Introduction

Hypertension is a syndrome associated with target organ damage and graded risk for developing cardiovascular complications. Sustained high blood pressure is both the cardinal feature of the syndrome and the operational definition for diagnosis of hypertension. The hypertensive syndrome, nevertheless, is more than just high blood pressure. The conventional blood pressure measurement cut-offs used to define hypertension remain somewhat arbitrary and interventions that lower blood pressure to the same degree do not confer equivalent reduction of cardiovascular morbidity and mortality.

The use of 24 h ambulatory blood pressure monitoring technique identifies different blood pressure patterns in normotensive and hypertensive individuals. By comparing data from ABP monitoring and OBP measurement, four major phenotypes of hypertension emerge: sustained normotension, white-coat hypertension, masked hypertension, and sustained hypertension [1]. By measuring the blood pressure during sleep, additional phenotypes of hypertension based on absence, exaggeration, and reversal of physiological nocturnal lowering of blood pressure (“dipping”) are also identified. These phenotypes are termed nondippers, extreme dippers, and reverse dippers, respectively.

What Are the Different Blood Pressure Measurement Techniques and Their Reliability?

Three commonly used outpatient BP measurement methods are office/clinic blood pressure readings (office blood pressure measurement, OBP), 24-h ambulatory blood pressure monitoring (ambulatory blood pressure measurement, ABP), and self-measurement at home (home blood pressure, HBP).

Most physicians continue to rely on OBP for diagnosis of hypertension and guidance of therapy. There is great variation in quality and accuracy of OBP. OBP can be imprecise and inconsistent due to poor measurement techniques such as the use of uncalibrated sphygmomanometers, incorrect cuff size, digit preference, talking during measurement, and anxiety [2]. OBP overestimates true BP readings for individuals with white coat hypertension while underestimating true BP readings for individuals with masked hypertension [3, 4].

ABP is lower than OBP in most hypertensive patients [5]. The diagnosis of white coat hypertension is made when an individual’s OBP is above the normal range and ABP is within the normal range. The diagnosis of masked hypertension is made when an individual’s OBP is within normal range and ABP records high BP readings. Many cross-sectional studies have related the extent of cardiovascular damage to both OBP and ABP, and most of them have shown that the correlation is higher for ABP [6]. As such, ABP is a better predictor of cardiovascular risk than OBP. However, it has not been widely adopted because of its cost, inconvenience, proficiency gap, and limited reimbursement by third party payors.

HBP produces lower BP readings and correlates better with target organ damage and cardiovascular mortality than OBP [7]. Like ABP, HBP eliminates both the white coat effect and white coat hypertension, and identifies masked hypertension. HBP has been more widely adopted over the past two decades, especially more recently in the context of pandemic-related growth of telehealth for chronic disease management [8•]. HBP allows for a larger number of measurements to be gathered over a longer period of time compared to OBP and ABP. Other advantages of HBP are its ease of use, accessibility (without requiring a doctor’s office visit), affordability, and active participation of patients in their own diagnosis and treatment [9].

Clinical practice guidelines recommend using ABP and/or HBP for diagnosis of hypertension, detection of white coat hypertension, and screening of masked hypertension [10••, 11•, 12•, 13].

Relationship Between Different Blood Pressure Measurements and Hypertension-Related Cardiovascular Morbidity and Mortality

Several studies have assessed the relationship between different BP measurement methods and cardiovascular morbidity and mortality. ABP and HBP characterize BP phenotypes that would not have been identified otherwise with OBP measurement alone [14••]. Findings from key studies are summarized below.

A systematic review of 23 studies found that HBP and ABP are better in predicting target organ damage assessed by the left ventricular mass index (LVMI) than OBP [15]. Another systematic review of 8 prospective longitudinal studies found that increasing systolic HBP was associated with increasing risk of all-cause mortality (hazard ratio [HR] 1.14, 95% CI 1.01–1.29 per 10 mm Hg increase), cardiovascular mortality (HR 1.29, 95% CI 1.02–1.64 per 10 mm Hg increase), and cardiovascular events (HR 1.14, 95% CI 1.09–1.2 per 10 mm Hg increase) [16]. A pooled analysis of 5 population-based cohort studies found that a 10 mm Hg increase in systolic HBP was associated with an increased risk of cardiovascular events in individuals with normal, optimal, or high normal HBP and mild hypertension [17]. This study also found that masked hypertension occurred in 54% of individuals with normal, optimal, or high normal OBP, and that masked hypertension was associated with increased risk of cardiovascular events (HR 2.29, 95% CI 1.52–3.45).

Continuous HBP parameters have been associated with significantly higher risks for total cardiovascular events, major adverse cardiovascular events, and both all-cause and cardiovascular mortality with hazard ratios varying from 1.7 to 2.1 after adjustments for OBP and ABP. Uncontrolled total mean morning HBP, but not evening HBP, has been associated with significantly higher risks of all outcomes [18••]. An elevated systolic ABP is associated with an increased risk of cardiovascular events and mortality, independent of OBP (HR 1.27, 95% CI 1.18–1.38 for a combined cardiovascular endpoint) [19]. ABP can predict long-term cardiovascular outcomes independently of OBP [20].

HBP is independently related to LVMI and predicts cardiovascular end organ damage better than OBP and ABP [21]. ABP, systolic HBP, and diastolic HBP are more reliable than OBP. A difference of 10 mm Hg higher systolic HBP and diastolic HBP are associated with 5 times and 4 times higher LVMI, respectively [22••]. A prospective 5-year follow-up study of nearly 300 treated hypertensives showed that LVMI positively correlated with both HBP and OBP, and the correlation tended to be stronger with HBP than OBP [23]. A small prospective observational study showed that HBP correlated significantly, and more tightly, with LVMI, albumin excretion rate, and global target organ damage than with OBP [24].

Analysis of the data from an observational study of more than 4,000 participants in Japan suggested that measuring the systolic blood pressure twice a day, in the morning and in the evening, at home, and calculating the difference between the two systolic blood pressures would be useful for risk stratification of cardiovascular disease outcomes since a higher morning-evening difference was associated with higher cardiovascular risks as compared to a medium morning-evening difference, independent of the average morning and evening systolic blood pressures [25••].

In patients with treated hypertension, a higher systolic ABP or diastolic ABP predicts cardiovascular events after adjustment for other cardiovascular risk factors, including OBP [26]. In a population-based cohort study of more than 10,000 participants, higher 24-h and nighttime blood pressure readings measured by ABP were significantly associated with greater risks of death and a composite cardiovascular outcome (HR 1.23, 95% CI 1.17–1.28 for total mortality; HR 1.36, 95% CI 1.30–1.43 for composite cardiovascular outcome) [27••].

Night Time Blood Pressure Measurement

At night, lack of pressor effect of physical activity, emotional stress, and other environmental factors that would occur during the day renders nocturnal blood pressure more reproducible and representative of one’s actual BP levels and target organ damage. Nocturnal blood pressure levels can better predict cardiovascular events, cardiovascular mortality, and overall mortality [28]. A morning blood pressure surge has been considered a predictor of all cardiovascular, cardiac, coronary, and cerebrovascular events [29].

Blood pressure normally falls 10–20% of daytime values during sleep, a physiological phenomenon termed dipping. Individuals with less than 10% fall in blood pressure at night are considered non-dippers. Reverse dipping occurs when nocturnal blood pressure increases instead of a physiological drop. Isolated nocturnal hypertension is defined as an elevated nighttime blood pressure with normal daytime blood pressure. ABP-derived non-dipping pattern is more closely associated with target organ damage and worsened cardiovascular outcomes than OBP [30].

BP variability (BPV) occurs when impaired sympathetic drive and baroreceptor dysfunction lead to fluctuations in BP over short term, mid term, or long term. Overall, BPV is typically assessed through standard deviation of average 24 h, daytime, or nighttime BP readings. BPV symbolizes an independent risk factor for cardiovascular diseases and mortality [31]. BPV is increased in diabetic hypertensives, rendering them at an increased risk for cardiovascular disease and mortality [32]. OBP is not a good method to assess BPV. It would be impractical to repeat ABP frequently, and therefore, HBP is the most appropriate method for assessment of long-term BPV [33].

Diabetics are likely to benefit from ABP and/or HBP because of the high prevalence of non-dipping, reverse dipping, nocturnal systolic hypertension, and BPV among them [34•]. ABP is a better predictor of cardiovascular events than OBP in diabetics since diabetics are more likely to be nondippers than nondiabetics; therefore, OBP would not reflect the true cardiovascular risk [35].

Discussion

ABP and HBP provide additional information that cannot be ascertained from OBP. This additional information can be useful for diagnosis, monitoring of therapy and prognosis in hypertensives. HBP is a reliable method of measurement and less costly than ABP. A validated device and standardized procedures are essential for HBP, and good patient information and training are warranted. Although HBP can conventionally be used for measurement of daytime blood pressure only, newer semi-automatic HBP devices can additionally measure nighttime BP at fixed intervals during sleep providing information related to nocturnal dipping and early morning surge, which are important prognosticators.

Out of office blood pressure measurement methods, both HBP and ABP provide useful supplemental information for diagnosis, monitoring of therapy and prognosis of hypertensive patients. Since HBP and ABP are generally lower than OBP, they can confirm the diagnosis of hypertension for individuals with high OBP by ruling out white coat hypertension. HBP and ABP can also establish the diagnosis of masked hypertension for those with normal OBP.

HBP with or without additional supportive measures lowers blood pressure compared with usual care involving only OBP, but it is unclear whether this beneficial effect would persist beyond one year [36]. A recent pooled analysis of individual patient data from four randomized controlled trials showed that one year of HBP increased the likelihood of intensification of antihypertensive drug therapy and improved blood pressure control at 5 years [37••]. Prognostic value of HBP is better than OBP and at least equal to ABP [38]. It is an important predictor of target organ damage, and cardiovascular and cerebrovascular morbidity and mortality.

Some hesitation exists with utilizing HBP in the diagnosis of hypertension in real life settings. Barriers from the patients’ perspectives include incorporating scheduled HBP to daily routine in personal life, cost of purchasing a device, and the anxiety of excessive monitoring associated with having a device at home [22••]. The national analysis of HBP coverage and reimbursement policies within selected public and private payors/insurers across the USA revealed that there are currently no national coverage determinations or local coverage determinations that address blood pressure devices for HBP [39•]. Such lack of coverage causes the blood pressure devices to be an out-of-pocket expense to the patient [40]. Lack of patients’ adherence to the monitoring schedule, poor technique such as using the wrong sized cuff or poor placement, the use of inaccurate devices, and reporting bias can impose major challenges for physicians when relying on HBP readings to decide whether or not to treat the patient or modify therapy [9, 22••].

A study focused on reliability of HBP values found that patients were unaware of their device’s capability for storage of readings. The concordance rate between patient-reported values and those obtained by the monitor was 78%, and the BP values reported were significantly lower and less variable than those in the devices’ stored memory. The same study also found a higher discrepancy between stored and patient-reported values in type 2 diabetics with poor glycemic control [41].

Patient education on proper HBP measurement technique and the use of validated devices and cuffs are essential for obtaining accurate, reliable blood pressure readings. Patients should bring their monitors to the medical offices for training on proper measurement technique, and be provided written instructions regarding appropriately sized cuff placement in the proper location and alignment on the upper arm, sitting in the upright position with their feet flat on the ground and their arm supported at the level of the heart and avoid talking for the duration of the measurement [10••]. The UK’s NICE clinical practice guidelines recommend that self-measurement of blood pressure should be done in the morning and evening, in two consecutive measurements one minute apart, for a total of 4 daily measurements. HBP should be measured for a minimum of 4 consecutive days; however, 7 days are ideal [42•]. The blood pressure recordings should be reviewed by the physician during the office visit for treatment recommendations. Misreporting can be minimized by crosschecking the patient’s records against the automated memory stored in the blood pressure device.

Conclusion

HBP and ABP provide useful information for diagnosis, treatment, and prognosis of hypertension that cannot be readily available from OBP alone. The existing body of evidence shows that HBP and/or ABP are essential for diagnosis and risk stratification in patients with hypertension. A recent study showing HBP is more reliable than OBP or ABP and correlates better with LVMI justifies its use for the diagnosis of hypertension and cardiovascular risk stratification [22••]. Additional comparative studies between ABP and HBP are still necessary since most studies compared between HBP and OBP or ABP and OBP but rarely compared between HBP and ABP or among all three methods. Intervention trials to determine if there is any difference in cardiovascular outcomes in hypertensive patients managed based on out-of-office BP readings, such as ABP and/or HBP, compared to those managed based on conventional monitoring of OBP alone are warranted. If the value of HBP has been firmly established, further studies to determine the optimum frequency of monitoring will be crucial.