Info For Researchers
Australian Ambulatory Blood Pressure Database (May 2011)
Ambulatory Blood Pressure Monitoring Working Group of The National Heart Foundation of Australia and the High Blood Pressure Research Council of Australia
G.A. Head (1), A.S. Mihailidou (2), B. P. McGrath (3) A. Wilson (4), M.R. Nelson (5), M. Schlaich (1), M. Stowasser (6), D. Cowley (6), A.A. Mangoni, (7) L. Ruta (3)
(1) Baker IDI Heart and Diabetes Institute, Melbourne-AUSTRALIA, (2) Royal North Shore Hospital, University of Sydney, Sydney-AUSTRALIA, (3) Monash University, Dandenong Hospital, Dandenong-AUSTRALIA, (4) National Heart Foundation of Australia, Melbourne-Australia. (5) Menzies Research Institute, Hobart-AUSTRALIA (6) Hypertension Unit, Univ Qld School of Medicine, Princess Alexandra Hospital, Brisbane-AUSTRALIA, (7) Division of Applied Medicine, University of Aberdeen, United Kingdom.
Like most international guidelines, existing national guidelines in Australia on the management of hypertension emphasise that necessity, choice and intensity of blood pressure (BP)-lowering treatment should be related to absolute cardiovascular disease (CVD) risk. However there is only limited information of how to factor ambulatory blood pressure (ABP) monitoring into this framework. An Australian Database for ABP was initiated to address the need for ABP equivalents for target BP for management of hypertension in low and high risk subjects. Data from over 8,000 subjects was submitted from 11 centres from all states in Australia. The resulting ABP equivalents for BP targets and treatment of hypertension enabled this data to be included into ABP monitoring guidance on best clinical practice and prompted an update of the 2002 National Heart Foundation of Australia evidence-based position statement supporting the use of ABP monitoring. The 2011 updated position statement due to be finalised mid year is based on expert opinion of the best available evidence from clinical trials and controlled observational studies. It outlines the rationale for the recommended upper limits for daytime, night time and 24 hour systolic BP/diastolic BP levels through ABP monitoring in preference to clinic measurements, together with indicators for use, interpretation of the data and factors to consider where further research is identified.
Clinic assessment of BP is traditionally the approach to the management of hypertension. Recent studies examining the reliability of BP assessments have demonstrated that clinic BP readings alone are not sufficiently accurate to base clinical decisions such as whether a treatment has the desired antihypertensive effect.1 Increasingly evidence suggests that measurement of BP outside the clinic by ABP and/or home BP devices provides a more robust indication of a patient’s actual BP2, particularly when repeated on more than one occasion.3 Importantly, prospective studies have shown that ABP is a stronger predictor of clinical outcomes than conventional clinic BP measurements.4-7 ABP is currently the only practical method to detect a lack of nocturnal dipping which has been shown to be associated with increased risk of stroke, end-organ damage and cardiovascular events, including death.5 8-10 Hypertension management guidelines are most often based on clinic BP measurements11 12 and the need for similar guidelines based on ABP are necessary to enable physicians to provide more effective hypertension management.13 However, it is important to note that the use of ABP for management of hypertension is very limited since there are no outcome trials to show superiority of ABP over clinic BP and therefore, powered clinical trials are needed. Guidelines for ABP thresholds that have been derived from population, multicentre studies and meta analyses for the hypertension threshold of clinic 140/90 mmHg have been available for many years.12 14-18 To date no study has attempted to define ABP levels that are equivalent to the various treatment targets used for hypertensive subjects with co-morbidity, or the BP levels which are used to classify the severity of hypertension. The PAMELA study demonstrated that this can be achieved statistically by finding the ABP equivalents for the desired level of clinic BP as defined in current guidelines.18 For these reasons the National Heart Foundation (NHF) of Australia, in partnership with the High BP Research Council of Australia (HBPRCA) would argue that for ABP to be more widely used, in conjunction with clinic and home based monitoring in the assessment and management of patients with suspected hypertension, a new study was required. While a population study (like PAMELA) was considered too expensive, the alternative was to prospectively gather a large number of ABP and clinic BP measurements from a multicentre approach.
Australian Ambulatory Blood Pressure Database
In 2008, the Australian Database for ABP was initiated to address this need for ABP equivalents for target BP for management of hypertension in low and high risk subjects. The database is the result of the cooperative effort of individuals and the financial and logistic support from the HBPRCA and NHF of Australia which has enabled this contribution to the appropriate use and interpretation of ABP monitoring and effective management of hypertension. 19 Seventeen members of the HBPRCA with expertise in ABP formed a working Group representing 11 hospitals or research institutes hypertension assessment centres from all states in Australia, contributed their de-identified data which included age, sex, ethnicity, clinic BP and ABP values. 19 ABP was recorded for at least a 24 hour period during a typical day using validated devices. We recruited 8,575 subjects (46% male, 54% female) from which we had both clinic and ABP measures from 5,327 patients (Table 1). Most subjects had been referred for ABP assessment and thus would include slightly more subjects suspected of white-coat hypertension, resistant or difficult to treat hypertension, or hypertension associated with renal disease. Some centres contributing a significant proportion of normative participants who were recruited by advertising from the general population. Participant average age was 56 years and body mass index was 28.9 kg/m2. Age, clinic and ABP measures were consistent across the centres with the exception of site 8 had a greater proportion of hypertensive subjects and hence higher clinic and ABP values (Table 1). In addition we also collected a cohort of participants from 4 centres where clinic measurements were taken by both physician and other health professional staff. Most were receiving antihypertensive medications (69%) and most (82%) were Caucasian.
Determining ABP equivalents for clinic BP thresholds and targets
A novel least product regression analysis was used which showed a high degree of association between seated clinic BP and 24 hour, day and night time ABP measurements. Slopes for least product regression in all cases were slightly less than one indicating some proportional bias and fixed bias. The least product regression proved superior to the least squares regression as they followed more closely the major axis of the ellipse of data points. The daytime ABP equivalents for staff measured clinic BP are shown in Table 2 and were published in 2010.19 20 Based on regression analysis, the daytime ambulatory equivalent for a clinic threshold of 140/90 mmHg is 136/87 mmHg, for target clinic 130/80 mmHg is 128/78 mmHg and for target clinic 125/75 mmHg is 124/74 mmHg. Target BP for patients with associated clinical conditions or end-organ damage is 2/2 mmHg lower and the equivalent for normal BP 1/1 mmHg lower. Thus clinic BP when assessed by clinic staff closely predicted the values obtained during daytime ABP recording (Table 2). 19 The closer the subject’s BP is to normal, the closer the agreement between daytime ABP and clinic BP. Also the more hypertensive, the more clinic BP measures exceed daytime ABP values.19 Other findings were that i) ambulatory values are 1/2 mmHg lower for females and 3/1 mmHg lower in older subjects ii) higher physician measurements of clinic BP gave inappropriate estimates of ABP treatment thresholds and iii) day time diastolic ABP equivalents were not affected by age but systolic equivalents were between 2-4 mmHg lower in older subjects compared to a younger group.19
Importantly, our findings for clinic BP equivalents for 140/90 mmHg were very similar to those which had been agreed internationally based on a number of different methods including large multicentre randomised population studies involving measurements from subjects considered normotensive.15 Further, when analysed in a comparable manner, the findings closely matched the findings in the PAMELA study18. A recognised limitation of the study was that it used a population not differentiated for co-morbidities or degree of cardiovascular risk in the regression analysis to predict target values. A future direction for the database would be to document and adjust for morbidity and to determine whether this makes a significant difference to the predictive values.
Recommendations, conclusions and policy implications: New position statement
The Australian ABP study provided for the first time a range of age and sex adjusted daytime ABP measurement equivalents for recognised diagnostic thresholds and target clinic BP, which the Australian working group suggested could be used to guide the management of hypertension. Based on this recommendation, the NHF of Australia, in partnership with the HBPRCA have reviewed the 2002 position statement that advocates ABP monitoring. The update is based on expert opinion of the best available evidence from clinical trials and controlled observational studies and has been an opportunity to include the latest research in ABP monitoring as well as introducing emerging concepts in BP variability. ABP monitoring is a specialised technique that requires training, skills and experience, validated and well-calibrated monitors, the use of correct cuff sizes and appropriate protocols. Thus only devices validated and approved (reaching grade A) by international standards21 22 should be used for ABP monitoring. The new consensus statement outlines the rationale for the recommended upper limits for daytime, night time and 24 hour systolic BP/diastolic BP levels through ABP monitoring in preference to clinic measurements, together with indicators for use, interpretation of the data and factors to consider where further research is identified. Updated flow diagrams are provided to guide clinicians in the investigation and management of white coat hypertension and the role of ABP monitoring among patients determined to be at high risk of developing CVD. As such the document advocates for increased provision of ABP monitoring among primary care services in Australia, using appropriately calibrated equipment, at a time when Government-led healthcare reform is under the spotlight. The draft position statement has been widely circulated and is undergoing review during this consultative phase during the first half of 2011 with the final document expected mid year. Although the wider use of ABP monitoring is justified, we recognize that in Australia, this is currently limited by its availability and cost due to lack of Australian Medicare subsidy except for veterans. Cost benefit analysis clearly shows cost savings in reduced numbers of inappropriate antihypertensive prescriptions and has been subsidized in many countries.
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