Search PubMed⌕ Search

SEARCH · Search PubMed

Results for “Limitations”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 757 records · Page 42Linked to original sources
PubMed1986

A limit cycle mathematical model of the REM sleep oscillator system.

A limit cycle mathematical model of the rapid-eye-movement (REM) sleep oscillator system has been developed from a structural model of interaction of populations of REM-on and REM-off neurons. The marked differences in latency, amplitude, and duration of the first REM sleep period seen with circadian variation and depressive pathology are modeled by beginning the REM oscillation at different initial points relative to the final position in the limit cycle. Beginning from a point that is graphically interior to the limit cycle produces a long-latency, short-duration, and less intense first REM period. Beginning from a point graphically exterior to the limit cycle produces a short-latency, long-duration, and more intense first REM period. In the model the determinant of whether the oscillation begins exterior or interior to the limit cycle is the time course of decay of the REM-off population discharge activity at sleep onset. When this time course is made to depend on circadian phase, the model produces a very close match to the empirically observed large shifts between the first and second REM periods in duration (often a 50% change) and intensity and also closely mimics the empirically observed shifts in REM latency as human sleep begins at different circadian phases. Although this variation in limit cycle entry accounts for the major changes in REM sleep over the night, the model also postulates a continuous but small circadian variation (of the order of +/- 5% change in REM parameters) acting throughout the course of a night's sleep. Because the model is derived from actual physiological data, rather than being a purely ad hoc or phenomenological construct, it offers the possibility of direct tests of its postulates through neurobiological studies in animals, by circadian phase-related manipulations of the sleep cycle, and through perturbations of the system in humans by the use of drugs. Indeed, an explicit phase-response curve of the system to cholinergic agonists has been developed; this will permit experimental tests of the model in both animals and humans.

Animals↗
PubMed1997

Respiratory effort sensation during exercise with induced expiratory-flow limitation in healthy humans.

Nine healthy subjects (age 31 +/- 4 yr) exercised with and without expiratory-flow limitation (maximal flow approximately 1 l/s). We monitored flow, end-tidal PCO2, esophageal (Pes) and gastric pressures, changes in end-expiratory lung volume, and perception (sensation) of difficulty in breathing. Subjects cycled at increasing intensity (+25 W/30 s) until symptom limitation. During the flow-limited run, exercise performance was limited in all subjects by maximum sensation. Sensation was equally determined by inspiratory and expiratory pressure changes. In both runs, 90% of the variance in sensation could be explained by the Pes swings (difference between peak inspiratory and peak expiratory Pes). End-tidal PCO2 did not explain any variance in sensation in the control run and added only 3% to the explained variance in the flow-limited run. We conclude that in healthy subjects, during normal as well as expiratory flow-limited exercise, the pleural pressure generation of the expiratory muscles is equally related to the perception of difficulty in breathing as that of the inspiratory muscles.

Adult↗
PubMed1987

Flow limitation in upper-airway obstruction. Theoretical analysis.

During a maximal forced expiration from total lung capacity (TLC) in normal human subjects flow limitation will occur (i.e., the flow will become independent of muscular effort) after the initial part of the expiratory manoeuvre. Flow limitation starts at the flow that causes a pressure drop in the bronchial tree large enough for the generation of a flow-limiting segment. In patients with upper-airway obstruction such as laryngeal obstruction or tracheal stenosis, the upper-airway resistance may keep the flow so low that flow limitation cannot arise. The present theoretical study confirms that glottic resistance does not prevent flow limitation from arising in normal human subjects. On the other hand, the mean glottic resistance characteristics measured for 19 patients with bilateral vocal-cord paralysis were found to be such as to prevent flow limitation. This means that in such patients and also in patients with other types of upper-airway obstruction flow may remain effort-dependent throughout the forced expiratory manoeuvre or in any case during much more of this manoeuvre than normal.

Airway Resistance↗
PubMed1996

Role of activation of protein kinase C in the infarct size-limiting effect of ischemic preconditioning through activation of ecto-5'-nucleotidase.

BACKGROUND: We have reported previously that ischemic preconditioning limits infarct size by increasing ecto-5'-nucleotidase activity. Since we have also reported that protein kinase C activation increases ecto-5'-nucleotidase activity in rat cardiomyocytes, we tested whether activation of protein kinase C during ischemic preconditioning contributes to the infarct size-limiting effect through augmentation of ecto-5'-nucleotidase activity in the canine heart. METHODS AND RESULTS: The coronary artery was occluded four times for 5 minutes with alternating 5-minute periods of reperfusion (ischemic preconditioning). Then the coronary artery was occluded for 90 minutes followed by 6 hours of reperfusion. Infarct size, normalized by the risk area, in the ischemic preconditioning group was smaller than in the control group (42.6 +/- 3.6% in the control group versus 7.9 +/- 1.8% in the ischemic preconditioning group, P < .001). Myocardial ecto-5'-nucleotidase activity was increased after the ischemic preconditioning procedure but the increase in ecto-5'-nucleotidase was attenuated by inhibitors of protein kinase C (polymyxin B and GF109203X). Both polymyxin B and GF109203X blunted the infarct size-limiting effect of ischemic preconditioning (infarct size 33.1 +/- 6.9% and 35.1 +/- 6.4%, respectively). The infarct size-limiting effect was also blunted by an inhibitor of ecto-5'-nucleotidase. Transient administration of methoxamine mimicked the increase in ecto-5'-nucleotidase activity and the infarct size-limiting effect, both of which were abolished by inhibitors of protein kinase C. CONCLUSIONS: We conclude that activation of ecto-5'-nucleotidase and protein kinase C contributes to the infarct size-limiting effect of ischemic preconditioning.

5'-Nucleotidase↗
PubMed2001

Evaluating auditory performance limits: II. One-parameter discrimination with random-level variation.

Previous studies have combined analytical models of stochastic neural responses with signal detection theory (SDT) to predict psychophysical performance limits; however, these studies have typically been limited to simple models and simple psychophysical tasks. A companion article in this issue ("Evaluating Auditory Performance Limits: I") describes an extension of the SDT approach to allow the use of computational models that provide more accurate descriptions of neural responses. This article describes an extension to more complex psychophysical tasks. A general method is presented for evaluating psychophysical performance limits for discrimination tasks in which one stimulus parameter is randomly varied. Psychophysical experiments often randomly vary a single parameter in order to restrict the cues that are available to the subject. The method is demonstrated for the auditory task of random-level frequency discrimination using a computational auditory nerve (AN) model. Performance limits based on AN discharge times (all-information) are compared to performance limits based only on discharge counts (rate place). Both decision models are successful in predicting that random-level variation has no effect on performance in quiet, which is the typical result in psychophysical tasks with random-level variation. The distribution of information across the AN population provides insight into how different types of AN information can be used to avoid the influence of random-level variation. The rate-place model relies on comparisons between fibers above and below the tone frequency (i.e., the population response), while the all-information model does not require such across-fiber comparisons. Frequency discrimination with random-level variation in the presence of high-frequency noise is also simulated. No effect is predicted for all-information, consistent with the small effect in human performance; however, a large effect is predicted for rate-place in noise with random-level variation.

Animals↗
PubMed1994

Assessment of respiratory limitation in activities of daily life among retired workers.

Epidemiologic studies of occupational lung diseases generally compare symptoms and pulmonary function between groups with differing exposure risks. However, the functional consequences (e.g., on activities of daily life) of statistically significant group differences in either symptoms or pulmonary function are rarely addressed. An understanding of this is important for both the scientific investigators and users of these public health investigations. To address this issue, we developed a 15-item questionnaire to assess breathing-related limitations in activities of daily life, and applied this in a study of 112 retired workers. For each subject, we calculated a limitation score as the number of items associated with breathing difficulty divided by the total number of applicable items. Results from this questionnaire were compared to respiratory symptoms, the Sickness Impact Profile (SIP), lung function test results, 6-min walking distance, and participants' assessment of breathing effort after the walk. The limitation score was significantly higher in subjects reporting dyspnea (22% versus 4%, p < 0.01) or occasional wheeze (27% versus 6%, p < 0.01), and correlated negatively with FEV1, MMEF, and FEV1/FVC (R2 = -0.47, -0.36, and -0.45, respectively, p < 0.0001) and breathing effort after the 6-min walk (R2 = 0.31, p < 0.01). Breathing related limitation in daily activities was also associated with a general health related limitation in household maintenance and ambulation in the SIP (p < 0.01). For each questionnaire item, we compared the responses among subjects with a normal FEV1 to those of subjects with an abnormal FEV1 (below the lower 95% confidence limit).(ABSTRACT TRUNCATED AT 250 WORDS)

Activities of Daily Living↗
PubMed1997

Expiratory flow limitation in stable asthmatic patients during resting breathing.

Application of negative pressure at the mouth during tidal expiration (NEP) provides a simple, rapid, noninvasive method for detecting expiratory flow limitation during spontaneous breathing. Patients in whom NEP elicits an increase in flow throughout expiration are not flow-limited (FL). In contrast, patients in whom application of NEP does not elicit an increase in flow during most or part of tidal expiration are considered FL. We have used the NEP technique to assess the prevalence of expiratory flow limitation during resting breathing in sable asthmatic patients in both the seated and supine positions. In patients in the sitting position, we have also assessed flow limitation with the conventional method, based on comparison of tidal and maximal expiratory flow-volume (MEFV) curves. We studied 13 patients (FEV1 range: 48 to 94% predicted) with both the NEP and conventional techniques. According to the NEP technique, none of the patients was FL in the seated and only two were FL in the supine position. By contrast, on the basis of the conventional method, six of the patients would have been classified as FL in the sitting position. We conclude that: (1) most stable asthmatic patients do not exhibit tidal expiratory flow limitation during resting breathing; and (2) the conventional method for assessing flow limitation may lead to erroneous conclusions.

Adolescent↗
PubMed1998

Severity of airflow limitation is associated with severity of airway inflammation in smokers.

To investigate the relationship between airflow limitation and airway inflammation in smokers, we examined paraffin-embedded bronchial biopsies obtained from 30 smokers: 10 with severe airflow limitation, eight with mild/moderate airflow limitation, and 12 control smokers with normal lung function. Histochemical and immunohistochemical methods were performed to assess the number of inflammatory cells in the subepithelium and the expression of CC chemokines macrophage inflammatory protein (MIP)-1alpha and -1beta in the bronchial mucosa. Compared with control smokers, smokers with severe airflow limitation had an increased number of neutrophils (p < 0.02), macrophages (p < 0.03), and NK lymphocytes (p < 0.03) in the subepithelium, and an increased number of MIP-1alpha+ epithelial cells (p < 0.02). When all smokers were considered together, the value of FEV1 was inversely correlated with the number of neutrophils (r = -0.59, p < 0.002), macrophages (r = -047, p < 0. 012), NK-lymphocytes (r = -0.51, p < 0.006) in the subepithelium, and with the number of MIP-1alpha+ epithelial cells (r = -0.61, p < 0.003). We conclude that in smokers the severity of airflow limitation is correlated with the severity of airway inflammation and that severe airflow limitation is associated with an increased number of neutrophils, macrophages, NK lymphocytes, and MIP-1alpha+ cells in the bronchial mucosa.

Aged↗
PubMed2001

Factors associated with persistent airflow limitation in severe asthma.

Persistent airflow limitation can develop in nonsmoking patients with asthma. However, the prevalence and risk factors for airways obstruction with incomplete reversibility in asthma are unknown. We assessed the prevalence of persistent airflow limitation (defined as postbronchodilator FEV(1) or FEV(1)/VC < 75% predicted) in 132 nonsmoking outpatients with severe asthma visiting chest physicians in general hospitals in The Netherlands. They had used inhaled corticosteroids (> or = 1,600 microg/d) and/or daily oral prednisone and long-acting bronchodilators for > 1 yr. In addition, we examined whether persistent airways obstruction in these patients was associated with specific clinical characteristics (age at onset, smoking history, atopic status, bronchodilator reversibility, provocative concentration of histamine causing a 20% decrease in FEV(1) [PC(20)histamine]) or markers of inflammation (exhaled nitric oxide [NO], blood eosinophils, total IgE; and eosinophilia or neutrophilia in induced sputum). Multiple logistic regression analyses were used to calculate adjusted odds ratios (OR). Persistent airflow limitation was observed in 49% of the patients in the study, and apart from older age and longer asthma duration, was strongly associated with a sputum eosinophils percent > or = 2% (OR = 7.7; confidence interval [CI]: 2.4 to 25), PC(20)histamine < or = 1.0 mg/ml (OR = 3.9; CI: 1.2 to 13), and adult onset (> or = 18 yr) of asthma (OR = 3.3; CI: 1.2 to 9). Only sputum eosinophilia appeared to be independently associated with persistent airflow limitation (OR = 8.9; CI: 1.3 to 59). In conclusion, persistent airflow limitation is common in adult patients with severe asthma, and is associated with adult onset of the disease, airway hyperresponsiveness, and most importantly, sputum eosinophilia. These findings suggest that eosinophilic airway inflammation contributes to persistent airflow limitation in severe asthma. Whether reduction of sputum eosinophils with more vigorous treatment leads to a better prognosis in severe asthma is still an open question.

Adolescent↗
PubMed1976

Mechanisms of maximal expiratory flow limitation in clinically unsuspected emphysema and obstruction of the peripheral airways.

We studied the determinants of maximal expiratory flow in 7 patients in whom surgical lung specimens showed evidence of mild emphysema or obstruction of the peripheral airways. All the patients showed frequency dependence of dynamic lung compliance and low maximal expiratory flow. Lung elastic recoil pressures were reduced at all lung volumes in 4 of 5 patients with emphysema but were normal in 2 patients with obstruction of the peripheral airways. Maximal flow-static recoil curves showed a high intercept in the pressure axis when the slope between 50 per cent and 30 per cent of vital capacity was extrapolated to zero flow (transmural pressure in the flow-limiting segment) in all patients. The slope of these curve represents the conductance of the segment that extends from the alveoli to the flow-limiting segment and includes the latter. Conductance of this segment was higher than normal in 4 patients with emphysema and borderline low in one, and low in one patient with obstruction of the peripheral airways and borderline low in the other. Increased transmural pressure in the flow-limiting segment was interpreted as being due to peripheral location of collapsible flow-limiting segment and, in part, limiting maximal expiratory flow. The results also suggested that flow limitation in emphysema may occur without physiologic evidence of increased resistance of peripheral airways when, in fact, bronchiolitis is present on histologic examination.

Adult↗
PubMed2005

Predicting in vivo clinical performance of anterior cruciate ligament fixation methods from in vitro analysis: industrial tests of fatigue life and tolerance limits are more useful than other cyclic loading parameters.

BACKGROUND: Anterior cruciate ligament graft fixations experience cyclic loads in vivo. Present cyclic loading studies testing fixation use "incremental cycling," "residual strength" protocols, or a combination. Industrial standards, however, rely on fatigue life testing and use tolerance limits to determine guaranteed minimum levels of cycles to failure. HYPOTHESIS: Industrial standards of fatigue life and lower tolerance limits provide a more conservative assessment of cycles to failure than do currently used cyclic loading models, and they facilitate interpretation of data toward clinical performance. STUDY DESIGN: Descriptive laboratory study. METHODS: Fatigue life curves and lower tolerance limits were calculated for porcine patellar tendon graft fixations that were tested to failure in single pulls or cycled to failure at 2 different load levels. A log curve was constructed so that the expected values at other load levels could be calculated. Comparison of a metal and a composite resorbable interference screw design was used as an example. RESULTS: Cycles to failure of both screw designs varied widely at each load level. The guaranteed minimum number of cycles calculated by tolerance limits was therefore much lower than the mean cycles to failure at any given load level. For example, at a load of 250 N, the predicted mean life for the composite screw and the metal screw was 2513 and 1490 cycles, respectively, whereas the 80/80 tolerance limits (the value that 80% could be expected to achieve with 80% confidence) were only 63 and 68 cycles, respectively. Small reductions in load level gave very large increases in minimally expected life. CONCLUSIONS: Fatigue testing of anterior cruciate ligament reconstructions better shows the wide variation between specimens and the large effect of load levels on expected life. Wide scatter makes mean levels over-optimistic and difficult to interpret. Tolerance limits give estimates that are more conservative and facilitate data interpretation. CLINICAL RELEVANCE: A method for testing and analyzing fatigue properties was presented, results of which are more readily interpreted to clinical practice.

Animals↗
PubMedDate not supplied

Role of the medial structures in the intact and anterior cruciate ligament-deficient knee. Limits of motion in the human knee.

We measured motion limits in human cadaveric knees before and after sectioning the anterior cruciate ligament and the medial structures. Sectioning the medial collateral ligament in an anterior cruciate ligament-deficient knee increased the anterior translation limit at 90 degrees of flexion but not at 30 degrees of flexion. The tibia displaced straight anteriorly without exhibiting the coupled internal rotation that occurred in intact and anterior cruciate ligament-deficient knees. A lateral 15 N-m abduction moment produced a coupled external rotation in the medial collateral ligament-deficient knee. This was in marked contrast to intact, anterior cruciate ligament-deficient, or combined medial collateral ligament and anterior cruciate ligament-deficient knees, in which an abduction moment produced a coupled internal rotation. Sectioning only the medial collateral ligament caused a small but significant increase in the abduction rotation limit, whereas larger increases in the abduction rotation limit occurred when the posterior oblique ligament and posterior medial capsule were cut in addition to the medial collateral ligament. Cutting the medial collateral ligament increased the external rotation limit. The increase was independent of whether the anterior cruciate ligament was intact or sectioned. Subsequent cutting of the posterior oblique ligament and posterior medial capsule further increased the external rotation limit.

Adult↗
PubMed2006

Oxygen limitation modulates pH regulation of catabolism and hydrogenases, multidrug transporters, and envelope composition in Escherichia coli K-12.

BACKGROUND: In Escherichia coli, pH regulates genes for amino-acid and sugar catabolism, electron transport, oxidative stress, periplasmic and envelope proteins. Many pH-dependent genes are co-regulated by anaerobiosis, but the overall intersection of pH stress and oxygen limitation has not been investigated. RESULTS: The pH dependence of gene expression was analyzed in oxygen-limited cultures of E. coli K-12 strain W3110. E. coli K-12 strain W3110 was cultured in closed tubes containing LBK broth buffered at pH 5.7, pH 7.0, and pH 8.5. Affymetrix array hybridization revealed pH-dependent expression of 1,384 genes and 610 intergenic regions. A core group of 251 genes showed pH responses similar to those in a previous study of cultures grown with aeration. The highly acid-induced gene yagU was shown to be required for extreme-acid resistance (survival at pH 2). Acid also up-regulated fimbriae (fimAC), periplasmic chaperones (hdeAB), cyclopropane fatty acid synthase (cfa), and the "constitutive" Na+/H+ antiporter (nhaB). Base up-regulated core genes for maltodextrin transport (lamB, mal), ATP synthase (atp), and DNA repair (recA, mutL). Other genes showed opposite pH responses with or without aeration, for example ETS components (cyo,nuo, sdh) and hydrogenases (hya, hyb, hyc, hyf, hyp). A hypF strain lacking all hydrogenase activity showed loss of extreme-acid resistance. Under oxygen limitation only, acid down-regulated ribosome synthesis (rpl,rpm, rps). Acid up-regulated the catabolism of sugar derivatives whose fermentation minimized acid production (gnd, gnt, srl), and also a cluster of 13 genes in the gadA region. Acid up-regulated drug transporters (mdtEF, mdtL), but down-regulated penicillin-binding proteins (dacACD, mreBC). Intergenic regions containing regulatory sRNAs were up-regulated by acid (ryeA, csrB, gadY, rybC). CONCLUSION: pH regulates a core set of genes independently of oxygen, including yagU, fimbriae, periplasmic chaperones, and nhaB. Under oxygen limitation, however, pH regulation is reversed for genes encoding electron transport components and hydrogenases. Extreme-acid resistance requires yagU and hydrogenase production. Ribosome synthesis is down-regulated at low pH under oxygen limitation, possibly due to the restricted energy yield of catabolism. Under oxygen limitation, pH regulates metabolism and transport so as to maximize alternative catabolic options while minimizing acidification or alkalinization of the cytoplasm.

Carrier Proteins↗
PubMed1996

The caudal limit of Otx2 gene expression as a marker of the midbrain/hindbrain boundary: a study using in situ hybridisation and chick/quail homotopic grafts.

Segmentation of the neural tube has been clearly shown in the forebrain and caudal hindbrain but has never been demonstrated within the midbrain/hindbrain domain. Since the homeobox-containing gene Otx2 has a caudal limit of expression in this region, we examined, mainly in chick embryos, the possibility that this limit could represent an interneuromeric boundary separating either two cerebellar domains or the mesencephalic and cerebellar primordia. In situ hybridisation with chick or mouse Otx2 probes showed the existence of a transient Otx2-negative area in the caudal mesencephalic vesicle, between stages HH10 and HH17/18 in chick, and at embryonic day 9.5 in mice. The first post-mitotic neurons of the mesencephalon sensu stricto, as labelled with an anti-beta-tubulin antibody, overlay the Otx2-positive neuroepithelium with a perfect match of the caudal limits of these two markers at all embryonic stages analysed (until stage HH20). Chick/quail homotopic grafts of various portions of the midbrain/hindbrain domain have shown that the progeny of the cells located in the caudal mesencephalic vesicle at stage HH10 are found within the rhombomere 1 as early as stage HH14. Furthermore, our results indicate that the cells forming the HH20 constriction (coinciding with the caudal Otx2 limit) are the progeny of those located at the caudal Otx2 limit at stage HH10 (within the mesencephalic vesicle). As a result, the Otx2-positive portion of the HH10 mesencephalic vesicle gives rise to the HH20 mesencephalon, while the Otx2-negative portion gives rise to the HH20 rostral rhombomere 1. Long-survival analysis allowing the recognition of the various grisea of the chimeric brains strongly supports the view that, as early as stage HH10, the caudal limit of Otx2 expression separates mesencephalic from isthmo/cerebellar territories. Finally, this study revealed unexpected rostrocaudal morphogenetic movements taking place between stages HH10 and HH16 in the mediodorsal part of the caudal Otx2-positive domain.

Animals↗
PubMed2004

Upper thermal tolerance and oxygen limitation in terrestrial arthropods.

The hypothesis of oxygen limitation of thermal tolerance proposes that critical temperatures are set by a transition to anaerobic metabolism, and that upper and lower tolerances are therefore coupled. Moreover, this hypothesis has been dubbed a unifying general principle and extended from marine to terrestrial ectotherms. By contrast, in insects the upper and lower limits are decoupled, suggesting that the oxygen limitation hypothesis might not be as general as proposed. However, no direct tests of this hypothesis or its predictions have been undertaken in terrestrial species. We use a terrestrial isopod (Armadillidium vulgare) and a tenebrionid beetle (Gonocephalum simplex) to test the prediction that thermal tolerance should vary with oxygen partial pressure. Whilst in the isopod critical thermal maximum declined with declining oxygen concentration, this was not the case in the beetle. Efficient oxygen delivery via a tracheal system makes oxygen limitation of thermal tolerance, at a whole organism level, unlikely in insects. By contrast, oxygen limitation of thermal tolerances is expected to apply to species, like the isopod, in which the circulatory system contributes significantly to oxygen delivery. Because insects dominate terrestrial systems, oxygen limitation of thermal tolerance cannot be considered pervasive in this habitat, although it is a characteristic of marine species.

Adaptation, Physiological↗
PubMed2001

Limits to sustainable human metabolic rate.

There is a limit to the performance of an organism set by energy intake and energy mobilization. Here, the focus is on humans with unlimited access to food and for whom physical activity can be limited by energy mobilization. The physical activity level (PAL) in the general population, calculated as doubly-labelled-water-assessed average daily metabolic rate as a multiple of basal metabolic rate, has an upper limit of 2.2-2.5. The upper limit of sustainable metabolic rate is approximately twice as high in endurance athletes, mainly because of long-term exercise training with simultaneous consumption of carbohydrate-rich food during exercise. Endurance athletes have an increased fat-free mass and can maintain energy balance at a PAL value of 4.0-5.0. High altitude limits exercise performance as a result of combined effects on nutrient supply and the capacity to process nutrients. Thus, trained subjects climbing Mount Everest reached PAL values of 2.0-2.7, well below the observed upper limit at sea level.

Altitude↗
PubMed1987

Principles of the International Commission on Radiological Protection system of dose limitation.

The current recommendations of the International Commission on Radiological Protection (ICRP), published in 1977, identify two types of effect against which protection is required. "Stochastic" effects are those for which the probability of an effect occurring, rather than its severity, is regarded as a function of dose without threshold, whereas "non-stochastic" effects are those for which the severity varies with the dose and for which a threshold may occur. The system of dose limitation recommended by the ICRP is based on the prevention of non-stochastic effects and limitation of the probability of stochastic effects to levels deemed to be acceptable. The prevention of non-stochastic effects is achieved by setting dose-equivalent limits at values such that no threshold dose would be reached, even following exposure for the whole of a lifetime or for the total period of a working life. The limitation of stochastic effects is achieved by keeping all justifiable exposures as low as is reasonably achievable, economic and social factors being taken into account, subject to the constraint that reductions in collective exposure do not cause unacceptably large individual exposures. The formulation of a quantitative system of dose limitation based on these principles requires that judgments be made on several factors including: relationships between radiation dose and the induction of deleterious effects for a variety of endpoints and radiation types; acceptable levels of risk for radiation workers and members of the public; and methods of assessing whether the cost of introducing protective measures is justified by the reduction in radiation detriment which they will provide. In the case of patients deliberately exposed to ionising radiations, the objectives of radiation protection differ somewhat from those applying to radiation workers and members of the public. For patients, risks and benefits relate to the same person and upper limits on acceptable risks may differ grossly from those appropriate to normal individuals. For these reasons, and because of its historical relationship with the International Congress of Radiology, the ICRP has given special consideration to radiation protection in medicine and has published reports on protection of the patient in diagnostic radiology and in radiation therapy.

Animals↗
PubMed1997

Expiratory flow limitation during spontaneous breathing: comparison of patients with restrictive and obstructive respiratory disorders.

BACKGROUND AND STUDY OBJECTIVES: Comparison of tidal and forced expiratory flow-volume (V-V) curves has inherent technical problems in the characterization of expiratory flow limitation. In addition, patients with neuromuscular disorders may be unable to perform forced expiratory maneuvers because of muscle weakness or poor coordination. A recently developed simple, noninvasive technique that avoids these problems was used to detect expiratory flow limitation at rest in 19 seated patients with restrictive respiratory (13 with musculoskeletal) disorders (RD) and 20 with chronic obstructive airway disease (COAD). SETTING: A large rehabilitation hospital for the care of patients with chronic musculoskeletal and respiratory disorders. INTERVENTIONS AND MEASUREMENTS: The method consisted of applying negative pressure of about 5 cm H2O at the airway opening during expiration and comparing the ensuing V-V curve to the preceding tidal V-V curve. RESULTS: While nine patients with COAD demonstrated flow limitation, only one patient with RD did so. Patients with expiratory flow limitation exhibited various contours of the control tidal expiratory V-V curve. Thus, inspection of the tidal V-V curve is not a reliable means of detecting expiratory flow limitation. CONCLUSIONS: We conclude that expiratory flow limitation during resting breathing is common in patients with COAD but not in patients with RD.

Adult↗