Info For Doctors
What is ambulatory Blood Pressure Monitoring?
Ambulatory Blood Pressure (ABP) Monitoring involves measuring blood pressure at regular intervals (usually every 20-30 minutes) over a 24 hour period while patients undergo normal daily activities and sleep. The portable monitor is worn on a belt connected to a standard cuff on the upper arm and uses an oscillometric technique to detect systolic, diastolic and mean blood pressure as well as heart rate.(1) When complete, the device is connected to a computer that prepares an analysis of the 24 hour, day time, night time as well as sleep and awake (if recorded) average systolic and diastolic blood pressure and heart rate. The accuracy of ABP monitoring has been validated in a wide range of patients including young, elderly, pregnant and obese subjects (provided the correct cuff is used).
What are the indications?
- Suspected white-coat hypertension (including in pregnancy).
- Suspected masked hypertension.
- Suspected nocturnal hypertension or lack of night time reduction in blood pressure (dipping).
- Hypertension despite appropriate treatment.
- Patients with a high risk of future cardiovascular events.
- Suspected episodic hypertension.
ABP monitoring may also be useful for
- Titrating antihypertensive therapy•Borderline hypertension.
- Hypertension detected early in pregnancy.
- Suspected or confirmed sleep apnoea.
- Syncope or other symptoms suggesting orthostatic hypotension, where this cannot be demonstrated in the clinic.
When is ABP monitoring not recommended?
ABP is contraindicated if it will delay antihypertensive treatment in patients with high cardiovascular risk, have hypertension grade 3 (SBP ≥ 180 mmHg and/or DBP ≥ 110 mmHg) or who have isolated systolic hypertension and widened pulse pressure (SBP ≥ 160 mmHg and DBP ≤ 70 mmHg). (see NHF 2010 guideline update)
ABP monitoring may not be accurate when there is irregular heart rate and arrhythmias.(1) For most patients ABP monitoring is appropriate being safe and not usually associated with complications, but occasionally petechiae of the upper arm or bruising under the inflating cuff may occur. Modern ABP devices are quiet, lightweight and relatively easy to wear but inflation of the cuff may cause some transient discomfort, particularly in people with hypertension or when multiple repetitions of the reading are triggered due to errors in measurement. ABP measurements during the night may disturb sleep which may require a repeat test if there is poor nocturnal BP measurements.
Where does it fit in a diagnostic approach?
ABP monitoring provides a more reliable measure of a patient’s blood pressure than isolated clinic measures and is not subject to the “white coat effect” which can overestimate blood pressure, particularly in susceptible patients. While clinic measurement of blood pressure will continue to be useful for screening in the management of suspected and true hypertension, ABP and home blood pressure measurements add considerably to the accurate diagnosis of hypertension and the provision of optimal care. Recent recommendations from expert groups such as the National Institute for Health and Clinical Excellence (UK) strongly advocate wider use of ABP monitoring in the diagnosis and management of hypertension
What should GPs tell patients about this test?
Patients should be given a written set of instructions to take home after the initial verbal description. Patients should be informed that the device will automatically inflate the cuff and measure BP periodically over a 24 hour period. They should continue with normal daily activities and if possible a work day should be chosen rather than a rest day. When the cuff starts to inflate the patient should stop moving and talking, keep the arm still and relaxed and breathe normally. They should be told to avoid activities that may interfere with the device such as vigorous exercise and instructed to keep a diary to record timing of activities, sleep, taking of medicines, posture and symptoms (e.g. dizziness) that may be related to BP. Importantly, no Medicare rebate is currently available and therefore costs need to be discussed with the patient.
How does the test work?
ABP monitors use cuff oscillometry, which relies on detection of cuff pressure oscillations and defines the maximal oscillations as mean arterial BP and then uses an algorithm to calculate systolic and diastolic BP. It is essential that the correct cuff size is used and if the patient has a very large upper arm circumference, a conical shaped cuff is necessary. To obtain valid results at the time the ABP monitoring device is fitted, at least three readings should be recorded simultaneously using a calibrated sphygmomanometer connected to the ABP monitoring device by a Y-connector. Average readings for ABP and sphygmomanometer should not differ by more than 5 mmHg. Measure BP in both arms and if the SBP difference is less than 10 mmHg, use the non-dominant arm. If the SBP difference is greater than 10 mmHg, use the arm with the higher pressure. ABP monitoring devices are usually programmed to take readings at set intervals of 15–30 minutes during the day and every 30–60 minutes at night, in order to avoid interfering with activity or sleep.
What do the results mean?
Measurements obtained from ABP monitoring must be interpreted carefully with reference to diary information and timing of medicines. Reference ‘normal’ ABP values for non-pregnant adults are
24-hour average < 115/75 mmHg (hypertension threshold 130/80 mmHg)
Daytime (awake) < 120/80 mmHg (hypertension threshold135/85 mmHg)
Night time (asleep) < 105/65 mmHg (hypertension threshold 120/75 mmHg).
ABP values above ‘normal’ and below thresholds for hypertension are considered ‘high-normal’. Night-time (sleeping) average systolic and diastolic BP should both be at least 10% lower than daytime (awake) average (1). BP load (percentage time during which BP readings exceed hypertension threshold over 24 hours) should be < 20%.BP variability, maximum systolic BP and morning BP surge should also be taken into account (and targeted by treatment). Treatment targets based on ABP are lower than for clinic BP readings. Importantly, ABP monitoring can be effectively used to manage antihypertensive treatment. (3)
What won’t the results tell you?
ABP monitoring will not provide any information relating to cardiac arrhythmias and may be inaccurate in cases of irregular cardiac rate such as in atrial fibrillation. ABP is not designed to detect postural hypotension as the measurements occur at a fixed interval and devices do not contain inclinometers which would be required to record a patient’s position. However, diary information can provide positional and event data. Moreover, ABP monitoring can be used to assess whether there is high variability of blood pressure which is often associated with orthostatic hypotension.(4)
What are the next steps if the test is negative or inconclusive?
For patients at low risk of cardiovascular disease with elevated clinic blood pressure but a normal ABP (i.e. day blood pressure is <135/85 mmHg) suggests a “white coat hypertension” condition. This should be confirmed with a second ABP. Subjects with white coat hypertension are at greater risk of developing true hypertension and also glucose intolerance and require continued assessment of absolute cardiovascular risk and continued monitoring with clinic and home blood pressure measurements. Repeat the ABP every 1-2 years. For patients with high cardiovascular risk or suspected masked hypertension it is appropriate to perform an initial ABP monitoring, even if clinic blood pressure is not elevated and lifestyle factors have been assessed and modified. This is to evaluate the 24h ABP profile and examine for nocturnal blood pressure dip.
1. McGrath BP. Ambulatory blood pressure monitoring. Med J Aust. 2002;176:588-592.
2. Head G, Mihailidou A, Duggan K, Beilin L, Berry N, Brown M, Bune A, Cowley D, Chalmers J, Howe P, Hodgson J, Ludbrook J, Mangoni A, McGrath B, Nelson M, Sharman J, Stowasser M. Definition of ambulatory blood pressure targets for diagnosis and treatment of hypertension in relation to clinic blood pressure: prospective cohort study. Brit Med J. 2010;340:c1104.
3. Staessen JA, Byttebier G, Buntinx F, Celis H, O'Brien ET, Fagard R. Antihypertensive treatment based on conventional or ambulatory blood pressure measurement. A randomized controlled trial. Ambulatory Blood Pressure Monitoring and Treatment of Hypertension Investigators. Jama. 1997;278:1065-1072.
4. Ejaz AA, Kazory A, Heinig ME. 24-hour blood pressure monitoring in the evaluation of supine hypertension and orthostatic hypotension. J Clin Hypertens (Greenwich). 2007;9:952-955.
NATIONAL HEART FOUNDATION AND HIGH BLOOD PRESSURE RESEARCH COUNCIL OF AUSTRALIA AMBULATORY BLOOD PRESSURE MONITORING CONSENSUS COMMITTEE.
Geoffrey A. Head(a), Barry P. McGrath (b), Anastasia S. Mihailidou (c), Mark R. Nelson (d), Markus P. Schlaich (a), Michael Stowasser (e), Arduino A. Mangoni, (f) Diane Cowley (e).
(a) Baker IDI Heart and Diabetes Institute, Melbourne-AUSTRALIA, (b) Southern Health, Melbourne-AUSTRALIA, (c) Kolling Institute of Medical Research, Royal North Shore Hospital & University of Sydney, Sydney-AUSTRALIA, (d) Menzies Research Institute, Hobart-AUSTRALIA (e) Hypertension Unit, University of Queensland School of Medicine, Princess Alexandra Hospital, Brisbane-AUSTRALIA, (f) Division of Applied Medicine, University of Aberdeen, Aberdeen-UK.
Extract from Aust Fam Phys 2011, 40:877-880.