This paper investigates the causal relations between stock return and volume based on quantile regressions. We first define Granger non-causality in all quantiles and propose testing non-causality by ...a sup-Wald test. Such a test is consistent against any deviation from non-causality in distribution, as opposed to the existing tests that check only non-causality in certain moment. This test is readily extended to test non-causality in different quantile ranges. In the empirical studies of three major stock market indices, we find that the causal effects of volume on return are usually heterogeneous across quantiles and those of return on volume are more stable. In particular, the quantile causal effects of volume on return exhibit a spectrum of (symmetric)
V-shape relations so that the dispersion of return distribution increases with lagged volume. This is an alternative evidence that volume has a positive effect on return volatility. Moreover, the inclusion of the squares of lagged returns in the model may weaken the quantile causal effects of volume on return but does not affect the causality per se.
Emerging evidence supports the view that (AQP) aquaporin water channels are regulators of transcellular water flow. Consistent with their expression in most tissues, AQPs are associated with diverse ...physiological and pathophysiological processes.
AQP knockout studies suggest that the regulatory role of AQPs, rather than their action as passive channels, is their critical function. Transport through all AQPs occurs by a common passive mechanism, but their regulation and cellular distribution varies significantly depending on cell and tissue type; the role of AQPs in cell volume regulation (CVR) is particularly notable. This review examines the regulatory role of AQPs in transcellular water flow, especially in CVR. We focus on key systems of the human body, encompassing processes as diverse as urine concentration in the kidney to clearance of brain oedema.
AQPs are crucial for the regulation of water homeostasis, providing selective pores for the rapid movement of water across diverse cell membranes and playing regulatory roles in CVR. Gating mechanisms have been proposed for human AQPs, but have only been reported for plant and microbial AQPs. Consequently, it is likely that the distribution and abundance of AQPs in a particular membrane is the determinant of membrane water permeability and a regulator of transcellular water flow.
Elucidating the mechanisms that regulate transcellular water flow will improve our understanding of the human body in health and disease. The central role of specific AQPs in regulating water homeostasis will provide routes to a range of novel therapies. This article is part of a Special Issue entitled Aquaporins.
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•Aquaporin (AQP) water channels are regulators of transcellular water flow.•AQPs are involved in mechanisms of cell volume regulation (CVR).•AQP translocation within the cell regulates AQP function.
Purpose:
To provide a comprehensive characterization of a novel respiratory surrogate that uses optical surface imaging (OSI) for accurate tidal volume (TV) measurement, dynamic airflow (TV′) ...calculation, and quantitative breathing pattern (BP) estimation during free breathing (FB), belly breathing (BB), chest breathing (CB), and breath hold (BH).
Methods:
Optical surface imaging, which captures all respiration-induced torso surface motion, was applied to measure respiratory TV, TV′, and BP in three common breathing patterns. Eleven healthy volunteers participated in breathing experiments with concurrent OSI-based and conventional spirometric measurements under an institutional review board approved protocol. This OSI-based technique measures dynamic TV from torso volume change (ΔV
torso = TV) in reference to full exhalation and airflow (TV′ = dTV/dt). Volume conservation, excluding exchanging air, was applied for OSI-based measurements under negligible pleural pressure variation in FB, BB, and CB. To demonstrate volume conservation, a constant TV was measured during BH while the chest and belly are moving (“pretended” respiration). To assess the accuracy of OSI-based spirometry, a conventional spirometer was used as the standard for both TV and TV′. Using OSI, BP was measured as BPOSI = ΔV
chest/ΔV
torso and BP can be visualized using BPSHI = SHIchest/(SHIchest + SHIbelly), where surface height index (SHI) is defined as the mean vertical distance within a region of interest on the torso surface. A software tool was developed for OSI image processing, volume calculation, and BP visualization, and another tool was implemented for data acquisition using a Bernoulli-type spirometer.
Results:
The accuracy of the OSI-based spirometry is −21 ± 33 cm3 or −3.5% ± 6.3% averaged from 11 volunteers with 76 ± 28 breathing cycles on average in FB. Breathing variations between two separate acquisitions with approximate 30-min intervals are substantial: −1% ± 34% (ranging from −64% to 40%) in TV, 4% ± 20% (ranging from −50% to 26%) in breathing period (T), and −1% ± 34% (ranging from −49% to 44%) in BP. The airflow accuracy and variation (between two exercises) are −1 ± 54 cm3/s and −5% ± 30%, respectively. The slope of linear regression between OSI–TV and spirometric TV is 0.93 (R
2 = 0.95) for FB, 0.96 (R
2 = 0.98) for BB, and 0.95 (R
2 = 0.95) for CB. The correlation between the two spirometric measurements is 0.98 ± 0.01. BP increases from BB, FB to CB, while TV increases from FB, BB, to CB. Under BH, 4% volume variation (range) on average was observed.
Conclusions:
The OSI-based technique provides an accurate measurement of tidal volume, airflow rate, and breathing pattern; all affect internal organ motion. This technique can be applied to various breathing patterns, including FB, BB, and CB. Substantial breathing irregularities and irreproducibility were observed and quantified with the OSI-based technique. These breathing parameters are useful to quantify breathing conditions, which could be used for effective tumor motion predictions.
Congenital diaphragmatic hernia (CDH) is a rare and life-threatening anomaly that needs surgical therapy after clinical stabilization of the neonate. Given an existing volume-outcome relationship for ...other high-risk, low volume procedures, we aimed at examining the relationship between hospital or surgeon volume and outcomes for surgery on CDH.
We conducted a systematic search in multiple databases in September 2019 and searched for additional literature. We assessed risk of bias of included studies using ROBINS-I and synthesized results in a structured narrative way using GRADE.
We included 5 cohort studies on hospital volume. Results for in-hospital mortality, one-year mortality and length of stay are inconclusive. The certainty of the evidence was very low for all outcomes, due to risk of bias, inconsistency and imprecision. We did not identify any study on surgeon volume.
Due to the very low certainty of the evidence it is uncertain whether higher hospital volume is associated with favorable outcomes for neonates undergoing surgery for CDH. There is no evidence on the relationship between surgeon volume and outcomes. Future studies should use more rigorous methodology and analyze additional outcomes to allow for more meaningful inferences.
III
PROSPERO (CRD42018090231).
Crop biomass estimation with high accuracy at low-cost is valuable for precision agriculture and high-throughput phenotyping. Recent technological advances in Unmanned Aerial Systems (UAS) ...significantly facilitate data acquisition at low-cost along with high spatial, spectral, and temporal resolution. The objective of this study was to explore the potential of UAS RGB imagery-derived spectral, structural, and volumetric information, as well as a proposed vegetation index weighted canopy volume model (CVMVI) for soybean Glycine max (L.) Merr. aboveground biomass (AGB) estimation. RGB images were collected from low-cost UAS throughout the growing season at a field site near Columbia, Missouri, USA. High-density point clouds were produced using the structure from motion (SfM) technique through a photogrammetric workflow based on UAS stereo images. Two-dimensional (2D) canopy structure metrics such as canopy height (CH) and canopy projected basal area (BA), as well as three-dimensional (3D) volumetric metrics such as canopy volume model (CVM) were derived from photogrammetric point clouds. A variety of vegetation indices (VIs) were also extracted from RGB orthomosaics. Then, CVMVI, which combines canopy spectral and volumetric information, was proposed. Commonly used regression models were established based on the UAS-derived information and field- measured AGB with a leave-one-out cross-validation. The results show that: (1) In general, canopy 2D structural metrics CH and BA yielded higher correlation with AGB than VIs. (2) Three-dimensional metrics, such as CVM, that encompass both horizontal and vertical properties of canopy provided better estimates for AGB compared to 2D structural metrics (R2 = 0.849; RRMSE = 18.7%; MPSE = 20.8%). (3) Optimized CVMVI, which incorporates both canopy spectral and 3D volumetric information outperformed the other indices and metrics, and was a better predictor for AGB estimation (R2 = 0.893; RRMSE = 16.3%; MPSE = 19.5%). In addition, CVMVI showed equal prediction power for different genotypes, which indicates its potential for high-throughput soybean biomass estimation. Moreover, a CVMVI based univariate regression model yielded AGB predicting capability comparable to multivariate complex regression models such as stepwise multilinear regression (SMR) and partial least squares regression (PLSR) that incorporate multiple canopy spectral indices and structural metrics. Overall, this study reveals the potential of canopy spectral, structural and volumetric information, and their combination (i.e., CVMVI) for estimations of soybean AGB. CVMVI was shown to be simple but effective in estimating AGB, and could be applied for high-throughput phenotyping and precision agro-ecological applications and management.
Time series momentum Moskowitz, Tobias J.; Ooi, Yao Hua; Pedersen, Lasse Heje
Journal of financial economics,
05/2012, Letnik:
104, Številka:
2
Journal Article
Recenzirano
Odprti dostop
We document significant “time series momentum” in equity index, currency, commodity, and bond futures for each of the 58 liquid instruments we consider. We find persistence in returns for one to 12 ...months that partially reverses over longer horizons, consistent with sentiment theories of initial under-reaction and delayed over-reaction. A diversified portfolio of time series momentum strategies across all asset classes delivers substantial abnormal returns with little exposure to standard asset pricing factors and performs best during extreme markets. Examining the trading activities of speculators and hedgers, we find that speculators profit from time series momentum at the expense of hedgers.
The f-cell ratio of 0.91 is a conversion factor between the hematocrit measured in peripheral blood and the hematocrit obtained by separate measurements of the red blood cell mass and plasma volume. ...The physiological background of the f-cell ratio is unclear.
Data were retrieved from 155 intravenous infusion experiments where 15–25 mL/kg of crystalloid fluid diluted the blood hemoglobin and plasma albumin concentrations. The hemodilution was converted to plasma dilution using the peripheral hematocrit, and the volume of distribution of exogenous albumin was calculated in 41 volunteers who received 20 % or 5 % albumin by intravenous infusion. Finally, the kinetics of plasma albumin was studied during 98 infusion experiments with 20 % albumin.
Plasma dilution based on hemoglobin and albumin showed a median difference of −0.001 and a mean difference of 0.000 (N = 2184), which demonstrates that these biomarkers indicate the same expandable vascular space. In contrast, exogenous albumin occupied a volume that was 10 % larger than the plasma volume indicated by the anthropometric equations of Nadler et al. and Retzlaff et al. The kinetic analysis identified a secondary compartment that was 450 mL in size and rapidly exchanged albumin with the circulating plasma.
The results suggest that the f-cell ratio is due to rapid exchange of albumin between the plasma and a non-expandable compartment located outside the circulating blood (possibly the liver sinusoids). This means that the hematocrit measured in peripheral blood correctly represents the ratio between the red cell volume and the circulating plasma volume.
•The f-cell ratio of 0.91 converts the peripheral hematocrit to the whole-body hematocrit.•This ratio is applied when measuring the blood volume, but its origin is unclear.•Plasma dilution data (N2184) ruled out maldistribution in the circulation as the cause.•Infused albumin distributed over a 10 % larger space than the expected plasma volume.•Infused albumin distributed within the plasma but also to a fast-exchange smaller space.
Background: Central venous pressure (CVP) is used almost universally to guide fluid therapy in hospitalized patients. Both historical
and recent data suggest that this approach may be flawed.
...Objective: A systematic review of the literature to determine the following: (1) the relationship between CVP and blood volume, (2)
the ability of CVP to predict fluid responsiveness, and (3) the ability of the change in CVP (ÎCVP) to predict fluid responsiveness.
Data sources: MEDLINE, Embase, Cochrane Register of Controlled Trials, and citation review of relevant primary and review articles.
Study selection: Reported clinical trials that evaluated either the relationship between CVP and blood volume or reported the associated between
CVP/ÎCVP and the change in stroke volume/cardiac index following a fluid challenge. From 213 articles screened, 24 studies
met our inclusion criteria and were included for data extraction. The studies included human adult subjects, healthy control
subjects, and ICU and operating room patients.
Data extraction: Data were abstracted on study design, study size, study setting, patient population, correlation coefficient between CVP
and blood volume, correlation coefficient (or receive operator characteristic ROC) between CVP/ÎCVP and change in stroke
index/cardiac index, percentage of patients who responded to a fluid challenge, and baseline CVP of the fluid responders and
nonresponders. Metaanalytic techniques were used to pool data.
Data synthesis: The 24 studies included 803 patients; 5 studies compared CVP with measured circulating blood volume, while 19 studies determined
the relationship between CVP/ÎCVP and change in cardiac performance following a fluid challenge. The pooled correlation coefficient
between CVP and measured blood volume was 0.16 (95% confidence interval CI, 0.03 to 0.28). Overall, 56 ± 16% of the patients
included in this review responded to a fluid challenge. The pooled correlation coefficient between baseline CVP and change
in stroke index/cardiac index was 0.18 (95% CI, 0.08 to 0.28). The pooled area under the ROC curve was 0.56 (95% CI, 0.51
to 0.61). The pooled correlation between ÎCVP and change in stroke index/cardiac index was 0.11 (95% CI, 0.015 to 0.21). Baseline
CVP was 8.7 ± 2.32 mm Hg mean ± SD in the responders as compared to 9.7 ± 2.2 mm Hg in nonresponders (not significant).
Conclusions: This systematic review demonstrated a very poor relationship between CVP and blood volume as well as the inability of CVP/ÎCVP
to predict the hemodynamic response to a fluid challenge. CVP should not be used to make clinical decisions regarding fluid
management.
anesthesia
blood volume
central venous pressure
fluid responsiveness
fluid therapy
hemodynamic monitoring
ICU
preload
stroke volume
Abstract Context Due to the complexity and challenging nature of radical prostatectomy (RP), it is likely that both short- and long-term outcomes strongly depend on the cumulative number of cases ...performed by the surgeon as well as by the hospital. Objective To review systematically the association between hospital and surgeon volume and perioperative, oncologic, and functional outcomes after RP. Evidence acquisition A systematic review of the literature was performed, searching PubMed, Embase, and Scopus databases for original and review articles between January 1, 1995, and December 31, 2011. Inclusion and exclusion criteria comprised RP, hospital and/or surgeon volume reported as a predictor variable, a measurable end point, and a description of multiple hospitals or surgeons. Evidence synthesis Overall 45 publications fulfilled the inclusion criteria, where most data originated from retrospective institutional or population-based cohorts. Studies generally focused on hospital or surgeon volume separately. Although most of these analyses corroborated the impact of increasing volume with better outcomes, some failed to find any significant effect. Studies also differed with respect to the proposed volume cut-off for improved outcomes, as well as the statistical means of evaluating the volume–outcome relationship. Five studies simultaneously compared hospital and surgeon volume, where results suggest that the importance of either hospital or surgeon volume largely depends on the end point of interest. Conclusions Undeniable evidence suggests that increasing volume improves outcomes. Although it would seem reasonable to refer RP patients to high-volume centers, such regionalization may not be entirely practical. As such, the implications of such a shift in practice have yet to be fully determined and warrant further exploration.
Current measures of tidal volume/forced vital capacity (VT/FVC) and VT/inspiratory capacity (VT/IC) at peak exercise cannot differentiate restrictive from obstructive ventilation patterns. This study ...aimed to investigate the utility of VT/total lung capacity (VT/TLC) as a marker for dynamic lung hyperinflation (DH) in patients with chronic obstructive pulmonary disease (COPD) and interstitial lung disease (ILD).
267 subjects were screened: 23 ILD, 126 COPD, and 33 healthy individuals were enrolled. Lung function tests and cardiopulmonary exercise tests with repeated IC maneuver were conducted and compared at three exercise efforts: unloaded, middle of exercise, and peak exercise.
During exercise, ILD patients demonstrated normal end-expiratory lung volume/TLC (EELV/TLC) ratios, but elevated end-inspiratory lung volume/TLC (EILV/TLC) ratios, except for peak exercise. COPD patients exhibited elevated ratios for both EELV/TLC and EILV/TLC during exercise with a larger EELV/TLC ratio compared to ILD patients at peak exercise (p < 0.05). The VT/TLC ratio distinguished ILD, COPD, and healthy controls at peak exercise (p < 0.05). A VT/TLC ratio of ≤ 0.22 or ≥ 0.30 indicated airflow obstruction with hyperinflation or normal lung expansion, respectively (AUC: 0.74 or 0.88). Furthermore, VT/TLC outperformed VT/FVC and VT/IC in differentiating lung expansion between ILD and COPD during exercise (all p < 0.05).
Exercise-induced DH was absent in ILD patients but observed in COPD patients. Excessive lung expansion occurred in all patients during exercise, except for limited expansion in ILD at peak exercise probably due to specific lung properties. VT/TLC can distinguish between restrictive, obstructive, and normal ventilatory patterns.
•VT/FVC and VT/IC at peak exercise cannot effectively differentiate between restrictive and obstructive ventilation defects.•VT/TLC is more reliable than VT/FVC and VT/IC in distinguishing lung expansion and EELV/TLC ratios.•A VT/TLC ratio of ≤ 0.22 or ≥ 0.30 indicates airflow obstruction with hyperinflation or normal lung expansion, respectively (AUC: 0.74 or 0.88). VT/TLC ≤0.22 or ≥0.30 indicates airflow obstruction with DH or normal lung expansion.•DH was not observed in ILD or healthy subjects but noted in COPD subjects.•All patients showed excessive lung expansion during exercise, except for limited expansion in ILD at peak exercise.•The above mentioned is probably due to the specific lung properties.
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