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The Use Of Ambulatory Blood Pressure Monitoring
In A Hypertension Clinic
(Manor Hospital, Walsall, United Kingdom)

Dr. K. M. Ali, Dr. S. Dean, Dr. B. Panayiotou, Dr. R. Roy

The traditional (clinic) method of blood pressure (BP) measurement has always formed the basis of the current operational definition of hypertension. It is a well known fact that BP is a continuum not a dichotomy. Riva-Rocci remarked on the variability of BP values depending on the state of mind and behavioural changes. In 1969 an intra-arterial technique for beat-to-beat monitoring of BP provided a faithful demonstration of BP variability⁽¹⁾. In 1983 Mancia et al demonstrated the marked alteration in BP as result of its measurement by a doctor or nurse ‘the white-coat effect’⁽²⁾.

Ambulatory BP Monitoring (ABPM) allows repeated measurements over an extended period of time (70-100 / 24 hours). It is also useful in certain categories of patients: elderly, pregnant ladies, resistant hypertension, nocturnal hypertension, in guiding anti-hypertensive medications, and in the diagnosis of secondary hypertension⁽³⁾. In some studies ABPM was found to be more predictive of end organ damage^(4,5). It is generally acceptable by patients⁽⁶⁾, and its readings are reproducible⁽⁷⁾. It also avoids observer bias. ABPM was found to be useful in identifying patients with ‘white coat’ hypertension, enabling more accurate screening and diagnosis⁽⁷⁾. In a comparative study, ABPM was found to be superior to clinic measurements in identifying hypertensive patients whose blood pressure is not controlled adequately or is uncontrolled⁽⁸⁾. Moreover the cost of ABPM is largely met by savings on drugs, and in the long term this technique may reduce the overall cost of management⁽⁹⁾. Some of its limitations are the reliability of the machine and the establishment of normal values for ambulatory BP, and a major drawback is sleep disturbance⁽¹⁰⁾.

White-coat hypertension has been defined as a persistently elevated clinic BP in combination with a normal ambulatory BP⁽¹¹⁾. Its prevalence has been estimated as approaching 20% among mild hypertensives, and increases with age⁽¹²⁾. In Caryn E. Lerman study white-coat hypertensives were older, had less angry dispositions, and reported less overt anger expression, as well as taking more antihypertensive medications⁽¹³⁾. White coat hypertension is a serious diagnosis in terms of management implications since drug treatment in not necessarily indicated but patients need to be followed up⁽¹¹⁾.

One of the most specific characteristics of ABPM is the possibility of being able to discover modification or alteration of the 24-hour BP cycle⁽¹⁴⁾. O’Brien et al were among the first to draw attention to the adverse prognostic significance of the absence of a night-time fall in BP and raised the concept of ‘dippers’ and ‘non-dippers’⁽¹⁵⁾.

The primary objective of the study was to survey the BP profile by using ABPM among newly referred patients to the hypertension clinic .The second objective was to assess the correlation between clinic BP and ambulatory BP, and explore its effect on management, as well as identifying ‘dippers’ and ‘non dippers’.

All patients referred to the hypertension clinic at the Manor Hospital in Walsall in the period from November 2000 till May 2001 were asked to participate in the study. Those who agreed to participate were enrolled. Patients with overt congestive cardiac failure, unstable angina, arrhythmias, dermatological conditions that would prevent them wearing the monitor, or refused to participate, were not included in the study.

Personal data, co-existing morbidity, clinic BP reading, ambulatory BP reading and evidence of end organ damage were obtained for all patients included in the survey. All patients had urine dipstick testing, blood sample for urea and electrolytes, an ECG, a CXR, and echocardiography. They all had fundal examination.

Normal clinic BP was taken as <140/90. Normal ambulatory BP was taken as 119-126 mm Hg systolic, and 75-80 mm Hg diastolic (3). Left ventricular hypertrophy (by ECG criteria, chest X-ray and echocardiography), confirmed proteinuria and/or renal impairment (normal values Na = 133-145 mmol/l, K = 3.3 – 5.2 mmol/l, urea = 2.5 – 8.9 mmol/l, and creatanine = 54 – 132 micro mol/l) and confirmed retinal changes of hypertension were all used as measures of end-organ damage.

Clinic systolic and diastolic BP recordings were performed by the clinic nurse twice on the non-dominant arm in the erect and supine position with the patient sitting after 10 and 20 minutes of rest according to the standard technique. Cuff size was selected on the basis of arm circumference. Each patient received a detailed explanation of the ABPM procedure from the clinic nurse. Ambulatory BP monitoring was performed with a portable non-invasive NuMed recorder on a day of typical activity after a clinic visit, and medications were not stopped prior to ABPM. Patients were given a diary sheet to record their activities and symptoms if any. Ambulatory BP readings were obtained automatically at 20-minute intervals from 6am to 12midnight, and at hourly intervals from midnight till 6am. On completion of ABPM, data were analysed by computer. Echocardiography was performed using Acuson ultrasound imaging system.

Primary outcome measures: average clinic BP readings, average day time ambulatory BP readings, and classification into dippers and non-dippers. Non-dippers were classofied as those who showed a reduction in BP of less than 10/5 mmHg or 10% between the day (06.00 - 22.00) and the night, or an elevation in BP.

Secondary outcome measures: evidence of end organ damage (left ventricular hypertrophy, confirmed proteinuria and/ or renal impairment, and confirmed retinal changes of hypertention).

Paired t test and chi square test were performed as appropriate. Values were represented as means + /- SD. The p value <0.05 level of significance was adopted in all tests.

A total of 118 patients (60 male and 58 female) were included. The mean age of the patients was 54 years, with a range of 25-88 years. 10 patients were diabetic, 5 had renal impairment, 3 had ischaemic heart disease and 64 patients had hyperlipidaemia. 103 patients (87%) were already on treatment for hypertension upon entering the study, and 15 patients (13%) were not on treatment.

4 of the 15 patients not on treatment (26%) were found to have ‘white-coat’ hypertension and consequently no treatment was initiated, but were followed up. 27 of the 103 patients (26%) already on treatment were found to have ‘white-coat’ hypertension and their treatment was not adjusted.

There was a significant correlation between clinic and AMBP systolic and diastolic readings, as shown by figures 1 and 2.

There was 27 dippers (23%), and 91 non dippers (77%). There was evidence of end-organ damage in 43% of non-dippers, and 7 % of dippers (chi square =10.58, p=0.001 highly significant, relative risk=5.8 ).The treatment for non-dippers was optimised, and they were offered more regular follow up appointments. The authors of this study agree with Helen M.C et al there is considerable scope for improving the treatement and control of hypertension in the English adult population⁽¹⁶⁾. The Health Survey for England 1996 reported that 94% of hypertensive subjects had not managed to achieve a BP of 140/ 90 mm Hg, the lower limit for definite hypertension according to recent quidelines⁽¹⁷⁾.

By utilising the valuable information from ABPM, we were able to rationalise treatment in our hypertensive population. This is in accordance with other studies in which ABPM led to less invasive drug treatment with preservative of blood pressure control, general well-being, and inhibition of left ventricular enlargement⁽¹⁸⁾. In our study we did not follow patients after seven months. In other studies, using ABPM, it was proven that if after 1 year of active theapy antihypertensive drug treatment was interrupted, left ventricular mass rose again in only 3 weeks time⁽¹⁹⁾. This shows the need for regular follow up using ABPM as a sensitive indicator of poor BP control.

There was a significant number of patients (26%) with ' white coat ' hypertension, and they were given appropriate advice regarding diet, smoking, alcohol, and were offered regular follow up appointments as recommended by Task Force 1V (12). This white-coat hypertension occurs in 20% or more of the hypertensive population⁽²⁰⁾.

ABPM was acceptable to all patients included in the study except for 2 patients who swithced the monitor off because of disturbed sleep and they were excluded from the study. However a recent article in the BMJ comparing different methods of measuring blood pressure in primary care, found that some measurements may be the most promising option, as they were the most acceptable method to patients and were preferrd to either readings in the surgery or ambulatory monitoring⁽²¹⁾. Alternatively an earlier study indicated that home blood pressure monitoring is not appropriate as an alternative to ABPM in the detection of white coat hypertension⁽²²⁾.

It can be argued that routine usage of ABPM can lead to a massive rise in cost, as suggested by the National High Blood Pressure Education Group ⁽²³⁾, but cost issues were not addressed in this study. Further studies will be needed to clarify this.

Ambulatory BP monitoring provides a well-tolerated, easy and effective way of managing hypertensive patients. We are not suggesting its use as a substitute for clinic BP measurement, but as demonstrated by this survey it identified a significant number (26%) of patient with white coat hypertention. It allowed judicious prescription of treatment as well as adjusting anti-hypertensive medications hence providing a cost-effective way of managing hypertention. It also identified an important category of patients (i.e non- dippers) who need to be treated aggresively.

The use of ambulatory blood pressure monitoring should not be restricted to specialised units, and we recommend its routine use in all hypertention clinics.