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Saudi Journal of Kidney Diseases and Transplantation
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ORIGINAL ARTICLE Table of Contents   
Year : 2006  |  Volume : 17  |  Issue : 3  |  Page : 326-337
High Prevalence of Masked Hypertension in Treated Hypertensive Patients with Type 2 Diabetes Mellitus

1 Department of Medicine, King Saud University, Riyadh, Saudi Arabia
2 Samuel Lunenfeld Research Institute at Mount Sinai Hospital, Toronto, Ontario, Canada
3 Medical Student, University of Western, Toronto, Ontario, Canada
4 Department of Medicine, University of Toronto, Toronto, Ontario, Canada

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This study was undertaken to determine whether self-measured home blood pressure (BP) readings were comparable to clinic visit BP readings in hypertensive type II diabetic patients. We measured the BP of 27 hypertensive patients at home and during the clinic visits over a three week period. The BP readings were analyzed using a mixed linear model with mean daytime ambulatory measure as a covariate. We found that, although there was no significant difference in the mean systolic BP between home and clinic readings (0.6 mm Hg), the mean home BP readings were significantly higher (difference = 6.8 mm p<0.0006). The proportion of masked hypertension, defined as elevated home systolic or diastolic BP (or both) values despite normal clinic visit BP values, was 40.7%. Three diastolic and one systolic BP measurement at home achieved a reliability coefficient of 0.8. Self-measurement of BP gave highly reliable readings when they were compared with blind readings taken by a trained professional using a mercury sphygmomanometer. We conclude that self-measured BP at home identifies a high prevalence of masked hypertension in treated hypertensive type 2 diabetic patients and that it represents a valuable management adjunct to ensure maximum benefit from antihypertensive drug therapy.

Keywords: Masked hypertension, Diabetes, Self-measured blood pressure.

How to cite this article:
Alsuwaida A, Parkes R, So J, Feig D, Logan A. High Prevalence of Masked Hypertension in Treated Hypertensive Patients with Type 2 Diabetes Mellitus. Saudi J Kidney Dis Transpl 2006;17:326-37

How to cite this URL:
Alsuwaida A, Parkes R, So J, Feig D, Logan A. High Prevalence of Masked Hypertension in Treated Hypertensive Patients with Type 2 Diabetes Mellitus. Saudi J Kidney Dis Transpl [serial online] 2006 [cited 2022 Jun 27];17:326-37. Available from: https://www.sjkdt.org/text.asp?2006/17/3/326/35764

   Introduction Top

Hypertension and diabetes frequently coexist. [1],[2] Among type II diabetics, 40 to 50% have hypertension, compared to 20% of matched non-diabetic patients. Hypertensive patients have a 2-fold excess risk of developing type II diabetes within their lifetimes. The coexi­stence of diabetes mellitus and hypertension has devastating consequences for the cardio­vascular system. The presence of hypertension in diabetic patients dramatically increases the rate of complications. In the United Kingdom Prospective Diabetes Study (UKPDS), for example, the incidence of any aggregate end­point related to diabetes was strongly and linearly associated with systolic BP; over the range of systolic BP from less than 120 mmHg to greater than 160 mmHg, the likeli­hood of a cardiovascular event increased two fold. [3] Additionally, the absolute event rate in diabetic patients is substantially higher than the rate in the non-diabetic patients at the same pressure level.

Physicians typically use office-based BP readings to make treatment decisions in hypertension. Validation is based on the use of this setting in the clinical trials to assess the benefits of antihypertensive therapy. Clinic visit readings are used as a surrogate measurement of the 'true' BP, which is thought to represent the average value over a prolonged period of time. 'True' BP can be more closely estimated from home BP readings or by ambulatory monitoring because more readings are recorded. Owing to habituation and regression to the mean, BP generally falls with repeated measurement. Thus, the use of a single measurement to define a patient's BP would overdiagnose hypertension in 20-30% of the population and miss a third of those who are truly hypertensive. [4],[5]

Cross-sectional studies have shown that home BP is indicative of hypertensive target organ damage. [6],[7],[8] Data also suggest that home pressures correlate more closely with cardiovascular mortality than do pressures found in the clinic. [9] The reduction in home BP during treatment also predicts the develop­ment of left ventricular hypertrophy. In the Tecumseh study, home BP was more predictive than clinic visit BP of hypertension and normotension after three years in untreated borderline hypertensives. [10] In the Ohasama study, home BP had a stronger predictive value than clinic visit BP for cardiovascular and overall mortality in the general popu­lation. [9],[11] Furthermore, home BP predicted first time stroke better than clinic visit BP. [12]

It has been demonstrated that home BP is consistently lower than clinic visit BP. [13],[14],[15],[16],[17],[18],[19] Since the latter is the basis of the Canadian guidelines for treatment of hypertension, the use of home readings may lead to undertreat­ment and may have serious prognostic impli­cations if the same target values of 130/80 are used for clinic visit and home BP. There­fore, a BP goal of 125/75 mm Hg is viewed as a more realistic threshold for self-monitoring of diabetic hypertensive subjects. Its validity, however, remains to be established by prospective studies.

Patients' self-measured BP levels can be used to supplement those measured by the doctor in his office. They may help doctors to have better informed therapeutic decisions. Diabetic patients tend to have greater BP variability. Therefore, it is important to address the concordance between home and office BP readings.

The objective of our study was to compare measurement of BP at home and office in patients with hypertension and type (II) diabetes mellitus to determine whether home readings can be used by health care providers to make treatment decisions.

   Patients and Methods Top

Hypertensive patients with type II diabetes who attended a tertiary care hypertension clinic at Mount Sinai Hospital were eligible to enter the study. Patients were at least 30 years old at onset of diabetes and had no previous history of diabetic ketoacidosis.

Patients with Type I diabetes, non-diabetic renal disease, secondary hypertension, self­identified substance abuse, atrial fibrillation and pregnant women were excluded from the study. The exclusion was based on medical history and chart review. During the study period no changes were made to the patients' antihypertensive medications. The study was in compliance with the Second Declaration of Helsinki and approved by ethics committee at Mount Sinai Hospital. Letters were sent to potential candidates, and were followed up by phone calls to discuss the details of the study.

Previous clinical trials showed that the correlation between office and ambulatory blood pressure monitoring (ABPM) was fairly strong for both systolic and diastolic pressure, ranging from 0.73 to 0.85. [20],[21],[22] Our study was designed to have 80% power to detect moderate correlation of 0.6 between office and home BP readings with 5% risk of type I error. Based on these specifications it was necessary to recruit 20 study subjects.

The study patients were taught to properly take BP readings using a semi automated machine and instructed to take two BP readings at home per day, five days a week for two weeks. The clinic visit BP readings were recorded at the beginning, after one week and at the end of the two week period. Daytime ABPM was performed once for each patient during the study. The patients were instructed not to change their medication prescription during the study.

Clinic visits' BP measurements were performed according to the 2001 Canadian guidelines using a mercury sphygmomanometer and stethoscope. [23],[24] Arterial BP (Korotkoff phases I and V) was measured after at least 5 minutes of rest in a quiet, calm environment using an appropriately sized cuff, with the patient sitting and the arm supported. Two measure­ments to the nearest 2 mm Hg were obtained two minutes apart and the average of the two readings was calculated. The clinic visits' BP measurements were performed by two examiners blinded to the home BP readings.

To validate the Omron BP device for home BP measurements, the readings were compared to those recorded using a mercury sphygmo­manometer and SpaceLab 90207 device. Fifteen patients were tested in 26 different settings. We compared the readings of the Omron device with an ambulatory BP monitor; four BP readings were obtained simultaneously in both arms over a 10 minute period. To test the accuracy of the Omron device against the sphygmomanometer, four consecutive readings over 2 minutes were performed with each device on the same arm. The arm used for the Omron device and the order of the device used were randomly determined. The Omron device was accurate to ± 2 mmHg for both systolic and diastolic pressures when compared to SpaceLab device and the sphygmomanometer. The study patients were lent Omron HEM-757 BP measuring devices and appropriately sized cuffs. Each patient attended a 20 minute teaching session that included explanations concerning the basic principles of BP self-measurement, the meaning of BP values, and the use of the monitoring device. Each patient measured his or her BP in the presence of the instructor, who verified in each case the correct use and functioning of the device.

The patients were instructed to record their own BP in the morning before they had taken any antihypertensive medication, and in the afternoon. This routine was conducted at least 5 days per week for 2 weeks. The patients were also instructed to measure the BP routinely at the same time of day, at least 30 minutes after any cigarette smoking, coffee drinking or stair climbing. In addition, the patients were instructed to rest comfortably in the seated position with their back supported, legs uncrossed, and the arm uncovered and supported/positioned at heart level while measuring the BP. The BP readings stored in the device memory were reviewed during the clinic visit.

Finally, the Space Lab 90207 BP monitor was used for the ABPM. This device recorded BP every 20 minutes during the day (6:00 P.M. to midnight) and every 30 minutes at night (midnight to 6:00 A.M.). The patients were asked to register the time they fell asleep and the time they woke up in the morning. The time periods was adjusted for shift workers. Data were expressed as 24-h, daytime, and nighttime mean BP values.

   Statistical Analysis Top

All data were entered into the Access database system and manually checked for errors. Statistical analyses were conducted using SAS software. [25] For all outcomes, a two-sided P value of 0.05 or less indicated statistical significance. We assessed for differ­rences in BP readings using a mixed linear model statistical procedure. Two variables were created to reflect the impact of Day and Time on BP measurements. [26] Day was the number of days after the first measurement for both home and clinic visits' BP readings. Time was considered as a dichotomous variable (A.M., P.M.) and was used to assess its effect on the variability of home BP readings. To minimize the effect of a systematic difference between the clinic visits' and home BP readings, the mean daytime ambulatory BP reading was used as a covariate to calibrate the data for the two methods of BP measurement. The ABPM was the gold standard for determining the 'true' BP of the patients.

The mixed model took into account the repeated measures aspect of the data and the use of individual readings for the clinic visits' measurements and the daily summary values for the home readings.

The final model used for the analysis of method effect contained three variables included in both Model and Random statement (method of measurement, Day and ABPM). For the categorical analysis of the BP status, a threshold value of 130/80 mm Hg for the clinic visit BP and 125/75 mm Hg for home BP were used to indicate hypertension. The number of measurements needed to create an adequate correlation coefficient for home BP was calculated by using the generalizability coefficient. [27]

   Results Top

Of 32 screened patients for the study, 27 patients were accepted. The reasons for participation refusal were: plans to travel (one patient); depression due to recent cancer diagnosis (one patient) and no specific reason (three patients).

[Table - 1] presents baseline characteristics for the 27 participants in the study. The mean age was 60 years; 8 (52%) were women, 25 (92.5%) never smoked or had quit smoking. The median number of antihypertensive medi­cations was 3 drugs. The patients' documentation of the home BP readings was not different from the stored data in the BP device. The study patients were compliant with home measurements and 95% completed data on self-recorded readings were obtained.

Throughout the study, both clinic visits' and home BP measurements showed no signi­ficant variability. [Table - 2] and [Figure - 1] show that the mean systolic BP at home was 0.6 mm Hg lower than that at the clinic visit, although the difference was not significant (F=0.03, P=0.86). On the other hand, [Table - 2] and [Figure - 2] show that the mean home diastolic BP was 6.5 mm Hg higher, and the difference was highly significant (F=13.7, P< 0.0006). The residual of the model for both the home and clinic visits' readings were approximately normally distributed. The mean daytime ambulatory systolic BP was 137.2 mm Hg (95% confidence interval, 134.8 to 138.9) and the diastolic BP was 76.1 mm Hg (95% confidence interval, 74.8 to 77.3 mm Hg).

The reliability coefficient is the proportion of true variability in the BP to the total obtained variability. A target reliability of 0.8 is often suggested as adequate, [28] which means that 80% of the variability in obtained BP could represent true individual differences and 20% of the variability is usually due to random error. [Table - 3] shows that three diastolic and single home BP readings were needed to achieve the sufficient reliability of 0.8.

The model used for analysis of day and time effect included three variables in fixed effect (day, time and method) and the random statement contained subject, day and time. The average time for the morning reading was 07:00 A.M. and for the afternoon readings it was 05:00 P.M. [Table - 4] shows that both day and time (morning vs. afternoon) BP readings had a minor contribution to the home BP variance component. Both Day and Time had a minor contribution to total variance. This indicates that there was no systematic diffe­rence in home BP values on different days or between morning and afternoon measurements. The residual variance for systolic BP was greater in the clinic visits' measurements, indicating that we needed to measure BP during the clinic visits more often than at home in order to obtain the reliability coefficients.

[Figure - 3] shows the classification of the study patients into one of the four systolic BP categories. The vertical line represents the threshold value for hypertension for the systolic BP (OSBP) during the clinic visits, and the horizontal line the threshold value for the systolic BP (HSBP) at home. There was a high degree of concordance between home and clinic visits' readings of systolic BP, with 81.5% having normal or high readings in both settings. Only one (3.7%) patient had the 'white-coat' effect, defined as high office and normal home systolic BP readings (WCH).

On the other hand, four (11.1%) patients had the 'reverse white-coat' effect, defined as normal clinic visits' and high home systolic BP readings. For diastolic BP, the degree of concordance between home and office BP readings was only 63.0%. [Figure - 4] shows that 10 (37.0%) discordant patients had the 'reverse white-coat' effect.

The most common adverse reactions observed were inconvenience and arm soreness resulting from the multiple cuff inflations with the ambulatory devices. For self-measurement devices, increased anxiety was detected in one patient with poorly controlled BP; home BP monitoring was stopped and appropriate adjustment in the treatment regimen was done.

   Discussion Top

The aim of the present study is to assess the degree of concordance between home and clinic visits' BP measurements in hyper­tensive type 2 diabetic patients. Most of the studies that have investigated this relation­ship had recruited patients with essential hypertension. Extrapolating the results of these studies to other groups of hypertensive patients may not always be valid. For example, diabetic patients have increased BP variability compared to non-diabetic patients. [29] Puig et al. reported dissociation between clinic visits' BP measurements and ABPM in patients with type 2 diabetes. [30] In our study, home diastolic BP was 6.5 mm Hg higher than measured during clinic visits. In addition, almost 40% of study subjects had an elevated mean diastolic BP with normal values during the clinic visits.

The difference in BP reading between home and clinic might be explained by random or systematic error. BP has considerable bio­logical variation. There are also numerous studies showing a systematic difference bet­ween average BP levels determined during a clinic visit versus at home. [31],[32],[33],[34],[35],[36],[37] In our study, we used a fixed-effects mixed-linear method for analysis that included the average daytime ambulatory BP as a covariate. This allowed us to estimate each component of variation separately; most previous studies determined correlation coefficients that measured asso­ciation rather than agreement. More recently, the Bland and Altman method has been used for assessing agreement between two methods of clinical measurement. [38] While this method provides a better estimate of agreement, it does not permit adjustments for important covariates or the incorporation of multiple measurements of a variable in the analysis.

Several factors including the technique of the measurer, the reproducibility and accuracy of the equipment, the characteristics of the study population, and the effects of medi­cation that should be considered as possible explanations for the significantly higher home diastolic BP detected in this study. It is unlikely that poor technique or faulty equipment accounted for this finding, since all patients were taught the correct procedure for measurement and were subsequently directly observed for errors as they measured their own BP. In addition, the Omron device is designed for self-measurement of BP and has been validated in the laboratory using the protocol of the British Hypertension Society and the criteria of the American Association for the Advancement of Medical Instrumentation. On the other hand, it is possible that patients' selection influenced the results. Our patients were recruited from a tertiary hypertension clinic in an academic center and had more severe or complicated forms of hypertension. They were used an average of three different types of antihyper­tensive medications daily and few demonstrated the 'white-coat' effect. Finally, the difference in the time that medications were administered in relation to the measurement of BP might account for the large number of patients with high home and normal clinic visits' BP readings. The patients were instructed to measure their BP in the morning, before taking their medi­cations, as well as in the afternoon; whereas all patients had taken their medications by the time of the clinic visit. It is also possible that the high percentage of patients with 'home hypertension' may indicate an alerting reaction that is analogous to the well-described 'white­ coat response'. [39]

Increasingly, hypertensive patients purchase home BP devices to monitor their BP response to antihypertensive drug therapy. [22] There are now many validated BP devices available at reasonable prices. Self-measurement of BP is easily taught and learned. In our study, approximately 20 minutes were required to teach and test the patients on the correct procedure to measure BP. The mean age for our study patients was 60 years, indicating that age is not a limiting factor in using home monitoring. Mengden and colleagues observed that, on average, 36% of the readings reported by patients were erroneous. [40] We, however, found few discrepancies, possibly because the patients were aware that our device could store the results of all their measurements.

Self-measurement of BP at home is often used by health care providers to identify and monitor patients with 'white-coat' hypertension or the 'white-coat' effect in those with treated hypertension. [41] Its superiority to the clinic visits' BP measurements is suggested by studies that demonstrated that home BP measurements correlated more closely with the echocardiographically-determined left ventricular mass or cardiovascular mortality. [42] Despite being a better predictor of risk, physicians have not generally used home BP measurements to manage the treatment of hypertension. In part there is a lack of large-scale prospective data on the health benefits of self-monitoring. There are also concerns about the reliability and validity of the readings. [43] Moreover, a randomized controlled trial of home management of hypertension was not significantly better than usual care in lowering BP, reducing the number of office visits related to hypertension or decreasing the cost of care. [44]

There are, however, many advantages to having hypertensive patients who have been adequately trained and assessed for their accuracy in measuring their own BP at home. [22],[41],[43] Home readings may assist physicians in making treatment decisions by providing better surveillance of daytime hypertension control and assessing the duration of action of antihypertensive medications. It allows better management of hypertension of patients demonstrating the 'white-coat' effect, or, possibly more importantly, those showing the reverse pattern - elevated home readings and normal clinic visits' BP measurements. [39] Finally, there is some evidence that the use of self-monitoring may improve patients' compliance with medication. [45],[46]

Using clinic visits and home BP measure­ments for control of hypertensive patients can be classified as being concordant (i.e. higher or normal in both settings) or discordant (i.e. high in one setting and normal in the other). Our results were comparable to other studies and showed a high level of concordance between BP results in the two settings, making management decisions quite straightforward. The situation is more difficult when there is discordance. 'White-coat' hypertension, in which there are high clinic visits' BP and normal home BP readings, is considered by some to represent a more benign form of hypertension, one that is usually associated with fewer cardiovascular complications than forms characterized by sustained hypertension. [34],[35],[36],[37],[38],[39],[40] A randomized controlled trial comparing treatment based on ambulatory pressures with that using clinic visits' readings showed no detrimental effects on BP control, left ventricular mass or the patient's general well­being with less intensive drug treatment in the ambulatory care group. [47] Despite the compelling nature of this evidence, others maintain that 'white-coat' hypertension is asso­ciated with a higher prevalence than normo­tensive individuals of target-organ damage and other cardiovascular risk factors. [32],[33] In their view, 'white-coat' hypertension cannot be considered a benign entity and it therefore requires antihypertensive treatment. This controversy, unfortunately, will not be resolved until there is new clinical trial that directly addresses this matter. The low prevalence of the 'white-coat' effect in our study may be related to patient selection. In another BP study that utilized a less well-selected popu­lation of type 2 diabetic patients, 54% were diagnosed as having 'white-coat' hypertension. Since more than half of the patients were on antihypertensive drug treatment at the time of study, it is misleading to consider the study to be a diagnostic one. [48] Nonetheless, the study does highlight the high prevalence of the 'white-coat' hypertensive effect on this patient population.

There are little clinical data to guide decision-making if BP readings are normal in the clinic visits but high at home. In a cohort study of 3200 Italians who had clinic visits and ambulatory BP monitoring, 9% had 'masked' or 'reverse white-coat' hyper­tension. Individuals with treated hypertension were excluded from this analysis. [37] In a study conducted in patients consecutively attending an academic family medicine clinic, 34.6% had systolic BP readings and 23.8%, diastolic pressures that were lower during the clinic visits than the pressures measured at home. The high percentage of patients with 'home hypertension' in our study was an unexpected finding and may simply be related to the time of BP measurement in relation to the time taking the medications. If this is not the case, however, our finding has important therapeutic implications for a group of patients who are extremely vulnerable to developing hypertension-related compli­cations. Pickering et al. reported that subjects with 'masked' hypertension appear to be at an increased cardiovascular risk. [49] A major issue is detecting this situation, since individuals without home readings would erroneously appear to be under good BP control based on clinic visits' readings. Accordingly, they would not receive more aggressive treatment.

Currently, no evidence exists to support the accuracy and feasibility of any particular home BP monitoring schedule over another. In meta-analysis of studies assessing the effect of different home BP monitoring schedules on the average BP, Brook et al. found that the accuracy of home BP measurements was maintained regardless of substantial variations in the monitoring schedule. [50] In our study, two BP measurements per day for four days gave 90% reliability. It would not be tedious for patients to follow this schedule monthly to provide an accurate record of their BP levels for long-term surveillance.

There are important limitations of this study. The results are applicable only to a highly selected group of hypertensive patients with type 2 diabetics patients treated with anti­hypertensive medications. This study did not examine the use of home BP monitoring to screen for hypertension. The threshold values used to define uncontrolled home BP readings were selected arbitrarily. There is currently no consensus among the different organizations making guidelines for hyper­tension management on the values that should be applied.

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Correspondence Address:
Abdulkareem Alsuwaida
Department of Medicine, King Saud University, P.O.Box 2925, Riyadh 11461
Saudi Arabia
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Source of Support: None, Conflict of Interest: None

PMID: 16970252

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  [Figure - 1], [Figure - 2], [Figure - 3], [Figure - 4]

  [Table - 1], [Table - 2], [Table - 3], [Table - 4]

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