ISSN:0091-2700    

DOI:10.1002/j.1552-4604.1994.tb04975.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

The high‐ and low‐pressure baroreceptor reflexes are integral to the control of blood pressure by the autonomic nervous system. Tests of the integrity of these baroreflexes make it possible to identify the site of autonomic dysfunction in patients with orthostatic hypotension. Clinical characteristics and typical results of autonomic testing in patients with autonomic failure, with carotid sinus hypersensitivity, and with hyperadrenergic autonomic dysfunction are described in this rev

ISSN:0091-2700    

DOI:10.1002/j.1552-4604.1994.tb04976.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

The authors' objective was to review previous studies of immediate (first 30 seconds) and stabilized (30 seconds to 20 minutes) hemodynamic responses of healthy adults to the head‐up posture, with particular reference to alteration of such responses in the elderly and the usefulness of such data in the diagnosis of orthostatic hypotension. The immediate response in healthy young adults is characterized by a prompt rise in heart rate, which peaks at about 8 to 15 seconds and then tapers; the arterial pressure and total vascular resistance decrease sharply at 5 to 10 seconds, followed by a rapid rebound and overshoot. Over the first 30 seconds there is a steady parallel decline of thoracic blood volume and stroke volume; there is also an initial surge of cardiac output followed by a steady decrease. During the stabilized response (30 seconds to 20 minutes), the hemodynamic variables are relatively steady, showing average increases in heart rate of about 15 to 30%, in diastolic pressure of 10 to 15%, and in total vascular resistance of 30 to 40%; during the 5th to 20th minutes there are also decreases in thoracic blood volume averaging about 25 to 30%, in cardiac output 15 to 30%, and in pulse pressure about 5 to 10%. It is evident that in normal human subjects, assumption of the upright posture results in profound hemodynamic changes, most of them occurring during the first 30 seconds. In elderly subjects (aged 60–69 years), there are, in the upright posture, lesser increments of heart rate and diastolic pressure, but no significant differences from younger age groups in the response of thoracic blood volume, cardiac output or total vascular resistance. However, beginning at about age 75, there is an increasing incidence of orthostatic hypotension, which averages about 14 to 20% at age 75 and older. The tendency toward orthostatic hypotension in the elderly is due (1) to the structural and functional changes in the circulation itself, (2) to a decline in autonomic function, and (3) to a probable functional deficiency of the skeletal muscle pump. The authors believe that closer hemodynamic monitoring of orthostatic hypotension patients would considerably increase our understanding and aid in the diagnosis of this condit

ISSN:0091-2700    

DOI:10.1002/j.1552-4604.1994.tb04977.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

Many astronauts experience intolerance to orthostatic stress after space flight, despite the ingestion of salt tablets and water equivalent to 0.9% saline just before their return to Earth. Previous research indicates that the ingestion of 1.07% saline solution increased plasma volume more than did 0.9% saline. Therefore, the authors hypothesized that the 1.07% saline would be more effective in reducing orthostatic stress during standing. In this study, six men (22–47 years) performed a 5‐minute “stand test” (5 minutes supine followed by 5 minutes standing) under four hydration conditions: 1) hypohydrated (HYPO, 20 mg intravenous [IV] Lasix), 2) euhydrated (EU), 3) rehydrated with 1 L 0.9% saline 2 hours after Lasix, or 4) rehydrated with 1 L 1.07% saline. Stand tests were done 4 to 5 hours after rehydration. Plasma volume was reduced 10% after Lasix, and was restored by both rehydration solutions. When subjects stood, their diastolic pressure, mean pressure, heart rate (HR), and peripheral resistance increased (P<.05), and their stroke volume (SV), cardiac output (CO), and thoracic fluid (TF, by impedance cardiography) decreased (P<.05). Systolic arterial pressure (SBP) increased when subjects stood after saline, but decreased if subjects were HYPO or EU (P<.05 for 1.07% versus HYPO and EU). Heart rate (HR), another indicator of orthostatic stress, did not differ among hydration states. During the last minute of the stand test, TF was greater if subjects had fluid countermeasures. Stroke volume, CO, and TF were significantly less during minute 5 of standing than during minute 3. Whether they would continue to fall in a longer stand test is not known. The results for SBP indicate that 1.07% saline may have advantages over 0.9% saline as a countermeasure to postspace‐flight or postbedrest orthostatic in

ISSN:0091-2700    

DOI:10.1002/j.1552-4604.1994.tb04978.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

The cardiovascular responses to postural change, and how they are affected by aging, are inadequately described in women. Therefore, the authors examined the influence of age and sex on the responses of blood pressure, cardiac output, heart rate, and other variables to change in posture. Measurements were made after 10 minutes each in the supine, seated, and standing positions in 22 men and 25 women who ranged in age from 21 to 59 years. Several variables differed, both by sex and by age, when subjects were supine. On rising, subjects' diastolic and mean arterial pressures, heart rate, total peripheral resistance (TPR), and thoracic impedance increased; cardiac output, stroke volume, and mean stroke ejection rate decreased; and changes in all variables, except heart rate, were greater from supine to sitting than sitting to standing. The increase in heart rate was greater in the younger subjects, and increases in TPR and thoracic impedance were greater in the older subjects. Stroke volume decreased less, and TPR and thoracic impedance increased more, in the women than in the men. The increase in TPR was particularly pronounced in the older women. These studies show that the cardiovascular responses to standing differ, in some respects, between the sexes and with age. The authors suggest that the sex differences are, in part, related to greater decrease of thoracic blood volume with standing in women than in men, and that the age differences result, in part, from decreased responsiveness of the high‐pressure baroreceptor syste

ISSN:0091-2700    

DOI:10.1002/j.1552-4604.1994.tb04979.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

Orthostatic hypotension after even short space flights has affected a significant number of astronauts. Given the need for astronauts to function at a high level of efficiency during and after their return from space, the application of pharmacologic and other treatments is strongly indicated. This report addresses the clinical problem of orthostatic hypotension and its treatments to ascertain whether pharmacologic or physiologic treatment may be useful in the prevention of orthostatic hypotension associated with space flight. Treatment of orthostatic hypotension in patients now includes increasing intravascular volume with high sodium intake and mineralocorticoids, or increasing vascular resistance through the use of drugs to stimulate alpha or block beta vascular receptors. Earlier treatment used oral sympathomimetic ephedrine hydrochloride alone or with “head‐up” bed rest. Then long‐acting adrenocortical steroid desoxycorticosterone preparations with high‐salt diets were used to expand volume. Fludrocortisone was shown to prevent the orthostatic drop in blood pressure. The combination of the sympathomimetic amine hydroxyamphetamine and a monoamine oxidase inhibitor tranylcypromine has been used, as has indomethacin alone. Davies et al. used mineralocorticoids at low doses concomitantly with alpha‐agonists to increase vasocontrictor action. Schirger et al used tranylcypromine and methylphenidate with or without a Jobst elastic leotard garment or the alpha‐adrenergic agonist midodrine (which stimulates both arterial and venous systems without direct central nervous system or cardiac effects). Vernikos et al established that the combination of fludrocortisone, dextroamphetamine, and atropine exhibited a beneficial effect on orthostatic hypotension induced by 7‐day 6° head‐down bed rest (a model used to simulate the weightlessness of space flight). Thus, there are numerous drugs that, in combination with mechanical techniques, including lower body negative pressure to elevate transmural pressure, could be studied to treat orthostatic hypotension

ISSN:0091-2700    

DOI:10.1002/j.1552-4604.1994.tb04980.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

It has been postulated that α2‐adrenergic receptors play a modulatory role in the regulation of blood pressure. Activation of α2‐receptors located in the central nervous system results in inhibition of sympathetic tone and decrease of blood pressure. This indeed may be the mechanism of action of central sympatholytic antihypertensives such as α‐methyldopa. Presynaptic α2‐receptors also are found in adrenergic nerve terminals. These receptors act as a negative feedback mechanism by inhibiting the release of norepinephrine. The relevance of α2‐adrenergic receptors for blood pressure regulation can be explored with yohimbine, a selective antagonist of these receptors. Yohimbine increases blood pressure in resting normal volunteers. This effect is associated with an increase in both sympathetic nerve activity, reflecting an increase in central sympathetic outflow, and in norepinephrine spillover, reflecting potentiation of the release of norepinephrine from adrenergic nerve terminals. These actions, therefore, underscore the importance of α2‐adrenergic receptors for blood pressure regulation even under resting conditions. Patients with autonomic failure, even those with severe sympathetic deprivation, are hypersensitive to the pressor effects of yohimbine. This increased responsiveness can be explained by sensitization of adrenergic receptors, analogous to denervation supersensitivity, and by the lack of autonomic reflexes that would normally buffer any increase in blood pressure. Preliminary studies suggest that the effectiveness of yohimbine in autonomic failure can be enhanced with monoamine oxidase inhibitors. Used in combination, yohimbine increases norepinephrine release, whereas monoamine oxidase inhibitors inhibit its degradation. Therefore, yohimbine is not only a useful tool in the study of blood pressure regulation, but may offer a therapeutic option in auto

ISSN:0091-2700    

DOI:10.1002/j.1552-4604.1994.tb04981.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

Atrial natriuretic factor, originally isolated from the atrium of the heart, has been found to consist of three major groups: atrial natriuretic peptide (ANP), B‐form natriuretic peptide (BNP), and C‐form natriuretic peptide (CNP). In addition, ANP exists in its precursor form, pro‐ANP, an active ANP with a longer peptide chain (urodilatin) and an antiparallel dimer of active ANP. Sites and production of these diverse forms of the peptides are also diverse, depending on pathologic states. Three major subtypes of ANP receptors exist; these include a clearance receptor and two types of a transmembrane receptor with guanylyl cyclase structures in their intracellular domain. The latter exists at least in two forms, one of which is found mainly in the brain. All the actions of ANP mediated by the transmembrane form of ANP receptors are mediated by cGMP generated by the guanylyl cyclase in the cytosolic domain of the receptor. Among the numerous effects of ANP, its major effects are stimulation of natriuresis and diuresis by the kidney through its hemodynamic and tubular effects. In addition, ANP causes vasodilatation and fluid volume reduction by direct actions on vascular smooth muscle cells, and inhibition of secretion of hormones, such as aldosterone, from adrenal cortex and norepinephrine from peripheral adrenergic neurons. Centrally mediated effects on the regulation of the fluid volume may also be impo

ISSN:0091-2700    

DOI:10.1002/j.1552-4604.1994.tb04982.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

In an effort to understand the interaction between acute postural fluid shifts and hypoxia on hormonal regulation of fluid homeostasis, the authors measured the responses to head‐down tilt with and without acute exposure to normobaric hypoxia. Plasma atrial natriuretic peptide (ANP), cyclic guanosine monophosphate (cGMP), cyclic adenosine monophosphate (cAMP), plasma aldosterone (ALD), and plasma renin activity (PRA) were measured in six healthy male volunteers who were exposed to a head‐down tilt protocol during normoxia and hypoxia. The tilt protocol consisted of a 17° head‐up phase (30 minutes), a 28° head‐down phase (1 hour), and a 17° head‐up recovery period (2 hours, with the last hour normoxic in both experiments). Altitude equivalent to 14,828 ft was simulated by having the subjects breathe an inspired gas mixture with 13.9% oxygen. The results indicate that the postural fluid redistribution associated with a 60‐minute head‐down tilt induces the release of ANP and cGMP during both hypoxia and normoxia. Hypoxia increased cGMP, cAMP, ALD, and PRA throughout the protocol and significantly potentiated the increase in cGMP during head‐down tilt. Hypoxia had no overall effect on the release of ANP, but appeared to attenuate the increase with head‐down tilt. This study describes the acute effects of hypoxia on the endocrine response during fluid redistribution and suggests that the magnitude, but not the direction, of these changes with posture is

ISSN:0091-2700    

DOI:10.1002/j.1552-4604.1994.tb04983.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

This paper reviews a series of studies that indicate that estrogens play an important role in blood volume regulation. The first study illustrates that the plasma volume (PV) of ambulatory women fluctuates during the menstrual cycle, increasing during periods of elevated estrogens. In the second study, it was shown that exogenous and endogenous elevations in blood estrogens attenuate the decrease in PV during bed rest. In the third study, the hypothesis was tested that women, who naturally have a higher blood estrogen content compared with men, will have a smaller loss of PV during bed rest. Ten men and ten women underwent a 13‐day, 6° head‐down bed rest. Plasma volume and red cell mass (RCM) were measured before and after bed rest using125I and51Cr labeling, respectively. Before bed rest, the men and women had similar blood volume (BV) and PV (mL/kg body weight), but the women had a smaller (P<.01) RCM (22.2 ± 0.9 versus 26.2 ± 0.8 mL/kg, mean ± SE). During bed rest, the decrease in RCM (mL/kg) was similar in men and women. However, the decrease in BV was greater in men (8.0 ± 0.8 mL/kg versus 5.8 ± 0.8 mL/kg), because of a greater reduction in PV (6.3 ± 0.6 mL/kg versus 4.1 ± 0.6 mL/kg). Because the decline in BV has been proposed to contribute to the cardiovascular deconditioning after bed rest, it is possible that women may experience less cardiac and circulatory strain on r

ISSN:0091-2700    

DOI:10.1002/j.1552-4604.1994.tb04984.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY