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Blood pressure is the pressure exerted by the blood on the walls of the blood vessels. Unless indicated otherwise, blood pressure is understood to mean arterial blood pressure, i.e. the pressure in the large arteries, such as the brachial artery (in the arm). The pressure of the blood in other vessels is lower than the arterial pressure.
The peak pressure in the arteries during the cardiac cycle is the systolic pressure, and the lowest pressure (at the resting phase of the cardiac cycle) is the diastolic pressure. Typical values for the arterial blood pressure of a resting, healthy adult are approximately 120 mmHg systolic and 80 mmHg diastolic (written as 120/80 mmHg), with large individual variations.
Last Updated - 24th November 2005
Blood pressure is not static, but undergoes natural variations from one heartbeat to another or in a circadian rhythm; it also changes in response to stress, nutritional factors, drugs, or disease.
Blood pressure is most accurately measured invasively by placing a cannula into a blood vessel and connecting it to a electronic pressure transducer. This invasive technique is regularly employed in intensive care medicine, anesthesiology, and for research purposes, but it is associated with complications such as thrombosis, infection, and bleeding. Therefore, the less accurate techniques of manual or oscillometric measurement predominate in routine examinations.
Most often, arterial blood pressure is measured manually using a sphygmomanometer. This is an inflatable cuff placed around the upper arm, at roughly the same vertical height as the heart in a sitting person, attached to a manometer. The cuff is inflated until the artery is completely occluded. Listening with a stethoscope to the brachial artery at the elbow, the examiner slowly releases the pressure in the cuff. When blood flow barely begins again in the artery, a "whooshing" or pounding sound (first Korotkoff sound) is heard. The pressure is noted at which this sound began. This is the systolic blood pressure. The cuff pressure is further released until no sound can be heard (fifth Korotkoff sound). This is the diastolic blood pressure.
Oscillometric methods are used in long-term measurement as well as in clinical practice. Oscillometric measurement (also termed NIPB = Non-Invasive Blood Pressure) is incorporated in many bedside patient monitors. It relies on a cuff similar to that of a sphygmomanometer, which is connected to an electric pump and a pressure transducer. The cuff is placed on the upper arm and is automatically inflated. When pressure is gradually released, the small oscillations in cuff pressure that are caused by the cyclic expansion of the brachial artery are recorded and used to calculate systolic and diastolic pressures.
Values are usually given in millimetres of mercury (mmHg). Normal ranges for blood pressure in adult humans are:
Systolic between 90 and 135 mmHg (12 to 18 kPa)
Diastolic between 50 and 90 mmHg (7 to 12 kPa)
In children the observed normal ranges are lower; in the elderly, they are more often higher. Clinical trials demonstrate that people who maintain blood pressures in 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 ends up being demonstrated to result in less disease/better outcomes long term.
The 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 mean blood pressure value is determined by the volume of blood the heart is pumping per minute, termed cardiac output, versus the resistance of the 20,000 to 30,000 arterioles, termed total peripheral resistance, through which the blood must flow to reach the capillaries and then veins.
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) and conduit vessels with high flow rates but producing very little pressure drop. For instance, about 5 mmHg mean pressure decrease in the blood flow traveling from the heart all the way to the toes is typical, assuming the individual is supine (horizontal with respect to gravity).
If the blood pressure exceeds these normal values, one speaks of arterial hypertension. Blood pressure that is too low is known as hypotension. The former is rarely an acute problem, while the latter may be a sign of severe disease and requires more urgent medical attention.
Any level of blood pressure puts mechanical stress on the arterial walls. The higher the pressure, the more stress that is present and the more atheroma tend to progress. Veins, when used as surgical bypasses, typically develop atheroma much more rapidly than arteries in the same individual.
When blood pressure and blood flow is very low, the perfusion of the brain may be critically decreased (i.e. the blood supply is not sufficient), causing lightheadedness, weakness and fainting. Sometimes the blood pressure drops significantly when a patient stands up from sitting. This is known as orthostatic hypotension. Other causes of low blood pressure include:
- 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. Low blood pressure is a sign of shock and can also contribute to further decreasing 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 is the blood pressure in a vein. It is much less than arterial blood pressure. e.g. typically about 5 mmHg in the right atrium, 8 mmHg 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.
Arterial hypertension Or High Blood Pressure
Arterial hypertension, or high blood pressure is a medical condition where the blood pressure is chronically elevated. Persistent hypertension is one of the risk factors for strokes, heart attacks and heart failure, and is a leading cause of chronic renal failure.
In patients with diabetes mellitus or kidney disease studies have shown that blood pressure over 130/80 mmHg should be considered a risk factor and may warrant treatment.
Etiology - Essential hypertension
- Age. Over time, the number of collagen fibres in artery and arteriole walls increases, making blood vessels stiffer. With the reduced elasticity comes a smaller cross-sectional area in systole, and so a raised mean arterial blood pressure.
- High salt intake
- Sedentary lifestyle
- Tobacco smoking
- Alcohol abuse
- High levels of saturated fat in the diet
- Obesity. In obese subjects, losing a pound in weight generally reduces blood pressure by 1mmHg.
- Low birth-weight
- Diabetes mellitus
- Various genetic causes
In essential hypertension
- Pregnancy. (See below for full details).
- Kidney disease or renal artery stenosis
- Certain cancers
- Drugs. In particular, alcohol, nasal congestants with adrenergic effects, NSAIDs, MAOIs, adrenoceptor stimulants, and the contraceptive pill (ethinyl-estradiol) can cause hypertension while in use.
- Malformed aorta
- Slow pulse
- Aortic valve disease
The mechanisms behind the factors associated with inessential hypertension are generally fully understood, and are outlined below. However, those associated with essential hypertension are far less understood. What is known is that cardiac output is raised early in the disease course, with total peripheral resistance normal; over time cardiac output drops to normal levels but TPR is increased. Three theories have been proposed to explain this:
- Inability of the kidneys to excrete sodium, resulting in natriuretic factor (note: the existence of this substance is theoretical) being secreted to promote salt excretion with the side-effect of raising total peripheral resistance.
- An overactive renin / angiotension system leads to vasoconstriction and retention of sodium and water. The increase in blood volume leads to hypertension.
- An overactive sympathetic nervous system, leading to increased stress responses.
- Pregnancy: unclear.
- Kidney disease / renal artery stenosis: the normal physiological response to low blood pressure in the renal arteries is to increase cardiac output to maintain the pressure needed for glomerular filtration. Here, however, increased CO can't solve the structural problems causing renal artery hypotension, with the result that CO remains chronically elevated.
- Cancers: tumours in the kidney can operate in the same way as kidney disease. More commonly, however, tumours cause inessential hypertension by ectopic secretion of hormones involved in normal physiological control of blood pressure.
- Drugs: anything with an adrenergic effect causes vasoconstriction at sites with alpha-adrenoceptors, increasing total peripheral resistance.
- Malformed aorta, slow pulse: these cause reduced blood flow to the renal arteries, with physiological responses as already outlined.
- Anemia: unclear.
- Fever: unclear.
- Aortic valve disease: unclear.
Signs and symptoms
Hypertension - Hypertension is usually found incidentally - "case finding" by healthcare professionals. It normally produces no symptoms.
Malignant hypertension (or accelerated hypertension) is distinct as a late phase in the condition, and may present with headaches, blurred vision and end-organ damage.
It is recognised that stressful situations can increase the blood pressure; if a normally normotensive patient has a high blood pressure only when being reviewed by a health care professional, this is colloquially termed white coat effect. Since most of what we know of hypertension and its outcome with or without modification is based on large series of readings in doctors' offices and clinics (eg Framingham) it is difficult to be sure of the significance of white-coat hypertension. Ambulatory monitoring may help determine whether traffic and ticket inspectors produce similar sustained rises.
Hypertension is often confused with mental tension, stress and anxiety. While chronic anxiety is associated with poor outcomes in people with hypertension, it alone does not cause it.
Hypertensive urgencies and emergencies - Hypertension is rarely severe enough to cause symptoms. These only surface with a systolic blood pressure over 240 mmHg and/or a diastolic blood pressure over 120 mmHg. These pressures without signs of end-organ damage (such as renal failure) are termed accelerated hypertension. When end-organ damage is present, but in absence of raised intracranial pressure, it is called hypertensive urgency. Hypertension under this circumstance needs to be controlled, but hospitalization is not required. When hypertension causes increased intracranial pressure, it is called malignant hypertension. Increased intracranial pressure causes papilledema, which is visible on ophthalmoscopic examination of the retina.
Complications - While elevated blood pressure alone is not an illness, it often requires treatment due to its short- and long-term effects on many organs. The risk is increased for:
- Cerebrovascular accident (CVAs or strokes)
- Myocardial infarction (heart attack)
- Hypertensive cardiomyopathy (heart failure due to chronically high blood pressure)
- Hypertensive retinopathy - damage to the retina
- Hypertensive nephropathy - chronic renal failure due to chronically high blood pressure
Pregnancy - Although few women of childbearing age have high blood pressure, up to 10% develop hypertension of pregnancy. While generally benign, it may herald three complications of pregnancy: pre-eclampsia, HELLP syndrome and eclampsia. Follow-up and control with medication is therefore often necessary.
The diagnosis of hypertension is by definition made by three separate measurements at least one week apart. Two caveats to this criteria is it must be in the presence mild elevations and in the absence of end organ damage. If either are not met, the diagnosis may be made without repeat measurements in some cases.
Obtaining reliable blood pressure measurements relies on following several rules and being cognizant of the many factors that influence blood pressure reading.
For instance, measurements should be at least 1 hour after caffeine, 30 minutes after smoking and without any stress. Cuff size is also important. The bladder should encircle and cover two-thirds of the length of the arm. The patient should be sitting for a minimum of five minutes. The patient should not be on any adrenergic stimulants, such as those found in many cold medications.
When taking manual measurements, the person taking the measurement should be careful to inflate the cuff at least 30 mmHg greater than systolic pressure. A stethoscope should be placed lightly over the brachial artery. The arm should be at the level of the heart and the cuff should be deflated at a rate of 2-3 mmHg/sec. Systolic pressure is the pressure reading at the onset of sounds. Diastolic pressure is then defined as the pressure at which the sounds disappear. Two measurements should be made at least 5 minutes apart and if there is a discrepancy of more than 5 mmHg, a third reading should be done. The readings should then be averaged. An initial measurement should include both arms. Also, in elderly patients, it is recommended to measure pressures in multiple postures as they are at risk for orthostatic hypotension.
Once the diagnosis of hypertension has been made it is important to attempt to identify reversible (secondary) causes. In the adult population over 90% of all hypertension has no known cause and is therefore called "essential/primary hypertension". Often, it is part of the metabolic "syndrome X" in patients with insulin resistance: it occurs in combination with diabetes mellitus (type 2), combined hyperlipidemia and central obesity. However, in the pediatric population the opposite is true, most cases have a secondary cause and these should be pursued more aggresively.
Important causes of secondary hypertension are:
- Heavy alcohol use
- Renal artery stenosis
- Obstructive sleep apnea
- Hyperaldosteronism (Conn's syndrome)
- Cushing's disease
- Steroid use
- Coarcation of the aorta
- Chronic renal failure
- Scleroderma renal crisis
- Liquorice (when consumed in excessive amounts)
Blood tests commonly performed in a newly diagnosed hypertension patient are:
- Creatinine (renal function)
- Electrolytes (sodium, potassium)
- Glucose (to identify diabetes mellitus)
- Being overweight
- Having a family history of high pressure
- Being of African-American descent
- Having an inactive lifestyle
- Drinking too much alcohol
- Having too much salt or not enough potassium, calcium or magnesium in your diet
- Certain medications, especially birth control pills, steroids, decongestants and anti-inflammatory drugs
- Maintain a healthy weight. If you are overweight, even losing 10 pounds can help lower your blood pressure
- Exercise regularly
- Drink alcohol only in moderation
- Use salt moderately
- Eat plenty of fruits, vegetables, legumes and low-fat dairy products to ensure you get enough potassium, calcium, and magnesium in your diet.
- Reduce the saturated fat in your diet (found in milk, cheese and meat)
- Stop using tobacco products - Learn how to check your own blood pressure.
When To Call A Physician
- Your blood pressure rises suddenly
- Your blood pressure is 180/100 or higher
- You have a sudden, severe headache that is worse than any headache you've had before
- If you develop uncomfortable or disturbing side effects from any medication
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