blood pressure(redirected from mean arterial (blood) pressure)
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In an emergency, blood cells and antibodies carried in the blood are brought to a point of infection, or blood-clotting substances are carried to a break in a blood vessel. The blood distributes hormones from the endocrine glands to the organs they influence. It also helps regulate body temperature by carrying excess heat from the interior of the body to the surface layers of the skin, where the heat is dissipated to the surrounding air.
Blood varies in color from a bright red in the arteries to a duller red in the veins. The total quantity of blood within an individual depends upon body weight; a person weighing 70 kg (154 lb) has about 4.5 liters of blood in the body.
Blood is composed of two parts: the fluid portion is called plasma, and the solid portion or formed elements (suspended in the fluid) consists of the blood cells (erythrocytes and leukocytes) and the platelets. Plasma accounts for about 55 per cent of the volume and the formed elements account for about 45 per cent. ( and table.)
Chemical analyses of various substances in the blood are invaluable aids in (1) the prevention of disease by alerting the patient and health care provider to potentially dangerous levels of blood constituents that could lead to more serious conditions, (2) diagnosis of pathologic conditions already present, (3) assessment of the patient's progress when a disturbance in blood chemistry exists, and (4) assessment of the patient's status by establishing baseline or “normal” levels for each individual patient.
In recent years, with the increasing attention to preventive health care and rapid progress in technology and automation, the use of a battery of screening tests performed by automated instruments has become quite common. These instruments are capable of performing simultaneously a variety of blood chemistry tests. Some of the more common screening tests performed on samples of blood include evaluation of electrolyte, albumin, and bilirubin levels, blood urea nitrogen (BUN), cholesterol, total protein, and such enzymes as lactate dehydrogenase and aspartate transaminase. Other tests include electrophoresis for serum proteins, blood gas analysis, glucose tolerance tests, and measurement of iron levels.
PaO2—partial pressure (P) of oxygen (O2) in the arterial blood (a)
SaO2—percentage of available hemoglobin that is saturated (Sa) with oxygen (O2)
PaCO2—partial pressure (P) of carbon dioxide (CO2) in the arterial blood (a)
pH—an expression of the extent to which the blood is alkaline or acidic
HCO3−—the level of plasma bicarbonate; an indicator of the metabolic acid-base status
These parameters are important tools for assessment of a patient's acid-base balance. They reflect the ability of the lungs to exchange oxygen and carbon dioxide, the ability of the kidneys to control the retention or elimination of bicarbonate, and the effectiveness of the heart as a pump. Because the lungs and kidneys act as important regulators of the respiratory and metabolic acid-base balance, assessment of the status of a patient with any disorder of respiration and metabolism includes periodic blood gas measurements.
The partial pressure of a particular gas in a mixture of gases, as of oxygen in air, is the pressure exerted by that gas alone. It is proportional to the relative number of molecules of the gas, for example, the fraction of all the molecules in the air that are oxygen molecules. The partial pressure of a gas in a liquid is the partial pressure of a real or imaginary gas that is in equilibrium with the liquid.
PaO2 measures the oxygen content of the arterial blood, most of which is bound to hemoglobin, forming oxyhemoglobin. The SaO2 measures the oxygen in oxyhemoglobin as a percentage of the total hemoglobin oxygen-carrying capacity.
A PaO2 of 60 mm Hg represents an SaO2 of 90 per cent, which is sufficient to meet the needs of the body's cells. However, as the PaO2 falls, the SaO2 decreases rapidly. A PaO2 below 55 indicates a state of hypoxemia that requires correction. Normal PaO2 values at sea level are 80 mm Hg for elderly adults and 100 mm Hg for young adults.
However, some patients with chronic obstructive pulmonary disease can tolerate a PaO2 as low as 70 mm Hg without becoming hypoxic. In caring for patients with this condition, it is important to know that attempts to elevate the PaO2 level to the normal level can be dangerous and even fatal. It is best to establish a baseline for each individual patient before supplementary oxygen is given, and then to assess his condition and the effectiveness of his therapy according to this baseline.
The PaCO2 gives information about the cellular production of carbon dioxide through metabolic processes, and the removal of it from the body via the lungs. The normal range is 32 to 45 mm Hg. Values outside this range indicate a primary respiratory problem associated with pulmonary function, or a metabolic problem for which there is respiratory compensation.
In the newborn the normal PaO2 is 50 to 80 mm Hg. At 40 to 50 mm Hg cyanosis may become apparent. Respiratory distress in an infant who is unable to ventilate the lungs adequately will produce a drop in PaO2 level. However, there is no marked increase in PaCO2 level in some infants as in adults with respiratory distress because many infants can still eliminate carbon dioxide from the lungs even though weakness prevents inhaling an adequate oxygen supply. All infants being ventilated and receiving oxygen therapy require frequent blood gas analyses and also pH, base excess, and oxygen saturation levels to avoid oxygen toxicity and acid-base imbalance.
Blood pH gives information about the patient's metabolic state. A pH of 7.4 is considered normal; a value lower than 7.4 indicates acidemia and one higher than 7.4 alkalemia.
Because the amount of CO2 in the blood affects its pH, abnormal PaCO2 values are interpreted in relation to the pH. If the PaCO2 value is elevated, and the pH is below normal, respiratory acidosis from either acute or chronic hyperventilation is suspected. Conversely, a PaCO2 below normal and a pH above normal indicates respiratory alkalosis. When both the PaCO2 and the pH are elevated, there is respiratory retention of CO2 to compensate for metabolic acidosis. If both values are below normal, there is respiratory elimination of CO2 (hyperventilation) to compensate for metabolic acidosis.
Abnormal levels of bicarbonate (HCO3−) in the plasma are also interpreted in relation to the pH in the diagnosis of disturbances in the metabolic component of the acid-base balance. The normal range for HCO3− is 22 to 26 mEq per liter. Abnormally low levels of both HCO3− and pH indicate acidosis of metabolic origin. Conversely, elevations of both of these values indicate metabolic alkalosis. The kidneys maintain bicarbonate levels by filtering bicarbonate and returning it to the blood; they also produce new bicarbonate to replace that which is used in buffering. Therefore, a decreased HCO3− and an increased pH level indicate either retention of hydrogen ions by the kidneys or the elimination of HCO3− in an effort to compensate for respiratory alkalosis. Conversely, if the HCO3− level is increased and the pH is decreased, the kidneys have compensated for respiratory acidosis by retaining HCO3− or by eliminating hydrogen ions.
The ABO blood group system was first introduced in 1900 by Karl Landsteiner; in 1920 group AB was discovered by van Descatello and Sturli. Identification of these four major blood groups represented a major step toward resolving the problem of blood transfusion reactions resulting from donor-recipient incompatibility. In 1938 Landsteiner and Weiner discovered another blood factor related to maternal-fetal incompatibility. The factor was named Rh because the researchers were using rhesus monkeys in their studies. Further research has uncovered additional factors in the Rh group.
Although more than 90 factors have been identified, many of these are not highly antigenic and are not, therefore, a cause for concern in the typing of blood for clinical purposes.
The term factor, in reference to blood groups, is synonymous with antigen, and the reaction occurring between incompatible blood types is an antigen-antibody reaction. In cases of incompatibility, the antigen, located on the red blood cells, is an agglutinogen and the specific antibody, located in the serum, is an agglutinin. These are so named because whenever red blood cells with a certain factor come in contact with the agglutinin specific for it, there is agglutination or clumping of the erythrocytes.
In determining blood group, a sample of blood is taken and mixed with specially prepared sera. One serum, anti-A agglutinin, causes blood of group A to agglutinate; another serum, anti-B agglutinin, causes blood of group B to agglutinate. Thus, if anti-A serum alone causes clumping, the blood is group A; if anti-B serum alone causes clumping, it is group B. If both cause clumping, the blood group is AB, and if it is not clumped by either, it is identified as group O.
The pumping action of the heart refers to how hard the heart pumps the blood (force of heartbeat), how much blood it pumps (the cardiac output), and how efficiently it does the job. Contraction of the heart, which forces blood through the arteries, is the phase known as systole. Relaxation of the heart between contractions is called diastole.
The main arteries leading from the heart have walls with strong elastic fibers capable of expanding and absorbing the pulsations generated by the heart. At each pulsation the arteries expand and absorb the momentary increase in blood pressure. As the heart relaxes in preparation for another beat, the aortic valves close to prevent blood from flowing back to the heart chambers, and the artery walls spring back, forcing the blood through the body between contractions. In this way the arteries act as dampers on the pulsations and thus provide a steady flow of blood through the blood vessels. Because of this, there are actually two blood pressures within the blood vessels during one complete beat of the heart: a higher blood pressure during systole (the contraction phase) and a lower blood pressure during diastole (the relaxation phase). These two blood pressures are known as the systolic pressure and the diastolic pressure, respectively.
It is generally agreed that a reading of 120 mm Hg systolic and 80 mm Hg diastolic are the norms for a blood pressure reading; that is, it represents the average blood pressure obtained from a large sampling of healthy adults. In general, a blood pressure of 95 mm Hg systolic and 60 mm Hg diastolic indicates hypotension. However, a reading equal to or below this level must be interpreted in the light of each patient's “normal” reading as determined by baseline data.
On the basis of validated research on the long-term effects of an elevated blood pressure, it is generally agreed that some degree of risk for major cardiovascular disease exists when the systolic pressure is greater than or equal to 140 mm Hg, and the diastolic pressure is greater than or equal to 90 mm Hg. Life expectancy is reduced at all ages and in both males and females when the diastolic pressure is above 90 mm Hg. (See accompanying table.)
A stethoscope is placed over the artery at the elbow and the air pressure within the cuff is slowly released. As soon as blood begins to flow through the artery again, Korotkoff sounds are heard. The first sounds heard are tapping sounds that gradually increase in intensity. The initial tapping sound that is heard for at least two consecutive beats is recorded as the systolic blood pressure.
The first phase of the sounds may be followed by a momentary disappearance of sounds that can last from 30 to 40 mm Hg as the gauge needle (or mercury column) descends. It is important that this auscultatory gap not be missed; otherwise, either an erroneously low systolic pressure or high diastolic pressure will be obtained.
During the second phase following the temporary absence of sound there are murmuring or swishing sounds. As deflation of the cuff continues, the sounds become sharper and louder. These sounds represent phase three. During phase four the sounds become muffled rather abruptly and then are followed by silence, which represents phase five.
Although there is disagreement as to which of the latter phases should represent the diastolic pressure, it is usually recommended that phase five, the point at which sounds disappear, be used as the diastolic pressure for adults, and phase four be used for children. The reason for this is that children, having a high cardiac output, often will continue to produce sounds when the gauge is at a very low reading or even at zero. In some adult patients whose arterioles have lost their elasticity, the fifth phase is also extremely low or nonexistent. In these cases, it is recommended that three readings be recorded: phase one and phases four and five. For example, the blood pressure would be written as 140/96/0. On most occasions, however, the blood pressure is written as a fraction. The systolic pressure is written as the top number, a line is drawn, and the diastolic pressure is written as the bottom number.
Errors in blood pressure measurement can result from failure of the cuff to reach and compress the artery. The cuff diameter should be 20 per cent greater than the diameter of the limb, the bladder of the cuff must be centered over the artery, and the cuff must be wrapped smoothly and snugly to ensure proper inflation. When a mercury gauge is used, the meniscus should be at eye level to avoid a false reading.
The blood volume in the pulmonary circulation is approximately 12 per cent of the total blood volume. Such conditions as left-sided heart failure and mitral stenosis can greatly increase the pulmonary blood volume while decreasing the systemic volume. As would be expected, right-sided heart failure has the opposite effect. The latter condition has less serious effects because the volume of the systemic circulation is about seven times that of the pulmonary circulation and it is therefore better able to accommodate a change in fluid volume.
Measurement of blood volume is accomplished by using substances that combine with red blood cells, for example, iron, chromium, and phosphate, or substances that combine with plasma proteins. In either case the measurement of the blood volume is based on the “dilution” principle. That is, the volume of any fluid compartment can be measured if a given amount of a substance is dispersed evenly in the fluid within the compartment, and then the extent of dilution of the substance is measured.
For example, a small amount of radioactive chromium (51Cr), which is widely used to determine blood volume, is mixed with a sample of blood drawn from the patient. After about 30 minutes the 51Cr will have entered the red blood cells. The sample with the tagged red blood cells is then returned by injection into the patient's bloodstream. About 10 minutes later a sample is removed from the patient's circulating blood and the radioactivity level of this sample is measured. The total blood volume is calculated according to this formula:
blood pres·sure (BP),
blood pressureThe force that blood in the circulation exerts on arterial walls, 2º to myocardial contraction, in response to various demands (e.g., exercise, stress, sleep), which is divided into systolic (due to heart contractions) and diastolic (relaxation phases). Blood pressure (BP) varies with age and sex.
Standard level for normal systolic BP
Adults—BP = 100 + age;
Children—BP = 2 x age + 80.
Standard level for normal diastolic BP
± 2/3 of above.
120/80 mm Hg.
blood pressureCardiology The force that blood in the circulation exerts on arterial walls, 2º to myocardial contraction in response to various demands–eg, exercise, stress, sleep, which is divided into systolic–due to heart contractions and diastolic–relaxation phases; BP varies with age and sex Rule of thumb for normal systolic BP–Adults BP = 100 + age; Children BP = 2 x age + 80; Diastolic BP should be ±2/3 Normal BP 120/80 mm Hg. See Hypertension, Hypotension, Sphygmomanometer–blood pressure cuff.
blood pres·sure(BP) (blŭd presh'ŭr)
Normal blood pressure is defined as a systolic BP between 100 and 120 mm Hg and a diastolic BP below 80 mm Hg (in adults over age 18). Prehypertension is present when measured blood pressures are between 120 and 140 mm Hg systolic or between 80 and 90 mm Hg diastolic. When either the systolic pressure exceeds 140 mm Hg or the diastolic exceeds 90 mm Hg, and these values are confirmed on two additional visits, stage I hypertension (high blood pressure) is present. See: illustration
Low blood pressure is sometimes present in healthy individuals, but it indicates shock in patients with fever, active bleeding, allergic reactions, active heart disease, spinal cord injuries, or trauma. Blood pressure should be checked routinely whenever a patient sees a health care provider because controlling abnormally high blood pressure effectively prevents damage to the heart and circulatory system as well as the kidneys, retina, brain, and other organs.
Elevated blood pressures should first be addressed by giving advice to patients about lifestyle modifications, such as limiting the intake of alcohol, following a diet approved by the American Heart Association, and increasing the level of physical exercise. Weight loss in obese patients is also advisable. Medications are added to lifestyle instructions most of the time. Antihypertensive medications are used according to evidence-based guidelines and the side effects these drugs may cause in particular patients. Diuretics, for example, are esp. helpful in blacks and elderly patients (but may be inadvisable in patients with gout); beta blockers are the drugs of choice in patients with a history of myocardial infarction (but would be contraindicated in patients with advanced heart block); alpha blockers are well suited for men with prostatic hypertrophy; and angiotensin-converting enzyme inhibitors prevent kidney disease in patients with diabetes mellitus. Other antihypertensive drug classes include the angiotensin II receptor antagonists, centrally active alpha antagonists, and calcium channel blockers. Low blood pressure is not treated in healthy patients; in patients with acute illnesses, it is often corrected with hydration or pressor agents.
augmented diastolic blood pressure
central blood pressure
chronic low blood pressure
diastolic blood pressure
direct measurement of blood pressure
high blood pressureHypertension.
indirect measurement of blood pressure
Palpation method: The same arm, usually the right, should be used each time the pressure is measured. The arm should be raised to heart level if the patient is sitting, or kept parallel to the body if the patient is recumbent. The patient's arm should be relaxed and supported in a resting position. Exertion during the examination could result in a higher blood pressure reading. Either a mercury-gravity or aneroid-manometer type of blood pressure apparatus may be used. The blood compression cuff should be the width and length appropriate for the size of the subject's arm: narrow (2.5 to 6 cm) for infants and children and wide (13 cm) for adults. The inflatable bag encased in the cuff should be 20% wider than one third the circumference of the limb used. The deflated cuff is placed evenly and snugly around the upper arm so that its lower edge is about 1 in above the point of the brachial artery where the bell of the electronic sensor will be applied. While feeling the radial pulse, inflate the cuff until the pressure is about 30 mm above the point where the radial pulse was no longer felt. Deflate the cuff slowly and record as accurately as possible the pressure at which the pulse returns to the radial artery. Systolic blood pressure is determined by this method; diastolic blood pressure cannot be determined by this method.
This method is used for both continuous and intermittent readings, and while it formerly was used primarily in ICUs, it now is used routinely by nursing assistants on units throughout health care agencies and in clinics and physicians' offices. Measuring blood pressure at the wrist is more comfortable than a conventional BP cuff because it derives readings without pumping a bladder full of air, and with accuracy rivaling direct measurement from an arterial catheter. The sensor is placed directly over the radial artery and connected to an electronic monitor. Pressure is monitored every 15 heartbeats and systolic, diastolic, mean arterial pressure, waveforms, and pulse rate are displayed. The first reading appears in 15 seconds, and the sensor measures pressures from 40 to 240 mm Hg, with preset alarms to alert the nurse to extreme highs and lows. Results are not affected by low cardiac output, arrhythmias, hypothermia, or obesity, and this method is being used increasingly on adults in hospital special care units where frequent serial readings are required.
Auscultatory method: Begin as above. After inflating the cuff until the pressure is about 30 mm above the point where the radial pulse disappears, place the bell of the stethoscope over the brachial artery just below the blood pressure cuff. Then deflate the cuff slowly, about 2 to 3 mm Hg per heartbeat. The first sound heard from the artery is recorded as the systolic pressure. The point at which sounds are no longer heard is recorded as the diastolic pressure. For convenience the blood pressure is recorded as figures separated by a slash. The systolic value is recorded first.
Sounds heard over the brachial artery change in quality at some point prior to the point the sounds disappear. Some physicians consider this the diastolic pressure. This value should be noted when recording the blood pressure by placing it between the systolic pressure and the pressure noted when the sound disappears. Thus, 120/90/80 indicates a systolic pressure of 120 with a first diastolic sound change at a pressure of 90 and a final diastolic pressure of 80. The latter pressure is the point of disappearance of all sounds from the artery. When the values are so recorded, the physician may use either of the last two figures as the diastolic pressure. When the change in sound and the disappearance of all sound coincide, the result should be written as follows: 120/80/80.
mean blood pressure
negative blood pressure
normal blood pressure
systolic blood pressure
blood pressureThe pressure exerted on the artery walls and derived from the force of the contraction of the lower chambers of the heart (the VENTRICLES). Blood pressure changes constantly. Peak pressure is called the systolic pressure and the running pressure between beats is called the diastolic pressure. Blood pressure in measured in millimetres of mercury. A typical normal reading is 120/80. See also HYPERTENSION and KOROTKOFF SOUNDS.
blood pressurethe force exerted by blood against the walls of the blood vessels, caused by heart contractions forcing a constant volume of blood round a closed system. Strong contraction of the left ventricle (SYSTOLE) ejects blood at high pressure into the AORTA, stretching the arterial walls. When the heart relaxes (DIASTOLE), force is no longer exerted on the arterial blood so pressure drops, although maintenance of pressure is helped by elastic recoil of the arterial walls. These oscillations of blood pressure are largest in the aorta, gradually diminishing as the blood flows along the arteries, becoming nonexistent in the CAPILLARIES.
The level of blood pressure also decreases from heart to tissue and back to the heart, these differences in pressure enabling the flow of blood around the system.
Blood in the veins is prevented from moving backwards by the presence of one-way valves. Venous circulation is also enhanced by activity of the skeletal muscles, hence leg and arm movements aid blood flow back to the heart.
Note that, although the comments above refer to the systemic circulation, a similar situation also applies in the smaller pulmonary system of mammals (see BLOOD CIRCULATORY SYSTEM).
Several factors control the exact level of blood pressure:
- heart action (rate of heartbeat, force per beat, volume per beat);
- peripheral resistance to blood flow in the capillary beds, caused by friction;
- elasticity of arteries;
- total blood volume (the higher the volume the higher the pressure);
- viscosity of blood (an increase in viscosity causes an increase in blood pressure but a decrease in flow rate).
blood pres·sure(BP) (blŭd presh'ŭr)
Patient discussion about blood pressure
Q. how can i reduce my blood pressure?
Q. what do i need to do to bring down my blood pressure? what cause a high blood pressure? what are the risks? of high blood pressure ? how can i deal with it effectively ?
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Q. What Are the Complications of High Blood Pressure? My wife suffers from high blood pressure. What are the possible complications that are so dangerous? Why is it important to keep high blood pressure under balance?