How to measure blood pressure: focus on general practice

Introduction

An accurate blood pressure (BP) assessment is needed to diagnose and treat hypertension. Oscillometric devices are increasingly used to estimate BP instead of the auscultation-based methods.¹ The estimation algorithms are device specific and only validated devices recommended by the British and Irish Hypertension Society should be used in health care. Meticulous choice of correct size of BP cuffs is crucial. For arms requiring an extra-large cuff, using a large cuff overestimates systolic BP by 9.6 mmHg and a regular cuff overestimates systolic BP by 19.5 mmHg.² Conversely, using too large a cuff underestimates BP. Circumference guides on cuffs must be observed and, if in doubt, arm circumference must be measured before BP measurement. BP can be measured with a range of techniques and settings. Each have their own pros and cons.

Office BP

Taken in clinical settings, office BP remains the most widely used approach for hypertension screening, BP assessment in acute settings, monitoring of BP control, and in cardiovascular risk calculators, such as QRISK3. The approach is prone to substantial measurement errors unless performed by trained staff who strictly adhere to measurement guidelines (Box 1).³ Failure to measure both arms and adopt results from the higher reading arm can misclassify up to 12% of people with hypertension and underestimates cardiovascular risk.⁴ Additional considerations are needed for BP measurement in pregnant women, patients with obesity, and patients with irregular heart rhythm (Table 1). Office BP may substantially differ from out-of-clinic BP, leading to overdiagnosis of sustained hypertension (white-coat hypertension) or its underdiagnosis (masked hypertension). Despite the limitations, office BP is recommended by the National Institute for Health and Care Excellence (NICE) for monitoring the response to lifestyle changes or antihypertensive pharmacotherapy.³

Table 1.

Blood pressure measurement in challenging clinical scenarios

Scenario	Challenge	Solution
Atrial fibrillation and other pulse irregularities	Fluctuations in cardiac output and cardiac cycle length Reduced fidelity of oscillometric algorithms	Palpate radial or brachial pulse before measuring BP to detect pulse irregularities Use an auscultatory method with a calibrated analogue sphygmomanometer Cuff deflation should be ≤2–3 mmHg per second Average ≥3 BP measurements
Obesity	Increase in arm circumference More conical arm shape	Measure upper arm circumference Choose cuff with the bladder length covering 80–100% of the circumference Standard cuff: 27–34 cm Large cuff: 35–44 cm Extra-large cuff: 45–52 cm Leg cuff may be needed for bigger arms Use wrist BP device if cuff of suitable size is not available; the wrist should be rested at heart level for measurement
Pregnancy	Haemodynamic changes and oedema, including preeclampsia Effects on fidelity of oscillometric algorithms	Use oscillometric devices validated for use in pregnancy

BP = blood pressure.

Box 1.

Recommendations for office blood pressure measurements

Ensure adequate initial training and periodic review of the healthcare staff measuring BP Use validated BP monitors listed by the British and Irish Hypertension Society Ensure BP devices are appropriately validated, maintained, and regularly recalibrated Measure BP in a relaxed, temperate setting, with the person quiet and seated, legs uncrossed, and their arm outstretched and supported at heart level Use BP cuff of size appropriate to the arm circumference On initial assessment, measure BP in both arms; repeat the measurements if the difference is >15 mmHg. If the difference remains, measure further BP in the arm with the higher reading If office BP is ≥140/90 mmHg, repeat the measurement. If the second measurement is substantially different from the first, take a third measurement and record the lower of the last two measurements as the office BP

BP = blood pressure.

Unattended automated oscillometric BP

Unattended automated oscillometric BP (AOBP) refers to taking BP measurements automatically at set intervals without an observer present. Usually, BP measurements start after 5 minutes, with three measurements taken 1 minute apart. This approach may help to overcome the overestimation of BP by office BP. However, when a BP measurement protocol is strictly followed, the difference in BP using unobserved and observed AOBP is small. NICE does not provide specific guidance on using unobserved AOBP; the choice remains with NHS service providers.³

Ambulatory BP monitoring

Ambulatory BP monitoring (ABPM) assesses daytime and night-time BP, helping identify white-coat hypertension, masked hypertension, or nocturnal hypertension. NICE recommends ABPM to confirm a new diagnosis of hypertension, using mean daytime ABPM values ≥135/85 mmHg.³ ABPM is accurate and reproducible, and a better predictor of future cardiovascular risk than office BP. However, ABPM monitors are expensive and not practical for routine long-term monitoring of BP in the NHS. NICE restricts recommendations for ABPM for hypertension diagnosis only. Although ABPM may cause discomfort and anxiety, the test could be completed by 88% of patients (96% for the daytime period). Night-time measurements could be affected by sleep disorders and nocturnal urination, making daytime measurements more reliable, as per NICE guidance.³

Home BP monitoring

Home BP monitoring (HBPM) refers to the patient repeatedly taking their own BP measurements at home. When using HBPM, two consecutive measurements should be taken, at least 1 minute apart, twice daily, ideally in the morning and evening. The recordings should continue for at least 4 (ideally 7) days. The measurements taken on the first day are discarded, and the remaining measurements are averaged. Like ABPM, the HBPM diagnostic threshold is ≥135/85 mmHg. NICE recommends using HBPM to confirm the diagnosis of hypertension if ABPM is unsuitable or the person cannot tolerate it.³ Because of limited access to ABPM in primary care, HBPM is widely used in general practice. HBPM, like ABPM, better predicts cardiovascular outcomes than office BP and is more cost-effective. HBPM is a method of choice for treatment monitoring in people with a white-coat effect or masked hypertension. Patients with sustained hypertension may also choose to use HBPM, which helps patient involvement and better BP control.⁵ There are, however, concerns about patients’ ability to correctly take and record BP measurements. Patients offered HBPM must be provided with in-person training with advice on correct timing, posture, arm positioning, recording of the results, and their interpretation. HBPM is also helpful for telemonitoring, providing that quality standards are met.

White-coat hypertension and masked hypertension

White-coat hypertension and masked hypertension are common and occur in 10–15% of people with hypertension. White-coat hypertension is defined by raised office BP when mean daytime or 24-hour BP are normal. Masked hypertension is defined by mean daytime, night-time, or 24-hour BP being in hypertension ranges when office BP is normal. Masked hypertension poses a significant cardiovascular risk that may be similar to the risk of sustained hypertension.⁶ The prognostic effect of white-coat hypertension is unclear. AOBP measurement is less likely to produce a white-coat effect than manual office BP. ABPM and HBPM are methods of choice for defining the two states, with ABPM having the advantage of assessing night-time BP.

Postural hypotension

Postural hypotension is defined as a fall of ≥20/10 mmHg in BP within 3 minutes of standing from lying. Postural hypotension is common, underreported, and a risk factor for falls, cognitive decline, and mortality.⁷ Postural hypotension affects 20% of older adults with hypertension.⁸ NICE recommends assessing postural BP changes in people with postural symptoms (for example, postural dizziness or falls) and those with type 2 diabetes and aged ≥80 years.³ When assessing postural BP changes, BP should be taken repeatedly over 1–3 minutes after standing from resting in the supine position. However, practicalities in primary care mean that it is usually measured on standing from sitting, which may underestimate a postural BP drop. If postural hypotension is suspected but an initial sit-to-stand measurement does not reach the diagnostic threshold, further BP assessment using lying and standing measurements should be made. Considering the time needed for accurate assessment of postural BP changes, the costs might be reduced by such measurements being taken by nurses or healthcare assistants, but no cost-effectiveness data exist to support this. Postural BP changes may fluctuate, and postural hypotension may be underdiagnosed based on office assessment (masked hypotension). Out-of-clinic methods for evaluating postural BP changes such as ABPM can provide valuable information in such situations; however, protocols for home postural BP measurement are not well established.

Nocturnal hypertension

Nocturnal hypertension predicts cardiovascular outcomes after accounting for daytime BP, even stronger than daytime BP. In healthy subjects, night-time BP decreases by 10–20% of daytime BP (normal dipper pattern). Reverse dipping is when night-time BP rises are common (3–39%) and not measuring night-time BP puts reverse dippers at risk for hypertension underdiagnosis. Emerging data suggest possible benefits of screening for nocturnal BP.⁹ However, the NICE guidance advises that, when using ABPM or HBPM to monitor the response to treatment, use BP obtained during waking hours.³ Although ABPM may cause discomfort and anxiety, the test could be completed by 88% of patients (96% for the daytime period). Night-time measurements could be affected by sleep disorders and nocturnal urination, making daytime measurements more reliable, as per NICE guidance.

New approaches

Several ‘wearable’ technologies have been introduced to estimate BP, including those using a calibrated radial pulse waveform and photoplethysmography, devices for nocturnal HBPM, and Intelli wrap cuff technology for HBPM. These technologies are being assessed and are not part of routine practice. There is no agreed international validation protocol for cuffless wearable technologies.¹⁰ The recommended use of wrist devices with cuffs is currently limited to people with obesity in whom upper-arm BP cannot be obtained.

Quality control and conclusions

Accurate BP measurements (Box 1) improve patient management and are cost-saving for GP surgeries, reducing overdiagnosis and overtreatment of hypertension, avoiding unnecessary repeat tests, and facilitating achievement of quality indicators. Time constraints often affect the accuracy of office BP measurements because of deviation from the clinical guidelines. Quality control metrics consistently recorded in electronic health records help audit BP measurements. BP assessments should be recorded as individual measurements rather than the average values to help quality assurance, making full use of codes for postural measurements and noting the higher-reading arm to guide future BP measurements. Similarly, individual ABPM and HBPM readings should ideally be recorded using automated data transfer processes. If BP is outside the target range during acute clinical presentations, this should be coded, and measurements repeated during a clinically stable state. GP practice policies should include staff training and assessments to standardised protocols. Better provision of primary care with ABPM is highly desirable.

Funding

None.

Provenance

Freely submitted; externally peer reviewed.

Competing interests

Christopher Clark is a member of the British and Irish Hypertension Society Standing Committee on Blood Pressure Measurement. He is a current National Institute for Health and Care Excellence (NICE) guideline committee member for hypertension, cardiovascular risk, and lipid modification guidelines. Eduard Shantsila is a current NICE guideline committee member for lipid modification guidelines.