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PubMed2003

Reflectivity of cholesteric liquid crystals with spatially varying pitch.

Solids with spatially varying photonic structure offer gaps to light of a wider range of frequencies than do simple photonic systems. We solve numerically the field distribution in a cholesteric with a linearly varying inverse pitch (helical wavevector) using equations we derive for the general case. The simple idea that the position where the Bragg condition is locally satisfied is where reflection takes place is only true in part. Here, reflection is due to a region where the waves are forced to become evanescent, and the rate of variation of structure determines over which distance the waves decay and therefore how complete reflection is. The approximate local Bragg-de Vries schemes are shown to break down in detail at the edges of the gap, and an analytical estimate is given for the transmission coefficient.

Cholesterol↗

PubMed1992

Limitations of laser-Doppler velocimetry and reflectance spectrophotometry in estimating gastric mucosal blood flow.

This study investigated the accuracy of laser-Doppler flowmetry (LDV) and reflectance spectrophotometry (RS) measurements as an index of blood flow in the gastric mucosa of the rat, in experimental conditions such as pharmacologically induced vasoconstriction, hypoxia, hyperoxia, and acute normovolemic anemia. Hydrogen gas clearance was used as a reference method. After vasopressin infusion, LDV signal and indexes of hemoglobin (IHb) and oxygen (ISO2) content in the gastric mucosa estimated by RS significantly decreased in parallel with the reduction of gastric mucosal blood flow (GMBF). Neither hypoxia (5% O2 administration) nor hyperoxia (100% O2) affected GMBF or LDV signal. However, both IHb and ISO2 significantly decreased or increased after hypoxia or hyperoxia, respectively. Acute normovolemic anemia induced a significant increase in GMBF, while LDV signal and ISO2 remained unchanged. IHb significantly decreased in linear relationship with the decrements in the hematocrit. It is concluded that 1) in pharmacologically induced GMBF changes, LDV and RS correlate with GMBF; 2) when changes in hemoglobin saturation are induced, LDV but not RS reflects GMBF; and 3) in acute normovolemic anemia, neither LDV nor RS reflects changes in GMBF.

Anemia↗

PubMed1984

Directional disparity of pulse reflection in the dog.

Local reflection coefficients were experimentally determined in the dog for the first time at the aortoiliac junction using characteristic impedances derived from phase velocity, fluid density, and the vessels' cross-sectional areas. Reflection coefficients were determined for both the normal trifurcation and with the segment of aorta between external and internal iliacs occluded. For the normal case the coefficients were as follows: antegrade, 0.7; retrograde, -0.74. For the occlusion case the coefficients were as follows: antegrade, 0.33; retrograde, -0.67. These results provide experimental support for the concept that the vascular system, at least in this region, favors antegrade and suppresses retrograde pulse transmission. In addition, global reflection coefficients were determined in the femoral artery using a three-point pressure technique. Coefficient magnitudes varied from low (1.6 Hz) to high (9.6 Hz) frequencies, i.e., control, 0.42-0.22; vasoconstriction, 0.65-0.33; vasodilation, less than 0.1 for all frequencies. Discrepancies between results appearing in the literature are evaluated and shown to be associated with the method utilized as well as with system nonlinearities enhanced by tying branches.

Animals↗

PubMed1989

Pulse reflection sites and effective length of the arterial system.

The concept of effective length (L) of the arterial system implies that it may be represented by a single viscoelastic tube terminated by an impedance, creating a single reflection site. Although the concept is straightforward, investigators for years have been unable to agree on the value of L. Proposed values range from a few millimeters to a few meters, confounding the identification of arterial reflection sites. This report shows analytically and illustrates with experimental data that the determination of the effective length leaves room for an infinite number of exact solutions for L and the corresponding terminal impedance if the input impedance of the tube is to match the measured input impedance of an arterial system. None of the possible values of L needs to bear any relationship to actual reflection sites.

Animals↗

PubMed1989

Influence of mean pressure on aortic impedance and reflections in the systemic arterial system.

The present investigation sought to determine the extent to which primary changes in mean arterial pressure (MAP) might influence the calculated aortic impedance. In seven open-chest, anesthetized, autonomically blocked dogs, we measured aortic impedance using white-noise analysis at various levels of MAP achieved by adjusting the height of a left atrial reservoir rather than by pharmacological intervention. Impedance spectra so obtained were parameterized according to best-fit wind-kessel models (Ra, Rc, Ca) and three parameters to characterize wave reflections: the frequency at which the phase of the impedance approached or crossed through 0 degree (phi 0), the frequency of the first minimum of the impedance modulus (fmin), and the amplitude of the first oscillation of the impedance modulus (a 1). The dependency of each of these parameters on MAP was investigated. In addition, we calculated the reflection coefficient spectrum (RCS). Results show that windkessel parameters were not significantly influenced by MAP and that the reflection parameters and the RCS depended on increased levels of MAP in a manner consistent with an increase in pulsewave velocity.

Animals↗

PubMed1991

LV-arterial coupling: interactive model to predict effect of wave reflections on LV energetics.

The interaction between the left ventricle (LV) and the arterial system was simulated using sequential convolution of the flow output generated by a time-varying elastance model of the LV with an impulse response calculated from a 128-element model of the arterial system. The model illustrates the effect of independent changes of components of the arterial load on LV performance and energetics. This report studies the response of the model LV to an increase in arterial resistance, a decrease in arterial compliance, and an increase in discrete vascular reflections. Although arterial resistance exerts the greatest effect on ventricular stroke output, a reduction of arterial compliance or an increase in early reflections resulted in less optimal coupling of the heart to the arteries and less efficient energy utilization by the LV. In addition, the earlier the reflections return, the greater the disturbance of ventricular arterial coupling.

Aging↗

PubMed1997

Protein osmotic pressure gradients and microvascular reflection coefficients.

Microvascular membranes are heteroporous, so the mean osmotic reflection coefficient for a microvascular membrane (sigma d) is a function of the reflection coefficient for each pore. Investigators have derived equations for sigma d based on the assumption that the protein osmotic pressure gradient across the membrane (delta II) does not vary from pore to pore. However, for most microvascular membranes, delta II probably does vary from pore to pore. In this study, we derived a new equation for sigma d. According to our equation, pore-to-pore differences in delta II increase the effect of small pores and decrease the effect of large pores on the overall membrane osmotic reflection coefficient. Thus sigma d for a heteroporous membrane may be much higher than previously derived equations indicate. Furthermore, pore-to-pore delta II differences increase the effect of plasma protein osmotic pressure to oppose microvascular fluid filtration.

Animals↗

PubMed1975

Reflection coefficients of dog lung endothelium to small hydrophilic solutes.

Step additions of small hydrophilic solutes to the perfusate of isolated dog lungs produce weight changes (Vargas-Johnson). The product: (observed reflection coefficient) times (observed filtration coefficient) for the lung endothelial barrier is obtained from the initial weight slope per unit osmolality change. The observed filtration coefficient determined from the sustained constant weight slope per increment of pulmonary outflow pressure was 0.038 plus or minus 0.011 cm/s (assumed exchange area 500 cm2/g lung). Correction to average transcapillary pressure (32% upward) and for osmotic buffering by resident solute (25% upward) gave 0.063 plus or minus 0.018 cm/s. The observed reflection coefficients were (mean plus or minus 2 SE): sodium chloride, 0.30 plus or minus 0.11; urea, 0.30 plus or minus 0.12; glucose, 0.48 plus or minus 0.26; sucrose, 0.39 plus or minus 0.17; raffinose, 0.35 plus or minus 0.16; hypotonic response: interstitial solute, 0.26 plus or minus 0.10. The observed reflection coefficient of sodium chloride, corrected for the preceding filtration coefficient effects (65% downward) and for combined flow limitation and osmotic buffering effects (50% upward) was 0.27. Transcapillary gradients of small hydrophilic solutes produce substantial osmotic effects for short times (seconds). Passive exchanges and net fluxes across the endothelium of the lung capillaries have parameters similar to those of the endothelium of skeletal muscle.

Animals↗

PubMed1989

Reflection coefficients and water permeability in rat proximal tubule.

Simultaneous microperfusion of proximal tubules and peritubular capillaries in kidneys of rats anesthetized with Inactin was used to measure reflection coefficients. All perfusates contained cyanide to inhibit active transport; the tubular perfusate was isotonic and the peritubular capillaries were perfused with solutions made hypertonic with NaCl, NaHCO3, L-glucose, or sodium ferrocyanide. Measurements of recollected fluid enabled a precise mean gradient and ionic fluxes to be calculated; net water flux was measured with inulin. Imposed gradients always partly dissipated. Reflection coefficients were 0.59 +/- 0.01 for NaCl, 0.87 +/- 0.04 for NaHCO3-, and 0.96 +/- 0.07 for ferrocyanide, assuming that L-glucose was 1. Water permeability of the proximal tubule was 1,030 microns/s. Ionic permeability of Cl- (21.6 +/- 1.3 X 10(-5) cm/s) was greater than that for Na+ (13.3 +/- 2.7 X 10(-5) cm/s); permeability for L-glucose was 5.4 +/- 1.3 X 10(-5), and for ferrocyanide ions 2.7 +/- 0.9 X 10(-5) cm/s. It is concluded that in rat proximal tubule both NaCl and NaHCO3 have reflection coefficients less than 1.0 and solute asymmetry across the epithelium is a significant driving force for fluid reabsorption. Furthermore the data suggest that there is a significant contribution of solvent drag to solute movement.

Animals↗

PubMed1996

Chest wall and lung volume estimation by optical reflectance motion analysis.

Estimation of chest wall motion by surface measurements only allows one-dimensional measurements of the chest wall. We have assessed on optical reflectance system (OR), which tracks reflective markers in three dimensions (3-D) for respiratory use. We used 86 (6-mm-diameter) hemispherical reflective markers arranged circumferentially on the chest wall in seven rows between the sternal notch and the anterior superior iliac crest in two normal standing subjects. We calculated the volume of the entire chest wall and compared inspired and expired volumes with volumes obtained by spirometry. Marker positions were recorded by four TV cameras; two were 4 m in front of and two were 4 m behind the subject. The TV signals were sampled at 100 Hz and combined with grid calibration parameters on a personal computer to obtain the 3-D coordinates of the markers. Chest wall surfaces were reconstructed by triangulation through the point data, and chest wall volume was calculated. During tidal breathing and vital capacity maneuvers and during CO2-stimulated hyperpnea, there was a very close correlation of the lung volumes (VL) estimated by spirometry [VL(SP)] and OR [VL(OR)]. Regression equations of VL(OR) (y) vs. VL(SP) (x, BTPS in liters) for the two subjects were given by y = 1.01x-0.01 (r = 0.996) and y = 0.96x + 0.03 (r = 0.997), and by y = 1.04x + 0.25 (r = 0.97) and y = 0.98x + 0.14 (r = 0.95) for the two maneuvers, respectively. We conclude spirometric volumes can be estimated very accurately and directly from chest wall surface markers, and we speculate that OR may be usefully applied to calculations of chest wall shape, regional volumes, and motion analysis.

Humans↗

PubMed2006

Preparatory activity in premotor and motor cortex reflects the speed of the upcoming reach.

Neurons in premotor and motor cortex show preparatory activity during an instructed-delay task. It has been suggested that such activity primarily reflects visuospatial aspects of the movement, such as target location or reach direction and extent. We asked whether a more dynamic feature, movement speed, is also reflected. Two monkeys were trained to reach at different speeds ("slow" or "fast," peak speed being approximately 50-100% higher for the latter) depending on target color. Targets were presented in seven directions and at two distances. Of 95 neurons with tuned delay-period activity, 95, 78, and 94% showed a significant influence of direction, distance, and instructed speed, respectively. Average peak modulations with respect to direction, distance and speed were 18, 10, and 11 spikes/s. Although robust, modulations of firing rate with target direction were not necessarily invariant: for 45% of neurons, the preferred direction depended significantly on target distance and/or instructed speed. We collected an additional dataset, examining in more detail the effect of target distance (5 distances from 3 to 12 cm in 2 directions). Of 41 neurons with tuned delay-period activity, 85, 83, and 98% showed a significant impact of direction, distance, and instructed speed. Statistical interactions between the effects of distance and instructed speed were common, but it was nevertheless clear that distance "tuning" was not in general a simple consequence of speed tuning. We conclude that delay-period preparatory activity robustly reflects a nonspatial aspect of the upcoming reach. However, it is unclear whether the recorded neural responses conform to any simple reference frame, intrinsic or extrinsic.

Algorithms↗

PubMed2006

Increases in alpha oscillatory power reflect an active retinotopic mechanism for distracter suppression during sustained visuospatial attention.

Human electrophysiological (EEG) studies have demonstrated the involvement of alpha band (8- to 14-Hz) oscillations in the anticipatory biasing of attention. In the context of visual spatial attention within bilateral stimulus arrays, alpha has exhibited greater amplitude over parietooccipital cortex contralateral to the hemifield required to be ignored, relative to that measured when the same hemifield is to be attended. Whether this differential effect arises solely from alpha desynchronization (decreases) over the "attending" hemisphere, from synchronization (increases) over the "ignoring" hemisphere, or both, has not been fully resolved. This is because of the confounding effect of externally evoked desynchronization that occurs involuntarily in response to visual cues. Here, bilateral flickering stimuli were presented simultaneously and continuously over entire trial blocks, such that externally evoked alpha desynchronization is equated in precue baseline and postcue intervals. Equivalent random letter sequences were superimposed on the left and right flicker stimuli. Subjects were required to count the presentations of the target letter "X" at the cued hemifield over an 8-s period and ignore the sequence in the opposite hemifield. The data showed significant increases in alpha power over the ignoring hemisphere relative to the precue baseline, observable for both cue directions. A strong attentional bias necessitated by the subjective difficulty in gating the distracting letter sequence is reflected in a large effect size of 2.1 (eta2 = 0.82), measured from the attention x hemisphere interaction. This strongly suggests that alpha synchronization reflects an active attentional suppression mechanism, rather than a passive one reflecting "idling" circuits.

Adult↗

PubMedDate not supplied

Does the hemoglobin concentration in fetal blood interfere with the accuracy of fetal reflection pulse oximetry?

OBJECTIVE: Our goal was to investigate the influence of the fetal hemoglobin (Hb) concentration on the accuracy of reflection pulse oximetry. METHODS: 179 fetuses were monitored by a reflection pulse oximetry system (RPOX MK2). Pulse oximetry measurements (SO2POX) were compared with the oxygen saturation and pH of umbilical artery and vein (UA-, UV-SaO2) immediately after delivery. Correlation and linear regression analyses were performed. Because it is unknown whether low or high Hb concentrations might interfere with the SO2POX measurements, the influence of the Hb level on the correlations was evaluated by weighting the linear regression in both ways; by Hb concentrations as weighting factor and as an inverse weighting factor (1/Hb). RESULTS: There was a significant correlation between SO2POX and UV-SaO2 (for the last 10 min of delivery, r = 0.45, p < 0.0005) but not with pH or UA-SaO2. Linear regression analysis between SO2POX and UV-SaO2 showed a multiple R of 0.45. Hb concentration in fetal blood ranged from 9.2 to 20.5 mg/dl. The weighted regression indicated a multiple RHb of 0.49, which did not differ by the inverse weighting (multiple Rinverse Hb: 0.49). CONCLUSIONS: Pulse oximetry measurements seem to reflect oxygen saturation in fetal blood, however further improvement is necessary. The Hb concentration in fetal blood does not interfere with the accuracy of fetal pulse oximetry and need not be taken into consideration in calibration curves.

Female↗

PubMedDate not supplied

Fast wavelength scanning reflectance spectrophotometer for noninvasive determination of hemoglobin oxygenation in human skin.

Oxygen saturation of hemoglobin (HbSO2) in skin vessels may be determined with photometric methods. However, the optical complexity of the skin makes quantitative measurements difficult. A possible approach is the analysis of reflectance spectra using the two-flux theory of Kubelka and Munk. The final equation of this theory which describes the transformation between absorbed and reflected light has been approximated by a hyperbola. Based on this approximation we evaluated skin spectra obtained from the forearm of 23 healthy subjects with a fast scanning reflection photometer (Oxyscan) applying visible light (535-620 nm). The hyperbola was used in a multicomponent analysis in which the measured spectrum is recalculated using reference spectra of oxygenated and deoxygenated hemoglobin (gaussian least-square method). A crucial requirement for the evaluation is the subtraction of the individual skin spectrum, obtained by clearing a spot of skin of hemoglobin exerting external pressure. At rest HbSO2 was in the range between 42 and 89% (mean +/- SD: 72.9 +/- 12.2%). Pharmacological and thermal generation of hyperemia combined with respiration of pure oxygen raised the values to 86-100% (97.9 +/- 4.6%). This was in good agreement with capillary ex vivo analysis yielding 96-100% (98.7 +/- 0.4%). Under arterial occlusion HbSO2 fell below 30% (14.5 +/- 7.8%). Our method allows rapid determinations of absolute HbSO2 values in the skin. The evaluation error is estimated to be between 5% for oxygenated and 10% for deoxygenated values.

Arterial Occlusive Diseases↗

PubMed1997

Clearance ratios of amylase isoenzymes and IgG subclasses: do they reflect glomerular charge selectivity?

The clearance ratios of endogenous plasma proteins with the same size but a different charge, such as the amylase isoenzymes and the immunoglobulin (Ig) G subclasses, have been used to assess glomerular charge selectivity in man. These proteins are, however, subject to tubular reabsorption. In this study we measured the IgG subclass/IgG clearance ratios for IgG1 (pI 8.0-9.5), IgG2 (pI 7.0-7.5) and IgG4 (pI < 6) in 6 healthy volunteers. Our results suggested a selective influence of tubular reabsorption: the IgG1/IgG clearance ratio was 0.68 +/- 0.14 (mean +/- SD) and lower than IgG2/IgG (2.02 +/- 1.06, p < or = 0.01). IgG4/IgG was 0.89 +/- 0.39. In addition, we studied the clearance ratios of pancreatic (PA, pI 7.0) and salivary amylase (SA, pI 5.9-6.4) and of IgG1 and IgG2 in 8 patients with minimal change nephrotic syndrome (MCNS), 11 patients recovering from acute tubular necrosis (ATN) and 9 healthy volunteers (controls). In MCNS glomerular charge selectivity is lost, while in recovering ATN tubular function is severely disturbed. The PA/SA clearance ratio was 3.25 +/- 0.89 in controls, reflecting intact glomerular charge selectivity. In MCNS patients the PA/SA clearance ratio had decreased to 1.21 +/- 0.23 (p < or = 0.001). In ATN patients the PA/SA clearance ratio was reduced as well: 1.55 +/- 0.41 (p < or = 0.001), although the aselective nature of the proteinuria and the modest albuminuria indicated intact glomerular charge selectivity. The IgG1/ IgG2 clearance ratio was 0.54 +/- 0.15 in controls, again suggesting preferential tubular reabsorption of IgG1. In MCNS patients the IgG1/IgG2 clearance ratio was 0.16 +/- 0.10 (p < or = 0.001); this probably reflects the relatively increased glomerular sieving of IgG2 when glomerular charge selectivity is lost. In ATN patients the IgG1/IgG2 clearance ratio was 1.07 +/- 0.47 (p < or = 0.001), which suggests a partial loss of preferential reabsorption of IgG1. It was concluded that the PA/SA clearance ratio is influenced by loss of tubular function and therefore does not reflect glomerular charge selectivity specifically. The IgG1/IgG2 ratio cannot be used to assess glomerular charge selectivity either because of the interference of selective tubular reabsorption of the subclasses. These findings put the assessment of glomerular charge using endogenous proteins in a new light and bring forward the necessity to interpret these ratios with the utmost cautiousness.

Adult↗

PubMed1984

Reflected reentry in nonhomogeneous ventricular muscle as a mechanism of cardiac arrhythmias.

Arrhythmogenesis in ventricular muscle exhibiting nonhomogeneous excitability was studied in isolated tissues from feline and canine hearts. Longitudinal bundles were mounted in a three-chambered bath and simultaneous transmembrane recordings were obtained from fibers in each chamber. Nonhomogeneous excitability was established by depressing only the central segment (1 to 2 mm wide) with high-K+ Tyrode's solution, which induced discontinuity of propagation associated with step delays mediated by electrotonic current flowing through the depressed zone. When transmission delays were long, activity distal to the site of block returned to proximal tissue as one of two forms of reflected reentry, each elicited by a different mechanism. Type I reflection, occurring with antegrade delays of 30 to 60 msec, was characterized by an early secondary depolarization due to electrotonic spread of currents from delayed responses in the depressed segment. Type II reflection evolved with delays greater than 90 msec and was manifest as a closely coupled regenerative action potential that developed independently of a pacemaker mechanism and of slow but continuous conduction. We conclude that delayed activation of excitable elements, which occurs when propagation is discontinuous, may lead to rhythm disturbances of focal origin that are mediated by electrotonic interactions across a zone of depressed tissue.

Action Potentials↗

PubMed1986

Mechanisms underlying the antiarrhythmic and arrhythmogenic actions of quinidine in a Purkinje fiber-ischemic gap preparation of reflected reentry.

The effects of therapeutic levels of quinidine were studied in an ischemic gap preparation of reflected reentry. The preparation consisted of a Purkinje fiber mounted in a three-compartment chamber. A narrow central compartment was perfused with a solution prepared to mimic the extracellular milieu at a site of ischemia. Quinidine in concentrations that exert little effect on normal Purkinje tissue, 1 to 2 micrograms/ml, greatly impaired conduction and markedly prolonged refractoriness across the ischemic gap. The drug effected these changes by (1) extending the inexcitable zone within the depressed region, (2) decreasing the amplitude of the input signal entering this zone, and (3) decreasing the excitability of the tissue beyond the depressed zone (evaluated by current clamp techniques). These actions of the drug produced both antiarrhythmic and proarrhythmic effects. When the initial level of conduction impairment was high, quinidine totally suppressed reflected reentry at all frequencies by precipitating complete anterograde conduction block. At intermediate levels of block, the drug generally caused a prominent shift of the frequency dependence of reentrant activity to lower stimulation rates. Finally, when conduction was relatively less impaired, quinidine created the conditions for reflected reentry to occur. Our results suggest that the heart rate dependence of reentrant arrhythmias might be of prognostic value in the administration of antiarrhythmic drugs.

Action Potentials↗

PubMed2003

Effects of isosorbide mononitrate and AII inhibition on pulse wave reflection in hypertension.

The aortic pulse wave contour in isolated systolic hypertension often shows a prominent reflection peak, which combines with the incident wave arising from cardiac ejection so as to widen pulse pressure. We investigated the effects of an extended-release nitrate preparation and of 2 angiotensin II (AII) inhibitors (an AII receptor antagonist and an ACE inhibitor) on the aortic pulse wave contour and systemic blood pressure in hypertensive subjects with high augmentation index caused by exaggerated pulse wave reflection. Two double-blind, randomized, placebo-controlled crossover studies were carried out in a total of 16 elderly patients with systolic hypertension resistant to conventional antihypertensive therapy. In 1 study, pharmacodynamic responses to single doses of placebo, isosorbide mononitrate, eprosartan, and captopril were determined; in the other, single-dose isosorbide mononitrate and placebo were compared in subjects treated with AII inhibitors at baseline. Blood pressure was measured by sphygmomanometry and pulse wave components by applanation tonometry at the radial artery. All 3 agents were shown to decrease brachial systolic blood pressure, aortic systolic blood pressure, and aortic pulse pressure. Qualitative effects on the aortic pulse wave contour differed: augmentation index was not significantly altered by either captopril or eprosartan but was decreased (P<0.0001) by approximately 50% of the placebo value with isosorbide mononitrate in both study groups. We propose that isosorbide mononitrate corrected the magnified wave reflection in systolic hypertension of these elderly patients by an effect that was distinct from that exercised by either acute or chronic AII inhibition.

Acrylates↗