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Hypotension, Orthostatic: A significant drop in BLOOD PRESSURE after assuming a standing position. Orthostatic hypotension is a finding, and defined as a 20-mm Hg decrease in systolic pressure or a 10-mm Hg decrease in diastolic pressure 3 minutes after the person has risen from supine to standing. Symptoms generally include DIZZINESS, blurred vision, and SYNCOPE.Hypotension: Abnormally low BLOOD PRESSURE that can result in inadequate blood flow to the brain and other vital organs. Common symptom is DIZZINESS but greater negative impacts on the body occur when there is prolonged depravation of oxygen and nutrients.Orthostatic Intolerance: Symptoms of cerebral hypoperfusion or autonomic overaction which develop while the subject is standing, but are relieved on recumbency. Types of this include NEUROCARDIOGENIC SYNCOPE; POSTURAL ORTHOSTATIC TACHYCARDIA SYNDROME; and neurogenic ORTHOSTATIC HYPOTENSION. (From Noseworthy, JH., Neurological Therapeutics Principles and Practice, 2007, p2575-2576)Postural Orthostatic Tachycardia Syndrome: A syndrome of ORTHOSTATIC INTOLERANCE combined with excessive upright TACHYCARDIA, and usually without associated ORTHOSTATIC HYPOTENSION. All variants have in common an excessively reduced venous return to the heart (central HYPOVOLEMIA) while upright.Hypotension, Controlled: Procedure in which arterial blood pressure is intentionally reduced in order to control blood loss during surgery. This procedure is performed either pharmacologically or by pre-surgical removal of blood.Dizziness: An imprecise term which may refer to a sense of spatial disorientation, motion of the environment, or lightheadedness.Tilt-Table Test: A standard and widely accepted diagnostic test used to identify patients who have a vasodepressive and/or cardioinhibitory response as a cause of syncope. (From Braunwald, Heart Disease, 7th ed)Posture: The position or attitude of the body.Intracranial Hypotension: Reduction of CEREBROSPINAL FLUID pressure characterized clinically by HEADACHE which is maximal in an upright posture and occasionally by an abducens nerve palsy (see ABDUCENS NERVE DISEASES), neck stiffness, hearing loss (see DEAFNESS); NAUSEA; and other symptoms. This condition may be spontaneous or secondary to SPINAL PUNCTURE; NEUROSURGICAL PROCEDURES; DEHYDRATION; UREMIA; trauma (see also CRANIOCEREBRAL TRAUMA); and other processes. Chronic hypotension may be associated with subdural hematomas (see HEMATOMA, SUBDURAL) or hygromas. (From Semin Neurol 1996 Mar;16(1):5-10; Adams et al., Principles of Neurology, 6th ed, pp637-8)Lower Body Negative Pressure: External decompression applied to the lower body. It is used to study orthostatic intolerance and the effects of gravitation and acceleration, to produce simulated hemorrhage in physiologic research, to assess cardiovascular function, and to reduce abdominal stress during childbirth.Autonomic Nervous System Diseases: Diseases of the parasympathetic or sympathetic divisions of the AUTONOMIC NERVOUS SYSTEM; which has components located in the CENTRAL NERVOUS SYSTEM and PERIPHERAL NERVOUS SYSTEM. Autonomic dysfunction may be associated with HYPOTHALAMIC DISEASES; BRAIN STEM disorders; SPINAL CORD DISEASES; and PERIPHERAL NERVOUS SYSTEM DISEASES. Manifestations include impairments of vegetative functions including the maintenance of BLOOD PRESSURE; HEART RATE; pupil function; SWEATING; REPRODUCTIVE AND URINARY PHYSIOLOGY; and DIGESTION.Syncope: A transient loss of consciousness and postural tone caused by diminished blood flow to the brain (i.e., BRAIN ISCHEMIA). Presyncope refers to the sensation of lightheadedness and loss of strength that precedes a syncopal event or accompanies an incomplete syncope. (From Adams et al., Principles of Neurology, 6th ed, pp367-9)Blood Pressure: PRESSURE of the BLOOD on the ARTERIES and other BLOOD VESSELS.Supine Position: The posture of an individual lying face up.Midodrine: An ethanolamine derivative that is an adrenergic alpha-1 agonist. It is used as a vasoconstrictor agent in the treatment of HYPOTENSION.Head-Down Tilt: Posture while lying with the head lower than the rest of the body. Extended time in this position is associated with temporary physiologic disturbances.Shy-Drager Syndrome: A progressive neurodegenerative condition of the central and autonomic nervous systems characterized by atrophy of the preganglionic lateral horn neurons of the thoracic spinal cord. This disease is generally considered a clinical variant of MULTIPLE SYSTEM ATROPHY. Affected individuals present in the fifth or sixth decade with ORTHOSTASIS and bladder dysfunction; and later develop FECAL INCONTINENCE; anhidrosis; ATAXIA; IMPOTENCE; and alterations of tone suggestive of basal ganglia dysfunction. (From Adams et al., Principles of Neurology, 6th ed, p536)Heart Rate: The number of times the HEART VENTRICLES contract per unit of time, usually per minute.Pure Autonomic Failure: A degenerative disease of the AUTONOMIC NERVOUS SYSTEM that is characterized by idiopathic ORTHOSTATIC HYPOTENSION and a greatly reduced level of CATECHOLAMINES. No other neurological deficits are present.Syncope, Vasovagal: Loss of consciousness due to a reduction in blood pressure that is associated with an increase in vagal tone and peripheral vasodilation.Bed Rest: Confinement of an individual to bed for therapeutic or experimental reasons.Baroreflex: A response by the BARORECEPTORS to increased BLOOD PRESSURE. Increased pressure stretches BLOOD VESSELS which activates the baroreceptors in the vessel walls. The net response of the CENTRAL NERVOUS SYSTEM is a reduction of central sympathetic outflow. This reduces blood pressure both by decreasing peripheral VASCULAR RESISTANCE and by lowering CARDIAC OUTPUT. Because the baroreceptors are tonically active, the baroreflex can compensate rapidly for both increases and decreases in blood pressure.Valsalva Maneuver: Forced expiratory effort against a closed GLOTTIS.Space Flight: Travel beyond the earth's atmosphere.Tachycardia: Abnormally rapid heartbeat, usually with a HEART RATE above 100 beats per minute for adults. Tachycardia accompanied by disturbance in the cardiac depolarization (cardiac arrhythmia) is called tachyarrhythmia.Autonomic Nervous System: The ENTERIC NERVOUS SYSTEM; PARASYMPATHETIC NERVOUS SYSTEM; and SYMPATHETIC NERVOUS SYSTEM taken together. Generally speaking, the autonomic nervous system regulates the internal environment during both peaceful activity and physical or emotional stress. Autonomic activity is controlled and integrated by the CENTRAL NERVOUS SYSTEM, especially the HYPOTHALAMUS and the SOLITARY NUCLEUS, which receive information relayed from VISCERAL AFFERENTS.Cardiovascular Deconditioning: A change in cardiovascular function resulting in a reduction in BLOOD VOLUME, and reflex DIURESIS. It occurs frequently after actual or simulated WEIGHTLESSNESS.Weightlessness Simulation: Condition under normal Earth gravity where the force of gravity itself is not actually altered but its influence or effect may be modified and studied. (From ASGSB Bull 1992;5(2):27)Hemodynamics: The movement and the forces involved in the movement of the blood through the CARDIOVASCULAR SYSTEM.Fludrocortisone: A synthetic mineralocorticoid with anti-inflammatory activity.Hypovolemia: An abnormally low volume of blood circulating through the body. It may result in hypovolemic shock (see SHOCK).Sympathetic Nervous System: The thoracolumbar division of the autonomic nervous system. Sympathetic preganglionic fibers originate in neurons of the intermediolateral column of the spinal cord and project to the paravertebral and prevertebral ganglia, which in turn project to target organs. The sympathetic nervous system mediates the body's response to stressful situations, i.e., the fight or flight reactions. It often acts reciprocally to the parasympathetic system.Vascular Resistance: The force that opposes the flow of BLOOD through a vascular bed. It is equal to the difference in BLOOD PRESSURE across the vascular bed divided by the CARDIAC OUTPUT.Weightlessness Countermeasures: Techniques and routines designed to prevent or reverse unwanted effects of weightlessness experienced during actual and simulated space flight, including physiologic changes related to removal of gravitational loading. Specific measures include creation of artificial gravity, exercise, low-level lower body negative pressure, and use of anti-deconditioning devices. (From Nicogossian, Space Physiology and Medicine, 2d ed, pp294-297)Plasma Volume: Volume of PLASMA in the circulation. It is usually measured by INDICATOR DILUTION TECHNIQUES.Primary Dysautonomias: Disorders of the AUTONOMIC NERVOUS SYSTEM occurring as a primary condition. Manifestations can involve any or all body systems but commonly affect the BLOOD PRESSURE and HEART RATE.Cardiac Output: The volume of BLOOD passing through the HEART per unit of time. It is usually expressed as liters (volume) per minute so as not to be confused with STROKE VOLUME (volume per beat).Blood Volume: Volume of circulating BLOOD. It is the sum of the PLASMA VOLUME and ERYTHROCYTE VOLUME.Cerebrovascular Circulation: The circulation of blood through the BLOOD VESSELS of the BRAIN.Dysautonomia, Familial: An autosomal disorder of the peripheral and autonomic nervous systems limited to individuals of Ashkenazic Jewish descent. Clinical manifestations are present at birth and include diminished lacrimation, defective thermoregulation, orthostatic hypotension (HYPOTENSION, ORTHOSTATIC), fixed pupils, excessive SWEATING, loss of pain and temperature sensation, and absent reflexes. Pathologic features include reduced numbers of small diameter peripheral nerve fibers and autonomic ganglion neurons. (From Adams et al., Principles of Neurology, 6th ed, p1348; Nat Genet 1993;4(2):160-4)Bradycardia: Cardiac arrhythmias that are characterized by excessively slow HEART RATE, usually below 50 beats per minute in human adults. They can be classified broadly into SINOATRIAL NODE dysfunction and ATRIOVENTRICULAR BLOCK.Gravity Suits: Double-layered inflatable suits which, when inflated, exert pressure on the lower part of the wearer's body. The suits are used to improve or stabilize the circulatory state, i.e., to prevent hypotension, control hemorrhage, and regulate blood pressure. The suits are also used by pilots under positive acceleration.Norepinephrine: Precursor of epinephrine that is secreted by the adrenal medulla and is a widespread central and autonomic neurotransmitter. Norepinephrine is the principal transmitter of most postganglionic sympathetic fibers and of the diffuse projection system in the brain arising from the locus ceruleus. It is also found in plants and is used pharmacologically as a sympathomimetic.Astronauts: Members of spacecraft crew including those who travel in space, and those in training for space flight. (From Webster, 10th ed; Jane's Aerospace Dictionary, 3d ed)Ephedrine: A phenethylamine found in EPHEDRA SINICA. PSEUDOEPHEDRINE is an isomer. It is an alpha- and beta-adrenergic agonist that may also enhance release of norepinephrine. It has been used for asthma, heart failure, rhinitis, and urinary incontinence, and for its central nervous system stimulatory effects in the treatment of narcolepsy and depression. It has become less extensively used with the advent of more selective agonists.Cardiovascular System: The HEART and the BLOOD VESSELS by which BLOOD is pumped and circulated through the body.Weightlessness: Condition in which no acceleration, whether due to gravity or any other force, can be detected by an observer within a system. It also means the absence of weight or the absence of the force of gravity acting on a body. Microgravity, gravitational force between 0 and 10 -6 g, is included here. (From NASA Thesaurus, 1988)Post-Exercise Hypotension: Transient reduction in blood pressure levels immediately after exercises that lasts 2-12 hours. The reduction varies but is typically 5-20 mm Hg when compared to pre-exercise levels. It exists both in normotensive and hypertensive individuals and may play a role in excercise related PHYSIOLOGIC ADAPTATION.Bionics: The study of systems, particularly electronic systems, which function after the manner of, in a manner characteristic of, or resembling living systems. Also, the science of applying biological techniques and principles to the design of electronic systems.Multiple System Atrophy: A syndrome complex composed of three conditions which represent clinical variants of the same disease process: STRIATONIGRAL DEGENERATION; SHY-DRAGER SYNDROME; and the sporadic form of OLIVOPONTOCEREBELLAR ATROPHIES. Clinical features include autonomic, cerebellar, and basal ganglia dysfunction. Pathologic examination reveals atrophy of the basal ganglia, cerebellum, pons, and medulla, with prominent loss of autonomic neurons in the brain stem and spinal cord. (From Adams et al., Principles of Neurology, 6th ed, p1076; Baillieres Clin Neurol 1997 Apr;6(1):187-204; Med Clin North Am 1999 Mar;83(2):381-92)Droxidopa: A precursor of noradrenaline that is used in the treatment of parkinsonism. The racemic form (DL-threo-3,4-dihydroxyphenylserine) has also been used, and has been investigated in the treatment of orthostatic hypotension. There is a deficit of noradrenaline as well as of dopamine in Parkinson's disease and it has been proposed that this underlies the sudden transient freezing seen usually in advanced disease. Administration of DL-threo-3,4-dihydroxyphenylserine has been claimed to result in an improvement in this phenomenon but controlled studies have failed to demonstrate improvement. (Reynolds JEF(Ed): Martindale: The Extra Pharmacopoeia (electronic version). Micromedex, Inc, Englewood, CO, 1995)Trimethaphan: A nicotinic antagonist that has been used as a ganglionic blocker in hypertension, as an adjunct to anesthesia, and to induce hypotension during surgery.Regional Blood Flow: The flow of BLOOD through or around an organ or region of the body.Pyridostigmine Bromide: A cholinesterase inhibitor with a slightly longer duration of action than NEOSTIGMINE. It is used in the treatment of myasthenia gravis and to reverse the actions of muscle relaxants.Heat Stress Disorders: A group of conditions that develop due to overexposure or overexertion in excessive environmental heat.Pressoreceptors: Receptors in the vascular system, particularly the aorta and carotid sinus, which are sensitive to stretch of the vessel walls.Blood Flow Velocity: A value equal to the total volume flow divided by the cross-sectional area of the vascular bed.Vascular Capacitance: The measure of a BLOOD VESSEL's ability to increase the volume of BLOOD it holds without a large increase in BLOOD PRESSURE. The vascular capacitance is equal to the change in volume divided by the change in pressure.Gravity, Altered: A change in, or manipulation of, gravitational force. This may be a natural or artificial effect.Fluid Shifts: Translocation of body fluids from one compartment to another, such as from the vascular to the interstitial compartments. Fluid shifts are associated with profound changes in vascular permeability and WATER-ELECTROLYTE IMBALANCE. The shift can also be from the lower body to the upper body as in conditions of weightlessness.Hyperventilation: A pulmonary ventilation rate faster than is metabolically necessary for the exchange of gases. It is the result of an increased frequency of breathing, an increased tidal volume, or a combination of both. It causes an excess intake of oxygen and the blowing off of carbon dioxide.Jogging: Running at a low rate of speed. It can be done as a means of conditioning or for general health and well being.Leg: The inferior part of the lower extremity between the KNEE and the ANKLE.History, Modern 1601-: The period of history from 1601 of the common era to the present.Early Ambulation: Procedure to accelerate the ability of a patient to walk or move about by reducing the time to AMBULATION. It is characterized by a shorter period of hospitalization or recumbency than is normally practiced.Sweating: The process of exocrine secretion of the SWEAT GLANDS, including the aqueous sweat from the ECCRINE GLANDS and the complex viscous fluids of the APOCRINE GLANDS.Cardiovascular Physiological Phenomena: Processes and properties of the CARDIOVASCULAR SYSTEM as a whole or of any of its parts.Central Venous Pressure: The blood pressure in the central large VEINS of the body. It is distinguished from peripheral venous pressure which occurs in an extremity.Anesthesia, Spinal: Procedure in which an anesthetic is injected directly into the spinal cord.Syndrome: A characteristic symptom complex.Anesthesia, Obstetrical: A variety of anesthetic methods such as EPIDURAL ANESTHESIA used to control the pain of childbirth.Reflex: An involuntary movement or exercise of function in a part, excited in response to a stimulus applied to the periphery and transmitted to the brain or spinal cord.Blood Patch, Epidural: The injection of autologous blood into the epidural space either as a prophylactic treatment immediately following an epidural puncture or for treatment of headache as a result of an epidural puncture.Plethysmography: Recording of change in the size of a part as modified by the circulation in it.Time Factors: Elements of limited time intervals, contributing to particular results or situations.Physical Conditioning, Human: Diet modification and physical exercise to improve the ability to carry out daily tasks and perform physical activities.Tremor: Cyclical movement of a body part that can represent either a physiologic process or a manifestation of disease. Intention or action tremor, a common manifestation of CEREBELLAR DISEASES, is aggravated by movement. In contrast, resting tremor is maximal when there is no attempt at voluntary movement, and occurs as a relatively frequent manifestation of PARKINSON DISEASE.Space Motion Sickness: Disorder characterized by nausea, vomiting, and dizziness, possibly in response to vestibular disorientation or fluid shifts associated with space flight. (From Webster's New World Dictionary)Vasoconstriction: The physiological narrowing of BLOOD VESSELS by contraction of the VASCULAR SMOOTH MUSCLE.Forearm: Part of the arm in humans and primates extending from the ELBOW to the WRIST.Epinephrine: The active sympathomimetic hormone from the ADRENAL MEDULLA. It stimulates both the alpha- and beta- adrenergic systems, causes systemic VASOCONSTRICTION and gastrointestinal relaxation, stimulates the HEART, and dilates BRONCHI and cerebral vessels. It is used in ASTHMA and CARDIAC FAILURE and to delay absorption of local ANESTHETICS.Electrocardiography: Recording of the moment-to-moment electromotive forces of the HEART as projected onto various sites on the body's surface, delineated as a scalar function of time. The recording is monitored by a tracing on slow moving chart paper or by observing it on a cardioscope, which is a CATHODE RAY TUBE DISPLAY.Phenylephrine: An alpha-1 adrenergic agonist used as a mydriatic, nasal decongestant, and cardiotonic agent.Blood Circulation: The movement of the BLOOD as it is pumped through the CARDIOVASCULAR SYSTEM.Nitroprusside: A powerful vasodilator used in emergencies to lower blood pressure or to improve cardiac function. It is also an indicator for free sulfhydryl groups in proteins.Hypertension: Persistently high systemic arterial BLOOD PRESSURE. Based on multiple readings (BLOOD PRESSURE DETERMINATION), hypertension is currently defined as when SYSTOLIC PRESSURE is consistently greater than 140 mm Hg or when DIASTOLIC PRESSURE is consistently 90 mm Hg or more.Vagus Nerve: The 10th cranial nerve. The vagus is a mixed nerve which contains somatic afferents (from skin in back of the ear and the external auditory meatus), visceral afferents (from the pharynx, larynx, thorax, and abdomen), parasympathetic efferents (to the thorax and abdomen), and efferents to striated muscle (of the larynx and pharynx).Methoxyhydroxyphenylglycol: Synthesized from endogenous epinephrine and norepinephrine in vivo. It is found in brain, blood, CSF, and urine, where its concentrations are used to measure catecholamine turnover.Veins: The vessels carrying blood away from the capillary beds.Vasoconstrictor Agents: Drugs used to cause constriction of the blood vessels.Norepinephrine Plasma Membrane Transport Proteins: Sodium chloride-dependent neurotransmitter symporters located primarily on the PLASMA MEMBRANE of noradrenergic neurons. They remove NOREPINEPHRINE from the EXTRACELLULAR SPACE by high affinity reuptake into PRESYNAPTIC TERMINALS. It regulates signal amplitude and duration at noradrenergic synapses and is the target of ADRENERGIC UPTAKE INHIBITORS.Shock: A pathological condition manifested by failure to perfuse or oxygenate vital organs.Hypohidrosis: Abnormally diminished or absent perspiration. Both generalized and segmented (reduced or absent sweating in circumscribed locations) forms of the disease are usually associated with other underlying conditions.Fluid Therapy: Therapy whose basic objective is to restore the volume and composition of the body fluids to normal with respect to WATER-ELECTROLYTE BALANCE. Fluids may be administered intravenously, orally, by intermittent gavage, or by HYPODERMOCLYSIS.Sympathomimetics: Drugs that mimic the effects of stimulating postganglionic adrenergic sympathetic nerves. Included here are drugs that directly stimulate adrenergic receptors and drugs that act indirectly by provoking the release of adrenergic transmitters.Blood Pressure Determination: Techniques for measuring blood pressure.Vasomotor System: The neural systems which act on VASCULAR SMOOTH MUSCLE to control blood vessel diameter. The major neural control is through the sympathetic nervous system.Antihypertensive Agents: Drugs used in the treatment of acute or chronic vascular HYPERTENSION regardless of pharmacological mechanism. Among the antihypertensive agents are DIURETICS; (especially DIURETICS, THIAZIDE); ADRENERGIC BETA-ANTAGONISTS; ADRENERGIC ALPHA-ANTAGONISTS; ANGIOTENSIN-CONVERTING ENZYME INHIBITORS; CALCIUM CHANNEL BLOCKERS; GANGLIONIC BLOCKERS; and VASODILATOR AGENTS.Antiparkinson Agents: Agents used in the treatment of Parkinson's disease. The most commonly used drugs act on the dopaminergic system in the striatum and basal ganglia or are centrally acting muscarinic antagonists.Stress, Physiological: The unfavorable effect of environmental factors (stressors) on the physiological functions of an organism. Prolonged unresolved physiological stress can affect HOMEOSTASIS of the organism, and may lead to damaging or pathological conditions.Anesthesia: A state characterized by loss of feeling or sensation. This depression of nerve function is usually the result of pharmacologic action and is induced to allow performance of surgery or other painful procedures.Independent Living: A housing and community arrangement that maximizes independence and self-determination.Adie Syndrome: A syndrome characterized by a TONIC PUPIL that occurs in combination with decreased lower extremity reflexes. The affected pupil will respond more briskly to accommodation than to light (light-near dissociation) and is supersensitive to dilute pilocarpine eye drops, which induce pupillary constriction. Pathologic features include degeneration of the ciliary ganglion and postganglionic parasympathetic fibers that innervate the pupillary constrictor muscle. (From Adams et al., Principles of Neurology, 6th ed, p279)Cardiography, Impedance: A type of impedance plethysmography in which bioelectrical impedance is measured between electrodes positioned around the neck and around the lower thorax. It is used principally to calculate stroke volume and cardiac volume, but it is also related to myocardial contractility, thoracic fluid content, and circulation to the extremities.Shock, Septic: Sepsis associated with HYPOTENSION or hypoperfusion despite adequate fluid resuscitation. Perfusion abnormalities may include, but are not limited to LACTIC ACIDOSIS; OLIGURIA; or acute alteration in mental status.Tyramine: An indirect sympathomimetic. Tyramine does not directly activate adrenergic receptors, but it can serve as a substrate for adrenergic uptake systems and monoamine oxidase so it prolongs the actions of adrenergic transmitters. It also provokes transmitter release from adrenergic terminals. Tyramine may be a neurotransmitter in some invertebrate nervous systems.Parkinson Disease: A progressive, degenerative neurologic disease characterized by a TREMOR that is maximal at rest, retropulsion (i.e. a tendency to fall backwards), rigidity, stooped posture, slowness of voluntary movements, and a masklike facial expression. Pathologic features include loss of melanin containing neurons in the substantia nigra and other pigmented nuclei of the brainstem. LEWY BODIES are present in the substantia nigra and locus coeruleus but may also be found in a related condition (LEWY BODY DISEASE, DIFFUSE) characterized by dementia in combination with varying degrees of parkinsonism. (Adams et al., Principles of Neurology, 6th ed, p1059, pp1067-75)Hypocapnia: Clinical manifestation consisting of a deficiency of carbon dioxide in arterial blood.Mineral Waters: Water naturally or artificially infused with mineral salts or gases.Renal Veins: Short thick veins which return blood from the kidneys to the vena cava.Splanchnic Circulation: The circulation of blood through the BLOOD VESSELS supplying the abdominal VISCERA.Stroke Volume: The amount of BLOOD pumped out of the HEART per beat, not to be confused with cardiac output (volume/time). It is calculated as the difference between the end-diastolic volume and the end-systolic volume.Treatment Outcome: Evaluation undertaken to assess the results or consequences of management and procedures used in combating disease in order to determine the efficacy, effectiveness, safety, and practicability of these interventions in individual cases or series.Adrenergic alpha-1 Receptor Agonists: Compounds that bind to and activate ADRENERGIC ALPHA-1 RECEPTORS.Carotid Sinus: The dilated portion of the common carotid artery at its bifurcation into external and internal carotids. It contains baroreceptors which, when stimulated, cause slowing of the heart, vasodilatation, and a fall in blood pressure.Ultrasonography, Doppler, Transcranial: A non-invasive technique using ultrasound for the measurement of cerebrovascular hemodynamics, particularly cerebral blood flow velocity and cerebral collateral flow. With a high-intensity, low-frequency pulse probe, the intracranial arteries may be studied transtemporally, transorbitally, or from below the foramen magnum.Renin: A highly specific (Leu-Leu) endopeptidase that generates ANGIOTENSIN I from its precursor ANGIOTENSINOGEN, leading to a cascade of reactions which elevate BLOOD PRESSURE and increase sodium retention by the kidney in the RENIN-ANGIOTENSIN SYSTEM. The enzyme was formerly listed as EC 3.4.99.19.Cross-Over Studies: Studies comparing two or more treatments or interventions in which the subjects or patients, upon completion of the course of one treatment, are switched to another. In the case of two treatments, A and B, half the subjects are randomly allocated to receive these in the order A, B and half to receive them in the order B, A. A criticism of this design is that effects of the first treatment may carry over into the period when the second is given. (Last, A Dictionary of Epidemiology, 2d ed)Compression Bandages: Strips of elastic material used to apply pressure to body parts to control EDEMA and aid circulation.Arterial Pressure: The blood pressure in the ARTERIES. It is commonly measured with a SPHYGMOMANOMETER on the upper arm which represents the arterial pressure in the BRACHIAL ARTERY.Middle Cerebral Artery: The largest of the cerebral arteries. It trifurcates into temporal, frontal, and parietal branches supplying blood to most of the parenchyma of these lobes in the CEREBRAL CORTEX. These are the areas involved in motor, sensory, and speech activities.Autoimmune Diseases of the Nervous System: Disorders caused by cellular or humoral immune responses primarily directed towards nervous system autoantigens. The immune response may be directed towards specific tissue components (e.g., myelin) and may be limited to the central nervous system (e.g., MULTIPLE SCLEROSIS) or the peripheral nervous system (e.g., GUILLAIN-BARRE SYNDROME).Ocular Hypotension: Abnormally low intraocular pressure often related to chronic inflammation (uveitis).Adrenergic alpha-Agonists: Drugs that selectively bind to and activate alpha adrenergic receptors.Altitude Sickness: Multiple symptoms associated with reduced oxygen at high ALTITUDE.Ganglia, Autonomic: Clusters of neurons and their processes in the autonomic nervous system. In the autonomic ganglia, the preganglionic fibers from the central nervous system synapse onto the neurons whose axons are the postganglionic fibers innervating target organs. The ganglia also contain intrinsic neurons and supporting cells and preganglionic fibers passing through to other ganglia.Homeostasis: The processes whereby the internal environment of an organism tends to remain balanced and stable.Skin Temperature: The TEMPERATURE at the outer surface of the body.Ferricyanides: Inorganic salts of the hypothetical acid, H3Fe(CN)6.Space Simulation: An environment simulating one or more parameters of the space environment, applied in testing space systems or components. Often, a closed chamber is used, capable of approximating the vacuum and normal environments of space. (From NASA Thesaurus, 1988) This also includes simulated EXTRAVEHICULAR ACTIVITY studies in atmosphere exposure chambers or water tanks.Headache: The symptom of PAIN in the cranial region. It may be an isolated benign occurrence or manifestation of a wide variety of HEADACHE DISORDERS.Prospective Studies: Observation of a population for a sufficient number of persons over a sufficient number of years to generate incidence or mortality rates subsequent to the selection of the study group.Diabetic Neuropathies: Peripheral, autonomic, and cranial nerve disorders that are associated with DIABETES MELLITUS. These conditions usually result from diabetic microvascular injury involving small blood vessels that supply nerves (VASA NERVORUM). Relatively common conditions which may be associated with diabetic neuropathy include third nerve palsy (see OCULOMOTOR NERVE DISEASES); MONONEUROPATHY; mononeuropathy multiplex; diabetic amyotrophy; a painful POLYNEUROPATHY; autonomic neuropathy; and thoracoabdominal neuropathy. (From Adams et al., Principles of Neurology, 6th ed, p1325)Catecholamines: A general class of ortho-dihydroxyphenylalkylamines derived from tyrosine.Physical Education and Training: Instructional programs in the care and development of the body, often in schools. The concept does not include prescribed exercises, which is EXERCISE THERAPY.Gravitation: Acceleration produced by the mutual attraction of two masses, and of magnitude inversely proportional to the square of the distance between the two centers of mass. It is also the force imparted by the earth, moon, or a planet to an object near its surface. (From NASA Thesaurus, 1988)Vagus Nerve Diseases: Diseases of the tenth cranial nerve, including brain stem lesions involving its nuclei (solitary, ambiguus, and dorsal motor), nerve fascicles, and intracranial and extracranial course. Clinical manifestations may include dysphagia, vocal cord weakness, and alterations of parasympathetic tone in the thorax and abdomen.Drinking: The consumption of liquids.Rest: Freedom from activity.Respiration: The act of breathing with the LUNGS, consisting of INHALATION, or the taking into the lungs of the ambient air, and of EXHALATION, or the expelling of the modified air which contains more CARBON DIOXIDE than the air taken in (Blakiston's Gould Medical Dictionary, 4th ed.). This does not include tissue respiration (= OXYGEN CONSUMPTION) or cell respiration (= CELL RESPIRATION).Laser-Doppler Flowmetry: A method of non-invasive, continuous measurement of MICROCIRCULATION. The technique is based on the values of the DOPPLER EFFECT of low-power laser light scattered randomly by static structures and moving tissue particulates.Perioperative Care: Interventions to provide care prior to, during, and immediately after surgery.Exercise: Physical activity which is usually regular and done with the intention of improving or maintaining PHYSICAL FITNESS or HEALTH. Contrast with PHYSICAL EXERTION which is concerned largely with the physiologic and metabolic response to energy expenditure.Fatigue Syndrome, Chronic: A syndrome characterized by persistent or recurrent fatigue, diffuse musculoskeletal pain, sleep disturbances, and subjective cognitive impairment of 6 months duration or longer. Symptoms are not caused by ongoing exertion; are not relieved by rest; and result in a substantial reduction of previous levels of occupational, educational, social, or personal activities. Minor alterations of immune, neuroendocrine, and autonomic function may be associated with this syndrome. There is also considerable overlap between this condition and FIBROMYALGIA. (From Semin Neurol 1998;18(2):237-42; Ann Intern Med 1994 Dec 15;121(12): 953-9)Adrenergic alpha-Antagonists: Drugs that bind to but do not activate alpha-adrenergic receptors thereby blocking the actions of endogenous or exogenous adrenergic agonists. Adrenergic alpha-antagonists are used in the treatment of hypertension, vasospasm, peripheral vascular disease, shock, and pheochromocytoma.Vasodilator Agents: Drugs used to cause dilation of the blood vessels.Dose-Response Relationship, Drug: The relationship between the dose of an administered drug and the response of the organism to the drug.Intraoperative Complications: Complications that affect patients during surgery. They may or may not be associated with the disease for which the surgery is done, or within the same surgical procedure.Tetralones: A group of TETRAHYDRONAPHTHALENES containing a keto oxygen.Subdural Effusion: Leakage and accumulation of CEREBROSPINAL FLUID in the subdural space which may be associated with an infectious process; CRANIOCEREBRAL TRAUMA; BRAIN NEOPLASMS; INTRACRANIAL HYPOTENSION; and other conditions.