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The measurement of effective conductivities is complicated by the traditionally used electrode geometry treatment genital warts order meclizine 25mg without a prescription. The bidomain model describes the electrical properties of cardiac muscle [Henriquez treatment action campaign proven 25 mg meclizine, 1993] treatment 2 prostate cancer purchase 25mg meclizine. It is essentially a two- or three-dimensional cable model that takes into account the resistance of both the intracellular and the interstitial spaces (Figure 21 medicine online purchase meclizine 25 mg. Thus, the concept of current redistribution, discussed earlier in the context of the longitudinal effective conductivity of a suspension of fibers, now applies in all directions. These properties make impedance measurements of cardiac muscle difficult to interpret [Plonsey and Barr, 1986; Le Guyader et al. The situation is complicated further because the intracellular space is more anisotropic than is the interstitial space (in the jargon of bidomain modeling, this condition is known as "unequal anisotropy ratios") [Roth, 1997]. Consequently, an expression for a single effective conductivity for cardiac muscle is difficult, if not impossible, to derive. In general, one must solve a pair of coupled partial differential equations simultaneously for the intracellular and interstitial potentials. The bidomain model characterizes the electrical properties of the tissue by four effective conductivities: giL, giT, geL, and geT, where i and e denote the intracellular and interstitial spaces, and L and T denote the the Electrical Conductivity of Tissues (a) (b) 21-7 0. Current, I, is passed through the outer two electrodes, and the potential, V, is measured between the inner two. We can relate these parameters to the microscopic tissue properties by using an operational definition of an effective bidomain conductivity, similar to the operational definition given earlier. To determine the interstitial conductivity, first dissect a cylindrical tube of tissue of length L and cross-sectional area A (one must be sure that L and A are large 21-8 Biomedical Engineering Fundamentals (a) 1 0. Next, apply a drug to the tissue that makes the membrane essentially insulating. This procedure must be performed twice, once with the fibers parallel to the axis of the cylinder, and once with the fibers perpendicular to it. To determine the effective intracellular conductivities, follow the above procedure but apply the voltage difference to the intracellular space instead of the interstitial space. The lower array of resistors represents the intracellular space, the upper array represents the extracellular space, and the parallel resistors and capacitors represent the membrane. Expressions have been derived for the effective bidomain conductivities in terms of the microscopic tissue parameters [Roth, 1988; Henriquez, 1993; Neu and Krassowska, 1993]. The effective conductivities in the direction parallel to the fibers are simplest. If the conductivity of the interstitial fluid is e, then the effective interstitial conductivity parallel to the fibers, geL, is simply geL = 1 - f. When the gap junctional resistance is not negligible compared to the myoplasmic resistance, the expression for giL is more complicated: 1 giL = (21. If the intercellular junctions contribute significantly to the intracellular resistance, the bidomain model only approximates the tissue behavior [Neu and Krassowska, 1993]. For sufficiently large junctional resistance, the discrete cellular properties become important, and a continuum model no longer represents the tissue well. Interestingly, as the junctional resistance increases, cardiac tissue behaves less like a syncytium and more like a suspension of cells. We started by considering a suspension of cells, then examined suspensions of fibers, and finally generalized to syncytia. Yet, when the intercellular junctions in a syncytium are disrupted, we find ourselves again thinking of the tissue as a suspension of cells. The Electrical Conductivity of Tissues 21-11 Defining Terms Anisotropy: Having different properties in different directions. Bidomain: A two- or three-dimensional cable model that takes into account the resistance of both the intracellular and the extracellular spaces. Cable theory: Representation of a cylindrical fiber as two parallel rows of resistors (one each for the intracellular and extracellular spaces) connected in a ladder network by a parallel combination of resistors and capacitors (the cell membrane). Conservation of current: A fundamental law of electrostatics, stating that there is no net current entering or leaving at any point in a volume conductor.
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Assist in more complex corneal surgery (eg treatment jalapeno skin burn purchase meclizine 25mg fast delivery, penetrating keratoplasty and lamellar keratoplasty) medications that cause weight loss safe 25 mg meclizine. Perform more complex and recurrent pterygium excision medications affected by grapefruit order meclizine 25 mg mastercard, including conjunctival grafting treatment wpw meclizine 25mg. Perform more complex corneal laceration repair (eg, stellate perforating laceration). Perform and interpret more complex stains of the cornea and conjunctiva (eg, calcofluor white, acid fast). Repair simple lacerations of the lacrimal drainage apparatus (eg, perform intubations and primary closure). Treat hyphema and microhyphema with associated increased intraocular pressure and/or blood staining (eg, surgical evacuation). Describe the most complex anatomy, embryology, physiology, histopathology, microbiology, immunology, genetics, epidemiology, and pharmacology of the cornea, conjunctiva, sclera, eyelids, lacrimal apparatus, and ocular adnexa. Understand the most complex corneal optics and refraction (eg, postkeratoplasty) and their methods of treatment (eg, contact lenses, refractive surgery). Describe the most complex and less common congenital abnormalities of the cornea, sclera, and globe (eg, cornea plana, keratoglobus). Recognize common and uncommon corneal and conjunctival neoplasms and degenerations (eg, spheroidal degeneration, carcinoma in situ). Describe less common and rare ocular infections, and describe the differential diagnosis of the most complicated corneal and conjunctival infections (eg, amoebas, leishmaniasis, nematodes). In endemic areas, define the etiology, vector (eg, black fly), and incidence, diagnostic features (eg, microfilariae, keratitis, iritis), diagnosis (eg, skin snip test), course and prognosis, treatment (eg, ivermectin, nodulectomy), and prevention (eg, vector control, environmental and behavioral changes) of onchocerciasis. Describe the most complex principles of ocular pharmacology of anti-infective, antiinflammatory, and immune modulating agents (eg, combination therapies of antiviral and anti-inflammatory agents). Describe the most complex differential diagnosis of red eye (eg, pemphigoid, pemphigus, Stevens-Johnson syndrome). Describe the differential diagnosis and the external manifestations of the most complex or uncommon anterior segment inflammations (eg, syphilitic keratouveitis). Diagnose and treat the most complex traumatic and toxic injuries to the anterior segment (eg, total lid avulsion, severe alkali burn). Recognize and treat complex corneal lacerations (eg, lacerations extending beyond the limbus, uveal involvement). Diagnose and treat the most severe corneal exposure cases (eg, conjunctival flap). Describe the indications for ocular surface transplantation, including conjunctival autograft/flap, amniotic membrane transplantation, and limbal stem cell transplantation. Describe the surgical indications (eg, Fuchs dystrophy, aphakic/pseudophakic bullous keratopathy, keratoconus), surgical techniques, and recognition and management of postoperative complications (especially immunologically-mediated rejection) of corneal transplantation (eg, penetrating, lamellar). Perform and interpret the most advanced corneal techniques (eg, endothelial microscopy, computerized corneal topography and tomography, anterior segment ocular coherence tomography). Perform specialized and complicated fitting of contact lenses (eg, postkeratoplasty, advanced keratoconus). Perform more complex corneal surgery (eg, penetrating or lamellar keratoplasty, keratorefractive procedures, and phototherapeutic keratectomy), and understand the postoperative management including postkeratoplasty astigmatism management and graft rejection. Manage and treat more complex neoplasms of the conjunctiva (eg, carcinoma, melanoma). Very Advanced Level Goals: Subspecialist Fellowship training requires more indepth education about the pathophysiology and management than can usually be obtained in residency training in ophthalmology. Fellowships include a continuous period of intense and focused training in developing and maintaining knowledge, skills, scholarship, and professionalism. A fellow should be knowledgeable and proficient in all the activities listed for residency training. Subspecialty fellowship training should include a more in-depth exposure and understanding of the diagnosis and medical management of diseases of the eyelids, conjunctiva, cornea/sclera, and anterior ocular segment, as well as recognition and treatment of posterior segment disease that may affect the anterior segment. Subspecialty fellowship training should include hands-on training covering surgery of the conjunctiva, cornea/sclera, anterior segment, lens, and anterior vitreous, with special emphasis on corneal transplantation and related procedures. Recognize acute and chronic blepharitis, including both infectious and noninfectious etiologies, with emphasis on microbial blepharitis, meibomian gland dysfunction, and rosacea. Recognize acute and chronic conjunctivitis, neonatal conjunctivitis, chlamydial disease, adenoviral conjunctivitis, allergic conjunctivitis, and bacterial conjunctivitis.
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Some possible applications of shape memory alloys are orthodontic dental archwire symptoms lymphoma purchase 25mg meclizine with amex, intracranial aneurysm clip treatment quadricep strain generic meclizine 25 mg on line, vena cava filter 714x treatment for cancer effective meclizine 25mg, contractile artificial muscles for an artificial heart medicine cabinet buy meclizine 25 mg on line, vascular stent, catheter guide wire, and orthopedic staple [Duerig et al. In order to develop such devices, it is necessary to understand fully the mechanical and thermal behavior associated with the martensitic phase transformation. A widely known NiTi alloy is 55-Nitinol (55 weight% or 50 atomic % Ni), which has a single phase and the mechanical memory plus other properties, for example, high acoustic damping, direct conversion of heat energy into mechanical energy, good fatigue properties, and low temperature ductility. Deviation from the 55-Nitinol (near stoichiometric NiTi) in the Ni-rich direction yields a second group of alloys which are also completely nonmagnetic but differ from 55-Nitinol in their ability to be thermally hardened to higher hardness levels. Shape recovery capability decreases and heat treatability increases rapidly as the Ni content approaches 60%. Both 55 and 60-Nitinols have relatively low modulus of elasticity and can be tougher and more resilient than stainless steel, NiCr, or CoCr alloys. Efficiency of 55-Nitinol shape recovery can be controlled by changing the final annealing temperatures during preparation of the alloy device [Lee et al. For the most efficient recovery, the shape is fixed by constraining the specimen in a desired configuration and heating to 482 to 510 C. If the annealed wire is deformed at a temperature below the shape recovery temperature, shape recovery will occur upon heating, provided the deformation has not exceeded crystallographic strain limits (8% strain in tension). The NiTi alloys also exhibit good biocompatibility and corrosion resistance in vivo. There is no significant difference between titanium and NiTi in the inhibition of mitosis in human fibroblasts. NiTi showed lower percentage bone and bone contact area than titanium and the Ti6Al4V alloy [Takeshita et al. The mechanical properties of NiTi alloys are especially sensitive to the stoichiometry of composition (typical composition is given in Table 38. Although much is known about the processing, mechanical behavior, and properties relating to the shape memory effect, considerably less is known about the thermomechanical and physical metallurgy of the alloy. The solid alloy is mixed with (liquid) mercury in a mechanical vibrating mixer and the resulting material is packed into the prepared cavity. One of the solid alloys is composed of at least 65% silver, and not more than 29% tin, 6% copper, 2% zinc, and 3% mercury. The phase diagram for the Ag-Sn-Hg system shows that over a wide compositional range all three phases are present. The final composition of dental amalgams typically contain 45% to 55% mercury, 35% to 45% silver, and about 15% tin after fully set in about one day. Gold and gold alloys are useful metals in dentistry as a result of their durability, stability, and corrosion resistance [Nielsen, 1986]. Cast restorations are made by taking a wax impression of the prepared cavity, making a mold from this impression in a material such as gypsum silica, which tolerates high temperature, and casting molten gold in the mold. Gold alloys are used for cast restorations, since they have mechanical properties which are superior to those of pure gold. Corrosion resistance is retained in these alloys provided they contain 75% or more of gold and other noble metals. Platinum also improves the strength, but no more than about 4% can be added, or the melting point of the alloy is elevated excessively. A small amount of zinc may be added to lower the melting point and to scavenge oxides formed during melting. Softer alloys containing more than 83% gold are used for inlays which are not subjected to much stress. Harder alloys containing less gold are chosen for crowns and cusps which are more heavily stressed. In this type of welding, the metal layers are joined by thermal diffusion of atoms from one layer to the other. Since intimate contact is required in this procedure, it is particularly important to avoid contamination.
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