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What is Blood Pressure?

Blood is carried from the heart to all parts of your body in vessels called arteries. Blood pressure is the force of the blood pushing against the walls of the arteries. Each time the heart beats (about 60-70 times a minute at rest), it pumps out blood into the arteries. Your blood pressure is at its highest when the heart beats, pumping the blood. This is called systolic pressure. When the heart is at rest, between beats, your blood pressure falls. This is the diastolic pressure.

Blood pressure is always given as these two numbers, the systolic and diastolic pressures. Both are important. Usually they are written one above or before the other, such as 120/80 mmHg. The top number is the systolic and the bottom the diastolic. When the two measurements are written down, the systolic pressure is the first or top number, and the diastolic pressure is the second or bottom number (for example, 120/80). If your blood pressure is 120/80, you say that it is "120 over 80."

Blood pressure changes during the day. It is lowest as you sleep and rises when you get up. It also can rise when you are excited, nervous, or active.

Still, for most of your waking hours, your blood pressure stays pretty much the same when you are sitting or standing still. That level should be lower than 120/80. When the level stays high, 140/90 or higher, you have high blood pressure. With high blood pressure, the heart works harder, your arteries take a beating, and your chances of a stroke, heart attack, and kidney problems are greater.

What causes it?
In many people with high blood pressure, a single specific cause is not known. This is called essential or primary high blood pressure. Research is continuing to find causes.

In some people, high blood pressure is the result of another medical problem or medication. When the cause is known, this is called secondary high blood pressure.

What is high blood pressure?
A blood pressure of 140/90 or higher is considered high blood pressure. Both numbers are important. If one or both numbers are usually high, you have high blood pressure. If you are being treated for high blood pressure, you still have high blood pressure even if you have repeated readings in the normal range.

There are two levels of high blood pressure: Stage 1 and Stage 2 (see the chart below).

Categories for Blood Pressure Levels in Adults*
(In mmHg, millimeters of mercury)

Category	Systolic (Top number)	Diastolic (Bottom number)
Normal	Less than 120	Less than 80
Prehypertension	120-139	80-89
High Blood Pressure	Systolic	Diastolic
Stage 1	140-159	90-99
Stage 2	160 or higher	100 or higher

* For adults 18 and older who:

• Are not on medicine for high blood pressure
• Are not having a short-term serious illness
• Do not have other conditions such as diabetes and kidney disease

Note: When systolic and diastolic blood pressures fall into different categories, the higher category should be used to classify blood pressure level. For example, 160/80 would be stage 2 high blood pressure.
There is an exception to the above definition of high blood pressure. A blood pressure of 130/80 or higher is considered high blood pressure in persons with diabetes and chronic kidney disease.

Hypertension & Women

Does menopause affect the blood pressure?

By: Thomas Pickering, MD, DPhil, FRCP, Director of Integrative and Behavioral Cardiology Program of the Cardiovascular Institute at Mount Sinai School of Medicine, New York. It has been uncertain whether there is a true increase of blood pressure associated with the menopause, or whether there is just a gradual upward trend that is the consequence of aging. A Belgian study of 315 healthy women aged between 30 and 70 checked their blood pressure three times over a 5-year period by conventional clinic measurements, and once by ambulatory (24 hour) monitoring. During that time 44 of the women went through the menopause. Both types of blood pressure measurement led to the same conclusion: that there is an increase of systolic pressure of about 5 mmHg that is the result of the menopause itself, that is distinct from the effects of aging and putting on weight, which also tends to happen at the time of the menopause. There appeared to be no effect of the menopause on diastolic pressure, and men of the same ages as the women showed no similar change of blood pressure. Doctor’s comments Part of the previous confusion as to whether the menopause directly affects blood pressure has been due to the fact that blood pressure (particularly systolic) tends to go up with age. What this study was able to show was that, over the same span of 5 years, women who went through the menopause showed a bigger increase of systolic pressure than those who did not. The fact that only systolic pressure was affected led the authors to suggest that the lack of estrogens, which accompany the menopause may result in an increased stiffness of the arteries. Where it was published JA Staessen and colleagues. Conventional and ambulatory blood pressure and menopause in a prospective population study. Journal of Human Hypertension 1997;11:507

Do oral contraceptives raise blood pressure?

By: Thomas Pickering, MD, DPhil, FRCP, Director of Integrative and Behavioral Cardiology Program of the Cardiovascular Institute at Mount Sinai School of Medicine, New York. Soon after oral contraceptives were first introduced, more than 30 years ago, there were a number of reports of high blood pressure associated with their use. The pills contain a mixture of estrogens and progestins, but the amount of each ingredient in the preparations used today is only about one fifth of the original dose, although there is considerable variation between different brands. The Nurses’ Health Study has investigated the risk of developing high blood pressure from taking the oral contraceptives now in use. The study followed the health of more than 100,000 American nurses, who were first contacted in 1989. Among other questions, they were asked about their use of oral contraceptives and their blood pressure on three occasions over a four year period. Those who reported having hypertension at the beginning of the study were excluded from this analysis. The principal findings were: The chances of developing hypertension were about 80% higher in the women who had used oral contraceptives in comparison with those who had not. The risk was slightly higher in women who had used them for more than six years. The risk did not vary significantly according to the amount of estrogens and progestins in the pills. Women with a family history of high blood pressure were at increased risk of developing it themselves, but they did not appear to be any more susceptible to the effects of oral contraceptives than those with no family history. There was no evidence of any racial difference in susceptibility to the effects of oral contraceptives on blood pressure; if anything, black women were at lower risk than white women. The number of women who were estimated to have developed hypertension as a result of taking the pills was small- the equivalent of 41 cases in 10,000 women over a period of one year, or 0.4%. Doctor’s comments This study is reassuring for women taking oral contraceptives, because although there is a definite risk of developing high blood pressure, it is quite small. Furthermore, other studies have shown that if the pressure does go up, it will usually go back to its previous level within three months of stopping the pills. One implication is that if you are taking oral contraceptives, it is important to get your blood pressure checked at regular intervals. Women who already have hypertension are generally advised not to take oral contraceptives. Where it was published Chasan-Taber L and colleagues. Prospective study of oral contraceptives and hypertension among women in the United States. Circulation.1996;94:483-489. High blood pressure and bone weakening By: Thomas Pickering, MD, DPhil, FRCP, Director of Integrative and Behavioral Cardiology Program of the Cardiovascular Institute at Mount Sinai School of Medicine, New York. There is a lot of evidence from both human and animal studies to show that high blood pressure leads to loss of calcium from the body, mainly as a result of increased amounts of calcium in the urine. Most of our calcium stores are in our bones, which leads to the question whether high blood pressure might contribute to the development of osteoporosis, a common cause of weak bones and fractures in older people, particularly women. A survey of 3,676 elderly white women living in four parts of the USA, whose average age was 73, had their blood pressures measured and also a bone density test, which was measured a second time after another three and a half years. Comparison of the bone densities over this period enabled the estimation of the rate of bone loss. The results showed that women with the highest blood pressures had nearly double the rate of bone loss in comparison with women with low blood pressure. Doctor’s comments Osteoporosis is a major health problem for older people and accounts for more than a million bone fractures a year in the US. It affects women more than men, and whites more than blacks (which is why this study did not include black women). The reason for the link with high blood pressure which this study shows is presumed to be the increased loss of calcium in the urine, which is a direct consequence of high blood pressure. A high salt diet also leads to more calcium loss. For people who have high blood pressure the good news is that thiazide diuretics (the sort normally used to treat high blood pressure in older people) actually reduce the calcium loss, and there is some evidence that they may help to prevent osteoporosis. Where it was published FP Cappuccio and colleagues. High blood pressure and bone-mineral loss in elderly white women: a prospective study. Lancet 1999;354:971. High Blood Pressure Isn't Well Controlled in Older Women Data from the Women's Health Initiative offers a glimpse of hypertension treatment in older women. Based on information from nearly 99,000 women aged 50 to 79, researchers found: 37.8% had high blood pressure (defined as 140/90 mmHg or being on medication for high blood pressure). While 64.3% of those were receiving medication, blood pressure was controlled in only 36.1%. More women on hormone replacement therapy were hypertensive than those not taking hormones. 44.3% of the hypertensive women used diuretics either alone or in combination with other classes of drugs. Women taking diuretics alone had better blood pressure control than those taking any of the other drugs by themselves. The researchers, writing in the journal Hypertension, concluded that hypertension in older women is not being treated aggressively enough, especially not in those most at risk for stroke and heart disease because of their age. Source: Wassertheil-Smoller S, Anderson G, Psaty BM, et al. Hypertension and its treatment in postmenopausal women. Hypertension, 2000;36:780. (Abs.) Symptoms of High Blood Pressure How do I know when my blood pressure is high? By: Thomas Pickering, MD, DPhil, FRCP, Director of Integrative and Behavioral Cardiology Program of the Cardiovascular Institute at Mount Sinai School of Medicine, New York. For the most part, you can't tell if you have high blood pressure, and most hypertensive people have no symptoms. So the only way to find out if it's high is to have it measured. This statement may come as a surprise, because many people are convinced that they can tell when their pressure is high. It's certainly true that when you get angry or anxious you may feel yourself tensing up, and your heart pounding. You may even go red in the face, something that's often erroneously associated with high blood pressure. And you're right: your pressure is likely to be high at such times. But that's perfectly normal, and unless you spend your life being permanently angry, which fortunately most of us don't, it doesn't mean much. High blood pressure is of concern only when it's still high when you are not angry or tense. What are the symptoms of high blood pressure? By: Thomas Pickering, MD, DPhil, FRCP, Director of Integrative and Behavioral Cardiology Program of the Cardiovascular Institute at Mount Sinai School of Medicine, New York. Usually, there are no specific symptoms which indicate that someone has high blood pressure. But some population surveys have shown that a wide variety of common symptoms, such as sleep disturbance, emotional upsets, and dry mouth, are slightly commoner in people with higher pressures. The differences are small, however. Going red in the face, or feeling flushed, is not indicative of high blood pressure. Headache and high blood pressure If you asked a hundred people what is the commonest symptom of high blood pressure, the chances are that the majority would say headache. In fact, not only do most people with high blood pressure not have headaches any more than the rest of us, but when they do, it's usually not from the blood pressure. Merely having a high level of blood pressure inside your head does not normally produce any symptoms; if you lift a heavy weight, your pressure may go up by 30 or 40 mm Hg, but you don't get a headache. What can cause headache is muscle tension. Any muscle that is tensed for long enough starts to hurt, and chronic tension in the scalp or neck muscles is a very common cause of headache. A study conducted many years ago shed some very interesting light on the relationship between headache and high blood pressure. Out of 104 people who had high blood pressure but were unaware of it, only three volunteered that they had headaches, although another 14 admitted it when asked. But of 96 people who had been told that they had high blood pressure, 71 said they had headaches. The simplest explanation for this finding is that being told that you have high blood pressure makes you start to worry, and that this in turn causes the headaches. There is a much smaller number of patients, mostly with very high pressures, in whom headaches are directly related to the height of the blood pressure. In such individuals treating the blood pressure will relieve the symptoms. Headaches & High Blood Pressure Is Headache Related to Increases of Blood Pressure? By: Thomas Pickering, MD, DPhil, FRCP, Director of Integrative and Behavioral Cardiology Program of the Cardiovascular Institute at Mount Sinai School of Medicine, New York. Headache is generally regarded as being a symptom of high blood pressure, although it is also stated that hypertension is the “silent killer”. Many people who have high blood pressure think that when they get a headache it means that their pressure has gone up. The problem here is that both headache and high blood pressure are very common, which means that it is certainly true that the two occur together, but what exactly is the connection? A study conducted in Poland has provided some interesting answers. One hundred and fifty patients, most of who were taking blood pressure-lowering medications, and who were referred to a hypertension clinic for evaluation, wore a 24-hour ambulatory blood pressure monitor. These monitors take a blood pressure reading every 20 minutes during the day, and 30 minutes during the night. The patients were asked to record in a diary whenever they had a headache, and in fact during the 24-hour monitoring period 43 of the patients (about one third) reported a headache, which lasted an average duration of 3 hours. In general, there was no connection between the occurrence of headaches and what was happening to the blood pressure, and in most cases the highest readings of blood pressure were not associated with headaches. In addition, there was no particular tendency for the blood pressure to go up or down just before the headache started. Whether or not the patients were taking blood pressure lowering medications also had no effect on the association between headache and blood pressure. Doctor’s comments Most of us, whatever our blood pressure levels, get headaches from time to time, and the relationships between blood pressure and headache have always been a bit uncertain. It is true that in a small number of patients with very high blood pressure, headache can occur as a consequence, but this is probably rare. One study actually found that headaches were more likely to occur after people had been diagnosed with high blood pressure, which may have been because of the anxiety associated with the awareness of high blood pressure, rather than because of the blood pressure itself. The clear finding of this study is that most of the headaches in people with high blood pressure, whether treated or not, do not occur because the blood pressure has gone up There is some evidence from other studies that people with hypertension are more likely to have migraine headaches, and also some reports that some of the blood pressure lowering drugs such as calcium channel blockers and beta-blockers are effective for treating these. Again, this may not necessarily a result of a lower blood pressure. The message here is that if you have hypertension and are getting headaches, it does not mean that your blood pressure out of control, and that the best treatment for the headache may be the traditional pain killers such as aspirin. Where it was published P Kruszewski and colleagues. Headache in patients with mild to moderate hypertension is generally not associated with simultaneous blood pressure elevation. Journal of Human Hypertension, 2000; 18:437. Study Proves Headaches Are Not Related to Blood Pressure Many people with high blood pressure believe there is an association between headaches and their pressure levels. There has been little direct evidence to support the relationship, and doctors repeatedly advise their patients that there are no symptoms of high blood pressure. Now a new study using ambulatory blood pressure monitoring has provided convincing data to back up the lack of any connection. Seventy-five people with mild hypertension had their blood pressure monitored for 24 hours and kept a log of their headaches. One-third of the study participants had a headache while they were being monitored, but there was no difference in their blood pressure curves from those of the people who did not have a headache. Blood pressure values registered during the headache and in the hours before and after it were not different from each other. Even in a subset of 8 patients who had a migraine, the analysis showed a similar pattern. This report underscores doctors' advice not to rely on the occurrence of a headache as a sign that your blood pressure is up and the importance of following your treatment program and taking medication as directed all of the time. Source: Gus M, Fuchs FD, Pimentel M, et al. Behavior of ambulatory blood pressure surrounding episodes of headache in mildly hypertensive patients. Archives of Internal Medicine (Jan.) 2001;161:252-255. (Abs) Measuring High Blood Pressure What are Korotkoff sounds? The 'Korotkoff sounds' are the sounds that a doctor or nurse listens for when they are taking your blood pressure. They are named after Dr. Nikolai Korotkoff, a Russian physician who described them in 1905, when he was working at the Imperial Medical Academy in St. Petersburg. They are detected by listening with a stethoscope placed over the brachial (arm) artery just below a blood pressure cuff, as it is gradually deflated from a pressure high enough to occlude the artery and stop the blood flowing. As the cuff pressure reaches systolic pressure, the blood starts to flow, and a whooshing sound is heard (the first phase of the Korotkoff sounds). As the cuff is deflated further, the artery opens and closes, and the flow of blood is turbulent, so sounds are still heard. When the cuff pressure is less than the diastolic pressure, the artery is open all the time, and the flow of blood is smooth, and the sounds disappear. Systolic pressure is registered as the pressure at which the sounds are first heard, and diastolic as the pressure at which they disappear (phase 5 of the Korotkoff sounds). What is an electrocardiogram? This test is used routinely in the evaluation of people with high blood pressure and is the traditional way of seeing if the hypertension has had any affect on the heart. When heart muscle contracts, it creates an electrical impulse, which can be detected on the surface of the body. The electrocardiogram (ECG or EKG) is performed by applying six electrodes to the skin on different parts of the chest. Another four are applied to each wrist and ankle. These pick up the electrical signal coming from the heart, which is amplified and written out on a paper strip. Each electrode looks at the heart from a different viewpoint, and the electrical waveform is slightly different in each one. The pattern of waveforms in the different leads provides a lot of information about the structure and function of the heart. For the evaluation of high blood pressure, the two most important things are whether the heart is enlarged, and whether there's any sign of damage from a heart attack. Do You Have 'White Coat Hypertension'? Some patients have what is called 'white-coat hypertension;' their blood pressure is consistently elevated in the doctor's office or clinic but normal at other times. It is called this because the patient is reacting to the white coat that the doctor or other caregiver is wearing. If your blood pressure is high when measured at the doctor's office or clinic but is low when you measure it yourself, you should mention that to your doctor so that a more thorough evaluation may be done. Sometimes in these cases ambulatory blood pressure (ABP) monitoring will be used to further measure blood pressure. These devices take readings every 15 to 30 minutes throughout the day and night while you go about your normal daily activities. An analysis of these readings will clarify the status of your blood pressure. Source: The Sixth Report of the Joint National Committee on Prevention, Detection, Evaluation, and Treatment of High Blood Pressure. NHLBI, November 1997. Stress & High Blood Pressure Panic attacks are more common in people with high blood pressure By: Thomas Pickering, MD, DPhil, FRCP, Director of Integrative and Behavioral Cardiology Program of the Cardiovascular Institute at Mount Sinai School of Medicine, New York. Panic attacks have only been recognized as a discrete entity since 1980, and it has been suggested that they may be related to hypertension. Now a study has come up with an answer. The definition of a panic attack is “ a discrete period of intense fear or discomfort involving at least four of the following symptoms: Shortness of breath or smothering symptoms Dizziness, unsteadiness or faintness Palpitations or rapid heart beats Trembling or shaking Sweating Choking Nausea or stomach upset Depersonalization or feeling of unreality Numbness or tingling Hot flushes or chills Chest pin or discomfort Fear of dying Fear of going crazy or losing control The study looked at two samples of patients with high blood pressure (all were taking blood pressure lowering medications or had a blood pressure of at least 160/90mmHg), one of which was selected from a primary care practice, and the other from a hospital clinic, and compared them with the same number (about 350 people) of patients with normal pressure from the same family practice. All were sent questionnaires asking them if they had the symptoms described above. The main finding was that panic attacks were significantly commoner in people with hypertension as opposed to those with a normal blood pressure: the percentages of people with panic attacks were 35% in the primary care practice, 39% in the hospital practice, and 22% in the normotensives. Both groups of hypertensives were more likely to have spontaneous panic attacks (ones that started for no obvious reason), and the attacks were more likely to be severe. The hypertensives were also more likely to report that they were generally anxious than the normotensives. The use of different types of antihypertensive drugs did not appear to influence whether or not the hypertensives had panic attacks. In most patients the panic attacks started after the hypertension. Whether hypertensive or not, women were more likely to report panic attacks than men. Doctor’s comments This study shows clearly that there is a connection between panic attacks and hypertension, but it does not tell us which is the chicken and which is the egg. The fact that the hypertension started before the attacks in most patients makes it unlikely that the attacks were the direct cause of the hypertension, however. Other studies have shown that blood pressure goes up during an attack, but it comes down again when the attack is over. What causes these attacks remains a mystery; although they have some relation with a generally anxious personality, this relationship is not very strong. Where it was published SJC Davies and colleagues. Association of panic disorder and panic attacks with hypertension. American Journal of Medicine 1999; 107:310 Does stress cause hypertension? There is no doubt that acute stress can cause a transient increase of pressure, but whether exposure to chronic stress raises pressure is controversial. When people move from a traditional village life to the big cities, their blood pressure goes up, but whether this is from stress or a change of diet is uncertain. Having a stressful job may raise the pressure in men but probably not so much in women. Fight or flight' response raises blood pressure -- even while you're asleep Many people experience sleep apnea; it occurs especially in those sleeping on their backs and snoring, when the tongue and soft parts of the upper airway may fall back and partially block the airway. This causes a period of intermittent breathing known as obstructive sleep apnea (OSA). From the University of Colorado Health Sciences Center comes a report on the effect of sleep apnea on hypertension. As the period between breaths increases, the amount of oxygen in the blood may fall. This sets off the body's 'fight or flight' mechanism, which is a reflex controlled by the sympathetic nervous system. When that happens, the vascular system clamps down and raises blood pressure in an attempt to get the body ready for action. When normal breathing resumes, blood pressure levels return to normal. Hypertension in awake patients is also associated with increased activity of the sympathetic nervous system. Individuals who have not been identified as OSA patients exhibit this same sympathetic response. A common respiratory therapy, called continuous positive airway pressure, or CPAP, has been shown to lower blood pressure in some hypertensive OSA patients. Effective CPAP therapy moderates this hypertensive effect even in awake patients with undiagnosed OSA. Source: Zwillich, CW. Obstructive sleep apnea causes transient and sustained systemic hypertension. Int J Clin Pract 1999;53(June):301-5. (Abs)

Blood pressure

Blood pressure (BP) is the pressure exerted by circulating blood upon the walls of blood vessels, and is one of the principal vital signs. During each heartbeat, BP varies between a maximum (systolic) and a minimum (diastolic) pressure. The mean BP, due to pumping by the heart and resistance to flow in blood vessels, decreases as the circulating blood moves away from the heart through arteries. Blood pressure drops most rapidly along the small arteries and arterioles, and continues to decrease as the blood moves through the capillaries and back to the heart through veins. Gravity, valves in veins, and pumping from contraction of skeletal muscles, are some other influences on BP at various places in the body.

The term blood pressure usually refers to the pressure measured at a person's upper arm. It is measured on the inside of an elbow at the brachial artery, which is the upper arm's major blood vessel that carries blood away from the heart. A person's BP is usually expressed in terms of the systolic pressure over diastolic pressure (mmHg), for example 140/90.

Classification

Classification of blood pressure for adults

Category	systolic, mmHg	diastolic, mmHg
Normal	<>	<>
Prehypertension	120 – 139	or 80 – 89
Stage 1 Hypertension	140 – 159	or 90 – 99
Stage 2 Hypertension	160 - 179	or 100 - 109
Hypertensive Crisis	≥ 180	or ≥ 110

The following US classification of blood pressure applies to adults aging 18 and older. It is based on the average of seated BP readings that were properly measured during 2 or more office visits. In the UK, hypertension is considered when a patient's reading is above 140/90 mmHg. According to the American Heart Association the following are the blood pressure categories]

Normal

While average values for arterial pressure could be computed for any given population, there is often a large variation from person to person; arterial pressure also varies in individuals from moment to moment. Additionally, the average of any given population may have a questionable correlation with its general health, thus the relevance of such average values is equally questionable. However, in a study of 100 subjects with no known history of hypertension, an average blood pressure of 112/64 mmHg was found, which are the normal values.

Various factors influence a person's average BP and variations. Factors such as age and gender influence average values. In children, the normal ranges are lower than for adults and depend on height. As adults age, systolic pressure tends to rise and diastolic tends to fall.In the elderly, BP tends to be above the normal adult range, largely because of reduced flexibility of the arteries. Also, an individual's BP varies with exercise, emotional reactions, sleep, digestion and time of day.

Differences between left and right arm BP measurements tend to be random and average to nearly zero if enough measurements are taken. However, in a small percentage of cases there is a consistently present difference greater than 10 mmHg which may need further investigation, e.g. for obstructive arterial disease.

The risk of cardiovascular disease increases progressively above 115/75 mmHg.In the past, hypertension was only diagnosed if secondary signs of high arterial pressure were present, along with a prolonged high systolic pressure reading over several visits. Regarding hypotension, in practice blood pressure is considered too low only if noticeable symptoms are present.

Clinical trials demonstrate that people who maintain arterial pressures at the low end of these pressure ranges have much better long term cardiovascular health. The principal medical debate concerns the aggressiveness and relative value of methods used to lower pressures into this range for those who do not maintain such pressure on their own. Elevations, more commonly seen in older people, though often considered normal, are associated with increased morbidity and mortality.

Average blood pressure in (mmHg):

1 year	6 – 9 years	adults
95/65	100/65	110/65 - 140/90]

Pathophysiology

There are many physical factors that influence arterial pressure. Each of these may in turn be influenced by physiological factors, such as diet, exercise, disease, drugs or alcohol, stress, obesity, and so-forth.

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 volume of blood flow from the heart is called the cardiac outputwhich 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). The higher the heart rate, the higher the arterial pressure, assuming no reduction in stroke volume.^{[citation needed]}
• Volume of fluid or blood volume, the amount of blood that is 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 arterial pressure, though this varies with the individual and is highly dependent on autonomic nervous system response and the renin-angiotensin system.^{[citation needed]}
• Resistance. In the circulatory system, this is the resistance of the blood vessels. The higher the resistance, the higher the arterial pressure upstream from the resistance to blood flow. Resistance is related to vessel radius (the larger the radius, the lower the resistance), vessel length (the longer the vessel, the higher the resistance), blood viscosity, as well as the smoothness of the blood vessel walls. Smoothness is reduced by the build up of fatty deposits on the arterial walls. Substances called vasoconstrictors can reduce the size of blood vessels, thereby increasing BP. Vasodilators(such as nitroglycerin) increase the size of blood vessels, thereby decreasing arterial pressure. Resistance, and its relation to volumetric flow rate (Q) and pressure difference between the two ends of a vessel are described by Poiseuille's Law.
• Viscosity, or thickness of the fluid. If the blood gets thicker, the result is an increase in arterial 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. It had been thought that aspirin and related "blood thinner" drugs decreased the viscosity of blood, but studies found that they act by reducing the tendency of the blood to clot instead.

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 arterial 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 BP from moment to moment.

Moreover, blood pressure is the result of cardiac output increased by peripheral resistance: blood pressure = cardiac output X peripheral resistance. As a result, an abnormal change in blood pressure is often an indication of a problem affecting the heart's output, the blood vessels' resistance, or both. Thus, knowing the patient's blood pressure is critical to assess any pathology related to output and resistance.

Mean arterial pressure

The mean arterial pressure (MAP) is the average over a cardiac cycle and is determined by the cardiac output (CO), systemic vascular resistance (SVR), and central venous pressure (CVP),

MAP can be approximately determined from measurements of the systolic pressure and the diastolic pressure while there is a normal resting heart rate,^[18]

Pulse pressure

The up and down fluctuation of the arterial pressure results from the pulsatile nature of the cardiac output, i.e. the heartbeat. The pulse pressure is determined by the interaction of the stroke volume of the heart, compliance (ability to expand) of the aorta, and the resistance to flow in the arterial tree. By expanding under pressure, the aorta absorbs some of the force of the blood surge from the heart during a heartbeat. In this way the pulse pressure is reduced from what it would be if the aorta wasn't compliant.^[19] The loss of arterial compliance that occurs with aging explains the elevated pulse pressures found in elderly patients.

The pulse pressure can be simply calculated from the difference of the measured systolic and diastolic pressures^]

Arm–leg gradient

The arm–leg (blood pressure) gradient is the difference between the blood pressure measured in the arms and that measured in the legs. It is normally less than 10 mmHg,^[20] but may be increased in e.g. coarctation of the aorta.

Vascular resistance

The larger arteries, including all large enough to see without magnification, are conduits with low vascular resistance (assuming no advanced atherosclerotic changes) with high flow rates that generate only small drops in pressure. The smaller arteries and arterioles have higher resistance, and confer the main drop in blood pressure along the circulatory system.

Vascular pressure wave

Modern physiology developed the concept of the vascular pressure wave (VPW). This wave is created by the heart during the systole and originates in the ascending aorta. Much faster than the stream of blood itself, it is then transported through the vessel walls to the peripheral arteries. There the pressure wave can be palpated as the peripheral pulse. As the wave is reflected at the peripheral veins it runs back in a centripetal fashion. Where the crests of the reflected and the original wave meet, the pressure inside the vessel is higher than the true pressure in the aorta. This concept explains why the arterial pressure inside the peripheral arteries of the legs and arms is higher than the arterial pressure in the aorta, and in turn for the higher pressures seen at the ankle compared to the arm with normal ankle brachial pressure index values.

Regulation

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

• Baroreceptor reflex: Baroreceptors detect changes in arterial pressure and send signals ultimately to the medulla of the brain stem, RVLM to be precise. The medulla, by way of the autonomic nervous system, adjusts the mean arterial pressure by altering both the force and speed of the heart's contractions, as well as the total peripheral resistance. The most important arterial baroreceptors are located in the left and right carotid sinuses and in the aortic arch.
• Renin-angiotensin system (RAS): This system is generally known for its long-term adjustment of arterial pressure. This system allows the kidney to compensate for loss in blood volume or drops in arterial 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, arterial 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 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; the antihypertensive effect of diuretics is 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 fainting.

]Measurement

A medical student checking blood pressure using a sphygmomanometer and stethoscope.

Arterial pressure is most commonly measured via a sphygmomanometer, which historically used the height of a column of mercury to reflect the circulating pressure. BP values are generally reported in millimetres of mercury (mmHg), though aneroid and electronic devices do not use mercury.

For each heartbeat, BP varies between systolic and diastolic pressures. Systolic pressure is peak pressure in the arteries, which occurs near the end of thecardiac cycle when the ventricles are contracting. Diastolic pressure is minimum pressure in the arteries, which occurs near the beginning of the cardiac cycle when the ventricles are filled with blood. An example of normal measured values for a resting, healthy adult human is 120 mmHg systolic and 80 mmHgdiastolic (written as 120/80 mmHg, and spoken [in the US and UK] as "one-twenty over eighty").

Systolic and diastolic arterial BPs are not static but undergo natural variations from one heartbeat to another and throughout the day (in a circadian rhythm). They also change in response to stress, nutritional factors, drugs, disease, exercise, and momentarily from standing up. Sometimes the variations are large.Hypertension refers to arterial pressure being abnormally high, as opposed to hypotension, when it is abnormally low. Along with body temperature,respiratory rate, and pulse rate, BP is one of the four main vital signs routinely monitored by medical professionals and healthcare providers.^[26]

Measuring pressure invasively, by penetrating the arterial wall to take the measurement, is much less common and usually restricted to a hospital setting.

Noninvasive

The noninvasive auscultatory and oscillometric measurements are simpler and quicker than invasive measurements, require less expertise, have virtually no complications, are less unpleasant and less painful for the patient. However, noninvasive methods may yield somewhat lower accuracy and small systematic differences in numerical results. Noninvasive measurement methods are more commonly used for routine examinations and monitoring.

Palpation

A minimum systolic value can be roughly estimated by palpation, most often used in emergency situations.^{[citation needed]} Historically, students have been taught that palpation of a radial pulse indicates a minimum BP of 80 mmHg, a femoral pulse indicates at least 70 mmHg, and a carotid pulse indicates a minimum of 60 mmHg. However, at least one study indicated that this method often overestimates patients' systolic BP.

A more accurate value of systolic BP can be obtained with a sphygmomanometer and palpating the radial pulse. The diastolic blood pressure cannot be estimated by this method.The American Heart Association recommends that palpation be used to get an estimate before using the auscultatory method.

Auscultatory

Auscultatory method aneroid sphygmomanometer with stethoscope

Mercury manometer

The auscultatory method (from the Latin word for "listening") uses a stethoscope and a sphygmomanometer. This comprises an inflatable (Riva-Rocci) cuffplaced around the upper arm at roughly the same vertical height as the heart, attached to a mercury or aneroid manometer. The mercury manometer, considered the gold standard, measures the height of a column of mercury, giving an absolute result without need for calibration and, consequently, not subject to the errors and drift of calibration which affect other methods. The use of mercury manometers is often required in clinical trials and for the clinical measurement of hypertension in high-risk patients, such as pregnant women.

A cuff of appropriate size is fitted smoothly and snugly, then 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, the turbulent flow creates a "whooshing" or pounding (first Korotkoff sound). The pressure at which this sound is first heard is the systolic BP. The cuff pressure is further released until no sound can be heard (fifth Korotkoff sound), at the diastolic arterial pressure.

The auscultatory method is the predominant method of clinical measurement.

Oscillometric

The oscillometric method was first demonstrated in 1876 and involves the observation of oscillations in the sphygmomanometer cuff pressure which are caused by the oscillations of blood flow, i.e., the pulse. The electronic version of this method is sometimes used in long-term measurements and general practice. It uses a sphygmomanometer cuff, like the auscultatory method, but with an electronic pressure sensor (transducer) to observe cuff pressure oscillations, electronics to automatically interpret them, and automatic inflation and deflation of the cuff. The pressure sensor should be calibrated periodically to maintain accuracy.

Oscillometric measurement requires less skill than the auscultatory technique and may be suitable for use by untrained staff and for automated patient home monitoring.

The cuff is inflated to a pressure initially in excess of the systolic arterial pressure and then reduced to below diastolic pressure 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. It is essential that the cuff size is correct: undersized cuffs may yield too high a pressure; oversized cuffs yield too low a pressure. 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 values of systolic and diastolic pressure are computed, not actually measured from the raw data, using an algorithm; the computed results are displayed.

Oscillometric monitors may produce inaccurate readings in patients with heart and circulation problems, which include 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 results as well as possible. Some equipment uses computer-aided analysis of the instantaneous arterial pressure waveform to determine the systolic, mean, and diastolic points. Since many oscillometric devices have not been validated, caution must be given as most are not suitable in clinical and acute care settings.

The term NIBP, for non-invasive blood pressure, is often used to describe oscillometric monitoring equipment.

White-coat hypertension

For some patients, BP measurements taken in a doctor's office may not correctly characterize their typical BP. In up to 25% of patients, the office measurement is higher than their typical BP. This type of error is called white-coat hypertension (WCH) and can result from anxiety related to an examination by a health care professional.The misdiagnosis of hypertension for these patients can result in needless and possibly harmful medication. WCH can be reduced (but not eliminated) with automated BP measurements over 15 to 20 minutes in a quiet part of the office or clinic.

Debate continues regarding the significance of this effect.^{[citation needed]} Some reactive patients will react to many other stimuli throughout their daily lives and require treatment. In some cases a lower BP reading occurs at the doctor's office.

Home monitoring

Ambulatory blood pressure devices that take readings every half hour throughout the day and night have been used for identifying and mitigating measurement problems like white-coat hypertension. Except for sleep, home monitoring could be used for these purposes instead of ambulatory blood pressure monitoring. Home monitoring may be used to improve hypertension management and to monitor the effects of lifestyle changes and medication related to BP.Compared to ambulatory blood pressure measurements, home monitoring has been found to be an effective and lower cost alternative.

Aside from the white-coat effect, BP readings outside of a clinical setting are usually slightly lower in the majority of people. The studies that looked into the risks from hypertension and the benefits of lowering BP in affected patients were based on readings in a clinical environment.

When measuring BP, an accurate reading requires that one not drink coffee, smoke cigarettes, or engage in strenuous exercise for 30 minutes before taking the reading. A full bladder may have a small effect on BP readings; if the urge to urinate exists, one should do so before the reading. For 5 minutes before the reading, one should sit upright in a chair with one's feet flat on the floor and with limbs uncrossed. The BP cuff should always be against bare skin, as readings taken over a shirt sleeve are less accurate. During the reading, the arm that is used should be relaxed and kept at heart level, for example by resting it on a table.

Since BP varies throughout the day, measurements intended to monitor changes over longer time frames should be taken at the same time of day to ensure that the readings are comparable. Suitable times are:

• immediately after awakening (before washing/dressing and taking breakfast/drink), while the body is still resting,
• immediately after finishing work.

Automatic self-contained BP monitors are available at reasonable prices, some of which are capable of Korotkoff's measurement in addition to oscillometric methods, enabling irregular heartbeat patients to accurately measure their blood pressure at home.

Invasive

Arterial blood pressure (BP) is most accurately measured invasively through an arterial line. Invasive arterial pressure measurement with intravascular cannulae involves direct measurement of arterial pressure by placing a cannula needle in an artery (usually radial, femoral, dorsalis pedis or brachial).

The cannula must be connected to a sterile, fluid-filled system, which is connected to an electronic pressure transducer. The advantage of this system is that pressure is constantly monitored beat-by-beat, and a waveform (a graph of pressure against time) can be displayed. This invasive technique is regularly employed in human and veterinary intensive care medicine, anesthesiology, and for research purposes.

Cannulation for invasive vascular pressure monitoring is infrequently associated with complications such as thrombosis, infection, and bleeding. Patients with invasive arterial monitoring require very close supervision, as there is a danger of severe bleeding if the line becomes disconnected. It is generally reserved for patients where rapid variations in arterial pressure are anticipated.

Invasive vascular pressure monitors are pressure monitoring systems designed to acquire pressure information for display and processing. There are a variety of invasive vascular pressure monitors for trauma, critical care, and operating room applications. These include single pressure, dual pressure, and multi-parameter (i.e. pressure / temperature). The monitors can be used for measurement and follow-up of arterial, central venous, pulmonary arterial, left atrial, right atrial, femoral arterial, umbilical venous, umbilical arterial, and intracranial pressures.

Other sites

view · talk · edit Site		Normal pressure range (in mmHg)
Central venous pressure		2–8
Right ventricular pressure	systolic	15–30
	diastolic	2–8
Pulmonary artery pressure	systolic	15–30
	diastolic	4–12
Pulmonary vein/ Pulmonary capillary wedge pressure		2–15
Left ventricular pressure	systolic	100–140
	diastolic	3–12

Blood pressure generally refers to the arterial pressure in the systemic circulation. However, measurement of pressures in the venous system and the pulmonary vessels plays an important role in intensive care medicine but requires an invasive central venous catheter.

]Venous pressure

Venous pressure is the vascular pressure in a vein or in the atria of the heart. It is much less than arterial pressure, with common values of 5 mmHg in the right atrium and 8 mmHg in the left atrium.

Pulmonary pressure

Main article: Pulmonary artery pressure

Normally, the pressure in the pulmonary artery is about 15 mmHg at rest.

Increased BP in the capillaries of the lung cause pulmonary hypertension, with interstitial edema if the pressure increases to above 20 mmHg, and to frank pulmonary edema at pressures above 25 mmHg.

Further information: Fetal circulation#Blood pressureit]

Fetal blood pressure

In pregnancy, it is the fetal heart and not the mother's heart that builds up the fetal BP to drive its blood through the fetal circulation.

The BP in the fetal aorta is approximately 30 mmHg at 20 weeks of gestation, and increases to approximately 45 mmHg at 40 weeks of gestation.
The average BP for full-term infants:
Systolic 65–95 mm Hg
Diastolic 30–60 mm Hg

Diseases

High

Main article: Hypertension

Overview of main complications of persistent high blood pressure.

Arterial hypertension can be an indicator of other problems and may have long-term adverse effects. Sometimes it can be an acute problem, for examplehypertensive emergency.

All levels of arterial pressure put 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 theheart muscle tends to thicken, enlarge and become weaker over time.

Persistent hypertension is one of the risk factors for strokes, heart attacks, heart failure and arterial aneurysms, and is the leading cause of chronic renal failure. Even moderate elevation of arterial pressure leads to shortened life expectancy. At severely high pressures, mean arterial pressures 50% or more above average, a person can expect to live no more than a few years unless appropriately treated.

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).

For those with heart valve regurgitation, a change in its severity may be associated with a change in diastolic pressure. In a study of people with heart valve regurgitation that compared measurements 2 weeks apart for each person, there was an increased severity of aortic and mitral regurgitation when diastolic blood pressure increased, whereas when diastolic blood pressure decreased, there was a decreased severity.

Low

Main article: Hypotension

Blood pressure that is too low is known as hypotension. The similarity in pronunciation with hypertension can cause confusion. Hypotension is a medical concern only if it causes signs or symptoms, such as dizziness, fainting, or in extreme cases, shock.

When arterial 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 or fainting.

Sometimes the arterial pressure drops significantly when a patient stands up from sitting. This is known as orthostatic hypotension (postural 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, the veins below their heart quickly constrict and the heart rate increases 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 aerobatic or combat pilots 'pulling Gs', greatly increases this effect. Repositioning the body perpendicular to gravity largely eliminates the problem.

Other causes of low arterial pressure include:

• Sepsis
• Hemorrhage - blood loss
• Toxins including toxic doses of BP medicine
• Hormonal abnormalities, such as Addison's disease
• Eating disorders, particularly anorexia nervosa and bulimia

Shock is a complex condition which leads to critically decreased 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 arterial pressure, especially low pulse pressure, is a sign of shock and contributes to and reflects decreased perfusion.

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