24-hour blood pressure monitoring: what are the benefits

Introduction
Non-invasive ambulatory blood pressure monitoring (ABPM) is being increasingly used to assess patients with hypertension. This trend is supported by evidence that 24-hour blood pressure profiles may be superior to isolated clinic pressures and the new monitors are acceptable to patients.¹ The development of such a blood pressure profile means that established correlations of clinic pressures with cardiovascular morbidity and mortality in treated and untreated patients must be reviewed and repeated with ABPM. Normal ABPM reference values have been devised and the association of ABPM profiles with surrogate cardiovascular end-points such as left ventricular hypertrophy is being established. ABPM has several indications (Table 1) but appears to be of particular value in detecting patients with 'white coat' hypertension who may not need treatment. It is also used to assess antihypertensive treatment in clinical trials. However, its cost-effectiveness in routine clinical practice has not been established.

Caution While the use of ABPM is increasing and the benefits of such devices are becoming more apparent, it must not be forgotten that these new methods have not been evaluated as comprehensively as clinic pressures. The benefits of drug treatment in significantly lowering morbidity and mortality have only been proven using clinic blood pressures.

How they work
Most devices use either brachial artery microphones to detect Korotkoff sounds or cuff oscillometry where cuff pressure oscillations are detected. The measurement frequency can be varied, but it is usually between 20-30 minutes while awake and 30-60 minutes while sleeping. Patients can start or stop recordings and they can read the displayed results if they wish. Patient diaries are encouraged so that the cause of sudden changes in blood pressure can be evaluated. The units function poorly during strenuous activity and work best if the patient slows or stops moving.

Patient acceptance
New generation devices are small (the size of a personal cassette player), light, and quiet, so patient acceptance is good. When recently evaluated in general practice, 49% of patients reported some interference with normal activity and 76% reported some disruption of sleep. Bed partners may also complain of interrupted sleep. The main adverse effect is bruising from the cuff and occasionally a petechial rash, particularly if the patient has fat arms or if the cuff inflation pressure is high. Ulnar nerve palsy is rare.

Table 1 Current indications for ABPM use* In patients with newly discovered 'hypertension' whose casual office or clinic blood pressure levels are in the mild category (diastolic blood pressure under 105 mmHg) with no evidence of target organ damage. In patients with borderline or labile hypertension. For blood pressure management in the compliant patient whose blood pressure is apparently poorly controlled, despite the use of appropriate antihypertensive drug therapy. In patients who exhibit worsening of end organ damage, despite adequate blood pressure control on office or clinic readings. In patients with a history suggestive of syncope or orthostatic hypotension. In such patients, ambulatory blood pressure monitoring may be best used in conjunction with Holter monitoring. Where there are symptoms or signs suggestive of episodic hypertension, as in phaeochromocytoma. In clinical research. Significant treatment effects can be seen with smaller numbers of patients. Duration of effects of antihypertensive therapy can be carefully assessed. Evaluation of ABP profiles in disease states, effects of therapies on ABP components and their significance are fertile areas for research. * These indications were presented in a paper given at the Australian Consensus Conference on the Management of Hypertension in 1993 (see Aust Prescr 1994;1 Suppl:67-70). They were prepared by a working party of the High Blood Pressure Research Council of Australia.

Device validation
Approximately 30 manufacturers market more than 40 devices and less than 50% have been validated for accuracy according to two current protocols (Association for the Advancement of Medical Instrumentation and the British Hypertension Society). Of these, only 9 fulfilled their criteria and achieved at least a B/B grading for systolic and diastolic blood pressure where the mean difference between the ABPM and a mercury standard was less than 5 mmHg with a standard deviation of <8 mmHg.² Apart from device validation, each recorder should be calibrated against a mercury column before each use and blood pressure cuffs should be the appropriate size for the arm circumference. How many doctors periodically check their mercury or aneroid sphygmomanometer and ensure that the correct cuff is used? Results from oscillometric and auscultatory devices appear to be comparable; however, most monitors are unreliable when used in patients with atrial fibrillation or frequent ectopics where the error rate can be from 5-20%. A loss of accuracy may occur in the elderly as well as in patients with very high blood pressure. In addition, problems can occur when evaluating results from devices that use ECG gating if fitted to patients with pacemakers. These devices relate Korotkoff sounds to the QRS complex to improve the accuracy of blood pressure monitoring.

Normal range
A number of studies, both large and small, have attempted to develop population reference values including a normal range for ABPM.^3,4 More data are required to develop more accurate population reference ranges. Clinic and 24-hour ambulatory daytime blood pressures are remarkably similar in normotensive patients, yet in hypertensives (borderline or definite), ABPM results are much lower. Also, work-day pressures are usually higher than non-work-day pressures in both hypertensives and normotensives.

'White coat' hypertension
The definition of 'white coat' hypertension is arbitrary.¹ A general definition is 'a persistently raised clinic blood pressure together with a normal ambulatory pressure'. This implies that several clinic visits have occurred to exclude the tendency of most clinic pressures to fall with repeated measures. All health professionals are aware of the marked variability of any individual's blood pressure including the effects of physical and mental stress. A clinic visit can probably provoke a press or response in some people that persists with time and subsequent readings. Unfortunately, this 'white coat' effect is not confined to subjects with 'white coat' hypertension and can occur in patients with severe hypertension.

'White coat' hypertension cannot be easily predicted by either the patient's personality (most patients deny anxiety and their pulse rate is not usually increased) or cardiovascular profile. It can occur in both young and old, and can alter both systolic and diastolic pressures; particularly the systolic. Demographic factors including gender and obesity can influence the prevalence of 'white coat' hypertension and it is even common in patients over the age of 65 with isolated systolic hypertension. The scale of this condition was revealed by a study where 4577 ABPM profiles from 24 centres world-wide were evaluated and compared with their clinic pressures.³ Between 24% and 30% of the patients who were considered to have mild hypertension (diastolic blood pressure 90-105 mmHg) on traditional clinic pressures had ABPM profiles within the 'normal' range. While this study may overestimate the 'white coat' effect since many patients with labile blood pressure may have been referred to these clinics, other hospital and private practice based studies have found a prevalence of 'white coat' hypertension in patients with mild to moderate hypertension of 20-40%.

While it is not yet clear how many of these patients with a 'white coat' syndrome will develop sustained hypertension, their prognosis appears favourable with a recent study of 1187 adults with essential hypertension, some followed for over 7 years. In the 20% who were considered to have 'white coat' hypertension, the numbers of fatal and non-fatal cardiovascular events were similar to a normotensive control group.⁵ Therefore, on current evidence, such patients are unlikely to benefit from immediate drug treatment. However, periodic review of such patients is prudent.

Dippers and non-dippers
Most people, including the majority of patients with hypertension, have a lower blood pressure while asleep (dippers). The systolic and diastolic falls in hypertensive patients are usually 10-15%, but do not fall to normal levels. However, in 30% of hypertensive patients, the nocturnal fall is smaller than usual, may not occur, or the pressure may even rise. It may be important to identify this group of 'non-dippers' because, from preliminary data, they appear to have a worse prognosis. This can be measured as cardiovascular morbidity and mortality as well as increased end organ damage including left ventricular hypertrophy. ABPM is the simplest way to detect 'non-dippers'. As a group, they appear to have more severe or complicated forms of hypertension and are often older. One study found that 5 fatal and non-fatal cardiovascular events per 100 patient years occurred in 'non-dippers' compared with two such events in 'dippers'.⁵ Other illnesses such as pre-eclampsia, heart failure and sleep apnoea can reverse the normal diurnal variation in blood pressure.

Left ventricular hypertrophy - a surrogate end-point
Left ventricular hypertrophy (LVH) is not only a consequence of hypertension, but also an independent risk factor for coronary artery disease and cardiac death. A number of studies have investigated if ABPM is a better predictor of LVH than clinic pressures. To date, most studies show a much greater predictive value of ABPM. In a recent meta-analysis of 19 studies, night-time blood pressures were no better than daytime pressures in predicting LVH.⁶ While this might suggest that daytime monitoring is probably all that is necessary in most patients, and this is clearly more convenient, only night-time monitoring will indicate 'non-dippers'.

As many pressures are recorded during a 24-hour period, attempts have been made to convert these data into more meaningful information. Consequently, most devices will calculate the blood pressure load from the proportion of systolic pressures >140 mmHg and diastolic pressures >90 mmHg while awake and 120 and 80 mmHg respectively while asleep.⁷ This approach recognises that fluctuations in blood pressure throughout the day and night are at least as important as the average pressures and appear to be an even better predictor of LVH.

Evaluation of antihypertensive drugs
ABPM has been particularly valuable in the testing of new (and also older) drugs. While trough-peak ratios calculated from ABPM have become a new surrogate treatment end-point and are now being used in marketing strategies, such an assessment of the ability of medications to lower blood pressure persistently over 24 hours when compared to placebo has been valuable. Unfortunately, many recent trough-peak ratio studies that have evaluated once daily medications are methodologically flawed and their conclusions dubious.

ABPM has also been valuable in the evaluation of several studies of life-style intervention.

Cost benefit analysis
A recent review concluded that 'Limited clinical applications of ABPM and blood pressure self-measurement in the diagnosis and management of hypertension appear to be warranted. Endorsement of these technologies for routine clinical use, however, will require more convincing evidence of their clinical effectiveness'.⁸ This review also suggested that if ABPM was in routine clinical use to diagnose and monitor all hypertensive patients, at a cost of $120.00 per service, its yearly cost could be $40 million. Clearly, financial considerations have been a factor in ABPM not yet being assigned a Medicare Benefit number. However, if at least $260 million is currently being spent on hypertensive drugs, the identification of up to 20% of patients with 'white coat' hypertension who may currently be on drug treatment and who may not need treatment, could make even widespread use of ABPM cost-neutral. Reduced visits to the doctor for these patients and improved wellbeing would be additional benefits. As more data on ABPM accumulate, a more informed decision on its cost-benefit will be possible. Clearly, the medical profession must use such a resource responsibly.

One method that could minimise costs is the greater use of self-home blood pressure monitoring, particularly in patients where sleeping pressures are not necessary. Simple to operate automatic devices are now available and are currently being evaluated and, in particular, being compared to ABPM.