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Alterations in Blood Pressure

By Renee Owens,2014-05-08 20:27
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Alterations in Blood Pressure

    Alterations in Blood Pressure “BP”

     Blood pressure is probably one of the most variable but best regulated functions of the body. The purpose of

    the control of BP is to keep blood flow constant to vital organs such as the heart, brain, & kidneys. Without constant flow to these organs, death ensues within seconds, minutes, or days. Although a decrease

    in flow produces an immediate threat to life, the continuous elevation of BP that occurs with hypertension is

    a contributor to premature death and disability due to its effect on the heart, blood vessels, and kidneys.

Control of Blood Pressure

    The arterial blood pressure reflects the rhythmic ejection of blood from the left ventricle into the aorta. It rises as the left ventricle contracts and falls as it relaxes pressure, ideally is less than 120 mm Hg, and the lowest

    pressure, called the diastolic pressure, is less than 80 mm Hg. The difference between the systolic and diastolic pressure (approximately 40 mm Hg) is the pulse pressure. The mean arterial pressure (approximately 90 to

    100 mm Hg),

Determinants of Blood Pressure

     The systolic and diastolic components of blood pressure are determined by the cardiac output and the

    peripheral vascular resistance and can be expressed as a product of the two (blood pressure = cardiac

    output X cardiac output is the product of the stroke volume (amount

    of blood ejected from the heart with each beat) and the heart rate.

     The peripheral vascular resistance reflects changes in the radius of the arterioles as well as the viscosity

    or thickness of the blood. The arterioles often are referred to as the resistance vessels because they can

    selectively constrict or relax to control the resistance to outflow of blood into the capillaries.

     The body maintains its BP by adjusting the cardiac output to compensate for changes in peripheral

    vascular resistance, & it changes the peripheral vascular resistance to compensate for changes in cardiac

    output.

Systolic Blood Pressure

     The systolic blood pressure reflects the rhythmic ejection of blood into the aorta. As blood is ejected into

    the aorta, it stretches the vessel wall and produces a rise in aortic pressure. The extent to which the

    systolic pressure rises or falls with each cardiac cycle is determined by the amount of blood ejected into

    the aorta with each heart beat (i.e., stroke volume), the velocity of ejection, and the elastic properties of

    the aorta.

     Systolic pressure increases when there is a rapid ejection of a large stroke volume or when the stroke

    volume is ejected into a rigid aorta. The elastic walls of the aorta normally stretch to accommodate the

    varying amounts of blood that are ejected into the aorta; this prevents the pressure from rising

    excessively during systole and maintains the pressure during diastole.

Diastolic Blood Pressure

     The diastolic blood pressure is maintained by the energy that has been stored in the elastic walls of the

    aorta during systole. The level at which the diastolic pressure is maintained depends on the elastic

    properties of the aorta and large arteries and their ability to stretch and store energy, the resistance of the

    arterioles that control the outflow of blood into the microcirculation, and the competency of the aortic

    valve.

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     The small diameter of the arterioles contributes to their effectiveness as resistance vessels because it

    takes more force to push blood through a smaller vessel than a larger vessel.

Pulse Pressure

     The pulse pressure is the difference between the systolic and diastolic pressures. It reflects the pulsatile

    nature of arterial blood flow and is an important component of blood pressure. During the rapid ejection

    period of ventricular systole, the volume of blood that is ejected into the aorta exceeds the amount that

    exits the arterial system. The pulse pressure reflects this difference.

     The pulse pressure rises when additional amounts of blood are ejected into the arterial circulation, and it

    falls when the resistance to outflow is decreased.

Mean Arterial Pressure

    The mean arterial blood pressure represents the average blood pressure in the systemic circulation. The mean arterial pressure can be estimated by adding one third of the pulse pressure to the diastolic pressure (i.e.,

     pressure/3).

    Humoral Mechanisms

     A number of hormones and humoral mechanisms contribute to blood pressure regulation, including the

    renin-angiotensin-aldosterone mechanism and vasopressin.

    ; The renin-angiotensin-aldosterone system plays a central role in blood pressure regulation. Renin

    is an enzyme that is synthesized, stored, and released by the kidneys in response to an increase in

    sympathetic nervous system activity or a decrease in blood pressure, extracellular fluid volume, or

    extracellular sodium concentration. Most of the renin that is released leaves the kidney and enters

    the bloodstream, where it acts enzymatically to convert an inactive circulating plasma protein

    called angiotensinogen to angiotensin I

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    ; Vasopressin, also known as antidiuretic hormone (ADH), is released from the posterior pituitary

    gland in response to decreases in blood volume and blood pressure, an increase in the osmolality of

    body fluids, and other stimuli.

     The antidiuretic actions of vasopressin are discussed in Chapter 6. Vasopressin has a direct

    vasoconstrictor effect on blood vessels, particularly those of the splanchnic circulation that supplies the

    abdominal viscera.

Hypertension

    ; Hypertension, or high blood pressure, is probably the most common of all health problems in adults

    and is the leading risk factor for cardiovascular disorders.

    ; Hypertension commonly is divided into the categories of primary and secondary hypertension. In

    primary, or essential, hypertension, which accounts for 90% to 95% of all hypertension, the chronic

    elevation in blood pressure occurs without evidence of other disease. In secondary hypertension, the

    elevation of blood pressure results from some other disorder, such as kidney disease. Malignant

    hypertension, as the name implies, is an accelerated form of hypertension.

Mechanisms of Blood Pressure Elevation

    ; Several factors, including hemodynamic, neural, humoral, & renal mechanisms, are thought to interact

    in producing long-term elevations in blood pressure. As with other disease conditions, it is probable that

    there is not a single cause of essential hypertension or that the condition is a single disease.

    ; Because arterial blood pressure is the product of cardiac output and peripheral vascular resistance, all

    forms of hypertension involve hemodynamic mechanismsan increase in either cardiac output or

    peripheral vascular resistance, or a combination of the two.

Contributing Factors

    - Family history of hypertension - Race - Diabetes Mellitus - Age-related increases in BP

Lifestyle Factors

     Lifestyle factors can contribute to the development of hypertension by interacting with the constitutional risk factors.

    1. Dietary fats and cholesterol are independent risk factors for cardiovascular disease, but there is no evidence

    that they raise blood pressure.

    2. Smoking

    3. Increased salt intake has long been implicated as an etiologic factor in the development of hypertension. 4. Excessive weight commonly is associated with hypertension.

    5. Fat distribution might be a more critical indicator of hypertension risk than actual overweight. 6. Oral contraceptives cause a mild increase in BP in many women & overt hypertension in approximately 5%. 7. Various contraceptive drugs contain different amounts & combinations of estrogen & progestational agents,

    & these differences may contribute to the occurrence of hypertension in some women

Manifestations

    ; Essential hypertension is typically an asymptomatic disorder. Hypertension is a major risk factor for

    atherosclerosis; it predisposes to all major atherosclerotic cardiovascular disorders, including heart failure,

    stroke, coronary artery disease, and peripheral artery disease.

    ; Hypertension increases the workload of the left ventricle by increasing the pressure against which the

    heart must pump as it ejects blood into the systemic circulation. As the workload of the heart increases,

    the left ventricular wall hypertrophies to compensate for the increased pressure work. Despite its

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    compensatory function, left ventricular hypertrophy is a major risk factor for ischemic heart disease,

    cardiac dysrhythmias, sudden death, and congestive heart failure. Hypertension also can lead to

    nephrosclerosis, a common cause of renal insufficiency

    ; The main objective for treatment of essential hypertension is to achieve and maintain arterial blood

    pressure of less than 140/90 mm Hg, with the goal of preventing morbidity and mortality. ; Antihypertensive medications and other measures supplement the treatment for the underlying disease.

    Includes lifestyle modification and, when necessary, guidelines for the use of pharmacologic agents to

    achieve and maintain systolic pressure below 140 mm Hg and diastolic pressure below 90 mm Hg.

Lifestyle Modification

    ; Lifestyle modification includes reduction in sodium intake, maintenance of adequate potassium intake,

    weight reduction if overweight, regular aerobic physical activity, and modification of alcohol intake. ; A reduction in dietary saturated fats and cholesterol is recommended for overall cardiovascular health.

    Smoking cessation should be encouraged for people who smoke.

Pharmacologic Treatment

    Drug selection is based on the stage of hypertension. Among the drugs used in the treatment of hypertension are diuretics, β-adrenergicblocking drugs, angiotensin-converting enzyme (ACE) inhibitors or angiotensin II receptor blockers, the calcium channel-blocking drugs, central α2-adrenergic agonists, α1-adrenergic receptor

    blockers, and vasodilators.

Orthostatic Hypertension

    ; Orthostatic or postural hypotension is an abnormal drop in blood pressure on assumption of the standing

    position. In the absence of normal circulatory reflexes or blood volume, blood pools in the lower part of the

    body; when the standing position is assumed, cardiac output falls, and blood flow to the brain is inadequate.

    Dizziness, syncope (i.e., fainting), or both may occur.

    ; In persons with healthy blood vessels & normal autonomic nervous system function, cerebral blood flow

    usually isn’t reduced on assumption of the upright position unless arterial pressure falls below 70 mm Hg.

Causes These include reduced blood volume

    1. Drug-induced hypotension

    2. Altered vascular responses associated with aging

    3. Bed rest

    4. Autonomic nervous system dysfunction.

Treatment

    Treatment of orthostatic hypotension usually is directed toward alleviating the cause or, helping people learn ways to cope with the disorder and prevent falls and injuries.

    1. Medications that predispose to postural hypotension should be avoided.

    2. Preventing or correcting the fluid deficit

    3. Avoidance of situations that encourage excessive vasodilatation (e.g.,exercising vigorously in a warm

    environment).

    4. Gradual ambulation (i.e., sitting on the edge of the bed for several minutes and moving the legs to

    initiate skeletal muscle pump function before standing).

    5. Elastic support hose or an abdominal support garment may help prevent pooling of blood in the lower

    extremities and abdomen.

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