Arterial blood pressure

Also known as: blood pressure, BP, mean arterial pressure, MAP, pulse pressure, hypertension, Korotkoff sounds

Arterial blood pressure is the pressure blood exerts on the arterial walls. It rises to a systolic peak and falls to a diastolic trough with each beat. Its mean value, mean arterial pressure (MAP ≈ cardiac output × total peripheral resistance), drives the perfusion of every organ.

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Overview

Optimal<120/80 mmHg

Mean arterial pressure≈70–105 mmHg (≈93)

Pulse pressure≈40 mmHg

Hypertension threshold≥140/90 (ESC) or ≥130/80 (ACC/AHA) mmHg

Definitions. Systolic pressure is the peak, about 120 mmHg. Diastolic pressure is the trough, about 80 mmHg. Pulse pressure is the difference between them, about 40 mmHg. It depends on stroke volume and on how stiff the arteries are. It widens with age, in aortic regurgitation and in hyperdynamic states. It narrows in aortic stenosis, tamponade, heart failure and shock.

Mean arterial pressure (MAP) is approximately the diastolic pressure plus one-third of the pulse pressure, about 93 mmHg. MAP also equals cardiac output × total peripheral resistance, plus central venous pressure. To keep organs perfused, MAP is usually kept at 65 mmHg or more, for example in sepsis.

Along the circulation. Pressure falls progressively: aorta (mean about 95 mmHg), arterioles, capillaries (about 35 falling to 15 mmHg), veins, right atrium (about 0–5 mmHg). In peripheral arteries the systolic pressure is actually higher than in the aorta, because reflected pressure waves add to it (pulse amplification).

Measurement. With a cuff, Korotkoff sounds are heard over the brachial artery as the cuff deflates. The first sound (phase I) marks systolic pressure, and their disappearance (phase V) marks diastolic pressure. The cuff must be the right size, as a cuff that is too small gives falsely high readings, and the arm must be at heart level. Ambulatory and home monitoring help identify "white-coat" and "masked" hypertension. Normally blood pressure falls by 10–20% during sleep (nocturnal dipping).

Function

Provides the driving pressure for flow through every organ.
Provides the hydrostatic force for capillary filtration and glomerular filtration.

Regulation

Blood pressure is regulated over three time scales.

Seconds. The baroreceptor reflex responds to stretch in the carotid sinus and aortic arch. Chemoreceptor reflexes and the CNS ischaemic response act when pressure falls very low.

Minutes to hours. The renin–angiotensin–aldosterone system acts on vascular tone and sodium retention. ADH (vasopressin) causes V1-mediated vasoconstriction and V2-mediated water retention. Natriuretic peptides (ANP and BNP) cause loss of sodium in the urine and vasodilation. Fluid also shifts between capillaries and tissues.

Days. Renal pressure natriuresis is the long-term control. When pressure rises, the kidney excretes more salt and water, so blood volume, and with it pressure, falls back. Guyton argued that the kidney therefore sets the long-term blood pressure.

Clinical significance

Hypertension

Hypertension is diagnosed at 140/90 mmHg or more in European guidelines, or 130/80 mmHg or more in US (ACC/AHA) guidelines. A hypertensive emergency is blood pressure of about 180/120 mmHg or more with acute organ damage. About 90–95% of cases are primary (essential).

Secondary causes include kidney disease, renal artery stenosis, primary aldosteronism (Conn syndrome, the commonest endocrine cause), phaeochromocytoma, Cushing syndrome, coarctation, obstructive sleep apnoea and drugs such as NSAIDs and oral contraceptives.

Hypertension damages the heart (LVH, heart failure, coronary disease), brain (ischaemic and haemorrhagic stroke), kidneys (CKD), eyes (retinopathy) and aorta (aneurysm and dissection). Treatment combines lifestyle change, particularly less salt, with drugs: ACE inhibitors or ARBs, calcium channel blockers and thiazide-like diuretics.

Orthostatic hypotension

Orthostatic hypotension is a fall in blood pressure of at least 20 mmHg systolic or 10 mmHg diastolic within 3 minutes of standing. Causes include hypovolaemia, drugs and autonomic failure (diabetes, Parkinson disease, multiple system atrophy). It causes dizziness on standing, falls and fainting.

Test yourself

Oesophageal varices in portal hypertension develop at an anastomosis between which veins?

Left gastric vein and oesophageal tributaries of the azygos vein
Superior rectal vein and inferior rectal veins
Paraumbilical veins and superficial epigastric veins
Splenic vein and left renal vein

Answer: Left gastric vein and oesophageal tributaries of the azygos vein

The left gastric (coronary) vein belongs to the portal system and the oesophageal veins drain into the azygos (systemic) system. When portal pressure rises they dilate into oesophageal varices, which can bleed catastrophically. The other options describe the anorectal, paraumbilical and retroperitoneal anastomoses.

A patient’s blood pressure is 120/80 mmHg. What is their approximate mean arterial pressure?

80 mmHg
93 mmHg
100 mmHg
107 mmHg

Answer: 93 mmHg

MAP ≈ diastolic + ⅓ pulse pressure = 80 + 40/3 ≈ 93 mmHg. MAP sits closer to diastolic than systolic pressure because diastole takes up about two-thirds of the cardiac cycle at rest.