Detailed recommendations for home BP monitoring were published in the first international guidelines for home BP monitoring in 2000 (6) and incorporated thereafter in the 2003 European Society of Hypertension recommendations for BP measurement. The latest guidelines on the topic have come out of the European Society of hypertension in 2010 and form the backbone of this article.
I - The oscillometric method - calibration and validation
The pressure sensor should be calibrated periodically to maintain accuracy. Most manufacturers service and calibrate their own machines but this is costly. The details of service centres can be found in the instruction manual that comes with the devices. Calibration usually requires two standard mercury sphygmomanometers, two observers and a supervisor. On an average, 10-12 subjects are required for validation. The detailed process for the validation of blood pressure measuring devices in adults has been given in the European Society of Hypertension International Protocol revision 2010 (7). To pass the validation process, a device must achieve all minimum pass requirements.The cuff is inflated to a pressure initially in excess of the systolic arterial pressure and then reduced to below diastolic pressure over a period of about 30 seconds.
When blood flow is nil (cuff pressure exceeding systolic pressure) - or unimpeded (cuff pressure below diastolic pressure), cuff pressure will be essentially constant.
When blood flow is present, but restricted, the cuff pressure, which is monitored by the pressure sensor, will vary periodically in synchrony with the cyclic expansion and contraction of the brachial artery, i.e., it will oscillate.
The values of systolic and diastolic pressure are computed, not actually measured from the raw data, using an algorithm; the computed results are displayed.
II - Indications
- Uncover white-coat hypertension. "White-coat" hypertension is known as elevated BP in the office with low home BP. Subjects with white-coat hypertension are at a marginally increased cardiovascular risk and an increased risk of developing sustained hypertension.
- Evaluate "masked" hypertension. Conversely, normal BP in the office with elevated home BP has been termed masked hypertension (8,9). Masked hypertension is associated with increased risk of cardiovascular events, similar to that of uncontrolled hypertension (10).
- Improve compliance with long term treatment. It is thought that HBPM may increase the involvement of patients in hypertension management.
- Improve conditions where strict blood pressure control is mandatory (high-risk patients and pregnancy).
- Enable a more precise initial diagnosis of hypertension. HBPM provides clinically useful information on BP level and profile to practising doctors.
III - Advantages
- Annulling the white coat effect. Since the measurements are made in the usual environment of each individual, away from the physician’s office, the possibility of white coat hypertension is largely taken care of.
- Indicating target organ damage and predicting cardiovascular events. Cross-sectional studies have shown that home BP is indicative of hypertensive target organ damage (11). The reduction in home BP during treatment also predicts regression of left ventricular hypertrophy (12). In the Tecumseh study, home BP was more predictive than clinic BP of hypertension and normotension after 3 years in untreated borderline hypertensives (13). In the Ohasama study, home BP had a stronger predictive power than clinic BP for cardiovascular and overall mortality in the general population (14,15), and predicted first-time stroke better than clinic BP (16). In a study, home BP predicted cardiovascular events in elderly, treated hypertensive patients, whereas office BP did not (specific studies are needed to compare the prognostic values of clinic, home, and ambulatory BP measurements).
- Performing multiple measurements in series, over a longer period.
- Detecting the masked phenomenon.
- Cost - Less expensive and more widely available and convenient (compared to ABPM).
- Improving patients’ compliance with treatment and hypertension control rates.
- Possibility of digital storage, printout, PC download or teletransmission of blood pressure values (some devices).
IV- Difficulties and pre-requisites
- Patient training (however, it is simple for automated devices)
- Inaccurate devices, not properly calibrated
- Reliability of patient-reported data
- Anxiety resulting in excessive monitoring
- Treatment changes made by patients on the basis of casual home measurements without doctor’s guidance
- Ongoing debate over normality thresholds and therapeutic targets mainly in patients at high cardiovascular risk
- Unlike ABPM it does not allow the assessment of BP during sleep or at work, or the quantification of short-term BP variability (17,18)
- Patient-training under medical supervision. Trained nurses and/or pharmacists can have an important part in the implementation of HBPM in daily practice. Training should include information regarding hypertension, BP variability, conditions and procedure for self-monitoring, advice on equipment choice and its proper use, and interpretation of results. The provision of telemonitoring facilities may be of further advantage (19).
- Support of nurse or family member-. In some patients, in particular the elderly with motor or cognitive impairment and in young children- the support of a trained nurse or family member may be needed. Adequate blood pressure measurements schedule and data reporting by patient, or patient's aide is always necessary.
- Devices - The use of auscultatory devices (mercury, aneroid or other) is not generally recommended for HBPM except for special cases (for example, patients with arrhythmias trained in auscultatory BP measurement). Patients only rarely master the auscultatory technique required for measurement of BP using these devices.
- Choice among validated devices. Specific validation of devices destined to use in special populations - the elderly, children, normal pregnancy, pre-eclampsia, end-stage renal disease and arrhythmias - is required. Use of Manufacturer-facilitated devices with a range of cuffs for varying arm sizes and capable of automatically calculating average BP are priveleged in these instances (20).
- Selection of the appropriate size of the cuff to fit the arm of each individual is essential for an accurate BP measurement (the inflatable bladder of the cuff should cover 80–100% of the individual’s arm circumference). The use of a small cuff for the size of the arm can result in overestimation of BP, whereas a too large one in its underestimation. Although standard cuffs are appropriate for most patients, in those with small (<24 cm) or large (>32 cm) arm circumference only the devices equipped with appropriate sized cuffs should be used.
V- Readings, data-processing and interpretation
With its inflatable bladder centred on the anterior surface of arm (most cuffs have an indication of proper placement) and the lower edge of the cuff approximately 2–3 cm above the bend of the elbow, reading should be taken. Morning measurement before drug intake and evening reading before meal is preferred. Two measurements should be taken per occasion (1–2 min apart) (21). Results should be immediately reported in a specific logbook or stored in device memory.
In rare cases of significant (>10 mm Hg) and consistent BP difference between arms, the patient should be advised to use the arm with higher BP values for HBPM.
How to proceed with the data obtained?
Morning and evening blood pressure should be measured for atleast 5 days and ideally 7. In each sitting, a minimum of 2 readings should be taken 1-2 minutes apart. Measurements of the first monitoring day are usually higher and unstable and are excluded. This is because of the anxiety induced from using the technique but the patient generally tends to settle over the next day.
The average of a series of measurements taken as described should be used for the clinical decisions based on HBPM readings. The users should be informed that BP may vary between measurements and be instructed not to be alarmed by high or low BP measured on a single occasion, unless an important elevation or reduction persists or is associated with symptoms of clinical relevance (for example, dyspnoea, chest pain).
For follow-up patients: less frequent measurements (for example, once or twice per week) could be regularly performed aimed at reinforcing compliance, although isolated readings should never be used for diagnostic purposes. Overuse of the method and self-modification of treatment on the basis of HBPM should be avoided (22). Casual, isolated home measurements can be very misleading and should not by themselves constitute the basis for clinical decisions (23).
Average systolic home BP >135 mm Hg and/or diastolic>85 mm Hg indicates elevated BP. The levels of ‘normal’ and "optimal" home BP are still under investigation, provisionally suggested values being <130/80 mm Hg for normal home BP (24). In high-risk subjects (for example, those with diabetes or chronic kidney disease) lower home BP values should probably be achieved but the targets have not yet been defined.
In most patients, there is a concordance in the blood pressure values measured at home or in the hospital settings; making it easy to categorize them as normotensives or hypertensives. However, cases of discrepancies between office and home (or ambulatory) BP measurements are not uncommon in both untreated and treated subjects.
Patient training, knowledge of devices and cuffs and mode remain important considerations; if used casually, home blood pressure monitoring may cause unnecessary anxiety and unwarranted medications. Home BP monitoring offers advantages over clinic BP measurements provided the calibration, validation process, readings, data processing and interpretation are performed as described and that its indications and pre-requisites are met. Home BP monitoring should form a part of management of all hypertensive patients.