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At least 37 records · Page 2Linked to original sources
PubMed1986

Theoretical effects of a distribution of capillary dimensions on glomerular ultrafiltration.

Simple network models were developed to examine the effects of a distribution of capillary dimensions on the filtration of fluid in the renal glomerulus. Two idealized networks were considered, one with parallel capillaries of varying length, and the other with parallel capillaries of varying radius. It was found that for a fixed value of the ultrafiltration coefficient (Kf), the product of hydraulic permeability and total capillary surface area per glomerulus, any assumed heterogeneity of capillary dimensions lowered the single nephron glomerular filtration rate (SNGFR) below that predicted for uniform capillaries in parallel. This implies that, to the extent that the capillaries are not identical, a given filtering surface area is used less efficiently. A consequence of this is that for a given set of measured glomerular pressures and flows, a model which assumes identical capillaries will underestimate the value of Kf for the actual (heterogeneous) glomerular network. The effects of a distribution in capillary lengths were found to be more pronounced, in general, than those of a distribution in radii. Considering only heterogeneity in capillary length, and characterizing the effective length distribution using available data in the literature, it was found that calculating Kf using the customary assumption of identical capillaries will underestimate the true value by approximately 30%, under typical micropuncture conditions in the rat. The predicted dependence of SNGFR on afferent plasma flow rate, transmural hydraulic pressure difference, and afferent protein concentration was quite similar, however, whether capillary dimensions were assumed to be uniform or heterogeneous.

Animals↗
PubMed1998

Influence of pulse oximeter settings on the frequency of alarms and detection of hypoxemia: Theoretical effects of artifact rejection, alarm delay, averaging, median filtering or a lower setting of the alarm limit.

OBJECTIVE: The potential benefit of a reduced frequency of false pulse oximeter low oxyhemoglobin saturation (SpO2) alarms is that the attention of personnel is only directed to patients who experience hypoxemia. The present study was undertaken to better understand the effects of different settings of the pulse oximeter on false (artifact) and true (hypoxemia) alarms. METHODS: Using the original SpO2 data of 200 postoperative patients, we calculated off-line the effects of five methods (artifact rejection, alarm delay (2-44 s, 2 s increments), averaging (10-90 s), median filtering (10-90 s) and decreasing the alarm limit from 90% to 85%) on the number of (true- and false) alarms. RESULTS: 830 episodes of hypoxemia (SpO2 < or = 90%) and 73 episodes of severe hypoxemia (SpO2 < or = 85%) occurred. With a SpO2 alarm limit of 90%, the alarm was triggered 1535 times (830 true, 705 false). Artifact rejection reduced alarms by almost 50%. An alarm delay of 6 s or an averaging or median filtering epoch of 10 s resulted in an alarm reduction of almost 50%. No differences were found in the reduction of alarms between averaging and median filtering. Changing the alarm limit to 85% reduced the number of alarms by 82%. A similar reduction of alarms was obtained with either an alarm delay of 18 s or an averaging or median filtering epoch of 42 s. However, an alarm limit of 85% reduced the number of false alarms less than the other three algorithms (p < 0.01). CONCLUSIONS: The data from the present study suggest that in order to effectively suppress false alarms caused by pulse oximeter artifacts, it may be preferable to use a longer filtering epoch of approximately 40 s, rather than to decrease the lower alarm limit.

Artifacts↗
PubMed1998

Anatomical variation of cerebral venous drainage: the theoretical effect on jugular bulb blood samples.

Recent studies have demonstrated significant variation in bilateral jugular venous oxygen saturation measurements which may be of clinical significance. We have therefore measured variations in normal dural sinus venous drainage to assess the possible effects of normal anatomical variations on measured jugular venous oxygen saturation. Normal volunteers (n = 25) were imaged using magnetic resonance venography to demonstrate variations in venous anatomy. Flow was measured in the superior sagittal sinus and bilaterally in the transverse sinus, sigmoid sinus proximal to the jugular bulb and proximal jugular vein using phase difference magnetic resonance imaging. Examination of magnetic resonance venogram images showed considerable variability in the symmetry of transverse sinus flow. Complete absence of one transverse sinus was seen in four cases and significant asymmetry in the size of the transverse sinuses was present in 13. Quantitative flow studies demonstrated that the ratio of superior sagittal sinus to combined jugular bulb flow showed remarkably little variation (0.46 +/- 0.06). Measurements of transverse sinus flow showed significant asymmetry (< 40% of superior sagittal sinus flow in one transverse sinus) in 21 of 25 volunteers. The effect of the observed asymmetry on jugular venous oxygen saturation was modelled based on the assumption of either a supratentorial or infratentorial lesion. This model predicted significant asymmetry in jugular venous oxygen saturation measurements (> 10%) in 65% of cases with a supratentorial lesion which is in close agreement with clinical observations. This study suggests that normal variations in venous drainage may account for observed asymmetry in jugular venous oxygen saturation measurements.

Adult↗
PubMed1984

Theoretical effects of transmembrane electroneutral exchange on membrane potential.

Transmembrane electroneutral transport mechanisms [e.g., Na/H exchange, Cl/HCO3 exchange, (K + Cl) cotransport] have recently been identified in a wide variety of cell types. If these exchanges sum to give a net electroneutral Na/K exchange, they may hyperpolarize the membrane potential beyond the value calculated from the Mullins-Noda equation, provided the cell maintains steady state intracellular ionic concentrations. In extreme circumstances, the membrane potential could hyperpolarize beyond the potassium reversal potential. This effect is mediated by the electrogenic Na/K pump. If either Na or K exchanges electroneutrally against a third ion (e.g., Na/Ca exchange), then the exchange may depolarize the membrane potential.

Bicarbonates↗
PubMed1993

Theoretical effect of refractive error and accommodation on longitudinal chromatic aberration of the human eye.

Simple formulas based on reduced eyes have been developed to predict the variation in longitudinal chromatic aberration with variation in ametropia or accommodation. Two formulas were developed, one for axial ametropia and one for refractive ametropia. The latter also served as a model for accommodation. The results using the formulas are in close agreement with results obtained using raytracing through more sophisticated models. Combining the results of different methods gives the following predictions of change in chromatic difference of focus, between wavelengths of 400 and 700 nm, with change in each diopter of refractive error or accommodation: axial ametropia 0.012 to 0.017 D (0.6 to 0.9%), refractive ametropia 0.05 D (2.2 to 2.4%), and accommodation 0.04 to 0.05 D (2.1 to 2.6%). The chromatic aberration effects of correcting lenses with low dispersion are intermediate in effect and opposite in sign to the effects of corresponding degrees of axial ametropia and refractive ametropia.

Accommodation, Ocular↗
PubMed1986

Early oral contraceptive use and breast cancer: theoretical effects of latency.

Many cancers and other chronic diseases are associated with a long delay between exposure to a putative risk factor and subsequent diagnosis. This presents well recognised problems in the elucidation of suspected risk factors by epidemiological methods. In this paper we discuss the interpretation in epidemiological studies of the effect of a possible risk factor when population exposure is recent and rapidly changing. An important contemporary example concerns the study of early oral contraceptive (OC) use in relation to the subsequent risk of breast cancer. Computer simulations reported here indicate that plausible delays in the manifestation of any effect on breast cancer incidence make it difficult to exclude early OC use as a risk factor for breast cancer, even when large well conducted epidemiological studies show no apparent increased risk. Methods for detecting a 'latent' effect are discussed.

Breast Neoplasms↗
PubMed2003

Theoretical effects of UTB urea transporters in the renal medullary microcirculation.

A mathematical model of transport in the renal medullary microcirculation was used to investigate the role of the UTB urea transporter expressed in descending vasa recta (DVR) endothelia and red blood cell (RBC) membranes. Our simulations suggest that UTB raises RBC and plasma and interstitial urea concentrations by facilitating radial diffusion of the solute and therefore serves to increase the contribution of urea to the corticomedullary osmolality gradient, assuming no secondary effects on tubular transport. However, by lowering transmural urea concentration gradients, UTB reduces water efflux from DVR through aquaporin-1 (AQP1) water channels, thereby decreasing plasma sodium concentration. The net result of these competing effects on the osmolality gradient depends on the fraction of filtered urea that is reabsorbed by vasa recta. We also found that the contribution of UTB to water transport across DVR and RBCs is negligible, even in the absence of AQP1. Our model predicts that UTB plays a significant role, however, in reducing the shrinking and swelling of RBCs as blood flows along the medulla.

Animals↗
PubMed1989

Theoretical effects of network structure on glomerular filtration of macromolecules.

A parallel network model was developed to examine the effects of a distribution of capillary lengths on the filtration of macromolecules by the glomerulus. When networks having different distributions of capillary lengths (but similar values of single nephron glomerular filtration rate) were compared, the filtrate-to-plasma concentration ratio (theta s) for neutral macromolecules was found to increase as the vessel lengths became less uniform. Because anatomical studies have demonstrated that the glomerulus is in fact a heterogeneous network, this implies that the conventional modeling assumption of identical capillaries in parallel leads to an overestimation of effective pore sizes. However, simulations employing various pore-size distributions demonstrated that the expected errors in estimating membrane-pore parameters are generally negligible. Furthermore, the dependence of theta s on hemodynamic inputs such as glomerular plasma flow rate and transmembrane hydraulic pressure difference was insensitive to the assumed distribution of capillary lengths. We conclude that models based on dimensionally uniform capillary networks remain valid for interpreting clearance data for macromolecules.

Capillaries↗
PubMedDate not supplied

Theoretical effects of surgery on length tension relationships in extraocular muscles.

The main effect of recession or resection of a rectus muscle is thought to be due to an alteration of its length-tension curve (Starling's Law). Because the subsequent straightening of the eye produces a movement of the new insertions of the rectus muscles either anteriorly or posteriorly in the orbit, the actual amount of tightening or shortening of the muscle is not equal to the amount of recession or resection. How much this phenomenon alters the true amount of tightening or shortening of a muscle has important implications in the determination of safe amounts of surgery, which may differ if one is operating on the fixing or non-fixing eye. In addition, the size of the globe should have an important role in determining the response per millimeter of surgery.

Biomechanical Phenomena↗