Refractory Characteristics Research Articles

Actions of Halothane, Isoflurane, and Enflurane on the Regional Action Potential Characteristics of Canine Purkinje Fibers

The effects of halothane, isoflurane, and enflurane on proximal (false tendon) and distal (apical) canine Purkinje fibers were measured in vitro to assess their comparative effects on fibers exhibiting characteristically long (proximal) and short (distal) action potential duration. High- and low-dose anesthetic effects were evaluated in three groups of six left ventricular preparations and were compared with the changes occurring at identical times in six control preparations using analysis of variance with repeated measures. Under control conditions in all groups, the action potential duration, measured at 90% repolarization (APD90, mean +/- SEM), of proximal fibers was longer than that of distal fibers (320 +/- 16 vs 252 +/- 11 ms, P less than or equal to 0.01). Halothane (0.3 and 0.7 mM), isoflurane (0.4 and 0.8 mM), and enflurane (0.6 and 1.0 mM) produced a dose-dependent decrease (P less than or equal to 0.01) of proximal fiber APD90 with less (P less than or equal to 0.01) change of distal fiber APD90 and reduced (P less than or equal to 0.05) regional differences of APD90 at the higher dose. The decreases of proximal fiber APD90 were greater (P less than or equal to 0.01) for 1.0 mM (1.7 MAC) enflurane (-66 +/- 7 ms) and 0.8 mM (3.0 MAC) isoflurane (-69 +/- 9 ms) than for 0.7 mM (2.9 MAC) halothane (-33 +/- 8 ms). We conclude that the regional actions of anesthetics on Purkinje fiber repolarization may influence conduction during the relative refractory period and the occurrence of arrhythmias associated with disparity of regional refractory characteristics in the ventricular conduction system.

Rate dependence of action potential duration and refractoriness in canine ventricular endocardium differs from that of epicardium: Role of the transient outward current

Previous studies have provided evidence for an important contribution of the transient outward current to the electrical activity of canine ventricular epicardium, but not endocardium. The present study examines the characteristics of action potential duration and refractoriness in these two tissue types. The time and rate dependence of changes in action potential duration and refractoriness observed in epicardium were significantly more accentuated than in endocardium. The restitution of action potential duration in epicardium paralleled the restitution of phase 1 amplitude of the action potential in this tissue. The correlation between phase I amplitude and action potential duration recorded from a large number of epicardial and endocardial preparations was significant under both steady state and restitution conditions. 4-Aminopyridine, a transient outward current blocker, decreased the time dependence of phase I amplitude and concomitantly decreased the time dependence of action potential duration in epicardium. 4-Aminopyridine abbreviated the action potential duration of epicardium at slow stimulation rates but had little effect or prolonged it at fast rates or after premature stimulation. (The availability of a transient outward current is relatively small after premature stimulation.)The data support the hypothesis that the prominent presence of a transient outward current in epicardium, but not endocardium, contributes to the differences in the time and rate dependence of action potential duration and refractoriness in the two tissue types. The results also demonstrate the effect of an outward current to prolongthe action potential and the effect of an outward current blocker to abbreviatethe action potential. These observations may also aid in our understanding of the basis for rate-dependent changes in the T wave and 1 wave of the electrocardiogram, the rate dependence of cardiac arrhythmias, the greater sensitivity of epicardium to ischemia and the differential effects of drugs on muscle tissues spanning the ventricular wall.

Comparative Electrophysiological Effects of Propafenone, 5-Hydroxy-Propafenone, and N-Depropylpropafenone on Guinea Pig Ventricular Muscle Fibers

Propafenone (Pf) is a class I antiarrhythmic drug that can be given both orally and intravenously. In order to examine whether its two major metabolites [5-hydroxypropafenone (5-OH-Pf) and N-depropylpropafenone (N-DP-Pf)] possess pharmacodynamical properties, we compared their electrophysiological effects to those of the parent drug on papillary muscle fibers from guinea pig ventricular myocardium. After baseline action potential and refractory period characteristics were measured at different pacing rates, the tissue preparations were superfused with either Pf, 5-OH-Pf, or N-DP-Pf at five different concentrations and electrophysiological characteristics were studied again. The maximal rate of depolarization (Vmax) was depressed by the three compounds only at the highest concentration, although the effect of N-DP-Pf was slightly less than the other two. Refractory periods were altered only by the highest concentration of 5-OH-Pf. Propafenone and N-DP-Pf exhibited equally slow on-set/off-set kinetics of the sodium channel block, whereas those of 5-OH-Pf were twice as long, which seems to suggest a slower rate of dissociation of the latter from the inactivated sodium channels. Thus, 5-OH-Pf and N-DP-Pf comply with the definition of the class IC antiarrhythmic drugs. The cumulative in vivo effects of the two metabolites and of the parent drug could have far reaching clinical implications, especially in the genetically predisposed extensive metabolizing subject.

The geochemistry of fallout plutonium in the North Atlantic: II. [formula omitted] ratios and their significance

A systematic decrease in the 240Pu 239Pu ratio in marine sediments is found with increasing water depth along a transect of cores between Woods Hole and Bermuda. The 240Pu 239Pu atom ratios range from ≅O.18 on the shelf to ≅O.10 at 5000 m but do not change with depth in individual cores. A model is presented which can account for the range of 240Pu 239Pu ratios found in this and other similar studies ( Noshkin and Gatrousis, 1974; Scott et al., 1983). We propose that there have been at least two distinct sources of fallout Pu to this region. The major source of Pu is global stratospheric fallout, characterized by a 240Pu 239Pu ratio of 0.18 and a relatively long residence time in seawater. The second source is characterized by a much lower 240Pu 239Pu ratio, and relative to global fallout it must have been much more efficiently removed from the water column to deep-sea sediments. We suggest that surface-based low yield testing at the Nevada Test Site is the only source of low ratio fallout Pu which could account for the timing, inventories, and refractory characteristics of this second component of fallout Pu inputs to the North Atlantic.

An apparatus for determining the thermophysical characteristics of refractories based on automatic processing of the experimental results

An apparatus for determining the thermophysical characteristics of refractories based on automatic processing of the experimental results

Electrophysiological mechanisms underlying rate-dependent changes of refractoriness in normal and segmentally depressed canine Purkinje fibers. The characteristics of post-repolarization refractoriness.

Tissues from diseased hearts are known to exhibit post-repolarization refractoriness and rate-dependent changes of the refractory period that are often inconsistent with changes in action potential duration. To examine the electrophysiological mechanisms responsible for such rate-dependent changes of the refractory period, a narrow inexcitable zone was created by superfusing the central segments of Purkinje fibers with an "ion-free" isotonic sucrose solution. The degree of conduction impairment could be finely regulated by varying the resistance of the extracellular shunt pathway. At intermediate or low levels of block, the refractory period remained unchanged or decreased, respectively, as the rate was increased. At relatively high levels of block, however, we observed marked increases of the refractory period in response to increases in the stimulation rate. The disparity of refractoriness between normally conducting fibers and fibers exhibiting discontinuous conduction characteristics and post-repolarization refractoriness increased dramatically as a function of increasing stimulation rate. With the aid of current clamp techniques, we demonstrate that the differential behavior is due to the interplay between rate-dependent changes in the restitution of excitability at the site beyond the depressed zone secondary to changes in passive and active membrane properties and in the intensity of local circuit current provided to that site by activity generated in the segment proximal to the zone of block. Our data suggest that rate-dependent changes of refractoriness in Purkinje tissue are principally governed by attendant changes in membrane resistance.

On the Characteristics of Some Fusion Cast Refractories for High Temperature Glass Melting

Fusion cast refractories for high temperature (>1400°C) glass melting are Al2O3-ZrO2-SiO2 system, Al2O3-ZrO2-Cr2O3-SiO2 system and ZrO2 system. The corrosion resistance, stoning potential and bubble potential of fusion cast refractories has been studied in the temperature range over 1400°C. Five commercial glasses are used for testing. Glasses are soda-lime glass, panel glass, al-silicate glass, alkali-resistant glass and boro-silicate glass. Corrosion resistance of fusion cast refractories to five commercial glasses has different results by glass composition. ZrO2 system refractory is the best corrosion resistance to al-silicate glass and boro-silicate glass. Al2O3-ZrO2-SiO2 system refractory is the best corrosion resistance to panel glass and alkali-resistant glass. Al2O3-ZrO2-Cr2O3-SiO2 system refractory is the best corrosion resistance to soda-lime glass. ZrO2 system refractory is the best stoning and bubble potential.

High quality chrome-bearing basic refractories from brine magnesia

Abstract Pure magnesia (>96% MgO) was recovered from marine brine of high MgCl2 content using calcined dolomitic magnesite as alkali precipitating agent. High quality chrome-bearing refractories were obtained from clinkers produced by joint fine grinding and firing at 1550°C of the dried Mg(OH)2 paste with different amounts (10, 20, 35 wt. %) of common chrome ore and chemically enriched ore. The technical characteristics of the basic refractories were improved as the chrome content is increased up to 20% common ore and 35% chemically enriched ore. On substituting the common ore by the chemically enriched one the properties of the refractories were remarkably improved due to lower silicate level (about 2%) and higher chrome spinellide content. The properties of the basic refractories were correlated with their different constituent phases.

Bidirectional nerve refractory characteristics in simulations of direct and remote stimulation.

The effects of remote stimulation on the refractory characteristics of myelinated nerve fibers were investigated using computer simulations of nerve action potentials, in response to spatially separated conditioning and test stimuli. The behavior of the test action potential was strongly influenced by its direction of propagation relative to that of the conditioning action potential. Under certain conditions, the variation of relative refractory period with conduction velocity (CV) changed from inverse, for propagation in opposing directions, to direct, for propagation in the same direction. A similar directionally dependent result occurred in the study of relative refractory period as a function of stimulus intensity. At certain interstimulus intervals, the test stimulus elicited action potentials which would conduct in the direction opposite to the conditioning action potential, but would not conduct in the wake of that conditioning action potential. These results are explained in terms of the spatial spread of stimulus current resulting from distant placement of the stimulating electrode in a volume conductor. Clinical repercussions of these results for correction of refractory period in collision neurography are discussed.

Enhanced atrioventricular nodal conduction in man: electrophysiologic effects of pharmacologic autonomic blockade.

Enhanced atrioventricular nodal conduction (EAVC) comprises a portion of the spectrum of atrioventricular node electrophysiology in which physiologic slowing of atrioventricular conduction is diminished. To evaluate the basis of EAVC in humans, we compared the effect of pharmacologic autonomic blockade on electrophysiologic characteristics of atrioventricular nodal conduction and refractoriness in 10 patients with EAVC and 12 patients without EAVC (non-EAVC). The effects of autonomic blockade (0.2 mg/kg iv propranolol and 0.04 mg/kg iv atropine) on sinus cycle length and atrial refractory period were similar in both EAVC and non-EAVC patients. After autonomic blockade, minimum atrial paced cycle length with 1:1 atrioventricular conduction lengthened in EAVC patients (control, 268 +/- 25 msec; blockade, 307 +/- 34 msec; p less than .01), but remained unchanged in non-EAVC patients (control, 392 +/- 69 msec; blockade, 382 +/- 49 msec; p = NS). Similarly, atrioventricular nodal functional refractory periods tended to lengthen in EAVC patients, while remaining unchanged or shortening in non-EAVC patients. Furthermore, although autonomic blockade reduced differences between EAVC and non-EAVC patients, both minimum cycle length with 1:1 atrioventricular conduction and atrioventricular functional refractory period remained shorter in EAVC than in non-EAVC patients (p less than .02). Thus, baseline differences in autonomic atrioventricular nodal control differ in EAVC and non-EAVC patients, but do not fully account for differences in atrioventricular nodal electrophysiologic properties in these patients. Consequently, underlying structural functional factors may play an important role in determining the range of atrioventricular nodal electrophysiologic characteristics in man.

Results of studying the effect of reactor radiation on the refractory characteristics of 03Kh20N45M4-type high-nickel alloys

Results of studying the effect of reactor radiation on the refractory characteristics of 03Kh20N45M4-type high-nickel alloys

Mechanism of cycling of migrating myoelectric complexes: effect of morphine.

Morphine was injected intravenously at various phases of the migrating myoelectric complex (MMC) cycle to study the oscillatory characteristics of MMCs by the premature initiation of phase IIIs. All injection timings were represented as a percentage of the normal MMC period at the most proximal duodenal electrode. During the initial 20% of the MMC cycle, the mechanism of initiation of MMCs was in an absolutely refractory state in the sense that a supramaximal dose of morphine (200-300 micrograms/kg) did not initiate a premature phase III. During the remainder of the MMC cycle, the control mechanism was in a relatively refractory state. As this state progressed, premature phase III activity was initiated with diminishing doses of morphine. This was called the relatively refractory state. The initiation of a premature phase III by morphine did not affect the phase III already in progress, except that its propagation velocity was increased. Truncal vagotomy did not affect the refractory characteristics of MMCs or the action of morphine. Only large doses of naloxone (2 mg/kg) blocked the above action of morphine. The study shows that the MMC cyclic phenomenon has the characteristics of relaxation oscillators that may result from enteric neural biological clocks. The period of these oscillators can be altered by stimulants such as morphine.

Structural features of high-zirconia refractories

Industrially produced fused baddeleyite--corundum refractories with a high concentration of ZrO2 (up to 41%) are already well known [i, p. ii]. The excellent corrosion resistance makes them irreplaceable in glass production. However, speeding up the glass-melting process and raising the specifications in relation to the quality of the glass make it necessary continually to improve the user characteristics of the refractories which are to a significant extent determined by the mineralogical composition of the materials. At the State Institute of Glass we have carried out some studies on their synthesis and investigated the properties of refractory materials based on Zr02, A1203, SIC2, etc. The present article deals with a study of the structural features of fused high-zirconia refractories with 55-85% ZrO= and 11-13% SIC2. The structural characteristics of refractories containing 33-45% ZrO2 and 12-15% Si02 have previously been studied in [2]. The experimental specimens were obtained by melting a batch of materials of industrial purity in semi-industrial electric-arc equipment.* By creating oxidizing conditions during melting [3], it was possible to obtain materials with a carbon concentration of <0.01%. The properties of the materials were studied using specimens cut from a cast measuring 125  200  250 mm. The characteristics of the specimens of synthesized refractories are given in Table i. The x-ray patterns of the specimens were obtained in an URS-50IM diffraction meter using Cu K s radiation (Ni filter) at 18-22~ The phases were identified from the x-ray card index [4]. *The synthesis of the specimens was carried out with the cooperation of V. I. Markin. TABLE i. Characteristics of Fused Specimens* Chemical composition, v1~ . ~ Specimen ZrO~ A1203 SiC= Na=O ~ ~

High quality chrome-bearing basic refractories from brine magnesia

Pure magnesia (>96% MgO) was recovered from marine brine of high MgCl 2 content using calcined dolomitic magnesite as alkali precipitating agent. High quality chrome-bearing refractories were obtained from clinkers produced by joint fine grinding and firing at 1550°C of the dried Mg(OH) 2 paste with different amounts (10, 20, 35 wt. %) of common chrome ore and chemically enriched ore. The technical characteristics of the basic refractories were improved as the chrome content is increased up to 20% common ore and 35% chemically enriched ore. On substituting the common ore by the chemically enriched one the properties of the refractories were remarkably improved due to lower silicate level (about 2%) and higher chrome spinellide content. The properties of the basic refractories were correlated with their different constituent phases.

Monophasic pulse pair analysis of intracranial self-stimulation loci

Monophasic pulse pair stimulation, traditionally used to determine the refractory characteristics of peripheral nervous system neurons, has been modified for the same purpose in intracranial self-stimulation (ICSS) behavior. When the interval between pulses in a pair (C-T interval) is systematically varied, ICSS response rates change as a function of this variation. The present study evaluated alterations in the C-T interval/ICSS response rate function across ICSS loci and across stimulation conditions. Diencephalic sites supported peak ICSS responding at significantly shorter C-T intervals than met-mesencephalic or telencephalic sites. Despite previously cited differences in ICSS behavior observed within diencephalic and within met-mesencephalic loci, no differences in ICSS responding across C-T intervals were observed within diencephalic and met-mesencephalic sites. Moreover, diencephalic sites produced peak ICSS responding at shorter C-T intervals when cathodal and anodal sources were distant than when they were adjacent; the relative proximity of cathodal and anodal sources did not affect met-mesencephalic ICSS behavior. These results suggest that alterations in ICSS responding across C-T intervals are due to differences in the locus and density of ICSS neurons in the cathodal field and the presence and/or absence of adjacent anodal influences as well as the refractory characteristics of the ICSS neurons.

Characteristics of atrioventricular conduction and the spectrum of arrhythmias in lown-ganong-levine syndrome.

Electrophysiological characteristics of atrioventricular (A-V) conduction and refractoriness were examined in 12 patients with Lown-Ganong-Levine (LGL) syndrome referred for assessment of the following arrhythmias: (group I) regular narrow QRS tachycardia 6/12 (50%), (group II) atrial fibrillation (AF) 2/12 (17%), group III) ventricular tachycardia (VT) 4/12 (33%). A-V node refractory periods were shorter, and enhanced A-V conduction more frequent (7/12, 58% vs 7/28, 25%) in LGL patients compared to similar studies in 28 normal controls. During laboratory study reciprocating tachycardia (RT) due to re-entry within the A-V node occurred in 4/12 (33%) LGL patients, and exhibited a shorter cycle length (294 +/- 60.4 msec) than did the same arrhythmia in 11/28 (39%) controls 372 +/- 51.8 msec, P less than 0.05). Similarly, RT utilizing a concealed accessory pathway had a shorter cycle length (228 +/- 3.5 msec) in 2/12 (17%) LGL patients than in 11/28 (39%) controls (314 +/- 24.3 msec, P less than 0.001). In AF, the shortest R-R intervals in 4/12 (33%) LGL patients (2 group I, 2 group II) were shorter than in 15/28 (54%) control patients (254 +/- 42.2 msec vs 325 +/- 64.2 msec, P less than 0.05). The mean R-R internvals did not differ significantly (LGL 372 +/- 89 msec vs control 428 +/- 82.6 msec). This study suggests that the characteristics of A-V conduction and refractoriness may permit development of more rapid heart rates during certain arrhythymias in LGL patients compared to normal controls. Furthermore, the occurrence of VT in patients with LGL sydrome indicates that symptomatic arrhythmias require specific diagnosis.

Effect of Particle-size Distribution on Characteristics of Castable Refractory

4mm以下の〓焼フリントクレイ骨材とアルミナセメントからなるキャスタブル耐火物において, その作業性, 充填性, 施工体と加熱体の強度および加熱による容積安定性などにおよぼす粒度分布の影響を, セメント添加量および可塑性粘土添加の影響と比較しながら研究をおこなった. その結果, 耐火物坏土のグレイディングにおける著者らの式, n成分系での任意のi成分以下の篩下量=20.5(1-n)q+{1-20.5(1-n)q}{(δ0-δ)n-i-1}/{(δ0-δ)n-1-1}において, 分布係数qが0.3で偏位係数δが-0.2の場合に適当なフロー値を示し, 比較的に高充填高強度となり容積安定性にすぐれたものがえられた. また微粉部骨材の微細度の向上およびセメント量の増大は上記ほどの効果を上げる事が出来ず, また可塑性粘土の添加は, 単に作業性中の保形性を向上させる効果しかみとめられなかった. したがってキャスタブル耐火物製造上, 骨材の粒径グレイディングが最も重要な因子であると結論した.

Effect of phase composition on the binding characteristics of unfired stable dolomite refractories

AbstractThree batches of stable dolomite cements with different mineral compositions and fineness were prepared. The hydraulic properties of each sample were measured after curing at 100% relative humidity for 1, 3, 7 and 28 days. The results were interpreted in the light of the phase composition, fineness and curing time. The cement with the highest hydraulic properties was used as a binder for the preparation of unfired stable dolomite bodies. The changes in properties of the prepared bodies at temperatures up to 1550°C were correlated with the phase compositions.

Characteristics of heat-insulating refractories produced by some foreign companies

Studies were made of heat insulating aluminosilicate products produced by certain companies. The articles are divided into four groups. It is found that within the limits of any one group there is a substantial spread in the properties: strength, apparent density, and temperature of deformation.

Effect of Forming Pressure on the Physical Properties and High Temperature Load Bearing Characteristics of Basic Refractories

The effect of forming pressure on the physical properties and high temperature load bearing characteristics of magnesite, chrome-magnesite and magnesite-chrome bricks prepared under industrial conditions has been discussed. It has been suggested that the physical properties of these refractories and their resistance to creep under compressional load at high temperature in particular, showed considerable improvement when the forming pressure exceeded 800 kg/cm2.