MD121 Blood Pressure Lab Background PDF

MD121 Blood Pressure Lab – Background The learning objectives for this lab are: To reinforce and develop your understanding of systolic and diastolic blood pressure. To further develop your understanding of some of the factors that determine blood pressure. To observe how blood pressure measur...

MD121 Blood Pressure Lab – Background The learning objectives for this lab are: To reinforce and develop your understanding of systolic and diastolic blood pressure. To further develop your understanding of some of the factors that determine blood pressure. To observe how blood pressure measurement can be applied in a clinical scenario. Apply your understanding of the physiology and anatomy of the heart to explain the patient's symptoms and clinical findings. Reflect on the impact this condition has on the patient's life. Blood pressure is necessary to push blood around the body. It is generated by the contraction (systole) of the heart’s ventricles. All blood vessels have a blood pressure though we usually talk about systemic arterial blood pressure. Blood flows from higher to lower pressure. The pressure falls progressively as the blood flows through the arteries, arterioles, capillaries, venules, and veins, and then returns to the atria (see figure below). Many students make the mistake of thinking that pressure is lowest in the capillaries because they are the narrowest and thinnest walled vessels. The pressure in the arteries varies during the cardiac cycle. The ventricles contract (systole) to push blood into the arterial system, and then relax to fill with blood before contracting again. This intermittent ejection of blood into the arteries is balanced by a constant movement of blood from the arterial system through the capillaries. When the heart pushes blood into the arteries there is a sudden increase in pressure, which slowly declines until the heart contracts again. Arterial blood pressure is at its highest immediately after the ventricle contracts (systolic pressure) and at its lowest as the ventricle relaxes (diastolic pressure) immediately prior to the next contraction. Pulse pressure The difference between systolic and diastolic pressure in the arteries is called the pulse pressure. For example: 120 mmHg systolic – 80 mmHg diastolic = 40 mmHg pulse pressure The difference between systolic and diastolic pressures gets smaller as vessel size decreases. Thus, in small arterioles and capillaries there is no difference (that is, no systolic or diastolic), just one blood pressure. Note that left ventricular systolic pressure is higher than systolic arterial pressure, but ventricular diastolic pressure is almost zero (0 mmHg). Diastolic pressure in the arteries does not drop Copyright © 2018 ADInstruments Limited All Rights Reserved to zero, because blood flow from the aorta into the ventricles is stopped when the aortic valve shuts and traps the blood under pressure in the aorta and arteries. Blood pressure measurement Historical determination of blood pressure Systolic and diastolic pressures can be measured by inserting a small catheter into an artery and attaching the catheter to a pressure gauge. This direct measurement may be accurate, but it's invasive, and often inconvenient and impractical. This was the method Rev. Stephen Hales first used to measure blood pressure in 1714 on a horse. The direct method helps us understand what blood pressure means, and the units in which it is measured. Pressure can be measured as the height of liquid it can push up against gravity. As blood and water have similar densities, we can measure blood pressure in units of centimeters of water, inches of water, or feet of water. The horse’s systolic arterial blood pressure was ~180 cm (6 feet or 72 inches) of water. This is the height blood would spurt from a severed carotid artery without being caught in a glass tube. Using a glass tube contained the blood and prevented the horse from bleeding to death. In humans, blood would squirt to a height of ~163 cm (5 feet 4 inches) during systole and fall to ~109 cm (3 feet 7 inches) during diastole. The use of mercury Mercury is 13.5 times heavier than water (or blood) for the same volume. If Hales had filled his tube with mercury, the blood pressure of the horse would only have pushed the mercury up the tube to one thirteenth of the height: 12 cmHg or 120 mmHg (Hg is the chemical symbol for mercury). We use units of mmHg so we don't have to use a water-filled sphygmomanometer 13.5 times taller! Blood pressure measurements today Direct measurements Arterial blood pressure is usually measured indirectly with a cuff, but we can (and still do) directly measure venous blood pressure in patients in intensive care. This is achieved with an apparatus similar to the one Hales used; a plastic tube filled with saline connected to the vein. This pressure is called the central venous pressure (CVP). It is normally around 8–15 cmH2O or 6–11 mmHg. We can’t measure arterial pressure as the apparatus would have to be about two meters tall! Indirect measurements Simple estimates of blood pressure can be made with good accuracy using noninvasive, indirect methods. One traditionally-used method is the auscultatory method. In this method a stethoscope is used to listen to the heart sounds, and a blood pressure cuff is connected to a mercury sphygmomanometer so that cuff pressure can be measured. The pressure at which the cuff stops the blood flow is the systolic pressure in the artery. 1. The cuff is placed on the upper arm and inflated to stop arterial blood flow from the brachial artery. When the pressure in the cuff exceeds the pressure in the artery it collapses the artery. After this point no pulse will be felt below the cuff. We can approximate the systolic pressure to the pressure measured in the cuff when the pulse disappears. Copyright © 2018 ADInstruments Limited All Rights Reserved 2. The cuff pressure is then slowly released. When the cuff pressure begins to fall below the systolic pressure, the blood flows only intermittently into the arm. The blood flow is turbulent rather than streamlined, and tapping sounds called Korotkoff sounds can be heard through the stethoscope. When blood flow is first heard, the cuff pressure approximates systolic pressure. 3. As the cuff pressure continues to decrease and the artery regains its normal diameter, flow becomes streamlined and the sounds become muffled and then disappear. The cuff pressure at the point of the muffling sound approximates diastolic pressure, but the disappearance of sound is easier to detect than muffling. Since the two occur within a few millimeters of mercury pressure, the disappearance of sound is commonly used to determine diastolic pressure. 4. Note that in some healthy people, the Korotkoff sounds can still be heard at pressures appreciably below the true diastolic pressure. In these people, it is not possible to define their diastolic pressure accurately. How blood pressure is described? It is conventional to measure the blood pressure at the level of the heart. This is achieved by measuring the pressure in the brachial artery with the upper arm by the side. The peak systolic pressure and minimum diastolic pressure are often written as systolic/diastolic (for example, 120/80) with the units mmHg (which reflects the fact that early sphygmomanometers, and many still in use today, used mercury). What is normal blood pressure? It is essential to realize that there is no "normal" value for blood pressure. Some textbooks state "normal" values are 120/80 mmHg. But as with all other physiological variables, there is a normal distribution of blood pressures. Most healthy people's pressures range between 100/60–140/90 mmHg in a "one-off" measurement. In each individual it is not uncommon to find differences of as much as 5– 10 mmHg between the pressures in the two arms. The relationship between blood pressure and blood flow Each heartbeat ejects enough blood at a sufficient pressure to ensure that blood flow to the tissues is fast enough to provide the oxygen and nutrients required by cells. Waste products of metabolism must also be removed constantly so that they do not accumulate in body tissues. The amount of blood flowing is proportional to the blood pressure that drives the flow. That is, if the width of a tube remains the same, the greater the pressure, the greater the flow. Narrower tubes provide more resistance to flow. The blood flows through the arteries, arterioles, capillaries, and then back to the heart through the venules and veins. These vessels provide resistance to the flow. The greatest increase in resistance, and therefore the greatest decrease in blood pressure, occurs in the Copyright © 2018 ADInstruments Limited All Rights Reserved arterioles. Therefore, the arterioles make the greatest contribution to the vascular peripheral resistance. Resistance can be increased by vasoconstriction and lowered by vasodilatation in order to regulate blood pressure. Peripheral circulation Blood leaves the arterial system continuously through the capillaries but enters only intermittently from the heart. When the ventricles contract during systole the semilunar valves open, and blood flows into the arterial system. At this point, the arteries are stretched, and the blood pressure increases. "Systolic pressure" is defined as the peak pressure reached during the cardiac cycle. The period of relaxation of the ventricles is called "diastole". During diastole, the ventricles fill with blood returning from the veins and blood continues to flow out of the arterial system into the capillaries. This flow is driven by the elastic recoil of the major arteries. Consequently, the arterial pressure decreases. "Diastolic pressure" is when the arterial blood pressure is at its lowest, immediately before the contracting ventricle pushes blood into the arteries again. What determines arterial blood pressure? Arterial blood pressure is the product of cardiac output (CO) and peripheral resistance (PR). BP = CO × PR In turn, CO is determined by heart rate (HR) and the volume of blood ejected by the ventricles in each beat (stroke volume, SV). BP = HR × SV × PR Anything that alters any of these variables may affect arterial blood pressure. The regulation of arterial blood pressure The heart and blood vessels of the circulatory system can regulate blood pressure by altering cardiac output and peripheral resistance. But in order for the body to regulate its own blood pressure, it must be capable of measuring it. Arterial blood pressure is monitored by pressure receptors in the aortic arch and the first part of the internal carotid artery: the carotid sinus. These baroreceptors monitor the degree of stretch of the Copyright © 2018 ADInstruments Limited All Rights Reserved arterial wall. Their positions are such that the aortic arch baroreceptors monitor the pressure of blood flowing through the systemic arterial system, and the carotid sinus receptors monitor the pressure of the blood flowing to the brain. Acute changes in arterial blood pressure result in compensatory changes to return blood pressure to the normal range. For example, acute blood loss will decrease arterial blood pressure. This is detected by the baroreceptors which activate the cardiovascular control centers in the brain. In turn, these stimulate the autonomic nerves to vasoconstrict the peripheral blood vessels, and increase the heart rate and force of contraction. The vasoconstriction increases peripheral resistance, and the increased rate and force of cardiac contraction increases cardiac output; so blood flow to the brain and other vital organs is maintained. An acute increase in blood volume (as may occur with excess blood transfusion) will increase arterial blood pressure. This will cause peripheral vasodilation and a decreased heart rate. To understand the regulation of blood pressure you can think about what happens when you take a hot bath. Pale skin turns red as peripheral blood vessels dilate to encourage heat loss. This reduces peripheral resistance, and blood pressure starts to go down. You might notice this if you stand up to get out of the bath, and begin to feel dizzy. The baroreceptors quickly detect the falling blood pressure and increase stimulation of heart rate and force of contraction. This is felt as pounding in the chest. Effects of position on the measured arterial blood pressure Convention is to reference all arterial blood pressure measurements to the position of the heart. If we measure the pressure in an artery that is below this level, then the pressure will be increased. This is because of the effect of gravity on the column of blood in the vessels, which contributes to hydrostatic pressure. This effect is quite large. For example, if we measure the blood pressure in a femoral artery in the thigh with the person lying down, the artery is at the same level as the heart, so there is no extra pressure contributed by gravity. But if we measure this with the person sitting or standing up, then the height of the column of blood below the heart contributes around an additional 50 mmHg to the pressure. That is, if the person has a blood pressure measured at heart level of 120/80 mmHg, then the pressure in the femoral artery in the thigh will be 170/130 mmHg. Similarly, if we were to measure the blood pressure in the arm when the arm was raised above the head, it would be appreciably lower than it is at heart level. It is essential to realize that this hydrostatic pressure affects all fluids at the same level. Therefore, the interstitial fluid pressure, and the pressures of the blood in the capillaries and veins are all increased to the same extent. Thus, the pressure difference between interstitial fluid and adjacent capillaries is exactly the same whether a person is standing or lying down. The effect of gravity on pressure also affects veins. Because the veins are distensible, the increased pressure tends to cause blood to pool in the veins, thereby decreasing the return of the blood to the heart (venous return). This is counteracted by valves in the veins, which prevent blood backflow when standing or sitting. In addition, skeletal muscle contraction presses on the veins and helps to increase Copyright © 2018 ADInstruments Limited All Rights Reserved venous pressure in legs to push blood back towards the heart. This works well as long as we walk about, but not when we sit still. For example, sitting still for hours in airplane seats can result in slowing of blood flow and this contributes to deep vein thrombosis. In the head and brain, gravity has the opposite effect to veins as the head and brain are above the heart when standing. Since perfusion of the brain with blood is essential to life, it is not surprising that the pressure of the blood flowing to the brain is monitored separately (via the carotid sinus baroreceptors) from that of the blood going to the rest of the body. Remember that blood pressure is constantly measured and regulated, so lying down should lead to greater pressure being detected in the carotid baroreceptors with consequent slowing of heart rate. Position therefore affects not only the measurement of BP but also its regulation. Effect of cuff size on measured arterial blood pressure The measurement of arterial blood pressure using a sphygmomanometer requires that the artery be uniformly compressed, and that the pressure in the cuff is accurately transferred to the artery wall. It is essential that the right sized cuff is used for this. If the cuff is too narrow, values will be too high. If the cuff is too wide, values will be too low. However, note that the error from using a cuff that is too narrow is appreciably greater than that from using a cuff that is too wide. The most common mistake is to use a cuff that is too small for the arm of an overweight person. In practice, a range of cuff sizes should be available. For adults, official guidelines specify the following cuff sizes: Arm circumference 22 to 26 cm, "small adult" cuff, 12 x 22 cm. Arm circumference 27 to 34 cm, "adult" cuff, 16 x 30 cm. Arm circumference 35 to 44 cm, "large adult" cuff, 16 x 36 cm. Arm circumference 45 to 52 cm, "adult thigh" cuff, 16 x 42 cm. Also note that a whole range of smaller cuffs is required for measuring blood pressure in children. Why is blood pressure important clinically? Blood pressure is routinely measured as part of a physical assessment, and in many situations, it is essential to know a person's blood pressure in order to provide the most appropriate care. In acute situations, such as after an accident where someone is bleeding, we need to know the blood pressure and how it is changing in order to manage the situation correctly. Is the patient in shock with low blood pressure? Is the pressure rising or falling? Should we transfuse the patient? Should we give drugs that might constrict blood vessels? Is it possible that kidney failure may occur? Accurate and repeated measures of arterial blood pressure provide guidance and clues to answer these questions. Before, during and after anesthetics, blood pressure measurements are important. Some anesthetic agents, in addition to other drugs, may lower blood pressure acutely. A relatively small increase in blood pressure with aging is very common and may not require treatment. But persistently elevated blood pressure – hypertension – is potentially life threatening and is associated with heart disease and strokes. This is why life insurance companies like to know your blood pressure. Effective management and treatment of hypertension is now possible. This can be through lifestyle change, medication or both. Therefore, it is essential that health professionals can measure blood pressure accurately so that people with hypertension are identified early and managed correctly. Copyright © 2018 ADInstruments Limited All Rights Reserved Hypertension Increased arterial blood pressure is often seen clinically. Hypertension refers to a chronic condition characterized by a resting blood pressure of 140/90 mmHg or greater. It is estimated that as many as 30% of the adult population in developed countries are hypertensive. Although, many people have an occasional blood pressure reading greater than 140/90 mmHg; they are not necessarily hypertensive. To establish the diagnosis of hypertension, repeated measurements at rest are often made. However, a better approach is to monitor blood pressure regularly over 24 hours, particularly because blood pressure during sleep should always be lower than blood pressure when awake. A blood pressure of 120/80 mmHg (or greater) during sleep indicates hypertension. Caution is required when answering the question: “Is my blood pressure OK?” It depends on whether the person is adequately rested, sitting, standing, or lying down, and on the cuff size. Even then, the so-called normal values may be different depending on the medical viewpoint of the country you live in. Notably, a person with a blood pressure of 141/91 mmHg is at no greater risk than someone with 139/89 mmHg. Essential hypertension In around 90% of adults with hypertension, there are no recognized causes. This group is referred to as having essential hypertension. As people age, they are more likely to develop this condition. Many factors are thought to contribute to essential hypertension, including lack of exercise, stress, alcohol intake, and obesity (more than 85% of the cases are seen in people with a BMI > 25). There may be a family history of high blood pressure. The pathophysiology is unclear but the prime problem is an increased arteriolar resistance, although the causes for this increase remains to be established. Secondary hypertension In the remaining 10% or so of people with hypertension, it is often possible to identify an underlying cause. This group is referred to as having secondary hypertension. Recognized causes of chronically raised blood pressure include: increased adrenal corticoid production (Cushing's syndrome, primary aldosteronism); increased epinephrine (adrenaline) production by adrenal medullary tumors (pheochromocytoma); chronic renal disease; and coarctation of the aorta. It is essential to eliminate secondary causes of hypertension before concluding that a person has essential hypertension. This is particularly important in younger people in whom essential hypertension is less common. Adrenal steroids and hypertension The adrenal cortex makes a number of steroid hormones that are described as mineralocorticoids (for example, aldosterone which mainly affects salt excretion) and glucocorticoids (for example, cortisol and corticosterone which mainly affect glucose and protein metabolism). Together these hormones are often referred to as cortical steroids or "corticoids". Increased production of corticoids (aldosterone in particular) by the adrenal cortex, causes increased salt reabsorption by the kidneys. This is accompanied by fluid retention which expands blood volume, raises blood pressure, and also increases stroke volume. Copyright © 2018 ADInstruments Limited All Rights Reserved Copyright © 2018 ADInstruments Limited All Rights Reserved Epinephrine and hypertension Increased epinephrine (adrenaline) production raises blood pressure by causing arteriolar vasoconstriction, and hence increased peripheral resistance. In addition, epinephrine increases the heart rate and increases cardiac output. Chronic renal disease and hypertension Chronic renal diseases often result in fluid retention and poor perfusion of the kidneys. This stimulates increased renin production, resulting in increased angiotensin II levels. Angiotensin II is the most potent vasoconstrictor in our bodies, producing profound arteriolar vasoconstriction. Thus, both stroke volume and peripheral resistance may be increased. Renin is an enzyme secreted by specialized kidney cells (granular cells of the juxtaglomerular apparatus). It breaks down angiotensinogen (produced in the liver and secreted into the blood) to angiotensin I, which is physiologically inactive. In the lungs, angiotensin I is converted to angiotensin II by angiotensin converting enzyme (ACE). Because Angiotensin II is such an important vasoconstrictor, both ACE inhibitors and Angiotensin II antagonists are now used to treat hypertension. 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