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During any OSCE examination, understand the pathology and reasoning behind each of the signs and symptoms you elicit. Do this even if the patient you are examining is ‘normal’. This article explains how to perform a blood pressure measurement and the key findings you should look for, showing you what each sign means and what conditions it may indicate.
Blood pressure measurement is a core clinical skill, and it is deceptively easy to get wrong. A measurement is only useful if the technique is correct, so understanding why each step matters is just as important as the steps themselves.
General Inspection and Equipment
Before measuring, take a moment for a general inspection of the patient and the bedside. Note whether the patient appears comfortable or distressed, flushed, sweaty or pale, as anxiety, pain and recent exertion all temporarily raise blood pressure. Look around the bed for clues such as fluid balance charts, an observation chart, antihypertensive medications (medicines that lower blood pressure) or a GTN spray (glyceryl trinitrate, used to treat angina, which is chest pain from a poor blood supply to the heart). These hint at an existing diagnosis such as hypertension or angina.
Choose the arm carefully. Avoid an arm with any of the following:
- an arteriovenous fistula (an artery and a vein joined by surgery to give access for dialysis)
- lymphoedema (swelling from poor lymph drainage) after surgery to the armpit (axillary surgery), such as a mastectomy (removal of a breast)
- an intravenous cannula or an infusion running
- any area of injury or infection.
Inflating a cuff over these can cause harm or give an unreliable result.
Gather and check your equipment: a sphygmomanometer (a blood pressure machine), an appropriately sized cuff, and a stethoscope if measuring manually. The sphygmomanometer is either a manual aneroid (dial) device or a validated automated device (one that has been tested for accuracy). Mercury column devices have not been available to buy in the UK for healthcare use since the start of 2021, so you are unlikely to meet one.
Image - A clinical aneroid sphygmomanometer with its cuff and inflation bulb, used to measure blood pressure manually by auscultation
Creative commons source by Michael V Hayes [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
Cuff size is critical and a common source of error. Match the cuff to the patient’s upper-arm circumference: the inflatable bladder inside the cuff should be 80–100% of the arm circumference in length and at least 40% of it in width. A cuff that is too small requires more pressure to compress the artery and will overestimate blood pressure, whereas a cuff that is too large will underestimate it. Keep small, standard and large cuffs available and choose deliberately rather than defaulting to whichever is to hand.
Measuring Systolic and Diastolic Pressure by Auscultation
Place the diaphragm of the stethoscope gently over the brachial artery in the antecubital fossa. Avoid pressing too hard or tucking the stethoscope under the cuff, as this can produce turbulent (disordered, noisy) flow and artefactual (false) sounds.
Re-inflate the cuff to around 20–30 mmHg above the palpated systolic estimate, then slowly deflate it at about 2 mmHg per second (or per heartbeat if the pulse is very slow). Deflating faster than this makes it hard to identify the true pressures reliably, so the reading may be inaccurate.
As the cuff deflates you will hear the Korotkoff sounds. These arise because the cuff partially blocks (occludes) the brachial artery. As the pressure falls below systolic, blood begins to spurt through the narrowed vessel in turbulent, pulsatile (pulse-like) bursts, which are audible. The classification is as follows:
- Phase I – the first appearance of clear, repetitive tapping sounds. The pressure at this point is the systolic blood pressure.
- Phase II – softer, sometimes swishing sounds.
- Phase III – crisper and louder sounds as more blood flows through.
- Phase IV – an abrupt muffling of the sounds.
- Phase V – the point at which the sounds disappear completely. This is taken as the diastolic blood pressure.
The sounds disappear at diastole for a simple reason. Once the cuff pressure falls below the lowest (diastolic) pressure in the artery, flow becomes continuous and laminar (smooth) rather than turbulent. Laminar flow is silent. In adults, including in pregnancy, and in children, Phase V is taken as the diastolic value. If the sounds carry on until the cuff pressure is almost zero, the muffling at Phase IV is used instead.
Image - The principle behind manual measurement. The cuff pressure is shown falling over time; turbulent flow produces the Korotkoff sounds, which begin at systolic pressure and disappear at diastolic pressure
Creative commons source by Cmglee [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
Record the result as systolic over diastolic, for example 120/80 mmHg, and note which arm was used and the patient’s position. Continue deflating fully and remove the cuff once finished.
Automated Blood Pressure Measurement
Validated automated (oscillometric) devices are commonly used to measure blood pressure in clinics, hospitals and at home. Rather than detecting Korotkoff sounds, these devices measure the oscillations in cuff pressure produced by the pulsating artery. The point of maximal oscillation corresponds to the mean arterial pressure (the average pressure in the arteries over a heartbeat), and the device uses an algorithm to estimate the systolic and diastolic values from this.
The same principles still apply: the patient should be rested and seated, the cuff size must be correct, and the arm supported at heart level. Automated devices can be unreliable in atrial fibrillation (a common irregular heart rhythm) and other irregular rhythms because the algorithm assumes a regular pulse; in these patients a manual measurement is preferred. If an automated reading is unexpectedly high or low, it should always be confirmed manually.
Image - An automated (oscillometric) blood pressure monitor, a type commonly used in hospitals and general practice
Creative commons source by Belovaci [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
Interpreting the Reading
A normal adult blood pressure is generally considered to be around 120/80 mmHg. Using current UK (NICE) guidance, a clinic reading of 140/90 mmHg or higher does not diagnose hypertension on its own. It prompts ambulatory blood pressure monitoring (a portable monitor worn while the patient goes about their usual day) or home monitoring to confirm the diagnosis. Hypertension is confirmed when the clinic reading is 140/90 mmHg or higher and the average ambulatory daytime or home reading is 135/85 mmHg or higher. Hypotension is broadly defined as a blood pressure below 90/60 mmHg, though what matters clinically is whether the patient is symptomatic or under-perfused.
Persistent hypertension is a major risk factor for stroke, ischaemic heart disease, heart failure and chronic kidney disease. About 90% of cases are primary (essential) hypertension, which has no identifiable cause. It is still important to remember secondary causes such as renal disease, primary hyperaldosteronism (too much of the hormone aldosterone, made by the adrenal glands), phaeochromocytoma (a tumour of the adrenal gland that releases adrenaline and noradrenaline) and coarctation of the aorta (a narrowing of the aorta). Coarctation is worth bearing in mind in a younger patient, where it characteristically produces hypertension in the arms with relatively low pressures in the legs and a radio-femoral delay (a delay between the radial and femoral pulses).
Image - Coarctation of the aorta: narrowing of the aorta beyond the arch raises pressure proximally (the arms) while reducing it distally (the legs), a recognised secondary cause of hypertension in younger patients
SimpleMed original
Hypotension may reflect hypovolaemia (low blood volume), sepsis, cardiac failure, an arrhythmia (abnormal heart rhythm), or the effect of medication. A single low reading in a well patient is often of little concern. A low blood pressure in an acutely unwell patient is a red flag for shock and warrants urgent review.
It is also worth considering the pulse pressure, which is the difference between the systolic and diastolic values (normally around 40 mmHg). A wide pulse pressure can be seen in aortic regurgitation (a leaking aortic valve) and in stiff, calcified arteries in the elderly. A narrow pulse pressure can occur in aortic stenosis (a narrowed aortic valve), heart failure or significant hypovolaemia.
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