For Clinicians and Researchers: Ambulatory Blood Pressure Monitoring

 

Introduction  The use of 24 hour ambulatory blood pressure monitoring (ABPM) for diagnosing hypertension has been gradually gaining acceptance by official bodies, and is now endorsed by the latest guidelines of both the Joint National Committee in the US and the World Health Organization-International Society of Hypertension. Clinic measurement remains the mainstay of evaluation, although it is increasingly recognized that it may give readings that are unrepresentative of the patient's true pressure . There are three reasons for this: first, the inherent variability of blood pressure coupled with the small number of readings that can be taken means that the estimate of the average level will be statistically unreliable .

Second, physicians' blood pressure measurement techniques are notoriously poor, and many show terminal digit preference (reading to the nearest 10 mm Hg instead of to the nearest 2) ; and third, the “white coat effect”, which results in a transient elevation of blood pressure in the clinic, and which varies greatly in magnitude from one patient to another.

.Information Provided by ABPM

 There are three types of information that ambulatory monitoring can provide:

•  First, it can give an estimate of the average or true level of blood pressure, which is the measure that is generally thought to be responsible for most of the adverse effects of high blood pressure.

•  Second, it can describe the diurnal rhythm of blood pressure.

•  Third, it can give an estimate of the short term variability of blood pressure.

Most of its clinical utility at the present time rests on the first of these.

Evidence for the Prognostic Significance of ABPM

 In the past few years several studies have examined the prognostic significance of ABPM in comparison with clinic BP measurement. All have related the two measures of BP to cardiovascular morbid events. The results all point in the same direction, namely that ambulatory pressure gives a better prediction of prognosis after controlling for clinic pressure, the corollary of which is that patients with white coat hypertension have a more benign prognosis than those with sustained hypertension.

Clinical Indications for Ambulatory Monitoring

 Some specific clinical situations in which ambulatory monitoring may be clinically indicated are listed below.

Newly Diagnosed Hypertensives Without Target Organ Damage - In many patients, particularly those with mild hypertension, the only detectable abnormality is an elevated blood pressure, so that the therapeutic decisions that are finally reached will depend on how the blood pressure is evaluated. While the first approach should be to obtain clinic readings on more than one occasion, it may be helpful to supplement these with ambulatory recordings. Some of these patients may turn out to have white coat hypertension.

White Coat Hypertension - This is the most important, but also the most controversial indication. The broad definition of white coat hypertension is a persistently elevated clinic blood pressure and a normal pressure at other times. One commonly used criterion is a clinic pressure which remains elevated (above 140/90 mm Hg) on the second and third visits. To establish that the blood pressure is normal outside the clinic requires the use of ambulatory monitoring, and the definition of a cutoff point for the upper limit of the normal range. The upper limit of normal for the daytime blood pressure is usually taken as 135/85mmHg. The prevalence of white coat hypertension among patients with mild to moderate hypertension is about 20 per cent . White coat hypertension tends to be somewhat commoner in women than in men, and is surprisingly common in patients over the age of 65, including those with isolated systolic hypertension . White coat hypertension is usually considered to have a relatively benign prognosis.

"Resistant" Hypertension - The patient whose clinic pressure remain obdurately high despite being prescribed multiple medications presents a not uncommon clinical problem. While in some cases this may be the result of a genuinely resistant hypertension, in others it may be due to an exaggerated white coat effect. Such patients have a relatively benign prognosis. Another cause of refractory hypertension is the sleep apnea syndrome. A clue to this may be that although the average level of blood pressure and heart rate fall during the night, their variability increases.

Intermittent Symptoms Possibly Related to Blood Pressure - Episodes of lightheadedness, particularly in patients who are on antihypertensive medication, may be a manifestation of transient hypotension. This can potentially be detected by ambulatory monitoring.

Episodic Hypertension - Episodic symptoms accompanied by transient elevations of blood pressure may occur in a variety of conditions. The use of ambulatory monitoring has been reported in patients with pheochromocytoma and panic attacks.

Episodic Hypotension and Autonomic Neuropathy - In cases of orthostatic hypotension (e.g. patients with diabetes) ambulatory monitoring may be extremely helpful, because many patients who are orthostatic during the day are hypertensive during the night. There are huge swings of blood pressure during the day, depending to a large extent on changes of posture and physical activity, and relatively stable but high pressures at night, when the patient is supine

What is a “Normal” Ambulatory Blood Pressure?

The upper limit of normal for the daytime pressure is 135/85 mmHg, and for the 24 hour pressure 130/80 mmHg.

How should Ambulatory Monitoring be used in routine clinical practice?

Most third party payers do not reimburse the costs of ABPM, but Medicare has approved its use in patients with suspected white coat hypertension and no evidence of target organ damage.

A schema for using ABPM to evaluate hypertensive patients is shown in the figure.

It is important to note that patients in whom it is decided to withhold antihypertensive treatment should continue to monitor their blood pressure at home.