How to measure blood pressure accurately

Blood pressure is the reflection of the performance of the heart and responsiveness of the vascular system. It is the product of cardiac output and peripheral vascular resistance. However, isolated systolic hypertension occurs when only systolic pressures are elevated, with a normal or lower mean arterial pressure, effected by increased stiffness of large arteries. To obtain accurate blood pressure measurements, it is useful to understand the influencing physiological factors and use proper recording techniques. Recording blood pressure accurately is at the top of the list of medical student proficiencies.

In 1708, English clergyman Stephen Hales was the first person to measure blood pressure, by connecting a 9‐foot tall glass manometer to a horse's sural artery using brass tubes and goose trachea.1 The indirect method of measuring blood pressure was introduced in 1855 by German physiologist Karl von Vierordt, based on the principle that blood pressure is equal to the external pressure necessary to obliterate the artery, causing the distal pulse to disappear. This was not widely accepted until 1896, as indirect measurements required cumbersome mechanical devices. The scenario changed when Scipione Riva‐Rocci introduced the blood pressure cuff.2 In 1905, Nikolai Korotkoff described sounds which enabled clinicians to record systolic and diastolic pressures, and in 1907, Janeway3 showed that monitoring blood pressure is clinically useful. Prior to these developments, clinicians were not universally using blood pressure as an important vital sign.

Blood pressure measurement guidelines

The 2017 American College of Cardiology/American Heart Association guidelines for prevention, detection, evaluation and management of high blood pressure in adults propose a checklist for accurate measurement of blood pressure, involving a 6‐step process comprising 19 specific instructions4 (Box 1). There are also specific Australian guidelines, which include graded recommendations for blood pressure management.5

Pathogenesis of Korotkoff sounds

Korotkoff sounds are produced underneath the distal half of the blood pressure cuff, when the cuff pressure is between the systolic and diastolic pressures. The underlying artery is collapsing completely and reopening with each heartbeat. The sudden deceleration of the rapidly opening arterial walls produces a snapping or tapping sound. When the pressure falls below the diastolic pressure, the sound disappears as the vessel wall no longer collapses but instead gently recedes and expands with each beat, being held open by the diastolic pressure.6

There are five Korotkoff phases (Box 2), numbered in their order of appearance during deflation of the cuff:

• phase 1: initial tapping sound;
• phase 2: swishing murmur phase;
• phase 3: reappearance of a soft tapping sound;
• phase 4: muffling or softer murmur; and
• phase 5: disappearance of all sounds.

It was a matter of great debate among clinicians as to whether phase 4 or phase 5 better indicates the diastolic pressure, although expert consensus now favours phase 5.

Common measurement errors

Blood pressure variations are common in response to biological stimuli. Physical activity, emotional state, smoking, caffeine, temperature and season are some of the factors that affect blood pressure. Inaccurate measurements can also be caused by incorrect measurement techniques, inappropriate equipment, or observer biases, which include the differences in the way Korotkoff sounds are interpreted, the differences in the perception of changes in the sounds, terminal digit preferences, and pre‐judgement against certain blood pressure values.7

Wrong cuff size

The bladder of a standard cuff measures 12 × 23 cm and is appropriate only for arm circumferences up to 28 cm. However, one‐third of the population have arm circumferences greater than 28 cm.

Smaller cuff sizes overestimate the blood pressure. As the transmission of pressure into the tissue is inefficient, higher pressures are necessary to obliterate the arterial flow. This results in overdiagnosis of hypertension. The error is greater if the centre of the bladder is placed further from the brachial artery.8

However, the magnitude of measurement errors because of a larger sized cuff is small. Box 3 summarises the mean errors resulting from using cuffs that are too small or too large.9

Auscultatory gap

This is the phenomenon of disappearance of Korotkoff phase 1 sounds for variable periods of time before reappearance above the diastolic pressure. If the cuff is inflated above the level of disappearance of the sounds, this underestimates the true systolic blood pressure.10 The error can be avoided by estimating the disappearance of the pulse by palpation to estimate the systolic pressure, as the pulse will persist during the auscultatory gap. The pathophysiology of the auscultatory gap is not fully understood, but is somehow related to arterial stiffness.

Positioning of the arm

To accurately record blood pressure, the elbow should be at heart level. If the arm is elevated more than 6–7 cm (manubrium level), both systolic and diastolic blood pressure will be about 5 mmHg lower. If the arm is lowered to xiphoid process level (7–8 cm), the measured pressures will be 5–6 mmHg higher than the true value.11

Special situations

Korotkoff sounds may not be reliably heard in children aged under 5 years, and especially so in the first year. Other methods such as plethysmography, oscillometry or Doppler ultrasound may be used in such patients. Cuff size should be appropriately selected from those available for children (4 × 13 cm, 10 × 18 cm, 12 × 26 cm, adult size [for adolescents or children with arm size comparable to adults]).

In pregnancy, systolic blood pressure is recorded as for all patients; ie, Korotkoff phase 1 sound. The current consensus for recording diastolic blood pressure is phase 5 (disappearance of sounds). However, if sounds persist to 0 mmHg, phase 4 should be used to record diastolic blood pressure.

In patients with atrial fibrillation, multiple recordings may be needed and the values averaged out. Oscillometric (automated) methods may be unreliable in such patients. In those with slow heart rates, the cuff should be deflated slowly to avoid overestimation of diastolic blood pressure. Newer device algorithms can overcome this problem. Also, in most situations, a read error is shown by the device, allowing the clinician to diagnose a previously unrecognised cardiac arrhythmia such as atrial fibrillation or premature ventricular contractions, and allowing manual auscultatory measurement.

Automated blood pressure recorders

Traditional mercury sphygmomanometers are being phased out in several countries as automated devices become more popular. The automated recorders available for hospital and home use detect the oscillations in the cuff when deflated either automatically or manually. The appearance of the oscillations corresponds to the systolic blood pressure, and the phase with peak amplitude of oscillations indicates mean arterial pressure. However, diastolic blood pressure is often derived by using algorithms. The steps are otherwise similar, as in the case of mercury sphygmomanometer. These devices need periodic calibration, and clinicians should also be aware of the special situations in which these machines may not be accurate, such as for patients with irregular heart rhythms and clinical conditions with low diastolic blood pressure.12

Automated office blood pressure measurement

Automated office blood pressure measurement (AOBP) involves using an automated device to take repeated blood pressure measurements with no medical staff present. The technique generally yields lower readings than conventional clinic blood pressure measurement and has been shown to correlate well with out‐of‐clinic measures.5 AOBP measurements can minimise auscultation‐related errors (hearing defects, digit‐rounding preferences).

The 2016 Canadian hypertension guidelines13 recommend AOBP (with validated electronic oscillometric devices) as the preferred method of blood pressure measurement over manual measurement in clinical practice,4 as it reduces overdiagnosis of hypertension resulting from the white coat phenomenon. When AOBP is used, the threshold for diagnosis of hypertension is 135/85 mmHg.

Conclusion

Blood pressure measurement is an essential part of vital signs assessment. Errors occur because of inappropriate measurement techniques as well as patient‐related factors affecting the dynamic nature of the blood pressure. Every effort should be made to minimise measurement errors that might otherwise result in potentially serious patient management problems.

Box 2 – Five phases of Korotkoff sounds

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Korotkoff sounds are caused by the turbulence created by the passage of pulsatile blood flow through the narrowed artery caused by the inflated sphygmomanometer cuff. When the cuff pressure is above the systolic pressure, there is complete occlusion of the artery and there is no flow through the vessel and auscultation over the artery is therefore silent. When the cuff pressure is between the systolic and diastolic pressure, pulsatile flow through the narrow artery creates the Korotkoff sounds (phases 1‐4). When the cuff pressure is lower than the diastolic pressure, the artery is no longer compressed and thus it is silent to auscultation (phase 5).