Part 1: Should out-of-clinic blood pressure monitoring be a routine component of cardiovascular risk management?
Elevated blood pressure is perhaps the most common modifiable risk factor for cardiovascular and kidney disease encountered
by general practitioners and practice nurses. Patients with increased blood pressure are at higher risk of stroke, myocardial
infarction, heart failure, atrial fibrillation, kidney disease and cognitive decline.1 An elevated systolic
blood pressure has been calculated to account for almost two-thirds (62%) of an individual’s stroke risk and half (49%)
of their risk for coronary heart disease.2
A survey of New Zealand adults found that a significant number of people were taking antihypertensive medicines or had
a systolic blood pressure that was ≥ 140 mmHg or a diastolic blood pressure ≥ 90 mmHg, i.e. the diagnostic criteria
for hypertension.3 With advancing age these numbers rise to almost 70% of males and 80% of females aged over
70 years.3 Hypertension is more prevalent in New Zealand adult Māori males (36%) than males of Pacific (29%)
or New Zealand European ancestry (28%).3 In New Zealand adult females, hypertension is also more common among
Māori (30%) and less common in those of New Zealand European ancestry (21%).3
Out-of-clinic testing increases the accuracy of blood pressure measurements
Once begun, antihypertensive treatment is generally life-long and often involves multiple medicines, thereby increasing
the prevalence of polypharmacy and the risk of adverse effects. It is therefore important that treatment decisions for
blood pressure are based on the best available information.
Clinic-based measurements often over-estimate blood pressure
Out-of-clinic blood pressure assessments can provide patients and clinicians with increased confidence that pharmacological
treatment for elevated blood pressure is appropriate. This is because clinic-based measurements tend to over-estimate
a patient’s blood pressure (see: “Blood pressures vary depending on when and where measurements are
taken”).
In a group of 200 patients in the United Kingdom, blood pressure measurements were taken in primary care. The systolic
blood pressure measured in the clinic was found to be on average 19 mmHg higher and the diastolic blood pressure 11 mmHg
higher than that calculated by ambulatory measurements over a twenty-four hour period.4
The benefits of out-of-clinic blood pressure testing include:5
- A large number of reproducible measurements are recorded that are not affected by the presence of a health professional
or the clinical setting
- The results of out-of-clinic blood pressure testing more accurately reflect the patient’s day-to-day blood pressure
than clinic measurements6
- Out-of-clinic blood pressure testing is more reliable at predicting cardiovascular morbidity and mortality
- Patients are provided with more accurate risk estimates upon which they can decide how to manage their health
- A reduction in the over and under-treatment of elevated blood pressure
Improving cardiovascular risk assessment
The intensity of cardiovascular interventions should be proportional to a patient’s cardiovascular risk.7 Cardiovascular
risk assessments that incorporate out-of-clinic blood pressure measurements provide a better starting point for discussions
about lifestyle and, where appropriate, pharmacological treatment, than do risk assessments based solely on clinic-based
measurements. Out-of-clinic blood pressure testing also allows clinicians to more accurately assess blood pressure in
patients who they suspect of being at increased cardiovascular risk, e.g. a young Māori adult with a strong family history
of chronic kidney disease.
Calculating cardiovascular risk from out-of-clinic blood pressure measurements
Current cardiovascular risk prediction models are based on clinic-based blood pressure measurements. This can be problematic
for clinicians using out-of-clinic blood pressure measurements with cardiovascular risk calculators, as out-of-clinic
blood pressures are generally lower than clinic-based measurements and it has been suggested that clinicians adjust for
this. There is little evidence to guide this practice, however, an Australian group of experts recommends adding 5 mmHg
to systolic and diastolic measurements.5
Detecting white-coat hypertension
Out-of-clinic blood pressure testing can reduce inappropriate blood pressure treatment by detecting patients with white-coat
hypertension. White-coat hypertension occurs when a patient with otherwise normal blood pressure has elevated blood pressure
due to the anxiety associated with measurement in a clinical setting.
White-coat hypertension occurs in 13% of the general population,5 and is more common in females and in
people who do not smoke.8 As many as one-third of adult patients diagnosed with hypertension, using clinic-based
measurements, can be expected to have normal blood pressure on reassessment with out-of-clinic testing.5
Patients with white-coat hypertension are recommended to have annual blood pressure assessments, depending on their
cardiovascular risk,5 and regular HbA1c measurements.8 These patients are at increased
risk of left ventricular hypertrophy and type 2 diabetes,8 largely due to metabolic abnormalities, e.g. impaired
glucose metabolism and elevated body mass index (BMI).9
Detecting masked hypertension
Masked hypertension is the reverse of white-coat hypertension and occurs when out-of-clinic blood pressure measurements
are ≥ 135/85 mmHg and clinic-based measurements are < 140/90 mmHg.10 People with masked hypertension often
have subclinical cardiovascular disease.8
The prevalence of masked hypertension is reported to be between 10 and 17% of the general population, up to 29% of people
with untreated diabetes and as many as half of people with treated hypertension or exercise hypertension may have masked
hypertension.5
Clinical suspicion of masked hypertension should be increased in any patient with normal blood pressure in the clinic
but with a family history of early-onset cardiovascular disease, e.g. onset prior to age 60 years, or evidence of target
organ damage on investigation consistent with hypertension,5 e.g.:
- Albuminuria or proteinuria
- Left ventricular hypertrophy
- Peripheral artery disease
- Retinal haemorrhage
Using out-of-clinic monitoring to guide treatment for hypertension
Out-of-clinic monitoring can be used to titrate doses of antihypertensive medicines, once treatment has been initiated,
in patients who are confirmed to be adherent to treatment.
A pharmacist-led study of 348 patients who were randomised to receive home-monitoring of blood pressure or usual care
reported that significantly more patients (54%) who received home-monitoring achieved treatment goals compared to patients
receiving usual care (35%).11 Furthermore, the patients who undertook home monitoring of blood pressure achieved,
on average, 12 mmHg larger reductions in systolic blood pressure and 6 mmHg larger reductions in diastolic pressure, compared
with patients receiving usual care.11 Patients who participated in home-monitoring received more intensive
treatment for hypertension.11 Patient satisfaction with treatment, on average, was higher among those who
participated in home-monitoring compared with patients who received usual care.11
Investigating treatment resistant hypertension
Resistant hypertension can have a variety of causes such as excessive alcohol consumption, high sodium intake, underlying
endocrine disorders, or pseudohypertension, i.e. elevated blood pressure readings caused by arterial incompressibility
due to atherosclerosis. Out-of-clinic blood pressure testing can confirm the presence of sustained elevations in blood
pressure in patients who are confirmed to be adherent to treatment, thus signifying the need for further investigation.
Pseudohypertension should be considered in older patients who appear to have treatment resistant hypertension but develop
symptoms consistent with hypotension with increasing doses of antihypertensive medicine. Osler’s manoeuvre has been proposed
to detect pseudohypertension. If the patient’s radial artery is firm on palpation, despite the blood pressure cuff being
above systolic pressure, this is a positive Osler’s manoeuvre which is suggestive of pseudohypertension. However, the
ability of Osler’s manoeuvre to detect pseudohypertension in clinical practice is poor.12
For information on the management of elevated blood pressure see: “Hypertension
in adults: The silent killer”, BPJ 54 (Aug, 2013).
Blood pressures vary depending on when and where measurements are taken
Hypertension in adults is generally defined as a clinic-based systolic blood measurement that is ≥ 140 mmHg and/or a
diastolic blood pressure measurement ≥ 90 mmHg.1, 6 With advancing age, however, increasing arterial stiffness
and decreased peripheral resistance mean that diastolic pressure cut-offs are not recommended when diagnosing hypertension
in older patients.13
Single measurements of blood pressure are often taken in primary care due to time pressures. However, it is recommended
that when clinic blood pressure measurements are used to calculate a patient’s cardiovascular risk that the average of
two seated measurements be taken,7 at least two minutes apart, ideally from both arms.
As clinic-based blood pressure measurements are generally higher than out-of-clinic measurements a lower threshold is
used to stratify blood pressure with ambulatory or home-based testing (Table 1).6
Table 1: Definitions of hypertension by measurement method1, 6
| Measurement method |
Systolic (mmHg) |
|
Diastolic (mmHg) |
Clinic |
≥ 140 |
and/or |
≥ 90 |
Ambulatory |
|
|
|
• Daytime or awake |
≥ 135 |
and/or |
≥ 85 |
• Night-time or asleep |
≥ 120 |
and/or |
≥ 70 |
• 24-hour |
≥ 130 |
and/or |
≥ 80 |
Home-based |
≥ 135 |
and/or |
≥ 85 |
Part 2: How to perform out-of-clinic blood pressure testing
Patient education is essential before using out-of-clinic of blood pressure testing. The procedure should be explained
to patients, training with the device provided and written instructions given for the patient to take away.
The auscultatory method of determining blood pressure is preferred in patients with unstable atrial fibrillation as
variations in ventricular filling time, stroke volume and contractility may result in blood pressure variability.14 Some
automated blood pressure monitoring devices, however, are able to detect the presence of atrial fibrillation and these
appear to have a high level of accuracy.14
Deciding whether to use ambulatory or home monitoring of blood pressure testing
Ambulatory blood pressure testing is considered to be the gold standard for confirming elevated blood pressure,1 and
detecting white-coat or masked hypertension.5 Studies also report that left ventricular hypertrophy and carotid
arterial wall thickness and other markers of organ damage correlate more closely with elevated ambulatory blood pressure
than clinic-based measurements.6 Ambulatory blood pressure testing is subject to variability, however, like
other techniques for blood pressure testing.
Home-based testing of blood pressure is recommended when ambulatory blood pressure testing is not available.5 Home-based
testing also has several advantages, compared to ambulatory blood pressure testing (Table 2).
Table 2: Comparison between 24-hour ambulatory and home-based blood pressure testing5, 15
| 24-hour ambulatory blood-pressure is able to detect |
The advantages of home-based blood pressure testing |
- Surges in morning blood pressure
- Short-term variations in blood pressure
|
- Better tolerated by patients
- Home-monitoring devices cost less than ambulatory devices
- Increasing patient engagement resulting in improved motivation and adherence to treatment
|
How to perform ambulatory blood pressure testing
Ambulatory blood pressure devices are normally worn for 24 hours on a belt or in a pouch with a tube connecting to a
sphygmomanometer on the patient’s dominant upper arm.6 Blood pressure measurements are taken at regular intervals,
often every fifteen minutes during the day and every 30 minutes overnight.6 Patients are asked to wear the
device during a normal day, but to refrain from strenuous exercise, and, when the cuff is inflated to stop moving
and talking and to keep their arm still with the cuff at the level of the heart.6 The height of
the cuff is especially important if it necessary to use a device with a wrist cuff. The patient should note any
events that might affect their blood pressure as well as meals, medicines, rising and going to bed and any symptoms
they might experience, e.g. dizziness.6
How to perform home-based blood pressure testing
Home blood pressure testing involves patients measuring their own blood pressure in their home, with the direction of
a health professional. Advise patients to take measurements at approximately the same time in the morning and evening,
over the course of a week.
The optimal conditions for home blood pressure measurements are a quiet room, following five minutes of seated rest,
with the patient’s feet flat on the floor, legs uncrossed, upper arm bare, back and arm supported in a relaxed position
with the cuff at heart level.5 Advise patients to take measurements after voiding and before any medicines,
food or vigorous exercise.5 Caffeine and tobacco smoke can increase blood pressure and measurements are best
at least 30 minutes before or after these stimulants.5 Two consecutive measurements for systolic and diastolic
pressure should be recorded, one minute apart. Patients should note anything that may affect blood readings, e.g. a poor
night’s sleep.
If orthostatic hypotension is suspected request that patients perform a baseline recording and then two blood pressure
measurements 1 minute and 3 minutes after standing.5
Blood pressure measurements should not be taken when patients are:5
- Stressed
- Uncomfortable
- In pain
- Affected by extremes of temperature, e.g. in a poorly heated home during winter
Which devices can be used to take out-of-clinic measurements?
Only validated devices (see below) should be used to measure out-of-clinic blood pressure. Devices for home monitoring
will ideally have:5
- Automatic inflation
- Memory storage to eliminate recording errors
- An appropriate cuff size for the patient; a cuff that is too small will overestimate blood pressure and a cuff that
is too large will underestimate blood pressure
Devices which measure blood pressure at the brachial artery are considered to be more reliable than those which record
at the patient’s wrist or finger.5 It may be necessary to use a validated wrist device if the patient has
a large arm circumference.5
Mobile phone apps are available that claim to be able to measure blood pressure without the use of a blood pressure
cuff. These apps are not validated and are not recommended for the diagnosis or management of hypertension.16
Home-monitoring blood pressure devices validated for use by the British Hypertension Society can be purchased from:
http://omronhealthcare.co.nz/product-category/omron-blood-pressure-monitors/.
Different cuff sizes are available and devices will typically alert users if measurements
are usable or need to be repeated. The devices provide an average for the last three readings taken within a ten minute
period. 24-hour ambulatory devices also validated by the British Hypertension Society can be purchased from: https://online.ebos.co.nz
The possible adverse effects of out-of-clinic blood pressure testing
Home blood pressure testing may cause adverse responses in some patients. For example, patients with elevated blood
pressure may become anxious due to continually high readings which may adversely affect subsequent readings. Other patients
may become obsessed with their blood pressure and take an excessive number of recordings. There is also the possibility
that some patients may adjust their treatment regimen in response to readings without consulting with a health professional.8 The
high cuff pressure may cause discomfort for patients with either undiagnosed severe hypertension or severe hypertension
that is resistant to treatment.