ABC of hypertension: Blood pressure measurement

Abstract

Setting up an ambulatory blood pressure measurement service Which monitor to buy? A large variety of ambulatory blood pressure measurement devices are now available on the market, and the number will increase as the technique of ambulatory blood pressure measurement becomes more widespread. A number of factors should influence this choice, among which the most important is to ensure that the device has been validated independently according to either the protocol of the British Hypertension Society (BHS),2 and/or that of the Association for the Advancement of Medical Instrumentation (AAMI).3 Which monitor to choose Check for independent validation by BHS/AAMI protocols How much will it cost? How expensive is the software? Is the software what you need? Are the instructions adequate? How much will maintenance cost? How expensive are consumables—batteries, etc? Have you adequate computer facilities? Is the technical/nursing back up available? Are training facilities available? Is the warranty adequate? Is there an adequate servicing facility? What type of service? Doctors in practice may establish their own ambulatory blood pressure measurement service, refer patients to a hospital ambulatory blood pressure measurement service, or refer patients to a blood pressure clinic for full evaluation, which includes ambulatory blood pressure measurement. Often an open access referral service is used, with referral of problem or complicated cases for fuller evaluation in a blood pressure clinic. Training requirements The technique of ambulatory blood pressure measurement is specialised, and should be approached with care. An understanding of the principles of traditional blood pressure measurement, cuff fitting, monitor function and analysis and interpretation of ambulatory blood pressure measurement data as presented in the BHS Working Party CD Rom on blood pressure measurement is recommended.4 A nurse with an interest and experience in hypertension can master the use of ambulatory blood pressure measurement devices after relatively brief training. However, the analysis and interpretation of ambulatory blood pressure measurement profiles requires experience in the technique, which is best acquired by the doctor in charge of an ambulatory blood pressure measurement service. View larger version: In this window In a new window SpaceLabs 90207 ABPM monitor Details of ABPM device manufacturers Additional information about manufacturers can be found on the BMJ's website: http://www.bmj/.com. See also O'Brien et al. BMJ 2001;322:531-6 Using an ambulatory blood pressure measurement monitor Time needs to be given to fitting the monitor and preparing the patient for the monitoring period if good results are to be obtained.1 The key to successful ambulatory blood pressure measurement is educating the patient in the process of monitoring and instructions should be explained and printed on a diary card. In clinical practice measurements are usually made at half hourly intervals so as not to interfere with activity during the day and with sleep at night, but measurements can be made more frequently if indicated. There are a number of ways of analysing blood pressures recorded during the 24 hour cycle.5 One simple and popular method is to assess the time of awakening and sleeping from diary card entries. Another is to use a fixed time method in which the retiring (2101 to 0059) and rising (0601 to 0859) periods during which blood pressures are subject to considerable variation are eliminated, with the daytime period being from 0900 to 2100 and night time from 0100 to 0600; in this way the variations that may exist between the young and the old and in different cultures are to some extent eliminated from the analysis. Using an ambulatory blood pressure monitor 15-30 minutes needed Relax patient in a quiet room Enter patient details into monitor Measure BP in both arms If SBP difference 14 SBP and DBP measurements Night >7 SBP and DBP measurements Causes of poor ABPM Poor technique Arrhythmias Small pulse volume Inability of automated devices to measure blood pressure Editing data Restrict editing to physiologically impossible pressures, eg DBP=SBP Displaying data Plot data (see figure) Statistics to include: Mean daytime SBP and DBP and heart rate Mean night time SBP and DBP and heart rate Mean 24 hour SBP and DBP and heart rate Ambulatory blood pressure measurement devices are usually sold with individual software packages, which present data in a variety of ways. It would facilitate practice if the graphic presentation of ambulatory blood pressure measurement data were standardised, much as is the case for ECG recordings. Such a standardised approach might provide a graphic display of ambulatory blood pressure measurement data (on screen or printout) with a visual time/pressure graph with blood pressure plotted on the vertical axis and time on the horizontal axis, and levels of normality can also be shown. 6 7 One program (DABL®, Cardiovascular 2000 ECF Medical, Dublin, Republic of Ireland) provides a printed report derived from the ambulatory blood pressure measurement data.6 The issue of normality/abnormality in ambulatory blood pressure measurement is controversial, but the levels shown in the table below are commonly used.8 The evidence from ongoing longitudinal studies gives some support to lower levels of normality for ambulatory blood pressure measurement, and we appreciate that these levels may be regarded as conservative by some. Recommended levels of normality for ambulatory blood pressure measurement View this table: In this window In a new window Recommended levels of normality for ambulatory blood pressure measurement The evidence supporting these demarcation levels is based on firm evidence from a number of studies; the evidence is not yet available to make recommendations for the intermediate pressure ranges between the “normal” and “abnormal” levels, nor for...