Blood pressure

Blood pressure is the pressure exerted by the blood on the walls of the blood vessels. Unless indicated otherwise, blood pressure refers to systemic arterial blood pressure, 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. Blood pressure values are universally stated in millimetres of mercury (mm Hg), and are always given relative to atmospheric pressure—the absolute pressure of the blood in an artery with mean arterial pressure stated as 100 mm Hg, on a day with atmospheric pressure of 760 mm Hg, is 860 mm Hg.

The systolic pressure is defined as the peak pressure in the arteries during the cardiac cycle; the diastolic pressure is the lowest pressure (at the resting phase of the cardiac cycle). The mean arterial pressure and pulse pressure are other important quantities.

Typical values for a resting, healthy adult are approximately 120 mm Hg systolic and 80 mm Hg diastolic (written as 120/80 mm Hg), with large individual variations. These measures of blood pressure are not static, but undergo natural variations from one heartbeat to another or throughout the day (in a circadian rhythm); they also change in response to stress, nutritional factors, drugs, or disease.

Measurement

Invasive measurement

Arterial blood pressure (BP) is most accurately measured invasively by placing a cannula into a blood vessel and connecting it to an electronic pressure transducer. This invasive technique is regularly employed in intensive care medicine, anesthesiology, and for research purposes, but it is, rarely, associated with complications such as thrombosis, infection, and bleeding.

Non-invasive measurement

The non-invasive auscultatory (from the Latin for listening) and oscillometric measurements are simpler and quicker, require less expertise in fitting, have no complications, and are less unpleasant and painful for the patient, 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, and where it just permits unrestricted flow. These are the methods more commonly used for routine examinations and monitoring.

Since blood pressure varies throughout the day, measurements should preferably be taken at the same time of day to ensure the readings taken are comparable. Suitable times are:

a) immediately after awaking (before washing/dressing and taking breakfast/drink), while the body is still resting

b) immediately after finishing work

Clearly it is difficult to meet these requirements at the doctor's surgery; also, some patients become nervous when their BP is taken at the surgery, causing readings to increase: white coat hypertension.

Taking blood pressure levels at home or work with a home blood pressure monitoring device may help determine a person's true range of blood pressure readings and avoid false readings from the white coat hypertension effect. More formal assessment may be made with an ambulatory blood pressure device that takes regular blood pressure readings every half an hour throughout the course of a single day and night. Aside from the white coat effect, blood pressure readings outside of a clinical setting are usually slightly lower in the majority of people. However the studies that looked into the risks from hypertension and the benefits of lowering the blood pessure in affected patients were based on the one-off clinic readings.

Basic digital blood pressure monitors are relatively inexpensive, making it easy for patients to monitor their own blood pressure. 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

The auscultatory method uses a stethoscope and a sphygmomanometer. This comprises an inflatable (Riva Rocci) cuff placed around the upper arm at roughly the same vertical height as the heart, attached to a mercury or aneroid manometer. The cuff is inflated manually by repeatedly squeezing a rubber bulb until the artery is completely occluded. Listening with the stethoscope to the brachial artery at the elbow, the examiner slowly releases the pressure in the cuff. When blood just starts to flow in the artery, a "whooshing" or pounding sound (first Korotkoff sounds) is heard. The pressure at which this sound is first heard is the systolic blood pressure. The cuff pressure is further released until no sound can be heard (fifth Korotkoff sound), at the diastolic blood pressure. Samples of the sounds can be found at ^*. With a mercury manometer this is simple technology which gives accurate pressure readings without issues of calibration.

Oscillometric methods

Oscillometric methods are used in long-term measurement as well as in clinical practice. The equipment is functionally the same as for the auscultatory method, but with an electronic pressure sensor (transducer) fitted in the cuff to detect blood flow, instead of using the stethoscope and the expert's ear. In practice, the manometer is a calibrated electronic device with a numerical readout of blood pressure, instead of a mercury tube; calibration must be checked periodically. In most cases the cuff is inflated and released by an electrically operated pump, and it may be fitted on the wrist (elevated to heart height), although the upper arm is preferred.

Oscillometric measurement requires less skill than auscultatory, and is suitable for use by non-trained staff and for automated patient monitoring.

The cuff is inflated to a pressure in excess of the systolic blood pressure. The pressure is then gradually released over a period of about 30 seconds. When blood flow is nil (cuff pressure exceeding systolic pressure) or unimpeded (cuff pressure below diastolic pressure), cuff pressure will be essentially constant. When blood flow is present, but restricted, the cuff pressure, which is monitored by the pressure sensor, will vary periodically in synchrony with the cyclic expansion and contraction of the brachial artery, i.e., it will oscillate. The cuff pressure at which oscillations start is the systolic pressure; pressure when oscillations cease is dyastolic pressure.

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 blood pressure waveform to determine the systolic, mean, and diastolic points.

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

Normal values of blood pressure

Normal ranges for blood pressure in adult humans are:

Systolic between 90 and 135 mm Hg (or 90 and 135 Torr, 12 to 18 kPa)
Diastolic between 50 and 90 mm Hg (or 50 and 90 Torr, 7 to 12 kPa)

In children the observed normal ranges are lower; in the elderly, they are often higher, largely because of reduced flexibility of the arteries. Clinical trials demonstrate that people who maintain blood pressures at the low end of these pressure ranges have much better long term cardiovascular health and are considered optimal. The principal medical debate is the aggressiveness and relative value of methods used to lower pressures into this range for those who don't maintain such pressure on their own. Elevations, more commonly seen in older people, though often considered normal, are associated with increased morbidity and mortality. The clear trend from double blind clinical trials (for the better strategies and agents) has increasingly been that lower BP is found to result in less disease.

Physiology

The mean arterial pressure (MAP) or mean blood pressure 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 blood pressure results from the pulsatile nature of the cardiac output. The pulse pressure is determined by the interaction of the stroke volume versus the volume and elasticity of the major arteries.

The larger arteries, including all large enough to see without magnification, are low resistance (assuming no advanced atherosclerotic changes) conduits with 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-mm Hg drop in mean pressure.

Regulation of blood pressure

The endogenous regulation of blood pressure is not completely understood. Currently, three mechanisms of regulating blood pressure have been well-characterized:

Baroreceptor reflex: Baroreceptors in various organs can detect changes in blood pressure, and adjust the mean arterial pressure by altering both the force and speed of the heart's contractions, as well as the total peripheral resistance.

Renin-angiotensin system (RAS): This system is generally known for its long-term adjustment of blood pressure. This system allows the kidney to compensate for loss in blood volume or drops in blood pressure by activating an endogenous vasoconstrictor known as angiotensin II.

Aldosterone release: This steroid hormone is released from the adrenal cortex in response to angiotensin II or high serum potassium levels. Aldosterone stimulates sodium retention and potassium excretion by the kidneys. Since sodium is the main ion that determines the amount of fluid in the blood vessels by osmosis, aldosterone will increase fluid retention, and indirectly, blood pressure.

These different mechanisms are not necessarily independent of each other, as indicated by the link between the RAS and aldosterone release. Currently, the RAS system is targeted pharmacologically by ACE inhibitors and angiotensin II receptor antagonists. The aldosterone system is directly targeted by spironolactone, an aldosterone antagonist. The fluid retention may be targeted by diuretics; however, the antihypertensive effect of diuretics is not due to its effect on blood volume. Generally, the baroreceptor reflex is not targeted in hypertension because if blocked, individuals may suffer from orthostatic hypotension and suffer from fainting.

Pathophysiology

Effects of high blood pressure

Blood pressure exceeding normal values is called arterial hypertension. It itself is only rarely an acute problem; see hypertensive crisis. But because of its long-term indirect effects (and also as an indicator of other problems) it is a serious worry to physicians diagnosing it.

All level of blood pressure puts mechanical stress on the arterial walls. Higher pressures increase heart workload and progression of unhealthy tissue growth (atheroma) that develops within the walls of arteries. The higher the pressure, the more stress that is present and the more atheroma tend to progress and the heart muscle tends to thicken, enlarge and become weaker over time.

Persistent hypertension is one of the risk factors for strokes, heart attacks, heart failure, arterial aneurysms, and is the second leading cause of chronic renal failure after diabetes mellitus.

In the past, most attention was paid to diastolic pressure; but nowadays it is recognised that both high systolic pressure and high pulse pressure (the numerical difference between systolic and diastolic pressures) are also risk factors. In some cases, it appears that a decrease in excessive diastolic pressure can actually increase risk, due probably to the increased difference between systolic and diastolic pressures (see the article on pulse pressure).

Effects of low blood pressure

Blood pressure that is too low is known as hypotension. The similarity in pronunciation with hypertension can cause confusion.

Low blood pressure may be a sign of severe disease and requires more urgent medical attention.

When blood pressure and blood flow decrease beyond a certain point, the perfusion of the brain becomes critically decreased (i.e., the blood supply is not sufficient), causing lightheadedness, dizziness, weakness and fainting.

However, people who function well while maintaining low blood pressures have lower rates of cardiovascular disease events than people with normal blood pressures.

Factors influencing blood pressure

The physics of the circulatory system, as of any fluid system, are very complex. That said, there are many physical factors that influence blood pressure. Each of these may in turn be influenced by physiological factors, such as diet, exercise, disease, drugs, etc.

Some physical factors are:

Rate of pumping. In the circulatory system, this rate is called heart rate, the rate at which blood (the fluid) is pumped by the heart. The higher the heart rate, the higher (potentially, assuming no change in stroke volume) the blood pressure.
Volume of fluid. In the case of the circulatory system, this is blood volume, the amount of blood present in the body. The more blood present in the body, the higher the rate of blood return to the heart and the resulting cardiac output. There is some relationship between dietary salt intake and increased blood volume, potentially resulting in higher blood pressure, though this varies with the individual and is highly dependent on autonomic nervous system response.
In cardiac physiology, the rate and volume of flow are accounted for in a combined fashion by cardiac output, which is the heart rate, the rate of contraction, multiplied by the stroke volume, the amount of blood pumped out from the heart with each contraction. It represents the efficiency with which the heart circulates blood throughout the body.
Resistance. In the circulatory system, this is the resistance of the blood vessels. The higher the resistance, the higher the blood pressure. Resistance is related to size (The larger the blood vessel, the lower the resistance), as well as the smoothness of the blood vessel walls. Smoothness is reduced by the buildup of fatty deposits on the arterial walls. Substances called vasoconstrictors can reduce the size of blood vessels, thereby increasing blood pressure. Vasodilators (such as nitroglycerin) increase the size of blood vessels, thereby decreasing blood pressure.
Viscosity, or thickness of the fluid. If the blood gets thicker, the result is an increase in blood pressure. Certain medical conditions can change the viscosity of the blood. For instance, low red blood cell concentration, anemia, reduces viscosity, whereas increased red blood cell concentration increases viscosity. Viscosity also increases with blood sugar concentration—visualise pumping syrup. (It was thought that aspirin and other drugs decreased the viscosity of blood, but this has been found not to be so^*; "blood thinners" reduce the tendency of the blood to clot, not viscosity.)

In practice, each individual's autonomic nervous system responds to and regulates all these interacting factors so that, although the above issues are important, the actual blood pressure response of a given individual varies widely because of both split-second and slow-moving responses of the nervous system and end organs. These responses are very effective in changing the variables and resulting blood pressure from moment to moment.

Other causes of low blood pressure

Sometimes the blood pressure drops significantly when a patient stands up from sitting. This is known as 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.

When people are healthy, they quickly constrict the veins below the heart and increase their heart rate to minimize and compensate for the gravity effect. This is carried out involuntarily by the autonomic nervous system. The system usually requires a few seconds to fully adjust and if the compensations are too slow or inadequate, the individual will suffer reduced blood flow to the brain, 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.

Other causes of low blood pressure include:

Sepsis
Hemorrhage
Toxins including toxic doses of blood pressure medicine
Hormonal abnormalities, such as Addison's disease

Shock is a complex condition which leads to critically decreased blood perfusion. The usual mechanisms are loss of blood volume, pooling of blood within the veins reducing adequate return to the heart and/or low effective heart pumping. Low blood pressure, especially low pulse pressure, is a sign of shock and contributes to/reflects decreased perfusion.

If there is a significant difference in the pressure from one arm to the other, that may indicate a narrowing (e.g., due to aortic coarctation, aortic dissection, thrombosis or embolism) of an artery.

Venous pressure

Venous pressure is the blood pressure in a vein or in the atria of the heart. It is much less than arterial blood pressure, with common values of 5 mm Hg in the right atrium and 8 mm Hg in the left atrium. Measurement of pressures in the venous system and the pulmonary vessels plays an important role in intensive care medicine but requires invasive techniques.

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