Blood pressure: description and measurement

Principle

Blood pressure (BP) is the pressure exerted by the blood at right angles to the walls of the blood vessels P_i minus the environmental pressure P_e, so:

BP = P_i - P_e.

Unless indicated otherwise, BP refers to systemic arterial BP, i.e. the pressure in the large arteries delivering blood to body parts other than the lungs, such as the brachial artery (in the arm). The pressure of the blood in other vessels is lower than the arterial pressure. BP values are generally stated in mmHg, but can be converted to an SI-unit, i.e. in Pascals. The conversion is:

P = ρ_Hggh, (Pa)

where ρ_Hg is the specific density of Hg (kg/m³), g the gravitational acceleration (m/s²) and h the height of the Hg column (m).

Systolic and diastolic adult BP of the brachial artery are typically 120 (16 kPa) and 80 mmHg (10 kPa) respectively.

The mean arterial pressure (MAP), see More Info, and pulse pressure (see Blood pressure: pulse pressure) are other important quantities.

BP varies from one heartbeat to another and throughout the day (in a circadian rhythm); they also change in response to stress (exercise etc.), nutritional factors, drugs, or disease.

Measurement

Invasive measurement

Arterial BP is most accurately measured invasively by placing a cannula into a blood vessel and connecting it to an electronic pressure transducer. This is done in human and veterinary intensive care medicine, anesthesiology, and for research purpose.

Non-invasive measurement

The non-invasive auscultatory and oscillometric measurements are simpler and quicker, have no complications, and are less unpleasant and painful, at the cost of somewhat lower accuracy and small systematic differences in numerical results. These methods actually measure the pressure of an inflated cuff at the points where it just occludes blood flow (systolic BP) or just permits unrestricted flow (diastolic BP).

The classical auscultatory method uses a stethoscope (for listening to the so-called Korotkoff sounds), a sphygmomanometer (upper arm cuff attached to a mercury or aneroid manometer).

Basic digital BP monitors are relatively inexpensive, making it easy for patients to monitor their own BP. Their accuracy can vary greatly; most have not been certified for accuracy by an approved authority. Upper arm, rather than wrist, monitors usually give readings closer to auscultatory. Some meters are automatic, with pumps to inflate the cuff without squeezing a bulb.

Auscultatory method aneroid sphygmomanometer with stethoscope

Oscillometric methods are used in the long-term measurement. The equipment is the same as for the auscultatory method, but with an electronic pressure sensor (transducer) fitted in the electronically operating cuff. The manometer is an electronic device with a numerical readout and checked periodically.

The cuff is inflated to a pressure initially in excess of the systolic BP (BP_systolic), reducing to below BP_diastolic over a period of about 30 s. When blood flow is nil (pressure > BP_systolic) exceeding systolic pressure) or unimpeded (cuff pressure <PB_diastolic), cuff pressure will be essentially constant. When blood flow is present, but restricted, the cuff pressure will vary periodically in synchrony with the cyclic expansion and contraction of the brachial artery, i.e., it will oscillate. The values of PB_systolic and PB_diastolic are computed from the raw measurements and displayed.

Oscillometric monitors do not give entirely meaningful readings in certain “special conditions” such as arterial sclerosis, arrhythmia, preeclampsia, pulsus alternans, and pulsus paradoxus.

In practice the different methods do not give identical results; an algorithm and experimentally obtained coefficients are used to adjust the oscillometric results to give readings which match the auscultatory as well as possible. Some equipment uses computer-aided analysis of the instantaneous BP waveform to determine the systolic, mean, and diastolic points.

The term NIBP, for Non-Invasive BP, is often used to describe oscillometric monitoring equipment.

More Info

Regulation of BP

The endogenous regulation comprises the baroreceptor reflex, the renin-angiotensin system (RAS) and

aldosterone release. This steroid hormone stimulates Na-retention and K-excretion by the kidneys. Since Na is the main ion that determines the amount of fluid in the blood vessels by osmosis, aldosterone will increase fluid retention, and indirectly, BP.

Factors influencing BP

The physics of the circulatory system, as of any fluid system, are very complex (see e.g. Elasticity of the aorta, Navier-Stokes equations, Windkessel model and Blood pressure: models).

Many physical and physiological factors influence BP. Cardiac output is heart rate times stroke volume. It represents the efficiency with which the heart circulates blood throughout the body.

Some other physical factors are:

Blood volume The higher the blood volume, the higher the cardiac output.

Resistance The higher the resistance, the higher the BP. Resistance is related to size (the larger the blood vessel, the lower the resistance: see Poiseuille’s law), as well as the smoothness of the blood vessel walls. Various substances (vasoconstrictors and vasodilators) change vessel diameter, thereby changing BP.

Viscosity Increase results in increase of resistance and so of BP. Anemia reduces and hyperemia increases viscosity. Viscosity also increases with blood sugar concentration.

Mean arterial pressure (MAP)

The MAP in the arteries supplying the body is a result of the heart pumping blood from the veins back into the arteries. The up and down fluctuation of the arterial BP results from the pulsatile nature of the cardiac output (see Blood pressure: pulse pressure). The larger arteries are low resistance have high flow rates that generate only small drops in pressure. For instance, with a subject in the supine position, blood traveling from the heart to the toes typically only experiences a 5-mmHg drop in mean pressure.

Orthostatic hypotension

Sometimes BP drops significantly when a patient stands up from sitting. This is orthostatic hypotension; gravity reduces the rate of blood return from the body veins below the heart back to the heart, thus reducing stroke volume and cardiac output.

A few seconds are needed for recovery and if too slow or inadequate, the individual will suffer reduced blood flow to the brain, resulting in dizziness and potential blackout. Increases in G-loading, such as routinely experienced by supersonic jet pilots "pulling Gs", greatly increases this effect. Repositioning the body perpendicular to gravity largely eliminates the problem.