Rebound Values Research Articles

Urea as a marker of adequacy in hemodialysis: Lesson from in vivo urea dynamics monitoring

"Dialysis dose," a concept developed by Sargent and Gotch based on urea kinetic modeling, is a useful and recognized tool that is used to quantitate and optimize a dialysis-efficacy program. However, it has been shown that oversimplification of the "dialysis adequacy" concept to the Kt/V index might lead to dramatic underdialysis and subsequent deleterious consequences on morbidity and mortality of dialysis patients. With this perspective, the determination of Kt/V must be very cautious and rely on accurate measurement of postdialysis urea concentration and its use integrated as a tool in a quality-assurance process. In this study, we analyzed urea dynamics by means of a blood side (ultrafiltrate) continuous online urea monitoring system interfaced with a two-pool model hosted in a microcomputer. The study was designed to provide instantaneous dialysis performances (body and dialyzer clearances, dialyzer mass transfer coefficient) and to determine the in vivo functional permeability characteristics of the patient [intercompartment urea mass transfer coefficient (Kc)]. Thirteen end-stage renal disease patients (age 54 +/- 16 years; 12 male and 1 female) were studied during nine consecutive dialysis sessions (3 weeks). Urea kinetics obtained from the urea monitoring system fitted closely the urea kinetic modeling prediction, confirming the validity of the double-pool model structure. Effective in vivo urea mass transfer coefficient averaged 912 +/- 235 mL/min/1.73 m2, a value close to those reported with more invasive methods. Large variations ranging from 363 to 1249 mL/min were observed among patients, confirming very large interindividual patient permeability differences. Interestingly, the urea mass transfer coefficient was inversely correlated with the postdialysis rebound values. Intraindividual variations were also noted as a function of time denoting functional changes in urea mass transfer coefficient values. The urea distribution volume was 38.1 +/- 7, 8 L (53 +/- 8% body weight). V1 referring to the extracellular volume and V2 to the intracellular volume were 9 +/- 2 L (13 +/- 2% body weight) and 29.2 +/- 6.6 L (41 +/- 1.3% body wt), respectively. The extracellular/intracellular volume ratio was 0.31 (approximately one third) and was not as usually defined by the paradigm 1/2 ratio. Online double-pool urea kinetic modeling gave a new insight in urea kinetic modeling approach. Urea dynamics fit perfectly a double-compartment model structure. Accessible extracellular volume to hemodialysis is smaller than expected. The in vivo urea mass transfer coefficient must be considered as an individual and variable characteristic of ESRD patients that should be taken into consideration when prescribing the hemodialysis schedule.

Techniques for measuring rock weathering: application to a dated fan segment sequence in southern Tunisia

There are a number of techniques for estimating the amount of weathering a clast has undergone. These usually have the objective of establishing an ordinal chronology of geomorphological surfaces, or investigation of site-specific conditions affecting weathering rates. Three such techniques are applied to a dated sequence of alluvial fan segments in southern Tunisia. Two of these techniques depend on measuring surface roughness (the micro-roughness meter and a displacement approach) and one uses the structural weakening of the rock fabric (Schmidt hammer). The micro-roughness meter enables calculation of standard deviation of surface height variation, root mean square roughness and surface autocorrelation function. Of these techniques, Schmidt hammer rebound values, standard deviation, root mean square roughness and the displacement technique show systematic changes on the three fan segments which are statistically significant at the 0·05 level. However, the amount of variance in all datasets is very large, indicating the need for caution in application of these techniques for relative dating. © 1998 John Wiley & Sons, Ltd.

Screening for extreme postdialysis urea rebound using the smye method: Patients with access recirculation identified when a slow flow method is not used to draw the postdialysis blood

To look for patients with extreme urea rebound, we drew intradialytic samples one third of the way into dialysis during routine modeling for 3 months. The samples taken postdialysis were obtained after stopping the blood pump, without any slow flow period. Using the Smye equations, the intradialytic urea level was used to predict urea rebound, expressed as Kt/V-equilibrated minus Kt/V-single pool (ΔKt/V). Results were averaged for the 3-month period in 369 patients. Mean estimated ΔKt/V was −0.20 ± 0.13, which was similar to but slightly higher than the predicted value (−0.6 × K/V ± 0.03) of −0.19 ± 0.04. In 27 patients, extreme rebound (mean ΔKt/V < − 0.40) was found. Sixteen of these patients consented to further study, but only after access revision in four patients. In these patients, additional slow flow samples after 15 seconds and 2 minutes of slow flow, respectively, were drawn one third of the way into dialysis and postdialysis, and a sample was drawn 30 minutes after dialysis. On restudy, postdialysis rebound was still high with full flow samples ΔKtN = −0.40 ± 25, but was much lower (−0.18 ± 0.07) and similar to predicted rebound (−0.19 ± 0.05; P = NS) when based on 15-second slow flow samples. Eight of the 16 had marked (>15%) access recirculation by urea sampling, and ΔKtN based on full flow post samples correlated with access recirculation ( r = −0.91). The results suggest that the Smye method is valuable for identifying patients with aberrantly large postdialysis rebound values. When the postdialysis samples are drawn without an antecedent slow flow period, most patients with extreme rebound values turn out to have marked access recirculation.

Application of Relative-Age Dating Techniques on Rock Glaciers of the La Sal Mountains, Utah: An Interpretation of Holocene Paleoclimates

This paper applies a variety of relative-age dating techniques on rock glaciers of the La Sal Mountains, Utah to interpret the area's Holocene paleoclimates. Relative ages of rock glaciers and talus of the La Sal Mountains, Utah, were determined using several techniques: lichenometry, Schmidt-hammer rebound value, clast roundness, clast surface roughness, and weathering rind thickness. The techniques were compared against one another and evaluated as to their effectiveness. Lichenometry can be used to differentiate among depositional episodes of the past 2–3000 yrs, but lichens are sometimes adversely affected by environmental factors such as prolonged snowcover, which can periodically reduce or eliminate thalli. Rock hardness, as measured by the Schmidt hammer, and roundness did not prove to be particularly useful in differentiating sites. Weathering rind development is less subject to variations in rate, and its measurement is less subjective than most of the other methods. Weathering rinds were therefore used in conjunction with lichenometry to establish a Holocene paleoclimatic chronology. The combination of methods permits some potential disadvantages of lichenometry to be exploited to advantage and used for paleoenvironmental interpretation.Data support a threefold sequence of deposition during the late Holocene. Weathering rind data indicate deposition in early Holocene-late Pleistocene as well, but precise relative-dating of this older material is beyond the resolving abilities of these techniques. Our conclusions are in general agreement with radiometrically-based Holocene chronologies of the Colorado Front Range.

The determination of rock material properties to predict the performance of machine excavation in tunnels

Abstract The assessment of rock material properties is a critical element in any site investigation for the planning and construction of a tunnel through rock strata. This is irrespective of the number of rock strata involved and crucial to the determination of ground reference conditions as detailed in CIRIA Report No. 79 (1978). An extensive suite of laboratory tests is combined with a comprehensive literature search in order to investigate the interrelationships between some commonly cited rock parameters. The suite includes Shore Scleroscope Rebound Values and results from a modified form of the Cerchar Abrasivity Test. In addition, the benefits from mineralogical analyses of rock samples are considered. Rock material strength and hardness, with regard to common tunnel excavation techniques is found to be a combination of shear and tensile strength and can respectively be designated as cutting strength and abrasivity. An overall classification system is proposed which incorporates these parameters into an unambiguous notation in order to clarify test data and facilitate interpretation. By an appraisal of each testing technique in respect of relative costs and applicability to rock failure under machine excavation forces an incremental approach for rock testing programme is constructed in the form of a flow chart. The model proposes how the efficiency of current test programmes can be improved thus enabling more confident predictions of material properties in single or multi-strata ground along a proposed tunnel route.

Application of Relative-Age Dating Techniques on Rock Glaciers of the La Sal Mountains, Utah: An Interpretation of Holocene Paleoclimates

This paper applies a variety of relative-age dating techniques on rock glaciers of the La Sal Mountains, Utah to interpret the area's Holocene paleoclimates. Relative ages of rock glaciers and talus of the La Sal Mountains, Utah, were determined using several techniques: lichenometry, Schmidt-hammer rebound value, clast roundness, clast surface roughness, and weathering rind thickness. The techniques were compared against one another and evaluated as to their effectiveness. Lichenometry can be used to differentiate among depositional episodes of the past 2–3000 yrs, but lichens are sometimes adversely affected by environmental factors such as prolonged snowcover, which can periodically reduce or eliminate thalli. Rock hardness, as measured by the Schmidt hammer, and roundness did not prove to be particularly useful in differentiating sites. Weathering rind development is less subject to variations in rate, and its measurement is less subjective than most of the other methods. Weathering rinds were therefore used in conjunction with lichenometry to establish a Holocene paleoclimatic chronology. The combination of methods permits some potential disadvantages of lichenometry to be exploited to advantage and used for paleoenvironmental interpretation.Data support a threefold sequence of deposition during the late Holocene. Weathering rind data indicate deposition in early Holocene-late Pleistocene as well, but precise relative-dating of this older material is beyond the resolving abilities of these techniques. Our conclusions are in general agreement with radiometrically-based Holocene chronologies of the Colorado Front Range.

Effect of dialysate temperature on central hemodynamics and urea kinetics

Use of cool dialysate is associated with increased intradialytic blood pressure, but the hemodynamic mechanism is unknown. Whether changes in dialysate temperature affect muscle blood flow, which may the alter the degree of urea compartmentalization, also is unknown. We measured hemodynamics and blood and dialysate-side urea kinetic indices in nine hemodialysis patients during two cool (35.0 degrees C) versus two warm (37.5 degrees C) dialysate treatments. The % change in mean arterial pressure was different when using the cool (+6.5 +/- 9.7 mm Hg) versus the warm (-13.4 +/- 3.6) dialysate (P < 0.01), despite comparable amounts of fluid removal. Percent changes in cardiac output were similar with the two dialysates, and thus the blood pressure effect was due primarily to changes in total peripheral resistance (% delta TPR, cool +26 +/- 13.6, warm +8.6 +/- 14.5; P < 0.02). During cool dialysate use tympanic membrane temperature changed by -0.51 +/- 0.23 degree C, whereas body temperature increased by 0.52 +/- 0.14 degree C during use of warm dialysate. Measured urea recovery normalized to the predialysis urea nitrogen concentration was similar with the two treatments: cool 31.3 +/- 0.039 liter-1; warm 29.7 +/- 0.021; P = NS. In a second study, post-dialysis urea rebound values from 15 seconds to 30 minutes, expressed as the percent of the post-dialysis SUN, were similar after the two treatments: cool 11.79 +/- 1.4; warm 12.21 +/- 2.27, P = NS.(ABSTRACT TRUNCATED AT 250 WORDS)

Weathering of surface clasts as an indicator of the relative age of glacial deposits on Jameson Land, East Greenland

The relative age of glacial deposits on Jameson Land was investigated by weathering‐based techniques. Two related methods, measurement of rind thickness and of Schmidt hammer rebound values. were added to a dating programme of luminescence‐, amino acid‐ and 14C‐analyses. Rind thicknesses differ significantly between areas which represent three glacial phascs, spanning a time interval from late Saale to Late Weichselian. Rebound values are more susceptible to external factors than the rind values but may, if these external factors are known, serve as a complement to the interpretation of rind development.

Evaluation of schmidt rebound hammer testing: A critical approach

In the present paper, a critical approach to the evaluation of Schmidt hammer testing is attempted. Through a statistical analysis of field measurements on sandstone blocks from the archaeological site of Delphi (Greece), it was shown that there is an inconsistency in the correction method proposed by the manufacturer, in relation to the direction of rebound. The horizontally obtained rebound values, when corrected, yielded lower values than the vertically obtained ones. When examining the various factors potentially responsible for possible differences between the vertical and horizontal values, we conclude that the proposed correction method is not applicable to all cases and that there are many factors that may influence the end results.

Correlations between L and N-type Schmidt hammer rebound values obtained during field testing : Ayday, C; Goktan, R M Proc ISRM Symposium: Eurock'92, Chester, 14–17 September 1992P47–50. Publ London: Thomas Telford, 1992

Correlations between L and N-type Schmidt hammer rebound values obtained during field testing : Ayday, C; Goktan, R M Proc ISRM Symposium: Eurock'92, Chester, 14–17 September 1992P47–50. Publ London: Thomas Telford, 1992

Late Quaternary Glacial Fluctuations in the Langtang Valley, Nepal Himalaya, Reconstructed by Relative Dating Methods

Glacial fluctuations in the Langtang Himal are discussed on the basis of multiple relative dating (RD) methods in addition to the '4C method. The RD methods used for the classification of the moraine deposits include the Schmidt hammer rebound values, weathering rind thickness, percentage of clasts with oxidation stain, height of mineral projection, pit depth, percentage of weathered clasts, soil development, and soil color index. By using the RD data, we were able to divide the moraines into five stages (Gora Tabela, Langtang, Lirung, and Yala I and II); an older stage (Lama) is recognized only by the glacialtrough configuration extending down to an altitude of approximately 2600 m. The Gora Tabela Stage is defined by the deeply weathered Lower Till extending down to 3200 m a.s.l. The Langtang Stage (3650-3000 14C yr BP) probably corresponds to the greatest advance in the Holocene, followed by a series of smaller advances in the Lirung Stage (2800-550 '4C yr BP), and the two smaller Little Ice Age advances (Yala I and II Stages).

The age and origin of Neoglacial moraines in Jotunheimen, southern Norway: new evidence from weathering‐based data

This paper considers the controversial issue of the existence of pre‐‘Little Ice Age’ Neoglacial moraines in southern Norway. Schmidt hammer rebound values are combined with measures of boulder roundness and weathering rind thickness in an attempt to isolate moraines that include weathered boulders. A critical approach is used in distinguishing sites where boulders have weathered in situ from those where previously weathered clasts have been incorporated into relatively young moraines. The results confirm that possible pre‐‘Little Ice Age’ Neoglacial moraines seem to be restricted to small, high‐altitude glaciers in eastern Jotunheimen. It is concluded that at these glaciers a particularly large response to a short‐lived earlier Holocene climatic event is more likely to explain the survival of such moraines than a particularly subdued response to the climatic deterioration of the ‘Little Ice Age’. More refined dating techniques are required to determine the age of formation of the anomalous moraines, but before the palaeoclimatic significance of such dates can be assessed, a critical test is required to establish whether the moraines mark former ice‐front positions, and therefore reflect lowering of equilibrium line altitudes, or whether they have been displaced forwards by later and more extensive glacier advances.

The schmidt hammer, weathering and rock surface roughness

AbstractMeasures of the differential relief of adjacent feldspar and pyroxene grains provide the first clear negative correlation of Schmidt hammer rebound (R‐) values and degree of weathering. However, weathering and roughness are intimately related, so the latter cannot be seen simply as a source of error limiting the utility of the Schmidt hammer. Only where surfaces had similar textures prior to weathering can R‐values be compared directly. Even where surface texture is well controlled, differences in R‐values should be viewed critically rather than assigned indiscriminately to differences in degree of weathering.

Measurement and calibration of weathering using the schmidt hammer, on wave washed moraines on the upper norrland coast, Sweden

AbstractThe Schmidt hammer has for some years been used to measure the hardness of different rock surfaces and to date relatively boulders in moraine deposits. In this paper applications on isostatically raised boulder‐beaches and rocky shores are described. The research area is the northern Swedish Bothnian coast where present isostatic uplift is between 0.8‐1.0 cm year−1. Elevations above sea‐level can be converted to a timescale. Rebound values (R) of the Schmidt hammer are correlated with the altitude and the degree of rock weathering can be estimated for each site.Eight sites with boulder‐beaches and nine sites with rocky shores have been analysed. For boulder‐beaches correlation‐coefficients of 0.82‐0.97 for weathering values (R) and the elevations above sea‐level were obtained. For the rocky shores these values were calculated to 0.75‐0.97.This method has applications in both geomorphology and archaeology. For example, a deep layer of weathered dolerite in the Nordingrå region, which previously has been regarded as a post‐Weichselian formation, can be shown to be of pre‐Weichselian origin. Several archaeological applications are also mentioned in the paper.

An investigation of the re-expansion of unsaturated, structured soils during cyclic static loading

A procedure is presented that quantifies soil resilience to compressive stress, through elastic deformation or re-expansion after stress removal, with a single numerical index. This was achieved by comparing the three parameter coefficients of static-loaded and rebound compression lines (normal stress range = 0–1.0 MPa) which had been fitted to a non-linear density-stress model equation. The difference between the static-loaded and rebound values of one of these coefficients was significantly correlated to the clay and organic matter contents, the gravimetric moisture content and the initial dry bulk density of the 33 soils sampled as intact cores at field moisture content (coefficients of determination=0.533–0.973, P<0.05). The magnitude of the sample rebound observed varied between 0.018 and 0.075 Mg m −3 at the maximum applied stress of 1.0 MPa. This is likely to be a significant component of the error in prediction inherent in compaction models based on static-loaded compression data. The data further support the segregation of soils into groupings of comparable mechanical behaviour for soil compaction modelling purposes. The implications of these findings for improving soil resilience to compressive stress through soil and crop management are discussed.

Use of the schmidt test hammer to detect enhanced boulder weathering under late‐lying snowpatches

AbstractBallantyne et al. (1989) presented data that show a significant reduction in Schmidt hammer rebound values for boulders buried by late‐lying snow. McCarroll (1990) questions these findings on the grounds of sampling design, methodology and unexpectedly rapid weathering of boulders at one recently‐deglaciated site. However, analysis of the data using the procedure recommended by McCarroll yields identical results, and his methodological objections appear groundless. The absence of evidence for reduction of rebound values at snowpatch sites investigated by McCarroll may reflect lack of meltwater runoff beneath snowbeds in permafrost areas. The implied rapidity of weathering on recently‐deglaciated ground requires further investigation.

Potential and Limitations of the Schmidt Hammer for Relative-Age Dating: Field Tests on Neoglacial Moraines, Jotunheimen, Southern Norway

In order to assess the potential of the Schmidt hammer for relative-age dating, rebound (R) values were collected from more than 50 sites on two glacier forelands in Jotunheimen, southern Norway. Four factors are found to influence results: instrument errors, lithology, weathering, and boulder-surface roughness. The influence of each is examined and its potential as a source of error assessed. Particular attention is paid to roughness, which may reflect either transport history or the differential weathering of minerals. On Little Ice Age moraines, where differential weathering of minerals is insignificant, variation in R-values parallels variation in boulder-surface roughness, which in turn is related to transport history. Boulder roundness provides a surrogate measure of primary bouldersurface roughness (prior to weathering) and assists in interpreting sites which yield anomalously low R-values. Although the Schmidt hammer has some potential for measuring rock weathering, and thus for interpreting relative age, it should be viewed only as a rapid, inexpensive, and convenient technique for preliminary assessment.

シュミットロックハンマーによる宮古島南東部琉球石灰岩の工学的性質について

This article deals with engineering properties and diagenesis process of the Ryukyu Limestone belonging to the Pleistocene Ryukyu Group widely exposed on south-east part of Miyakojima of the Ryukyu Islands.Karst topography found within Miyakojima includes limestone wall and mound. Limestone wall is the row of ridges along normal faults.On the basis of fossils contained and matrix of the rocks, the Ryukyu limestone is classified principally into four sedimentary facies: coral detrital limestone which contains corals and detritus, coral algal-mat limestone which contains autochthonous corals and algal-mat, algal ball limestone containing algal ball or rhodolites, detrital limestone which consists of detritus of calcareous organic remains.Compressive strength of the Ryukyu limestone shows a wide scatter from 1 to 400 kgf/cm2. To know regional engineering properties of the Ryukyu limstone, we measured rebound value of limestone on outcrops in Miyakojima by a Schmid-Rock-Hammer and petropysical properties such as density, specific gravity, porosity and compressive strength.The results are summarized as follows:1) On the basis of frequency of rebound values, three patterns are recognized: Coral algal-mat limestone and algal ball limestone showe hihger rebound value. Coral detrital limestone shows lower rebound value. Detrital limestone shows a wide range rebound values.2) The higher rebound value zones correspond to the karst topography of limestone wall and distribution zones composed of coral algal-mat limestone and algal ball limestone. Whereas lower rebound value zones correspond to coral detrital limestone.3) Rebound values have relation with porosities taking into account of the cavities.These results led us to the following hypothesis: Diagenesis of the carbonate sediments is influenced by initial porosity of each sedimentary facies, cementation and solution caused by infiltration of meteoric water. Cementation would have occured in a early stage of diagenesis process, and brought on the decrease of primary porosity and the increase of strength. Formation of secondary porosity by solution would cause the some decrease of strength.

The effect of a shallow low-viscosity zone on the mantle flow, the geoid anomalies and the geoid and depth-age relationships at fracture zones

A two-dimensional FEM is used to investigate the flow driven by the horizontal temperature gradient at a fracture zone and to calculate the resulting geoid and topography anomalies. Using a three-layered viscosity structure for the upper mantle, results are presented for the effects of varying: (1) the viscosity contrast between the fluid layers: (2) the Rayleigh number based on the viscosity of the bottom layer; and (3) the thickness of the low-viscosity channel. Good agreement is obtained with the results of geoid anomalies over the Udintsev fracture zone when the viscosity of the top layer is greater than one order of magnitude less than post-glacial rebound values.

Schmidt hammer rebound data for estimation of large scale in situ coal strength

The paper reports an investigation to determine whether a correlation exists between the Schmidt hammer rebound and the in situ large-scale strength. The results showed a reasonable correlation between the large-scale in situ crushing strength of 0.3 m cubes of coal and the lower mean of rebound values obtained. The regression is seen to be linear within the range 2.75 - 13.14 MPa of in situ strength. In situ large-scale testing procedures are cumbersome and expensive, and the Schmidt hammer rebound method can offer a quicker and cheaper means of estimating this strength. Laboratory 25 mm cube coal strength shows a greater scatter with rebound values than the large-scale in situ strength, and gives a non-linear regression.