Avoiding Pitfalls in Clinic BP Measurement: How we can Measure BP Better
Table of Content
Blood pressure (BP) measurement is one of the most commonly performed clinical procedures. However, BP is a variable hemodynamic phenomenon, which is influenced by many factors - some known and some not so commonly known. These influences on BP can be significant, often accounting for rises in systolic BP (SBP) > 20 mmHg. An effect of such magnitude can result in erroneously labeling a healthy individual as 'hypertensive' for life. Conversely, incorrect BP measurement may also result in missing a diagnosis of hypertension. Also, good control of hypertension begins with accurate BP measurement. Hence it goes without saying that accurate BP measurement is imperative. This article highlights the possible errors and pitfalls in BP measurement and emphasizes the right techniques for accurate BP measurement.
- Ask the patient to avoid smoking, eating, consuming alcohol, caffeine-containing beverages or chocolate for at least an hour before BP measurement.
- Allow the patient to rest for at least 5 minutes.
- Ensure that the arms, back and feet of the subject are supported whilst measuring BP. The arm must be horizontal at the level of the heart as denoted by the midsternal level.
- Use a properly maintained, calibrated and validated device. A mercury manometer is preferable. If aneroid devices are used, they should be checked regularly against the mercury device.
- Use cuffs of the appropriate size such that the bladder encircles at least 80% of the subject's arm.
- Place the cuff on the bare arm; avoid tight or thick clothes under the cuff.
- For manual determinations, use palpated radial pulse obliteration pressure to estimate systolic BP - the cuff should then be inflated 20-30 mmHg above this level for the auscultatory determinations; the cuff deflation rate should be 22 mmHg per second.
- Use phase I and V (disappearance) Korotkoff sounds to identify systolic and diastolic BP, respectively.
- Note down the BP as soon as it has been measured. The arm in which the pressure is being recorded and the position of the subject should also be noted.
- Take a mean of at least two readings spaced by 1-2 minutes; take additional readings if marked differences (>5 mmHg) between initial measurements are found.
- Measure BP in both the arms at the initial visit. If significant difference (>20/10 mmHg) is found, measure BP routinely in the arm with the higher value.
- Measure BP at 1 and 5 minutes after assumption of the standing position in elderly subjects, diabetic patients, and in other conditions in which orthostatic hypotension may be frequent or suspected.
The above summarise the essential points to be borne in mind whilst measuring BP. However, it may be worthwhile to read the entire article for understanding the numerous factors - some known and some not so commonly known - that can influence accurate BP measurement
Scipione Riva-Rocci and Nikolaij Korotkoff were among the first to introduce the measurement of 'BP' during the morning round in their hospital wards. They collected and contributed to most of the information on BP levels in arterial hypertension over more than a century. The combination of the Riva-Rocci and Korotkoff techniques has given us the auscultatory sphygmomanometric BP measurement - a method that has allowed us to identify arterial hypertension as an important factor in cardiovascular (CV) risk and to demonstrate that its treatment can markedly improve a patient's prognosis.
BP can be measured directly (intra-arterially) or indirectly. The indirect method is widely used in both daily practice and research. It mostly involves devices that are dependent on one common feature, namely, occluding the artery of an extremity (arm, wrist, finger, or leg) with an inflatable cuff to measure BP either oscillometrically, or by detection of Korotkoff sounds.
The accurate measurement of BP in clinical practice is dependent on the individual , the equipment used and the observer.
A) Rest Period
- Average drops in the systolic BP (SBP) of 9 and 14 mmHg, respectively, have been reported after a rest period of 4 and 8 minutes prior to BP measurement.
- A longer rest period of >25 minutes was found to further slightly decrease the BP values, especially the SBP; but this may not be feasible in general practice.
Hence, it has been recommended that at least 5 minutes of rest should be allowed in a quiet room in a chair (rather than on an examination table) with feet on the floor before the measurement of BP
B) Seasonal Variability
Studies have suggested a seasonal variation of the BP, showing, on an average, 3-8 mmHg higher BP values during the winter than during the summer, even in patients living in a stable environmental temperature. These differences seem to be inversely associated with the body mass index, possibly due to the increased thermoregulatory requirements of leaner individuals.
Hypertensive patients may require a lower dose of antihypertensive medication during periods of fever or if they move to (sub) tropic countries (holidays, business).
- Smoking the first cigarette of the day may acutely induce a rise in BP that lasts for 15-30 minutes, which is likely due to the acute release of norepinephrine. On the other hand, chronic smoking induces tolerance.
- Ingestion of caffeine-containing beverages may induce an acute rise in BP; however,a certain degree of tolerance may occur with repeated consumption. Other ingredients in coffee apart from caffeine may also be responsible for the CV activation.
- Eating as an activity increases BP by 8-9 mmHg; however, a postprandial decrease in BP can also be noted, especially in elderly patients.
- Ingestion of alcohol can also acutely increase BP.
Hence, smoking, eating, consumption of alcohol or caffeine-containing beverages and chocolate should be avoided for at least an hour before the measurement of BP.
D) Sympathetic Stimulators
- Pain and anxiety acutely increase the BP, probably due to increased sympathetic activity.
- Explain the procedure adequately beforehand, especially in nervous patients.
- Inform patients about a minor discomfort caused by the inflation of the cuff.
- Advise patients to empty their bladder before BP measurement as a distended bladder has been reported to increase BP.
E) Anatomy: One Arm or Both the Arms?
- There has been much controversy in various studies as to whether there is a difference between the BP readings in the two arms (inter-arm difference). Further, there is evidence associating an inter-arm difference with peripheral vascular disease, raising the possibility that its presence may predict CV events.
- A reasonable policy is to measure BP at the initial visit in both arms.
- If, after at least three readings, significant systematic difference (≥ 20/10 mmHg) is found, measure BP routinely in the arm with the higher value.
F) 'White-Coat Hypertension' and 'White-Coat' Effect
- White-coat hypertension is a condition in which a normotensive subject becomes hypertensive during BP measurement, but pressures then settle to normal outside the medical environment.
- If the BP is higher when measured by a physician than by a nurse or a medical student, it is known as the white-coat effect. When this phenomenon is suspected, nurses rather than physicians should measure the BP.
- White-coat hypertension should be considered when clinic BP is consistently elevated, or resistant to treatment in the absence of target organ damage.
- Ambulatory BP monitoring (ABPM) for 24 hours or self-measurement at home can be employed before patients are labeled 'hypertensive' or treatment is initiated.
- Physicians should diagnose white-coat hypertension whenever the office BP is ≥ 140/90 mmHg at several visits while 24-hour ABPM is <125/80 mmHg.
G) Routine Activities
- There is a substantial diurnal BP variation, with a clear fall in BP during the night of up to 15%, as a result of both sleep and inactivity, reflecting the decrease in sympathetic tone.
- Various daytime activities induce increases in BP of different magnitude. Activities accompanied by a large increase in BP of between 10 and 20 mm Hg include meetings, physical work, transportation, walking and dressing. Activities accompanied by increases in BP of up to 5 mmHg include deskwork, reading and watching television.
- Talking results in approximately a 7 mmHg increase in BP and should, thus, be avoided during BP measurement.
Hence, activities that induce increases in BP should be avoided before/during BP measurement. Talking should be avoided during BP measurement.
Factors Related to the Instrument
A) Instrument Used
The measuring device used can induce large variations in BP. There is a wide range of BP measuring devices in the market but, unfortunately, only a few of these devices have been validated according to official standards.
The equipment - whether aneroid, mercury, or electronic - should be regularly inspected and validated.
- Use a properly maintained, calibrated and validated device.
- Maintain the equipment properly; regular maintenance concerns three points: adequate filling of the mercury reservoir, replacement of the glazed tube in case of mercury precipitation, and replacement of the rubber connections in case of leaks.
- Aneroid devices are used widely, although they are notoriously difficult to maintain in an accurate state over time, usually leading to falsely low readings with the consequent underestimation of BP.
- When calibrated against a mercury sphygmomanometer, a mean difference of 3 mmHg is considered to be acceptable; however, 58% of aneroid sphygmomanometers have been shown to have errors >4 mmHg, with about one-third of these having errors of >7 mmHg.
- They are susceptible to carelessness in maintenance. The jolts and bumps of everyday use affect their accuracy and, therefore, they are not recommended for routine use.
- If used, check the aneroid devices regularly against mercury.
B) Rubber Tubing
- Leaks due to cracked or perished rubber cause inaccurate BP measurement, as the fall in mercury cannot be controlled. The rubber should be in good condition and free from leaks.
- The minimum length of tubing should be 70 cm between the cuff and manometer and at least 30 cm in length between the inflation source and cuff. Connections should be airtight and easily disconnected.
C) Control Valve
- A very common source of error is the control valve, especially when an air filter rather than a rubber valve is used.
- Defective valves cause leakage, making control of pressure release difficult; this leads to underestimation of SBP and overestimation of DBP.
- Faults in the control valve may be corrected easily by simply cleaning the filter or replacing the control valve.
Factors Related to the Observer
The person who is measuring the BP (the observer) requires meticulous and repeated theoretical and practical training and validation of his/her ability to measure the BP accurately.
- Whilst taking the patient's BP, be in a comfortable and relaxed position, because if hurried, the pressure will be released too rapidly, resulting in underestimation of SBP and overestimation of DBP.
- If any interruption occurs, the exact measurement may be forgotten and an approximation made, so the BP should always be written down as soon as it has been measured. The arm in which the pressure is being recorded and the position of the subject should also be noted, e.g., BP 154/92, R arm, sitting position
- Provide to patients, verbally and in writing, their specific BP numbers and the BP goal of their treatment.
- A number of factors can cause a bias in the BP reading:
- Systematic error may be caused by lack of concentration, poor hearing, confusion of auditory and visual cues, etc. The observer may fail to interpret the Korotkoff sounds accurately, especially for DBP; or interpret the Korotkoff sounds differently.
- Observers very often have a terminal digit preference (observer rounds off the BP reading), which, in the majority of cases, is 0 (75%) or 5 (25%).
- Observers may be influenced by the knowledge of previous BP values during serial readings (observer prejudice or expectation bias) and simply adjust the BP to meet his/her preconceived notion of what the BP should be.
Factors Related to Techniques
A) Bladder Size/Cuff Placement
- There is unequivocal evidence that either too narrow or too short a bladder (undercuffing) will cause overestimation of BP, so called 'cuff hypertension,' and too wide or too long a bladder (overcuffing) may cause underestimation of BP. Undercuffing has the effect in clinical practice of over-diagnosing hypertension and overcuffing leads to hypertensive subjects being diagnosed as normotensive.
- Use cuffs of the appropriate size such that the bladder encircles at least 80% of the upper arm.
- If the bladder does not completely encircle the arm, its centre must be over the brachial artery. The lower edge of the cuff should be 2-3 cm above the point of brachial artery pulsation.
- The width of the bladder should be equal to about two-thirds the distance from the axilla to the antecubital space.
- As per the European Society of Hypertension 2003 guidelines for management of hypertension, a standard bladder, 12-13 cm long and 35 cm wide, should be used.
- The British Hypertension Society (BHS) recommends three different bladder sizes depending on arm circumference. The physician should have a larger and smaller bladder available for fat and thin arms, respectively. In children, the smallest bladder should be used.
- Place the cuff on the bare arm; tight or thick clothes under the cuff should be avoided.
- The rubber tubes from the bladder are usually placed inferiorly, often at the site of the brachial artery, but it is now recommended that they should be placed superiorly or, with completely encircling bladders, posteriorly, so that the antecubital fossa is easily accessible for auscultation.
- Inflate the bladder rapidly to avoid prolonged discomfort for the patient, but deflate it slowly at a rate of 2 mmHg per beat or per second, to accurately record BP to the nearest 2 mmHg. On the other hand, deflation can be speeded up in the second or third readings, especially when there is an increase in pulse pressure (e.g., 224/62 mmHg) since otherwise the procedure may become too painful and pain may increase the BP further.
B) Position of the Manometer
- The manometer should be no further than three feet (92 cm) away so that the scale can be read easily.
- The mercury column should be vertical (some models are designed with a tilt) - this is achieved most effectively with stand-mounted models, which can be easily adjusted to suit the height of the observer.
- View the scale straight on with the eye on a line perpendicular to the centre of the face of the gauge in order to avoid the parallax effect. According to this effect, a higher BP will be read if the observer is watching from below the scale; a lower BP will be read when watching from above the scale.
C) Stethoscope Placement
- During auscultatory measurement, hold the stethoscope firmly and evenly but without excessive pressure.
- Too much pressure might distort the artery, producing sounds below diastolic pressure.
- The stethoscope end-piece should not touch the cuff or rubber tubes to avoid friction sounds.
D) Position of the Body and Arm
Posture of Subject
- Most official guidelines recommend that BP should be routinely measured with the patient in the sitting position. However, it is unclear if there are significant differences between BP measured in the sitting versus the supine position, or which position best approximates intra-aortic pressure. It is more convenient in most instances to measure the BP in the sitting position, and virtually all the evidence related to hypertension and CV outcomes is derived from studies that measured sitting BPs.
- The arms, the back and feet of the patients should be supported to avoid any isometric physical exercise that might increase the BP.
- Measurement of BP in the standing position is indicated periodically, especially in those at risk for postural hypotension and in those who report symptoms consistent with reduced BP upon standing.
- Measure BP at 1 and 5 minutes after assumption of the standing position in elderly, diabetics and in other conditions in which orthostatic hypotension may be frequent or suspected.
- If the arm in which measurement is being made is unsupported, as tends to happen if the subject is sitting or standing, the DBP may be raised by as much as 10%. It is essential that the arm is supported during BP measurement and this is best achieved in practice by having the observer hold the subject's arm at the elbow.
- Dependency of the arm below heart level leads to an overestimation of BP and raising the arm above heart level leads to underestimation. The magnitude of this error can be as great as 10 mmHg for BP. Such errors can occur in a patient who is standing with his/her arm hanging parallel to the body or in a sitting patient whose arm is supported by the armrest of the chair or by a regular office desk. However, it has been demonstrated that even in the supine position, an error of 5 mmHg for DBP may occur if the arm is not supported at the heart level.
- The arm must be horizontal at the level of the heart as denoted by the midsternal level. The level of the fourth intercostal space or the midsternum have been proposed as practical approximation of the right atrium level in the sitting and standing positions.
E) Diastolic Dilemma
- For many years, recommendations on BP measurement have been uncertain about the diastolic endpoint.
- Phase IV (muffling) may coincide with or be as much as 10 mmHg higher than phase V (disappearance), but usually the difference is <5 mmHg; phase V correlates best with intra-arterial pressure.
- There has been resistance to general acceptance of the silent endpoint until recently, because the silent endpoint can be greatly below the muffling of sounds in some groups of patients - children, pregnant women, anemic or elderly patients. In some patients, sounds may even be audible when cuff pressure is deflated to zero.
- There is now a general consensus that disappearance of sounds (phase V) should be taken as diastolic pressure except in children, pregnant women, the anemic or the elderly.
F) Number of Measurements
- Decisions based on single measurements will result in erroneous diagnosis and inappropriate management due to variability of BP measurements.
- Take a mean of at least two readings spaced by 1-2 minutes; additional recordings are needed if marked differences between initial measurements are found.
- Perform BP measurements in a quiet environment, as noisy rooms make it difficult for the patient to relax and the observer to concentrate and adequately hear the Korotkoff sounds.
- Room temperature should not be too high or too low either.
- Repeated office BP measurements in standard conditions have a prognostic value similar to that of 24-hour ABPM.
- In cases of slight BP elevation, repeated measurements have to be obtained over several months before a final diagnosis of hypertension can be made, because of the possibility of a spontaneous regression, over time, to normal levels.
- If a patient has a more marked BP elevation, evidence of hypertension-related organ damage or a high/very high CV risk profile, repeated measurements should be obtained over shorter periods of time, such as weeks or days, before a clinical decision is taken.
Table 4 Recommendations for followup based on initial Blood pressure measurements for adults without acute end organ damage
Certain populations merit special consideration for BP measurement, either because of age, body habitus, or disturbances of BP related to hemodynamic alterations in other parts of the CV system.
1. Children and Adolescents
- Hypertension during childhood is not rare; the estimated prevalence is 1-2%, although it is often an under-recognized clinical entity. Also, elevated BP may be a sign of underlying disease or it may represent early onset of primary hypertension.
- The variability of BP is greater in children than in adults and, thus, any one BP reading is less likely to represent the true value.
- SBP is preferred to DBP because of greater accuracy and reproducibility.
- Cuff dimensions are most important and three cuffs with bladders measuring 4-13 cm, 10-18 cm, and the adult dimensions of 12-26 cm are required for the range of arm sizes likely to be encountered in the age range of 0-14 years.
- Korotkoff sounds are not reliably audible in all children aged under 1 year and in many under 5 years of age. In such cases, conventional sphygmomanometry is impossible and more sensitive methods of detection such as Doppler, ultrasound, or oscillometry must be used.
- Clinically relevant hypertension occurs in more than 10% of pregnant women. BP measurement in pregnancy represents one of the key points in the diagnosis of pre-eclampsia.
- A general consensus from obstetricians based on careful analysis of the evidence is that disappearance of sounds (fifth phase) is the most accurate measurement of DBP, with the proviso that in those rare instances in which sounds persist to zero, the fourth phase of muffling of sounds should be used.
3. Critical-Care Setting
- In an emergency situation, a traditional sphygmomanometric technique is sufficient (palpation is recommended whenever an auscultatory measurement is impossible; when prolonged monitoring is to be performed, automated methods are required).
- Whenever continuous BP monitoring is essential, use of intra-arterial catheters is a common procedure.
4. Patients with Arrhythmias
- The BP measurement can be particularly difficult in patients with arrhythmias, especially atrial fibrillation, soft Korotkoff sounds or a 'silent gap' due to marked beat-to-beat variability.
- In patients with arrhythmias, auscultatory measurements and multiple readings are recommended.
- In patients with bradyarrhythmias, when the heart rate is extremely slow, e.g., 40 beats/minute, it is important that the deflation rate used is less than for normal heart rates as too rapid deflation will lead to underestimation of SBP and overestimation of DBP.
5. Dialysis Patients
- In dialysis patients (hemodialysis and peritoneal dialysis), there are large day-to-day variations in body fluid status and, therefore, also in BP level. The timing of BP measurement in relation to dialysis, changes in interdialytic weight gain, and inconsistent BP measurement technique in dialysis units contribute to the variability of BP readings. In principle, these rapid changes might be best assessed by 24-hour ABPM.
- Interdialytic BP monitoring with an ambulatory BP monitor is the most reproducible method and is thought to best represent BP in dialysis patients.
- A composite of BP measurements over a period of 1-2 weeks rather than isolated readings should be used for guidance.
- However, when ABPM is not possible, the BP obtained in the dialysis unit can be used in a qualitative sense for prediction of hypertension in these patients.
- The elderly are subject to considerable BP variability, which can lead to a number of circadian BP patterns that are best identified using ABPM.
- It has been postulated that as a consequence of the decrease in arterial compliance and arterial stiffening with ageing, indirect sphygmomanometry becomes inaccurate. This has led to the concept of 'pseudohypertension' to describe patients with a large discrepancy between cuff and direct BP measurement.
7. Resistant Hypertension
Evaluation of the patient with resistant hypertension should include 24-hour ABPM or home measurements and a search for secondary causes.
2. Netherlands J Med 2004; 62:297-303
3. Curr Opin Nephrol Hypertens 2004; 13:343-57
4. J Hum Hypertens 2004; 18:139-85
5. J Hypertens. 2003; 21:1011-53
6. JAMA 2003; 289:2560-72