Showing papers in "Critical Reviews in Biomedical Engineering in 1989"

Muscle and tendon: properties, models, scaling, and application to biomechanics and motor control

Abstract: Skeletal muscles transform neural control signals into forces that act upon the body segments to effect a coordinated motor task. This transformation is complex, not only because the properties of muscles are complex, but because the tendon affects the transmission of muscle force to the skeleton. This review focuses on how to synthesize basic properties of muscle and tendon to construct models applicable to studies of coordination. After a review of the properties of muscle and tendon, their integrated ability to generate force statically and dynamically is studied by formulating a generic model of the "musculotendon actuator", which has only one parameter, the ratio of tendon length at rest to muscle fiber length at rest. To illustrate the utility of the model, it is analyzed to show how this one parameter specifies whether excitation-contraction or musculotendon contraction is the rate-limiting process of force generation, whether elastic energy is stored in tendon or muscle, and whether hip- and knee-extensor actuators function as springs or dashpots during walking.

Dielectric properties of tissues and biological materials: a critical review.

Abstract: We critically review bulk electrical properties of tissues and other biological materials, from DC to 20 GHz, with emphasis on the underlying mechanisms responsible for the properties. We summarize the classical principles behind dielectric relaxation and critically review recent developments in this field. Special topics include a summary of the significant recent advances in theories of counterion polarization effects, dielectric properties of cancer vs. normal tissues, properties of low-water-content tissues, and macroscopic field-coupling considerations. Finally, the dielectric properties of tissues are summarized as empirical correlations with tissue water content in other compositional variables; in addition, a comprehensive table is presented of dielectric properties. The bulk electrical properties of tissues are needed for many bioengineering applications of electric fields or currents, and they provide insight into the basic mechanisms that govern the interaction of electric fields with tissue.

Theory of oxygen transport to tissue.

Abstract: This review focuses on the theory of oxygen transport to tissue and presents the state of the art in mathematical modeling of transport phenomena. Results obtained with the classic Krogh tissue-cylinder model and recent advances in mathematical modeling of hemoglobin-oxygen kinetics, the role of hemoglobin and myoglobin in facilitating oxygen diffusion, and the role of morphologic and hemodynamic heterogeneities in oxygen transport in the microcirculation are critically discussed. Mathematical models simulate different parts of the pathway of oxygen molecules from the red blood cell, through the plasma, the endothelial cell, other elements of the vascular wall, and the extra- and intracellular space. Special attention in the review is devoted to intracapillary transport, which has been the subject of intensive theoretical research in the last decade. Models of pre- and postcapillary oxygen transport are also discussed. Applications to specific organs and tissues are reviewed, including skeletal muscle, myocardium, brain, lungs, arterial wall, and skin. Unresolved problems and major gaps in our knowledge of the mechanisms of oxygen transport are identified.

Fundamental and practical aspects of therapeutic uses of pulsed electromagnetic fields (PEMFs).

Abstract: The beneficial therapeutic effects of selected low-energy, time-varying magnetic fields, called PEMFs, have been documented with increasing frequency since 1973. Initially, this form of athermal energy was used mainly as a salvage for patients with long-standing juvenile and adult nonunions. Many of these individuals were candidates for amputation. Their clearly documented resistance to the usual forms of surgical treatment, including bone grafting, served as a reasonable control in judging the efficacy of this new therapeutic method, particularly when PEMFs were the sole change in patient management. More recently, the biological effectiveness of this approach in augmenting bone healing has been confirmed by several highly significant double-blind and controlled prospective studies in less challenging clinical circumstances. Furthermore, double-blind evidence of therapeutic effects in other clinical disorders has emerged. These data, coupled with well-controlled laboratory findings on pertinent mechanisms of action, have begun to place PEMFs on a therapeutic par with surgically invasive methods but at considerably less risk and cost. As a result of these clinical observations and concerns about electromagnetic "pollution", interactions of nonionizing electromagnetic fields with biological processes have been the subject of increasing investigational activity. Over the past decade, the number of publications on these topics has risen exponentially. They now include textbooks, speciality journals, regular reviews by government agencies, in addition to individual articles, appearing in the wide spectrum of peer-reviewed, scientific sources. In a recent editorial in Current Contents, the editor reviews the frontiers of biomedical engineering focusing on Science Citation Index methods for identifying core research endeavors. Dr. Garfield chose PEMFs from among other biomedical engineering efforts as an example of a rapidly emerging discipline. Three new societies in the bioelectromagnetics, bioelectrochemistry, and bioelectrical growth and repair have been organized during this time, along with a number of national and international committees and conferences. These activities augment a continuing interest by the IEEE in the U.S. and the IEE in the U.K. This review focuses on the principles and practice behind the therapeutic use of "PEMFs". This term is restricted to time-varying magnetic field characteristics that induce voltage waveform patterns in bone similar to those resulting from mechanical deformation. These asymmetric, broad-band pulses affect a number of biologic processes athermally. Many of these processes appear to have the ability to modify selected pathologic states in the musculoskeletal and other systems.(ABSTRACT TRUNCATED AT 400 WORDS)

Intelligent instrumentation in diabetic management

Abstract: This presentation describes the problem of insulin-dependent diabetes mellitus and the various approaches to creating intelligent instrumentation for management of this type of diabetes. The article traces closed-loop diabetes control from 1974 to the present. Development and implementation of a microprocessor device aimed at achieving this in a practical sense are described. Accordingly, a chronological description of the earliest devices and their shortcomings is given, focusing on the use of blood glucose sensors and vascular access devices. A description of open-loop diabetes control and alternative systems is presented, highlighting the fundamental limitations of subcutaneously injected or infused insulins. A practical system for optimizing conventional therapy is described, together with its design criteria and the creation of a clinical closed-loop that includes the patient. The system has to deal with the effects of diet and exercise while fulfilling the need for adjusting insulin dosages according to expert intervention. Algorithms for this purpose and a computerized approach based on a portable microprocessor expert system are highlighted.

Mechanical properties of the aorta: a review.

Abstract: Measurement and calculation of the mechanical properties of the aorta depend on a number of factors, including the degree of surgical invasion, vasomotor tone, tissue hydration, physical contact (i.e., the presence of a strain gauge), and the use of inside or outside diameter. Measurements on the aorta using angiography, transit-time determination, and echocardiographic techniques yield the most consistent values for parameters, including PVD, Ep, and Eo. Values of these parameters are listed in Table 7 and range from about 8 to 12.0% (PVD), 337 to 741 g/cm2 (Ep), and 0.33 to 3.6 x 10(6) dyn/cm2 (Eo) for human ascending aorta. Measurements made with invasive techniques tend to result in lower values of PVD and higher values of elastic moduli. Modeling studies focus on predicting the mechanical properties of a viscoelastic, anisotropic tube. Difficulty arises in modeling aorta because of hysteresis and the large deformations that are associated with physiologic loading patterns. Various forms of the strain-energy density function have been used to model relationships between stress and strain in three dimensions; however, they have limited value in understanding the mechanism of time-dependent stress transfer between fibrous and nonfibrous components found in aortic tissue. The ultimate utility of these models will include improved understanding of the physical basis of deformation of this important tissue and the mechanisms of premature failure associated with different disease processes.

Neural modeling in electrical stimulation.

Abstract: In general, complete mathematical modeling of electrical neurostimulation encompasses two separate problems; clear delineation of this article becomes important. Solutions are required for the time-varying macroscopic fields generated by the stimulating electrodes, and only then can biophysical analysis be brought to bear on neural structures within those fields. This article is focused on the second of these aspects, and provides a survey of mathematical representations including nerve cell bodies, myelinated and unmyelinated fibers of passage, branched systems, fiber terminals and composite neurons. Effects on nerve cells of fields generated by remote electrodes are given primary attention, although methods for obtaining field solutions in biological media are discussed in detail only where the issue is inextricably linked to the use of a particular neural model, or relates crucially to experimental validation. Within these guidelines, it is intended to show the significant role that current nerve cell models may play in attempts to understand mechanisms of neural stimulation, and in the development of more advanced strategies for electronic intervention and control of neural function.

Detection of early breast cancer: an overview and future prospects.

Abstract: Detection and treatment of breast cancer at an early stage is the only method with proven potential for lowering the death rate from this disease Detection of early breast cancer is promoted by the American Cancer Society, American College of Radiology, and Canadian Association of Radiologists by encouraging the regular use of three types of screening: breast self-examination, clinical breast examination, and mammography When all factors are considered, it has been convincingly demonstrated that the potential benefits of mammography far outweigh the minimal, clinically undetected radiation risk incurred by the examination New technologies, such as computed tomography, magnetic resonance imaging, transillumination diaphanography, ultrasound, thermography, and digital subtraction angiography might offer a wide selection for patient examination However, none of these procedures, in its present form, is expected to replace mammography as the first-line imaging technique for the detection and diagnosis of benign and malignant breast lesions Breast cancer is detected now, in most cases, via casual or informed breast self-examination This first-line of detection is not sufficient, since most tumors may metastasize before they reach a palpable size Mammography generally shows up tumors no smaller than 1-cm diameter, which in many cases have already metastasized The more advanced imaging modalities in their current forms suffer from a number of drawbacks that give them a lower overall detection rate than mammography Understandably, improving breast imaging modalities is a great challenge to diagnostic radiology The purpose of this article is to provide a comprehensive overview of the detection of early breast cancer It briefly discusses the understanding of breast cancer, its incidence, and the mortality and survival of patients with breast cancer, as well as screening programs for breast cancer We review the developments in mammography and other breast imaging modalities over the last several years Prospects for digital mammography, digital image enhancement, and three-dimensional digital subtraction mammography, which may someday supplant film mammography, are also discussed

Three-dimensional data visualization and biomedical applications

Abstract: Three-dimensional data visualization is an important tool in several medical, scientific, and engineering areas. Visualization methods are based on a primitive representational element: contour, surface, or volume. Methods often incorporate options to cut open, see around, or see through structures, and form images in multiple windows or with animation. To visualize and interpret two or three related 3D data sets, composite imaging methods are required. The appropriate method depends on the user needs, application area, and available hardware. Visualization of 3D medical data is described for cranium/face, musculoskeletal systems, spine, intracranial structures, cardiovascular system, and radiation therapy.

Clinical utilization of the artificial heart.

Abstract: During the past 30 years the artificial heart has evolved from a bioengineering concept to clinical reality. To date four patients have had an artificial heart implanted as a permanent device, while over 150 artificial hearts have been utilized temporarily as a bridge to cardiac transplantation. Increased use of this device requires that a number of issues be critically addressed: (1) criteria for patient selection; (2) operative techniques for implantation including size of device and its position in the mediastinum; and (3) management of the patient in the intensive care unit (ICU), in particular, regimens of anticoagulation, assessment of adequacy of organ perfusion, and prevention of sepsis. This chapter is a discussion of these bioengineering and clinical concerns with respect to the Jarvik total artificial heart (TAH). Clinical data are presented which highlight the current problems with these devices and the areas of future research that need to be undertaken.

The electrogastrogram and its analysis.

Abstract: In recent years interest in the electrogastrogram, the EGG, has manifested itself in an increasing number of articles. In this review a brief exposition of the EGG phenomenon is made and the more prominent methods of signal analysis relevant to the EGG are presented and discussed.

Measurement techniques for urodynamic investigations.

Abstract: Important measurement techniques for investigating lower urinary tract function are flow and pressure measurements. The demands on urinary flowmeters and the measurement principles of balance type, rotating disc, dipstick, and air-displacement type are described. Urological pressure measurements are performed in the bladder, in the urethra, and in the abdominal cavity. Various fluid-filled and microtransducer systems are reviewed and demands for performance given. Differences in measuring a mechanical pressure, like in the urethra, and a fluid pressure in the bladder are discussed. Electromyography (EMG) technique is used to investigate various neurological disturbances in the lower urinary tract. The electrode technique is also described. Furthermore, techniques for incontinence detection are reviewed.