite the relatively small sample size of this study, the 95% CIs excluded any difference between HBP and awake ABP measurements larger than 2.8mmHg for systolic BP or 2.2mmHg for diastolic BP, and a difference in pulse rate larger than 2.2 beats per minute. The variability of HBP assessed by different approaches (s.d. of mean BP, s.d. and coefficient of variation of repeated BP readings of individual subjects and range of BP values) was consistently lower than that of awake ABP. This is probably because of the fact that HBP measurements are taken under more standardized conditions of activity and environment (after a few minutes sitting rest and only at home) than ABP measurements (ambulatory conditions, at work, at home and elsewhere). The similarity in HBP and awake ABP levels found in this study justifies the European Society of Hypertension recommendation for using the same diagnostic threshold for both of these methods.1, 2, 19

HBP and awake ABP have important similarities because they both provide multiple BP measurements obtained away from the clinic or office setting and in the usual environment of each individual. On the other hand, they have important differences because home measurements are taken only in the sitting posture and at home, whereas ambulatory measurements are taken in ambulatory conditions, at work, at home and during other usual activities. Furthermore, ABP is monitored for 1 to maximum 2 days, whereas HBP is usually monitored for multiple days. Thus, ABP monitoring seems to be more appropriate for the initial assessment of subjects with elevated BP, whereas HBP is regarded as the optimal method2, 3 for the long-term follow-up of treated hypertension. In other words, these methods seem to be more complementary than competitive and, despite their similar BP levels shown in this study, provide similar, but different, information about the BP behaviour of an individual.

The validation study of the device used for both HBP and ABP monitoring in this study passed the criteria of the International Protocol for systolic and diastolic BP (Table 1). All the criteria were comfortably fulfilled apart from one of the two criteria of phase 2.2 that was marginally fulfilled for diastolic BP. The AAMI criterion of mean difference <5±8 (s.d.)mmHg was also fulfilled for both systolic and diastolic BP. Therefore, the device can be recommended for clinical use in the adults.

This dual HBP and ABP monitor is a challenging tool for complete out-of-office BP assessment. Owing to its relatively low cost compared with currently available ABP monitors, this device has the potential to facilitate the widespread application of both HBP and ABP monitoring in the management of hypertension in general practice.

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References

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Acknowledgements

GS is a consultant to Microlife for the design of blood pressure monitors. The validation study was funded by a grant from Microlife, Widnau, Switzerland and the application study by the Hypertension Center, Third Department of Medicine, University of Athens, Greece.

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