Ambulatory or Home Measurement of Blood Pressure?

Abstract

Ambulatory blood pressure monitoring (ABPM) and home blood pressure (HBPM) monitoring have been shown to be superior to conventional measurement of blood pressure in terms of reproducibility, relationship to the impact of high blood pressure on target organs, and the prediction of cardiovascular events. Nevertheless, these 2 techniques have yet to find their place in the diagnosis of hypertension and during evaluation of the efficacy of antihypertensive treatment. Although these 2 methods do not give identical results in approximately 20% of cases, their diagnostic performance and prognostic value are quite comparable. Although ABPM remains a valuable tool in clinical research, its utilization in routine clinical practice is limited by cost and availability. HBPM is increasingly employed for informed and well‐managed patients, and it can help to improve control of the patient’s blood pressure. Physicians involved in the management of hypertensive patients should be aware of its value in order to assist patients in their care.

The limitations of measuring blood pressure (BP) on consultation (office BP) are now well recognized. They stem essentially from inadequate sampling in view of the variability in BP and the variety of measurement bias (subjectivity of measurements, digit preference, white coat effect, masked hypertension…). ¹ Two methods have been developed over recent years to overcome these drawbacks: Home BP monitoring (HBPM) and ambulatory BP monitoring (ABPM). They are now widely used in clinical research and have demonstrated superiority over office BP in terms of reproducibility, relation to the impact on target organs, and ability to predict cardiovascular (CV) events. They have 3 advantages in common: (1) many measurements can be made, (2) measurements can be automated, and (3) measurements are made during the normal living conditions of the patient. Despite these advantages, they remain inadequately used for a variety of reasons: habits in practice, contradictions in their recommendations, difficulties of organization and implementation of these examinations, and also probably insufficient knowledge of the respective advantages and disadvantages of the 2 methods. Several points should be borne in mind.

Different Principles of ABPM and HBPM

Ambulatory BP Monitoring

ABPM enables measurement of BP over 24 hours during the normal living activities of the patient. The patient should thus depart as little as possible from his or her normal level of activity, which is not always possible. The patient is also instructed to keep the arm still during measurements and keep a diary of activities.

ABPM is the only method that can measure 2 parameters: BP during sleep and the variability of BP. This determination of variability is both an advantage and a disadvantage. An increase in variability of BP is a component of hypertension and it does appear to increase CV risk. But this noninvasive determination of BP is limited to samples every 15 minutes, which are not sufficient for an accurate estimation of variability. Furthermore, variations in BP are extremely dependent on physical activity, which complicates interpretation of the results. Although the activity diary sometimes helps explain certain excursions of BP and helps delineate periods of rest and activity, patients do not always keep an accurate diary. We have proposed the utilization of position detectors that can determine whether the patient is active or resting (sensor placed on the thorax ² ) or standing or not (sensor placed on a thigh for orthostatic indication) and to automatically trigger measurements at particular times (on rising for example). We use a thoracic sensor in routine practice and a thigh sensor if there is a suspicion of orthostatic hypotension. We have also been evaluating movement detectors. ³

All these elements make ABPM a valuable tool in clinical research, especially for evaluation of antihypertensive therapy. Furthermore, it can be combined with other techniques for diagnosis of the sleep apnea syndrome or the measurement of arterial stiffness. ⁴ However, in current practice, interpretation of ABPM tends to be restricted to determination of mean BP over the periods of activity and nocturnal rest, the search for a nondipper profile, episodes of orthostatic hypotension, and also for excessive elevation in BP on rising. ⁵ , ⁶

Home BP Measurement

HBPM is closer to the office determination. The objective is to obtain a series of measurements under standardized conditions rather than evaluate the variability in BP per se. The patient must be at rest, seated for at least 5 minutes, 2 times during the day: in the morning after rising but before taking medication or breakfast, and in the evening before supper. Its advantages stem mainly from the repetition of measurements (2 or 3 in the morning, 2 or 3 in the evening over at least 3 days, ideally 7 days, at least for an initial diagnosis of hypertension) and the absence of the white coat effect. On the other hand, the patient needs to be advised that measurements must only be made under such conditions. The patient’s natural desire to see how BP fluctuates or is affected by certain activities or related to certain symptoms may confound interpretation. The complete cooperation of the patient is required, which is a clear advantage for the management of high BP, but does require discipline based on detailed information about the nature of the patient’s BP and its variability. Finally, it should be borne in mind that the results will be interpreted by the patient’s physician and not by the patient who should not modify treatment as a function of short‐term alterations in BP.

ABPM and HBPM Do Not Produce the Same Results

In view of the different principles described above, it can be seen that the results of the 2 methods will not be the same. Nowadays, the main criteria employed in interpretation is the daytime mean BP for ABPM and the average of 4 to 6 daily measurements over at least 3 days for self‐measurement. The proposed cutoff for “elevated blood pressure” is the same for both methods: 135/85 mm Hg. ⁷ , ⁸

Few studies have compared the 2 methods in the same population. We carried out a study of 300 patients recruited consecutively for consultation in a cardiology unit. ⁹ With the same cutoff of 135/85 mm Hg, the 2 methods came to different conclusions in 20% of cases. A similar percentage of differences has been reported in a population of children and adolescents. ¹⁰ In our population, systolic HBPM was on average 5 mm Hg higher than the value of the daytime BP on ABPM. In other populations, especially untreated young individuals, the opposite phenomenon has been described. ¹⁰ We found that the difference between the mean BP on HBPM and daytime ABPM depended on age, specific antihypertensive treatment, and the office BP. This latter relationship may be indicative of a similar state of alert in certain patients during self‐measurement and during the office determination.

Similar Prognostic Value of ABPM and HBPM

Although ABPM and HBPM evaluate BP in somewhat different ways, they are both better predictors of the impact of hypertension on target organs than is office BP. In our study, the 2 methods presented similar correlations to the extent of left ventricular hypertrophy, carotid intima‐media thickness, and microalbuminuria. The superiority of these 2 methods compared with the office value of BP in terms of the prediction of CV complications is now well established, although in the published studies ABPM appears to present some advantage. ⁸ , ¹¹ The choice of ABPM or HBPM is not thus a question of respective diagnostic power, but rather one of availability, objectives, characteristics of the patients, and cost.

ABPM or HBPM: Technical Considerations

Equipment

For ABPM, as for HBPM, it is imperative to utilize reliable and validated devices. In ABPM, there is little difficulty as there are few machines on the market and they have all been subjected to clinical validation. The measurements should be checked against values taken with a mercury sphygmomanometer when the device is fitted.

There is a wide variety of devices for self‐measurement, however, and not all have been subjected to proper validation. In general, armband devices are more reliable and the armband can be adjusted to the circumference of the arm of the patient. However, even for validated devices, there may be some drift in reliability. The physician should advise the patient on the type of equipment to purchase and check that it is working properly at regular intervals.

Patients

For ABPM, the patient requires little information about the device, which can be readily provided during the fitting process. The limitations stem mainly from the patient’s professional environment and sleep quality. It is preferable not to employ this method for patients involved in intense physical activity. The influence on sleep tends to be more subjective than objective ¹² and notwithstanding the reservations of some physicians, the devices are generally well accepted by properly informed patients. However, the examination should not be repeated too often.

HBPM requires adequate patient training ideally using the equipment that will be used in practice. This can take as long as 30 to 60 minutes, a time that can be profitably employed to eliminate patients for whom it may not be suited or which may lead to anxiety or stress.

Cost

Although cost differs from country to country, ABPM is more costly than HBPM and is not covered by social security in all countries. For self‐measurement, situations vary considerably, but in the majority of cases, the patient bears the cost. If this is not possible, loan of a validated device is an option, although the cost is then borne by the physician or the health authority. Support for reimbursement of the cost of self‐measurement by health authorities or social security needs to be canvassed. ¹³

Practice

The office BP is the reference value in most recommendations for both historical reasons and availability. At the present time, however, there are good arguments for requesting systematic ABPM or HBPM before deciding on antihypertensive treatment. This is the case for the current French recommendations. ¹⁴ Suspicion of a white coat effect, as mentioned in some recommendations, is not a sufficient indication, as it cannot be reliably predicted. A procedure that may avoid or postpone prescription of medication in at least 20% of patients appears to be cost effective for both patients and society in general. ¹⁵

In the diagnosis of hypertension, ABPM and HBPM are of equal merit. Self‐measurement is often simpler and, with the above‐mentioned reservations, less costly. However, it does require a conscientious and active participation of the patient, based on clear and detailed information from health care professionals. It may be contraindicated in particularly anxious patients. Information provided by ABPM is more detailed, only gives information over a finite time, and does not necessarily aid decisions in practice that justify its extra cost. Once treatment has been started, there is even less reason to employ ABPM for evaluating its efficacy. By contrast, reasonable self‐measurement by an informed and cooperative patient over the 3 to 7 days prior to each consultation is an excellent way of evaluating the treatment and improving compliance. ¹⁶ In certain patients with symptoms and either complex or resistant hypertension, ABPM is invaluable.