Which error will result in false high diastolic reading while measuring a clients blood pressure during a physical examination?

Accurate and reliable blood pressure (BP) measurement is critical for the proper diagnosis and management of hypertension. So much so that a 5 mm Hg BP measurement error can lead to incorrect hypertension classification in 84 million individuals worldwide, according to a position statement published in the Journal of Hypertension. Understanding the ways BP measurement goes wrong, and how to tackle them, can improve diagnosis and management of hypertension.

The AMA has developed online tools and resources created using the latest evidence-based information to support physicians to help manage their patients’ high BP. These resources are available to all physicians and health systems as part of Target: BP™, a national initiative co-led by the AMA and American Heart Association.

Unfortunately, BP measurement is often suboptimally performed in clinical practice, which can lead to errors that inappropriately alter management decisions in 20% to 45% of cases. This inaccuracy has persisted despite extensive education and efforts to raise awareness on the adverse consequences of incorrect clinic BP measurement, according to the Lancet Commission on Hypertension Group position statement. That statement was co-written by AMA Vice President of Health Outcomes Michael Rakotz, MD, and Gregory D. Wozniak, PhD, who is director of outcomes analytics at the AMA.

“Many measurement errors can be minimized by appropriate patient preparation and standardized techniques. Validated semi-automated or automated upper arm cuff devices should be used instead of auscultation to simplify measurement and prevent observer error,” the position statement says.

The position statement cites multiple causes of inaccuracy in measurement. Here are four ways BP measurement goes wrong and how to address them.

There are instances in which the BP-measurement error is caused by the patient. Acute meal ingestion, caffeine or nicotine use can all negatively affect BP readings, leading to errors in measurement accuracy. If the patient has a full bladder, that can lead to an error in systolic BP of between 4 mm Hg and 33 mm Hg, compared with the white-coat effect can have an error of up to 26 mm Hg.

It is important for the patient to rest comfortably in a quiet environment for five minutes in a chair. The patient should also have an empty bladder and not have eaten, ingested caffeine, smoked or engaged in physical activity at least 30 minutes before the measurement.

Errors can also occur due to inaccuracies with the procedure. For example, having the patient’s arm lower than heart level can lead to an error of 4 mm Hg up to 23 mm Hg. Procedure related error might also occur if the patient’s legs are crossed at the knees or if the patient is allowed to talk during the BP measurement. A fast deflation rate also can harm accuracy.

If a cuff is too small or too large, errors in measurement can occur. Adding to inaccuracy is automated device variability, which can account for average error in systolic BP of between -4 mm Hg and -17 mm Hg.

“An important issue with automated devices is that many have not been clinically validated for measurement accuracy,” says the statement. “Clinical validation involves demonstrating that the device meets the accuracy requirements of international BP measurement standards.”

The process of clinical validation involves performing a protocol-based comparison using multiple measurements against blinded, two-observer auscultatory reference standard. For greater accuracy, only validated devices should be used.

One common error in the clinical setting is failure to include a five-minute rest period. Errors can also include talking during the measurement procedure, using an incorrect cuff size and failure to take multiple measurements.

Time constraints are also quite common for casual measurements. This is because a casual reading takes about two minutes to perform compared with eight minutes for a standardized measurement. Physician readings were also found to be higher than nurse readings, which is the white coat effect in action.

The physician, nurse or other health professional is responsible for performing proper BP measurement while ensuring—to the greatest extent possible—that all potential causes of inaccuracy are avoided.

Training programs can lead to short-term success in BP readings. These can be web-based or in-person. However, shorter web-based programs are preferred because of their practical advantages, lower cost and scalability.

Quality improvement programs that combine use of automated office BP measurement with physician and care team education on proper measurement, as well as advice on clinical workflow enhancement, can also improve readings.

Measurement of blood pressure is an important diagnostic and monitoring procedure. This article, the first in a two-part series, discusses the principles of blood pressure measurement; it was originally an early online publication and was updated on 22/06/2020

Abstract

Blood pressure measurement is a common diagnostic and monitoring procedure, and accuracy is essential if patients are to receive the appropriate treatment and care in a timely manner. This article, part 1 in a two-part series, discusses principles of blood pressure monitoring and the devices used. Part 2 will describe the procedures for monitoring blood pressure.

Citation: Jevon P (2020) Blood pressure 1: key principles and types of measuring equipment. Nursing Times [online]; 116: 7, 36-38.

Author: Phil Jevon is academy tutor, Manor Hospital, Walsall, and honorary clinical lecturer, School of Medicine, University of Birmingham.

• Scroll down to read the article or download a print-friendly PDF here (if the PDF fails to fully download please try again using a different browser)
• Read part 2 of this series here

Introduction

The accurate measurement of blood pressure (BP) is an important diagnostic and monitoring tool in a wide range of clinical conditions. Good practice is essential when measuring BP to ensure abnormalities are identified and patients receive the correct treatment and care in a timely manner. This article reviews the principles guiding the non-invasive measurement of BP. Part 2 of the series will explain the different procedures for measuring BP.

General principles of measurement

Terms used in the measurement of BP are outlined in Box 1.

It can be measured:

• Manually, using the auscultatory method – this involves listening to arterial sounds (named ‘Korotkoff sounds’, after Nicolai Korotkoff, a Russian surgeon who first described the auscultation method of measuring BP in 1905);
• Automatically, using the oscillometric method – this detects variations in pressure oscillations due to arterial wall movement.

Both methods use a measuring device attached to an inflatable cuff that is placed around the patient’s upper arm, inflated to occlude the artery under the cuff, then released in a controlled manner.

BP is a variable haemodynamic phenomenon, and can be influenced by a range of factors; these are outlined in Box 2. In some situations, for example when managing a patient with hypertension, it is advised to undertake three consecutive BP readings to improve accuracy. Posters and videos outlining how to measure BP can be downloaded here

Box 1. Definitions of terms

• Blood pressure – the pressure of blood against the walls of the main arteries
• Systolic blood pressure – peak blood pressure in the artery following ventricular systole (contraction)
• Diastolic blood pressure – level to which the arterial blood pressure falls during ventricular diastole (relaxation)
• Hypertension – high blood pressure: consistently >140/90mmHg
• Hypotension – low blood pressure: typically a systolic reading of <90mmHg

Sources: National Institute for Health and Care Excellence (2019); McFerran and Martin (2017)

Box 2. Factors that can cause a variation in blood pressure

• Emotional state
• Temperature
• Respiration
• Bladder distension
• Pain
• Exercise
• Age
• Food consumption
• Race/ethnicity
• Tobacco use
• Diurnal variation (blood pressure is at its lowest during sleep)
• Alcohol use
• ‘White coat’ hypertension (raised blood pressure when measured in clinical settings)
• Sudden change in posture
• Underlying medical conditions, such as renal failure, diabetes, anaphylaxis, hypovolaemia

Source: O’Brien (2015)

Arm selection and patient position

BP should initially be measured in both arms, after which the arm with the higher reading(s) should be used for subsequent measurements (O’Brien, 2015). Although a difference in BP measurements between the arms can be expected in 20% of patients, if this difference is >20mmHg for systolic or >10mmHg for diastolic measurement, BP should be measured on both arms for the next reading. If these differences are seen in three consecutive readings (with a one-minute gap between each), further investigation may be indicated (O’Brien, 2015).

The patient’s arm should be supported while BP is measured; if it is unsupported muscle contraction in the arm can lead to an erroneous increase in the BP reading by as much as 10% (O’Brien, 2015).

The arm should be positioned at heart level: if it is lower than the heart, this can lead to overestimation, while being above the level of the heart can lead to an underestimation. This error may be as great as 10mmHg (O’Brien, 2015). If seated, the patient should not cross their legs as this can lead to an increase in BP.

Cuff size

It is vital that the appropriate cuff size is used when measuring BP. Miscuffing – particularly using a cuff that is too small – can lead to inaccurate readings (O’Brien, 2015): if a cuff is too small, the BP will be overestimated and, if it is too big, the BP will be underestimated. In general, there are three cuff sizes:

• Children or small adults;
• Standard adults;
• Obese adults.

The inflatable bladder should encircle between 80% and 100% of the arm.

Manual auscultatory measurement

Manual BP measurement devices require the user to inflate the upper-arm cuff to occlude the brachial artery, then listen to the Korotkoff sounds through a stethoscope while the cuff is slowly deflated. When the cuff is slowly deflated, five different sound phases can be heard:

• Phase I – a thud;
• Phase II – a blowing or swishing noise;
• Auscultatory gap – in some patients, the sounds disappear for a short period;
• Phase III – a softer thud than in phase I;
• Phase IV – a disappearing blowing noise;
• Phase V – silence: all sounds disappear (O’Brien, 2015).

Practically, the systolic reading is when the Korotkoff sounds are first heard and the diastolic reading is when they disappear.

The patient’s systolic (phase I) and diastolic (phase V) BP are recorded from the readings on the sphygmomanometer. Although portable and generally reliable, manual BP devices require clinical skill and are prone to observer bias (Medicines and Healthcare products Regulatory Agency, 2019). Mercury sphygmomanometers are considered the ‘gold standard’ in BP measurement but, as mercury is a toxic substance, there are maintenance and disposal problems and environmental concerns. As a result, these devices are being phased out: their use has already been banned in some European countries, while in the UK a ban on their sale will be introduced from 31 December 2020 (MHRA, 2019). It would, therefore, be prudent for healthcare providers still using mercury sphygmomanometers to start planning to phase these out and to ensure health professionals are competent at using alternative devices.

Devices that are generally used for manual BP measurement include:

• Aneroid sphygmomanometer – this replaces the mercury manometer with an aneroid (liquid-free) gauge that registers pressure using a bellows and lever system (O’Brien, 2015), and requires use of a stethoscope;
• Electronic sphygmomanometer – this battery-powered device replaces the mercury manometer with a pressure sensor and electronic display. The display may be numerical, or a circular or linear bar graph. No stethoscope is needed.

Where local protocols dictate that mercury devices still need to be used, Control of Substances Hazardous to Health (COSHH) and health and safety procedures and regulations should be followed (MHRA, 2019). Health professionals should be trained in:

• Safe handling during normal use;
• Storage of the device;
• What to do in the event of a mercury spillage (mercury spillage kits should be available);
• What procedures to follow in the event of mercury disposal or when the device is discarded (MHRA, 2019).

Although aneroid sphygmomanometers are mercury free and easy to use, wear and tear or mechanical shock to the mechanism can lead to incorrect readings; this means regular calibration checks are required. Electronic sphygmomanometers are prone to observer bias and require clinical skill to use accurately (MHRA, 2019). Devices need regular maintenance in line with the manufacturer’s instructions and local policy.

Automated measurement

Automated electronic BP devices

Most automated BP measurement devices in current clinical practice use the oscillometric method. Each arterial pulse wave results in a small rise and fall in the volume of the limb which, in turn, causes an increase then a decrease in the pressure within the encircling cuff (Lewis, 2019). The oscillometric method relies on detection of variations in pressure oscillations due to arterial wall movement beneath an occluding cuff to calculate the systolic and diastolic BP readings (Lewis, 2019).

It is important to note that some automated oscillometric BP measurement devices are unreliable in patients with cardiac arrhythmia, such as atrial fibrillation (AF) (National Institute for Health and Care Excellence, 2019); this is because the pulse pressures can vary significantly with each pulse. Where possible, manual BP measurement should be used when a patient has AF (Clark et al, 2019).

Extreme bradycardias can also cause inaccurate BP readings (O’Brien, 2015); again, where possible, manual BP measurement is preferable, with slow cuff deflation.

A number of automated BP measurement devices are available including:

• Spot-check non-invasive BP monitor – this is probably the device most often used in secondary care settings on general wards. Both mains and battery powered, they are designed for routine clinical assessment. Many have the option to measure other vital signs, such as oxygen saturation levels, while some have an automatic-cycling facility to record a patient’s BP at set time intervals;
• Multiparameter patient monitors – generally used only in critical care areas, these devices enable a range of vital signs (typically including arterial BP, pulse, electrical activity of the heart, respiratory rate, oxygen saturation levels and end tidal carbon dioxide) to be monitored;
• Automated (spot-check) device – an electronic monitor with a pressure sensor, a digital display and an upper-arm cuff. Battery powered and very portable, this is often used in primary care settings;
• Wrist device – an electronic monitor with a pressure sensor (an electrically driven pump) attached to a wrist cuff. Function is similar to that of the automated (spot-check) device. Not commonly used in healthcare settings;
• Finger device – this includes an electronic monitor and a finger cuff, or the device itself may be attached to the finger. Not commonly used in healthcare settings;
• Lists of reliable/unreliable automated BP devices are available from the British and Irish Hypertension Society (BIHS).

Lying and standing measurement

In healthy patients, there is usually little difference between lying and standing BP readings. However, a significant fall in BP (≥20mmHg) can occur with a change of position, particularly in patients aged ≥65 years and those with diabetes or symptoms suggesting postural hypotension. The routine measurement of both lying and standing BP in these groups of patients is, therefore, advisable (National Institute for Health and Care Excellence, 2019).

Postural hypotension

Postural hypotension, sometimes termed orthostatic hypotension, is when an abnormally low BP occurs when a person suddenly assumes a standing position, typically inducing dizziness and syncope. The condition is more common in older people and its prevalence increases with age. It can also be caused by a number of medications including diuretics and anti-hypertensive therapy (Windsor et al, 2016). Postural hypotension can present with a clinical picture of dizziness, syncope and falls on changing position. Although it may seem to be a relatively harmless phenomenon, the patient’s safety and quality of life can be seriously affected.

Falls risk assessment

Measurement of lying and standing BP is part of a multifactorial patient risk assessment (Royal College of Physicians, 2017). A diagnosis of postural hypotension is indicated when there is a:

• Drop in systolic BP of ≥20mmHg (with or without symptoms);
• Drop in BP to <90mmHg on standing, even if the drop is <20mmHg (with or without symptoms);
• Drop in diastolic BP of 10mmHg with symptoms (but, clinically, much less significant than a drop in systolic BP).

NICE (2013) stated that the following groups of inpatients should be considered as at risk of falling in hospital and receive an individualised, multifactorial assessment, including lying and standing BP:

• All patients aged ≥65 years;
• Patients aged 50-64 years who are judged by a clinician to be at higher risk of a fall due to an underlying condition.

Errors in measurement

There are numerous causes of errors in BP measurements, including:

• Patient not being rested and relaxed when BP is measured;
• Defective equipment – for example, leaky tubing or a faulty valve;
• Too-rapid deflation of the cuff;
• Use of incorrectly sized cuff;
• Cuff not being on a level with the heart;
• Poor technique;
• ‘Digit preference’ – rounding a reading up to the nearest 5mmHg or 10mmHg;
• Observer bias – for example, expecting a young patient’s BP to be normal;
• Irregular pulse (as can occur with, for example, AF, bradycardia, muscle tremors, a weak pulse or profound shock) – in some automated devices, this can lead to inaccurate measurement (MHRA, 2019; O’Brien, 2015).

Checking and maintenance

All BP measuring equipment should be regularly checked and calibrated in accordance with the manufacturer’s instructions (MHRA, 2019). Cuffs and their hoses should be regularly inspected and replaced as necessary; excessive air leakage from damaged cuffs, hoses and tubing connectors may reduce the accuracy of the readings. If re-usable cuffs are used, they should be cleaned between patients in accordance with the manufacturer’s instructions, ensuring that cleaning fluid does not enter the cuff bladder or hoses.

Faulty devices can lead to inaccurate BP measurements, with significant effects on patient care. Healthcare providers have a responsibility to ensure adequate maintenance arrangements are in place (MHRA, 2019).

Aneroid devices are particularly prone to inaccuracies (Coleman et al, 2005); the MHRA (2019) recommends these are checked and calibrated at least twice a year. Hand-held devices used in the community are particularly prone to shocks and drops, but devices that incorporate anti-shock mechanisms may be more resilient to this type of wear and tear.

It is considered good practice to occasionally check the device against another validated device (BIHS, 2017).

Conclusion

Accurate measurement of BP is an important diagnostic and monitoring tool in a wide range of clinical conditions. Nurses must be able to carry out the procedure accurately and reliably, and should be aware of the common pitfalls that can lead to inaccuracies. Part 2 of this series will explain the procedure for measuring BP.

Key points

• Non-invasive measurement of blood pressure is a common diagnostic and monitoring technique
• A range of manual and automated devices are available to measure blood pressure
• Blood pressure devices should be properly maintained and calibrated to ensure accuracy

References

British and Irish Hypertension Society (2017) Blood Pressure Measurement: Using Automated Blood Pressure Monitors. Leicester: BIHS.

Clark CE et al (2019) Measurement of blood pressure in people with atrial fibrillation. Journal of Human Hypertension; 33: 763-765.

Coleman AJ et al (2005) Accuracy of the pressure scale of sphygmomanometers in clinical use within primary care. Blood Pressure Monitoring; 10: 4, 181-188.

Lewis P (2019) Oscillometric measurement of blood pressure: a simplified explanation. A technical note on behalf of the British and Irish Hypertension Society. Journal of Human Hypertension; 33: 5, 349-351.

McFerran T, Martin E (2017) Oxford Dictionary of Nursing (7th edn). Oxford: Oxford University Press.

Medicines and Healthcare products Regulatory Agency (2019) Blood Pressure Measurement Devices. London: MHRA.

National Institute for Health and Care Excellence (2019) Hypertension in Adults: Diagnosis and Management. London: NICE.

National Institute for Health and Care Excellence (2013) Falls in Older People: Assessing Risk and Prevention. London: NICE.

O’Brien E (2015) Blood pressure measurement. In: Beevers G et al (ed) ABC of Hypertension (6th edn). BMJ Books.

Royal College of Physicians (2017) Measurement of Lying and Standing Blood Pressure as Part of a Multifactorial Falls Risk Assessment. RCP.

Windsor J et al (2016) Orthostatic hypotension 1: effect of orthostatic hypotension on falls risk. Nursing Times; 112: 43/44, 11-13.

Which error will result in false high diastolic readings while measuring a clients blood pressure?

Which actions would the nurse perform when using the technique of palpation during the physical assessment of a patient select all that apply?

Which site would be monitored for a pulse to assess the status of circulation to the foot?