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  • 1
    Electronic Resource
    Electronic Resource
    Electric analogue model of the singleneedle extracorporal blood-circulatory system of an artificial kidney (1985)
    Springer
    Medical & biological engineering & computing 23 (1985), S. 517-524 
    Details
    ISSN: 1741-0444
    Keywords: Artificial kidney ; Electric analogue model ; Extracorporal blood circulation ; Single-needle
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Chemistry and Pharmacology , Medicine
    Notes: Abstract Using an electric analogue model, the flow conditions (the progress over time of blood pressure and blood flow) within the single-needle extracorporal bloodcirculatory system of an artificial kidney are investigated. Two different arrangements are taken into consideration: a system with a nonpulsatile pump (a roller pump) and a system with a pulsatile pump (two roller pumps, or one bellows pump). It is demonstrated that, using a pulsatile pump and an optimised pump frequency, it is possible to achieve a reduction in pressure variations as well as flow-velocity variations (with a correspondingly reduced haemolysis rate), and also to achieve an increase in the mean blood flow (comparable to a two-needle system).
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF02455305
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  • 2
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    Autoregulation of cerebral blood flow in orthostatic hypotension (1998)
    In:  Other Sources
    Details
    Publication Date: 2019-07-13
    Description: BACKGROUND AND PURPOSE: We sought to evaluate cerebral autoregulation in patients with orthostatic hypotension (OH). METHODS: We studied 21 patients (aged 52 to 78 years) with neurogenic OH during 80 degrees head-up tilt. Blood flow velocities (BFV) from the middle cerebral artery were continuously monitored with transcranial Doppler sonography, as were heart rate, blood pressure (BP), cardiac output, stroke volume, CO2, total peripheral resistance, and cerebrovascular resistance. RESULTS: All OH patients had lower BP (P〈.0001), BFV_diastolic (P〈.05), CVR (P〈.007), and TPR (P〈.02) during head-up tilt than control subjects. In control subjects, no correlations between BFV and BP were found during head-up tilt, suggesting normal autoregulation. OH patients could be separated into those with normal or expanded autoregulation (OH_NA; n=16) and those with autoregulatory failure (OH_AF; n=5). The OH_NA group showed either no correlation between BFV and BP (n=8) or had a positive BFV/BP correlation (R2〉.75) but with a flat slope. An expansion of the "autoregulated" range was seen in some patients. The OH_AF group was characterized by a profound fall in BFV in response to a small reduction in BP (mean deltaBP 〈40 mm Hg; R2〉.75). CONCLUSIONS: The most common patterns of cerebral response to OH are autoregulatory failure with a flat flow-pressure relationship or intact autoregulation with an expanded autoregulated range. The least common pattern is autoregulatory failure with a steep flow-pressure relationship. Patients with patterns 1 and 2 have an enhanced capacity to cope with OH, while those with pattern 3 have reduced capacity.
    Keywords: Aerospace Medicine
    Type: Stroke; a journal of cerebral circulation (ISSN 0039-2499); 29; 1; 104-11
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  • 3
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    Effect of perturbations and a meal on superior mesenteric artery flow in patients with orthostatic hypotension (1997)
    In:  Other Sources
    Details
    Publication Date: 2019-07-13
    Description: Our aims were to evaluate to role of superior mesenteric blood flow in the pathophysiology of orthostatic hypotension in patients with generalized autonomic failure. METHODS: Twelve patients with symptomatic neurogenic orthostatic hypotension and 12 healthy controls underwent superior mesenteric artery flow measurements using Doppler ultrasonography during head-up tilt and tilt plus meal ingestion. Autonomic failure was assessed using standard tests of the function of the sympathetic adrenergic, cardiovagal and postganglionic sympathetic sudomotor function. RESULTS: Superior mesenteric flow volume and time-averaged velocity were similar in patients and controls at supine rest; however, responses to cold pressor test and upright tilt were attenuated (p 〈 0.05) in patients compared to controls. Head-up tilt after the meal evoked a profound fall of blood pressure and mesenteric blood flow in the patients; the reduction of mesenteric blood flow correlated (r = 0.89) with the fall of blood pressure in these patients, providing another manifestation of failed baroreflexes. We make the novel finding that the severity of postprandial orthostatic hypotension regressed negatively with the postprandial increase in mesenteric flow in patients with orthostatic hypotension. CONCLUSION: Mesenteric flow is under baroreflex control, which when defective, results in, or worsens orthostatic hypotension. Its large size and baroreflexivity renders it quantitatively important in the maintenance of postural normotension. The effects of orthostatic stress can be significantly attenuated by reducing the splanchnic-mesenteric volume increase in response to food. Evaluation of mesenteric flow in response to eating and head-up tilt provide important information on intra-abdominal sympathetic adrenergic function, and the ability of the patient to cope with orthostatic stress.
    Keywords: Aerospace Medicine
    Type: Journal of the autonomic nervous system (ISSN 0165-1838); 67; 2-Jan; 15-23
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  • 4
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    Hypocapnia and cerebral hypoperfusion in orthostatic intolerance (1998)
    In:  Other Sources
    Details
    Publication Date: 2019-07-13
    Description: BACKGROUND AND PURPOSE: Orthostatic and other stresses trigger tachycardia associated with symptoms of tremulousness, shortness of breath, dizziness, blurred vision, and, often, syncope. It has been suggested that paradoxical cerebral vasoconstriction during head-up tilt might be present in patients with orthostatic intolerance. We chose to study middle cerebral artery (MCA) blood flow velocity (BFV) and cerebral vasoregulation during tilt in patients with orthostatic intolerance (OI). METHODS: Beat-to-beat BFV from the MCA, heart rate, CO2, blood pressure (BP), and respiration were measured in 30 patients with OI (25 women and 5 men; age range, 21 to 44 years; mean age, 31.3+/-1.2 years) and 17 control subjects (13 women and 4 men; age range, 20 to 41 years; mean age, 30+/-1.6 years); ages were not statistically different. These indices were monitored during supine rest and head-up tilt (HUT). We compared spontaneous breathing and hyperventilation and evaluated the effect of CO2 rebreathing in these 2 positions. RESULTS: The OI group had higher supine heart rates (P〈0.001) and cardiac outputs (P〈0.01) than the control group. In response to HUT, OI patients underwent a greater heart rate increment (P〈0.001) and greater reductions in pulse pressure (P〈0.01) and CO2 (P〈0.001), but total systemic resistance failed to show an increment. Among the cerebrovascular indices, all BFVs (systolic, diastolic, and mean) decreased significantly more, and cerebrovascular resistance (CVR) was increased in OI patients (P〈0.01) compared with control subjects. In both groups, hyperventilation induced mild tachycardia (P〈0.001), a significant reduction of BFV, and a significant increase of CVR associated with a fall in CO2. Hyperventilation during HUT reproduced hypocapnia, BFV reduction, and tachycardia and worsened symptoms of OI; these symptoms and indices were improved within 2 minutes of CO2 rebreathing. The relationships between CO2 and BFV and heart rate were well described by linear regressions, and the slope was not different between control subjects and patients with OI. CONCLUSIONS: Cerebral vasoconstriction occurs in OI during orthostasis, which is primarily due to hyperventilation, causing significant hypocapnia. Hypocapnia and symptoms of orthostatic hypertension are reversible by CO2 rebreathing.
    Keywords: Aerospace Medicine
    Type: Stroke; a journal of cerebral circulation (ISSN 0039-2499); 29; 9; 1876-81
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  • 5
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    Postural tachycardia syndrome: time frequency mapping (1996)
    In:  Other Sources
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    Publication Date: 2019-07-13
    Description: Orthostatic tachycardia is common but its specificity remains uncertain. Our preliminary work suggested that using autonomic function testing in conjunction with time-frequency mapping (TFM), it might be possible to characterize a subset of the postural tachycardia syndrome (POTS), that is due to a restricted autonomic neuropathy. We describe 20 patients (17 women and 3 men, aged 14-43 years) with florid POTS and 20 controls (14 women and 6 men, aged 20-41 years). Autonomic failure was quantified by its distribution (cardiovagal, adrenergic and sudomotor) and severity, a symptom profile was generated, and spectral indices, based on modified Wigner distribution during rest and head-up tilt (80 degrees) were evaluated. During tilt-up POTS patients differed from controls by an excessive heart rate (〉 130 bpm) (P 〈 0.001), and higher diastolic pressure (P 〈 0.01). During rest, cardiovagal oscillations (at respiratory frequencies [RF]) and slow rhythms at nonrespiratory frequencies (NONRF) (from 0.01 to 0.07 Hz) in R-R intervals (RRI) (P 〈 0.01) were reduced. Both RF and NONRF rhythms in RRI were further blunted with tilt-up (P 〈 0.001). Slow adrenergic vasomotor rhythms in blood pressure (BP) (approximately 0.07 Hz) surged with tilt-up and returned to normal levels afterwards. The index of sympatho-vagal balance (NONRF-Systolic BP (SBP)/RF-RRI) was dramatically increased in POTS (P 〈 0.001). Distal postganglionic sudomotor failure was observed, and impairment of the BP responses to the Valsalva maneuver (phase II) suggested peripheral adrenergic dysfunction. Persistent orthostatic dizziness, tiredness, gastrointestinal symptoms and palpitations were common in POTS patients. It is possible to identify a subset of POTS patients who have a length-dependent autonomic neuropathy, affecting the peripheral adrenergic and cardiovagal fibers, with relative preservation of cardiac adrenergic fibers.
    Keywords: Aerospace Medicine
    Type: Journal of the autonomic nervous system (ISSN 0165-1838); 61; 3; 313-20
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