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Preoperative preparation. Value, perspective, and practice in patient care.

Preanesthesia preparation will continue to stimulate creativity and debate. Strategies for process improvement will take various shapes and require tools previously unfamiliar to many medical managers. At UNC Health System, anesthesiologists currently are committed to the centralized preanesthesia clinic approach used in PreCare. To date, their strategies have been validated by their institutional measures of success: a 0.7% first-case AM work-up rate, a 5% no PreCare visit rate, a 5% consent problem rate, and a 0% rejected specimen rate, with a 43% blood-draw rate for all patients. As their health system expands, however, other strategies and preparation modalities may become necessary. Telemedicine and Internet-dependent processes are appealing in the highly educated and technologically sophisticated marketplace. As the region becomes increasingly urbanized, local employment patterns prevent easy access to services, and functional compromises, such as bypassing PreCare or reliance on telephone or on-line interviews for preparation, may become necessary. The need to expand PreCare in the near future is already evident. As was found during initial planning, process improvement and space planning are enhanced by computer modeling. UNC Health System employed a proprietary animated simulation modeling (ASM) tool, MedModel, (ProModel, Orem, UT), although other techniques exist for the same purpose. Use of ASM as a strategy management tool allowed generation of ideal space-time-personnel scenarios that could expose potential problems before resources and physical restructuring occurred. ASM also can be used to compare data obtained from real-time observations to any reference scenario, including any that looks at economic measures of process, to help refine strategic visions before instituting tactical solutions. Used in this manner, ASM can reveal physical, temporal, personnel, and policy-related factors not otherwise seen as exerting effects on overall preprocedural preparation processes. Coupled with other classic planning techniques, such as time-motion studies, modified Delphi techniques, customer satisfaction surveys, benchmarking studies, and financial analysis and growth projections, ASM has helped enhance total quality management efforts in PreCare and other planning projects at UNC Health System. Other researchers have used other simulation techniques to assess clinic functions. Operational challenges that exist in preanesthesia preparation clinics place these clinics on the same level as other kinds of medical service clinics. The lessons learned in most clinic settings may be applied in a cross-wise manner to the preanesthesia clinic. Dexter contends that efficient delivery of care that respects patients' time and needs does more than assist the procedural preparation process; it also supports the physician's ethical obligation to the patient to give the best possible care. Documenting patient medical information, education efforts, and maintaining the medical-legal integrity of the preanesthesia preparation materials is another area of increasing concern and research. When hand-held computers capable of linking by way of infrared technologies or that use rented downloadable software to run specific applications become commonly used, the complexity of ensuring privacy will grow. The complexity, however, will not be insurmountable. Privacy problems exist in any information management system, whether paper-based (i.e., reducing redundant forms, eliminating indiscriminant photocopying of records), electronic-based (i.e., encryption, server access, system failure), or both. Electronic conveyance of medical information faces more legal and economic than technical hurdles. The penetration in US households of Internet services presently is less than 40%, whereas the penetration of personal computers is just over 50%. These figures are compared with penetration of corded telephones, televisions, and radios at levels greater tha

Anesthesiology↗

Effects of sepsis on recovery of the heart from 50 min ischemia.

Gram-negative sepsis as well as administration of agents that simulate or occur naturally subsequent to a septic challenge, can present as an oxidant stress to the myocardium. These stresses may then induce the development of protection of the heart from future stresses. This protection of the heart may occur in spite of the fact that sepsis itself induces myocardial dysfunction. In the present study we determined if sepsis is protective of a 50 min ischemic episode, one in which some degree of irreversible damage may occur. In addition we determined if sepsis-induced protection was still present when this ischemic challenge was imposed upon the heart of the alcoholic septic animal in which the chronic alcoholic state can lead to a potentiation of sepsis induced cardiac depression. Thus animals were fed an ethanol containing diet or a control diet for 8-10 weeks and were then made septic by the administration of Escherichia coli into the dorsal subcutaneous space. Control animals received sterile saline. The following day, hearts were studied in the isovolumic beating preparation and, after basal function was assessed, hearts were made globally ischemic for 50 min and reperfused for 30 min. Left ventricular pressure was continuously monitored and coronary flow was measured at specific intervals. After ischemia and reperfusion, hearts from control- and alcohol-fed animals that were nonseptic showed significantly decreased left ventricular performance. Ventricular pressure development of hearts from septic and alcoholic septic rats was not significantly decreased after ischemia and reperfusion compared to preischemia although preischemic function was significantly lower in the sepsis groups compared to their nonseptic control groups.(ABSTRACT TRUNCATED AT 250 WORDS)

Adenosine Triphosphate↗

A mathematical analysis on public goods games in the continuous space.

We consider the population dynamics of two competing species sharing the same resource, which is modeled by the carrying capacity term of logistic equation. One species (farmer) increases the carrying capacity in exchange for a decreased survival rate, while the other species (exploiter) does not. As the carrying capacity is shared by both species, farmer is altruistic. The effect of continuous spatial structure on the performance of such strategies is studied using the reaction diffusion equations. Mathematical analysis on the traveling wave solution of the system revealed; (1) Farmers can never expel exploiters in any traveling wave solution. (2) The expanding velocity of the exploiter population invading the farmer population can be analytically determined and it depends only on a cost of altruism and the diffusion coefficients while it is independent of the benefit of altruism. (3) When the effect of altruism is small, the dynamics of the invasion of exploiters obeys the Fisher-KPP equation. Numerical calculations confirm these results.

Altruism↗

Na/K pump current and [Na](i) in rabbit ventricular myocytes: local [Na](i) depletion and Na buffering.

Na/K pump current (I(pump)) and intracellular Na concentration ([Na](i)) were measured simultaneously in voltage-clamped rabbit ventricular myocytes, under conditions where [Na](i) is controlled mainly by membrane transport. Upon abrupt pump reactivation (after 10-12 min blockade), I(pump) decays in two phases. Initially, I(pump) declines with little [Na](i) change, whereas the second phase is accompanied by [Na](i) decline. Initial I(pump) sag was still present at external [K] = 15 mM, but prevented by [Na](i) approximately 100 mM. Initial I(pump) sag might be explained by subsarcolemmal [Na](i) ([Na](SL)) depletion produced by rapid Na extrusion and I(pump). Brief episodes of pump blockade allowed [Na](SL) repletion, since peak postblockade I(pump) exceeded I(pump) at the end of previous activation (without appreciably altered global [Na](i)). The apparent K(m) for [Na](i) was higher for continuous I(pump) activation than peak I(pump) (14.1 +/- 0.2 vs. 11.2 +/- 0.2 mM), whereas that based on d[Na](i)/dt matched peak I(pump) (11.6 +/- 0.3 mM). [Na](SL) depletion (vs. [Na](i)) could be as high as 3 mM for [Na](i) approximately 18-20 mM. A simple diffusion model indicates that such [Na](SL) depletion requires a Na diffusion coefficient 10(3)- to 10(4)-fold below that expected in bulk cytoplasm (although this could be subsarcolemmal only). I(pump) integrals and [Na](i) decline were used to estimate intracellular Na buffering, which is slight (1.39 +/- 0.09).

Adaptation, Physiological↗

Two-dimensional Fourier representation used in the bioelectric forward problem.

The objective of this paper is the application of two-dimensional discrete Fourier transformation for solving the integral equation of the bioelectric forward problem. Therefore, the potential, the source term, and the integral equation kernel are assumed to be sampled at evenly spaced intervals. Thus the continuous functions of the problem domain can be expressed by their two-dimensional discrete Fourier transform in the spatial frequency domain. The method is applied to compute the surface potential generated by an eccentric dipole in a homogeneous spherical conducting medium. The integral equation for the potential is solved in the spatial frequency domain and the value of the potential at the sampling points is obtained from inverse Fourier transformation. The solution of the presented method is compared to both, an analytic solution and a solution gained from applying the boundary element method. Isoparametric quadrilateral boundary elements are used for modeling the spherical volume conductor in the boundary element solution, while in the two-dimensional Fourier transformation method the volume conductor is represented by a parametric boundary surface approximation.

Computer Simulation↗

Adaptive meshing technique applied to an orthopaedic finite element contact problem.

Finite element methods have been applied extensively and with much success in the analysis of orthopaedic implants. Recently a growing interest has developed, in the orthopaedic biomechanics community, in how numerical models can be constructed for the optimal solution of problems in contact mechanics. New developments in this area are of paramount importance in the design of improved implants for orthopaedic surgery. Finite element and other computational techniques are widely applied in the analysis and design of hip and knee implants, with additional joints (ankle, shoulder, wrist) attracting increased attention. The objective of this investigation was to develop a simplified adaptive meshing scheme to facilitate the finite element analysis of a dual-curvature total wrist implant. Using currently available software, the analyst has great flexibility in mesh generation, but must prescribe element sizes and refinement schemes throughout the domain of interest. Unfortunately, it is often difficult to predict in advance a mesh spacing that will give acceptable results. Adaptive finite-element mesh capabilities operate to continuously refine the mesh to improve accuracy where it is required, with minimal intervention by the analyst. Such mesh adaptation generally means that in certain areas of the analysis domain, the size of the elements is decreased (or increased) and/or the order of the elements may be increased (or decreased). In concept, mesh adaptation is very appealing. Although there have been several previous applications of adaptive meshing for in-house FE codes, we have coupled an adaptive mesh formulation with the pre-existing commercial programs PATRAN (MacNeal-Schwendler Corp., USA) and ABAQUS (Hibbit Karlson and Sorensen, Pawtucket, RI). In doing so, we have retained several attributes of the commercial software, which are very attractive for orthopaedic implant applications.

Biomechanical Phenomena↗

Circadian rhythm in glomerular transport of macromolecules through large pores and shunt pathway.

In previous studies we have demonstrated that the circadian rhythm in renal clearance of serum proteins is more pronounced than the variability in glomerular filtration rate, and that the highest day-night fluctuations are found for the largest proteins. To analyze whether additional circadian rhythmicity in size-selective glomerular transport could explain these phenomena, we measured renal clearances of inulin and dextrans in a range of 30 to 90 A over a period of one day and compared these data with renal clearance in proteins. Eight patients with nephrotic syndrome and a GFR > 60 ml/min and 6 healthy volunteers were studied during a protocol of bed rest and spaced protein and fluid intake. After administration of a loading dose, inulin and dextran were continuously infused. Blood and urine were sampled every three hours. In patients, but not in normals, fractional clearances of dextrans larger than 45 A showed a circadian rhythm with a peak in daytime and a close phase-relationship with the rhythm in GFR. The day-night differences were the most pronounced for the largest dextrans. Analysis of the day-night differences in a computer model showed circadian variability in transport through the shunt pathway and through large pores. These results can, to an important degree, explain our previous observations on circadian variability in renal clearance of proteins in patients with nephrotic syndrome.

Adult↗

Virtual endoscopy--comparison with colonoscopy in the detection of space-occupying lesions of the colon.

BACKGROUND AND STUDY AIMS: A new technique has been described which combines abdominal helical computed tomography (CT) scanning and virtual reality computer technology, known as virtual colonoscopy (VC); the reconstructed images provide a simulation of the interior of the colon as viewed by endoscopy. We compared VC with conventional colonoscopy in patients with suspected or known colonic neoplasia. PATIENTS AND METHODS: A total of 38 patients, in whom there was a high likelihood of colonic polyps or cancer, underwent a noncontrast helical CT scan of the abdomen and pelvis after regular colonoscopy bowel preparation. The images were reconstructed into a VC presentation and compared with the subsequent conventional colonoscopy in a blinded manner. RESULTS: Conventional colonoscopy identified a total of 24 polyps 5 mm or greater. VC correctly identified five of 13 polyps 5-9 mm in size, and ten of 11 lesions greater than or equal to 10 mm in diameter. The reasons for four missed lesions were identified as being secondary to a collapsed rectum in two patients and stool in the right colon in two patients. The sensitivity and specificity per patient of VC for lesions greater than or equal to 5 mm were 66.7% and 75.0% respectively, and for lesions greater than 1 cm were 90.0% and 82.1%, respectively. CONCLUSIONS: Virtual colonoscopy is feasible, well tolerated, and capable of detecting most lesions greater than 10 mm in diameter. This technique is continuing to be developed and warrants further evaluation as a diagnostic and screening tool in colorectal neoplasia.

Adult↗

Influence of intercellular clefts on potential and current distribution in a multifiber preparation.

A theoretical model is presented for current and voltage clamp of multifiber bundles in a double sucrose gap. Attention is focused on methodological errors introduced by the intercellular cleft resistance. The bundle is approximated by a continuous geometry. Voltage distribution, as a function of radial distance and time, is defined by a parabolic partial differential equation which is specified for different membrane characteristics. Assuming a linear membrane, analytical solutions are given for current step and voltage step conditions. The theoretical relations (based on Bessel functions) may be used to calculate membrane conductance and capacity from experimental clamp data. The case of a nonlinear membrane with standard Hodgkin-Huxley kinetics for excitatory Na current is treated assuming maximum Na conductances (gNa) of 120, 10, and 1 mmho/cm2. Numerical simulations are presented for potential and current distribution in a bundle of 60 microns diameter during depolarizing voltage steps. Adequate voltage control is restricted to the peripheral fibers of the bundle whereas the membrane potential of the inner fibers deviates from the command level during early inward current, tending to the Na equilibrium potential. In the peak current-voltage diagram the loss of voltage control is reflected by an increased steepness of the negative region and a decreased slope conductance of the positive region. With gNa = 120 mmho/cm2, the positive slope conductance is approximately 25% of the slope expected from ideal space clamping. With the lower values of gNa, the slope conductance ratio is in the order of 50%. Implications of the results for an experimental voltage clamp analysis of early inward current on multifiber preparations are discussed.

Animals↗

Effects of continuous positive airway pressure after oleic acid-induced lung injury in dogs.

The physiologic effects of continuous positive airway pressure (CPAP) of 5,10,15, and 20 cm H2O during spontaneous ventilation were studied in six anesthetized dogs with simulated respiratory distress syndrome (RDS) induced by iv infusion of oleic acid and in three normal controls. After oleic acid, mean PaO2 dropped to 63.6 +/- 3.1 mm Hg while breathing 100% oxygen and mean shunt fraction was 48.3 +/- 3.0%. PaO2 and shunt fraction improved significantly at the two highest levels of CPAP (e.g.,PaO2 271.3 +/- 41.3 mm Hg and shunt fraction 17.8 +/-2.2% at 20 cm H2O CPAP). Mean mixed venous PO2 rose from 37.4 +/- 1.5 mm Hg with no CPAP TO 60.8 +/- 3.1 mm Hg at 20 cm H2O CPAP. Tissue oxygenation appeared to improve during CPAP, since cardiac output, oxygen delivery, and serum lactate were not significantly affected and mixed venous PO2 rose significantly. However, significant hypoventilation occurred at all but the lowest level of CPAP, mean PaCO2 rising from 44.1 +/- 1.8 mm Hg with no CPAP to 77.6 +/-6.8 mm Hg at 20 cm H2O CPAP. The hypoventilation during CPAP is consistent with increased work of breathing due to a combination of decreased lung compliance and increased dead space ventilation due to rapid, shallow breathing.

Animals↗

Life-cycle experiments of medaka fish aboard the international space station.

Fish are the most likely candidates to be the first vertebrate to live their life cycle aboard the International Space Station (ISS). In the space-shuttle experiment using medaka, the fry born in space had the same number of germ cells as the ground control fish, and these germ cells later developed to produce the offspring on the ground. Fry hatched in space did not exhibit any looping behavior regardless of their strain, visual acuity, etc. The aquatic habitat (AQH) is a space habitat designed for long-term breeding of medaka, zebrafish and Xenopus, and recent advancements in this hardware also support fish life-cycle experiments. From the crosses between two strains, fish having good eyesight and less sensitivity to gravity were obtained, and their tolerance to microgravity was tested by parabolic flight using an airplane. The fish exhibited less looping and no differences in degree of looping between light and dark conditions. These are possible candidates for the first adult medaka (parent fish) to start a life cycle aboard ISS. Embryos were treated with a three-dimensional clinostat. Such simulated microgravity caused no differences in tissue architecture or in gene expression within the retina, nor in formation of cartilage (head skeleton). Otolith formation in embryos and fry was investigated for wild-type and mutant (ha) medaka. The ha embryos could not form utricular otoliths. They formed saccular otoliths but with a delay. Fry of the mutant fish lacking the utricular otoliths are highly light-dependent at the time of hatching, showing a perfect dorsal-light response (DLR). As they grow, they eventually shift from being light dependent to gravity dependent. Continuous treatment of the fry with altered light direction suppressed this shift to gravity dependence. Being less dependent on gravity, these fish can serve as model fish in studying the differences expected for the fish that have experienced a life cycle in microgravity.

Animals↗

Potential use of computer simulation in treatment of burns with special regard to oedema formation.

The potential usefulness of computer-based 'patient-simulators' in burn care is discussed and illustrated in the special case of oedema formation in three patients with severe thermal injuries. The present model was derived from a model by Wiederhielm (1978), and modified to be applicable to thermal injuries. The model seems to describe accurately the oedema formation as well as the distribution and composition of local and general oedema. Following thermal injuries the general oedema is characterized by typical disturbances in the circulatory state variables such as capillary pressure and plasma colloid osmotic pressure. The net water flow is increased because of an increase in filtration rate and a decrease in reabsorption rate. The resulting interstitial oedema leads to dilution of the free water proteins and a decrease in interstitial colloid osmotic pressure. The elevation of interstitial pressure leads to an increase in lymph flow. The local oedema is caused by changes in both the circulatory system and the interstitial space. There is an increase in both water and protein net flows. The latter is due to increased protein leakage to the interstitium. This results in an interstitial oedema with a higher protein concentration in free water than in general oedema. Because of the higher protein concentration in free water, the interstitial colloid osmotic pressure is at all times larger than the corresponding pressure in general oedema. In all three patients the general shape of the simulated and measured oedema curve were the same, but with minor differences in numerical values. It is interesting to note that the general oedema was larger than the local oedema in all three extensively burned patients. The results from the present investigation indicate the importance of monitoring either the total plasma protein concentration or the plasma colloid osmotic pressure and small vein pressure for guidance of a proper fluid resuscitation. The steady state 'oedema vs. venous pressure'-diagram obtained may also be used for evaluating the dynamic effects on general oedema formation due to changes in venous pressure and plasma protein concentration. It thus turned out that for a typical thermal injury with a decreased small vein pressure a continuous colloid infusion is to be preferred instead of an intermittent model of administration.

Blood Proteins↗

A mathematical model for fibroblast and collagen orientation.

Due to the increasing importance of the extracellular matrix in many biological problems, in this paper we develop a model for fibroblast and collagen orientation with the ultimate objective of understanding how fibroblasts form and remodel the extracellular matrix, in particular its collagen component. The model uses integrodifferential equations to describe the interaction between the cells and fibers at a point in space with various orientations. The equations are studied both analytically and numerically to discover different types of solutions and their behavior. In particular we examine solutions where all the fibroblasts and collagen have discrete orientations, a localized continuum of orientations and a continuous distribution of orientations with several maxima. The effect of altering the parameters in the system is explored, including the angular diffusion coefficient for the fibroblasts, as well as the strength and range of the interaction between fibroblasts and collagen. We find the initial conditions and the range of influence between the collagen and the fibroblasts are the two factors which determine the behavior of the solutions. The implications of this for wound healing and cancer are discussed including the conclusion that the major factor in determining the degree of scarring is the initial deposition of collagen.

Animals↗

Exploring the free-energy landscape of a short peptide using an average force.

The reversible folding of deca-alanine is chosen as a test case for characterizing a method that uses an adaptive biasing force (ABF) to escape from the minima and overcome the barriers of the free-energy landscape. This approach relies on the continuous estimation of a biasing force that yields a Hamiltonian in which no average force is exerted along the ordering parameter xi. Optimizing the parameters that control how the ABF is applied, the method is shown to be extremely effective when a nonequivocal ordering parameter can be defined to explore the folding pathway of the peptide. Starting from a beta-turn motif and restraining xi to a region of the conformational space that extends from the alpha-helical state to an ensemble of extended structures, the ABF scheme is successful in folding the peptide chain into a compact alpha helix. Sampling of this conformation is, however, marginal when the range of xi values embraces arrangements of greater compactness, hence demonstrating the inherent limitations of free-energy methods when ambiguous ordering parameters are utilized.

Alanine↗

Hybrid global optimization algorithms for protein structure prediction: alternating hybrids.

Hybrid global optimization methods attempt to combine the beneficial features of two or more algorithms, and can be powerful methods for solving challenging nonconvex optimization problems. In this paper, novel classes of hybrid global optimization methods, termed alternating hybrids, are introduced for application as a tool in treating the peptide and protein structure prediction problems. In particular, these new optimization methods take the form of hybrids between a deterministic global optimization algorithm, the alphaBB, and a stochastically based method, conformational space annealing (CSA). The alphaBB method, as a theoretically proven global optimization approach, exhibits consistency, as it guarantees convergence to the global minimum for twice-continuously differentiable constrained nonlinear programming problems, but can benefit from computationally related enhancements. On the other hand, the independent CSA algorithm is highly efficient, though the method lacks theoretical guarantees of convergence. Furthermore, both the alphaBB method and the CSA method are found to identify ensembles of low-energy conformers, an important feature for determining the true free energy minimum of the system. The proposed hybrid methods combine the desirable features of efficiency and consistency, thus enabling the accurate prediction of the structures of larger peptides. Computational studies for met-enkephalin and melittin, employing sequential and parallel computing frameworks, demonstrate the promise for these proposed hybrid methods.

Algorithms↗

Can free energy calculations be fast and accurate at the same time? Binding of low-affinity, non-peptide inhibitors to the SH2 domain of the src protein.

The usefulness of free-energy calculations in non-academic environments, in general, and in the pharmaceutical industry, in particular, is a long-time debated issue, often considered from the angle of cost/performance criteria. In the context of the rational drug design of low-affinity, non-peptide inhibitors to the SH2 domain of the (pp60)src tyrosine kinase, the continuing difficulties encountered in an attempt to obtain accurate free-energy estimates are addressed. free-energy calculations can provide a convincing answer, assuming that two key-requirements are fulfilled: (i) thorough sampling of the configurational space is necessary to minimize the statistical error, hence raising the question: to which extent can we sacrifice the computational effort, yet without jeopardizing the precision of the free-energy calculation? (ii) the sensitivity of binding free-energies to the parameters utilized imposes an appropriate parametrization of the potential energy function, especially for non-peptide molecules that are usually poorly described by multipurpose macromolecular force fields. Employing the free-energy perturbation method, accurate ranking, within +/-0.7 kcal/mol, is obtained in the case of four non-peptide mimes of a sequence recognized by the (pp60)src SH2 domain.

Binding Sites↗

Changes in myocardial metabolism and transcardiac electrolytes during simulated ventricular tachycardia: effects of beta-adrenergic blockade.

Myocardial ischemia, electrolyte changes, and fluctuations in autonomic tone may play an important role in the presentation of malignant ventricular arrhythmias. beta-Adrenoceptor blocking agents have been shown to decrease the incidence of ventricular fibrillation and sudden cardiac death in patients with coronary artery disease. Therefore we investigated the changes in myocardial metabolism and transcardiac electrolytes during simulated ventricular tachycardia before and after beta-adrenergic blockade. Six patients with normal coronary arteries (group 1) and 12 patients with documented coronary artery disease (group 2) were included in the study. The right ventricle was paced with electrode catheters to a constant cycle length of 400 msec for 3 minutes. Blood samples were withdrawn simultaneously from the coronary sinus and femoral artery to determine the transcardiac differences in metabolic variables and electrolytes before the pacing, at the end of the pacing, and 2 minutes thereafter. After pacing, the patients were given intravenous propranolol (0.15 mg/kg), and the protocol was repeated. Intraarterial blood pressure and electrocardiogram were monitored continuously. There was a rapid decline of the mean arterial blood pressures after initiation of the pacing in both study groups, whereafter the pressures began to rise. Propranolol somewhat blunted the blood pressure recovery, especially in group 2. Norepinephrine levels increased during the pacing in both patient groups, and the increase was accentuated by beta-adrenergic blockade. The femoroarterial coronary sinus difference in lactate turned negative, and pH, PCO2 and potassium differences increased in group 2 during pacing. However, the myocardial energy state remained relatively good as estimated from the nonsignificant change in the transcardiac differences of the plasma adenosine catabolites. There were no changes in the metabolic variables or transcardiac electrolytes in group 1 patients during pacing. Propranolol did not prevent the metabolic ischemia, but it did prevent the pacing-induced decrease in coronary sinus potassium and increase in transcardiac potassium difference. Propranolol also decreased arterial levels of free fatty acids and their extraction in group 2 patients during pacing. In conclusion, blood pressure decay during simulated ventricular tachycardia is followed by instantaneous sympathoadrenergic activation. In patients with coronary artery disease, this process is accompanied by metabolic ischemia and net transfer of extracellular potassium into the intracellular space. The metabolic and electrolyte changes may result in alterations of electrophysiologic millieau, thereby also modifying the clinical characteristics of ventricular tachycardia. Propranolol decreases arterial levels of free fatty acids and prevents changes in transcardiac electrolytes observed in coronary artery disease patients during simulated ventricular tachycardia. These effects of propranolol may be of clinical significance.

Adenine Nucleotides↗

Real-time tracking of vertebral body movement with implantable reference microsensors.

OBJECTIVE: In the spine, navigation techniques serve mainly to control and accurately target insertion of implants. The main source of error is that the spine is not a rigid organ, but rather a chain of semiflexible movement segments. Any intraoperative manipulation of the patient alters the geometry and volumetry as compared to the 3D volume model created from the image data. Thus, the objective of the study was to implement the theoretical principle of microsensor referencing in a model experiment and to clarify which anatomical structures are suitable for intermittent implantation of positional sensors, as illustrated with cervical vertebral bodies. MATERIALS AND METHODS: Laboratory tests were conducted using 70 models of human cervical vertebral bodies. The first experiment investigated whether arbitrary movements of vertebral bodies can be tracked with the positional information from the implanted microsensors. The accuracy of this movement monitoring was determined quantitatively on the basis of positional error measurement. In the second experiment, different ventral and dorsal surgical operations were simulated on five models of the cervical spine. Quantifiable measurement values such as the spatial extension of the intervertebral space and the relative positions of the planes of the upper plates were determined. RESULTS: With respect to the differing anatomy of the individual vertebral bodies of the cervical spine, the sensors could be placed securely with a 5x2 mm drill. The registration error (RE) was determined as a root mean square error. The mean value was 0.9425 mm (range: 0.57-1.2 mm; median: 0.9400 mm; SD: 0.1903 mm). The precision of the movement monitoring of the vertebral body was investigated along its three main axes. The error tolerance between post-interventional 3D reconstruction and direct measurement on the model did not exceed 1.3 mm in the distance measurements or 2.5 degrees in the angular measurements. The tomograms on the system monitor could be updated in close to real time on the basis of the positional information from the reference sensor. CONCLUSIONS: Motion sensors implanted into the vertebral bodies communicated any change in position to the navigation system in close to real time, thus enabling the preoperative image data set to be updated. The experiments described could ultimately show that continuous real-time visualization of individual vertebral body movements along the movement axes (flexion-extension, tilting and rotation) is possible with high accuracy using implantable microsensors. A future application of such microsensors might be the integration of robot systems into spinal microsurgery.

Cervical Vertebrae↗