Stability of halo orbits.

We predict new populations of trapped nonequatorial ("halo") orbits of charged dust grains about an arbitrary axisymmetric planet. Simple equilibrium and stability conditions are derived, revealing dramatic differences between positively and negatively charged grains in prograde or retrograde orbits. Implications for the Cassini mission to Saturn are discussed.

Customer service: the key to remaining competitive in managed care.

The health care industry is undergoing a rapid transformation to meet the ever-increasing needs and demands of its patient population. Employers and managed care organizations are demanding better service and higher quality care, while providers are trying to tackle reimbursement cutbacks, streamlining of services, and serving a diverse population. Providers have begun to realize that to overcome these obstacles and meet the needs of their health plans and consumers, they must focus on the demands of their customers. Health care organizations have found they can meet the demands of both the consumer and the managed care industry through initiating and maintaining a customer service program. This essay explains the importance of customer service and its link to success in the managed care environment.

HMO Web sites: what do they tell consumers?

Despite the growth of managed care, little is known about health plans' web sites. This paper reports on a review of the web sites of the 25 largest HMOs in the United States to identify whether their content included information deemed important by consumers: quality, price, provider network, personal health information, and customer service features. Findings show variation in the content of HMO web sites, with sites emphasizing customer service features, the provider network, and personal health information. Two-thirds of HMOs offer information on quality; however, pricing information is offered by only four plans. Implications for future differentiation of HMO web sites are discussed.

Monitoring blood levels of selected drugs. Remember to factor in the many confounding variables.

Appropriate use of various pharmacologic agents involves not only awareness of therapeutic indications and side effects but also familiarity with clinical use and timing of blood level monitoring. The effective as well as the toxic level of antiepileptic drugs varies widely among patients, so the patient's response is more important than the serum drug level. These agents may interact with other disease states, other drugs, and even other antiepileptic agents. Because of digoxin's long half-life and the effect of physical exercise on serum concentration, the timing of serum collection is important. The usefulness of measuring amiodarone serum concentrations is controversial, but findings may help identify patients at risk for side effects related to the drug. Procainamide has a very short half-life and concentrations change over a short period, so blood levels of this agent should be measured before administration of a dose. The dose of levothyroxine required to restore a normal thyroid hormone level varies with age, coexistent conditions, and use of other medications. After the appropriate dose is determined, follow-up monitoring yearly is necessary (more often in the elderly). Efficacy and toxicity of theophylline are directly related to serum concentrations, and a reduced target level of 5 to 15 micrograms/mL has recently been suggested. Proper monitoring is important, because metabolic changes and drug interactions can cause either subtherapeutic or toxic levels.

Thyroid disease among the Rongelap and Utirik population--an update.

In 1954, 253 Marshallese were accidentally exposed to fallout radiation from the hydrogen bomb, BRAVO. The Marshall Islands Medical Program (MIMP) was established by the Department of Energy in 1955 to monitor and treat radiation-related disease pursuant to this accident. Medical teams from Brookhaven National Laboratory, a federal institution, regularly visit the Marshall Islands to give medical care to the exposed population. The most significant complication of the exposure has been found to be thyroid disease due to the ingestion of radioactive iodides from the fallout. In 1963 the first thyroid nodules were found in Rongelap subjects and in 1969 in Utirik. Non-neoplastic adenomatous nodules were associated with higher doses of radiation and neoplastic nodules developed in individuals receiving lower doses of radiation. Women were more susceptible to the development of palpable thyroid nodules than men. In 1994 the MIMP initiated examination of the thyroid by ultrasound to supplement the clinical examination. One hundred and sixty-four patients were evaluated. No significant differences were found in the incidence of thyroid nodules or the mean nodule count between the three groups of Rongelap and Utirik exposed and a comparison patient population. There was no significant difference in the incidence of thyroid nodules in males vs. females. Five exposed patients were referred for surgical excision of a nodule detected only by ultrasound. These ultrasound findings are unexpected in that females are known to have a higher incidence of thyroid disease than males and we expected that the incidence of ultrasound nodules would be higher in the exposed population.

Analysis of gastrulation: different types of gastrulation movement are induced by different mesoderm-inducing factors in Xenopus laevis.

In this paper we analyze the control of gastrulation in Xenopus laevis. Our approach takes advantage of the observation that mesoderm-inducing factors such as activin, FGF and BMP-4 induce presumptive ectodermal cells to undergo gastrulation-like movements. Activin, for example, makes intact animal pole regions undergo convergent extension and causes individual cells to spread and migrate on a fibronectin-coated substrate. By varying the concentrations of the growth factors to which animal pole cells are exposed, and by applying them in different combinations, we show how graded distributions of a combination of factors could establish the correct spatial and temporal patterns of gastrulation in the Xenopus embryo. The distributions we propose support and develop the model previously suggested by Green et al. (1992) to account for the spatial patterns of gene activation in the early embryo.

SEM studies of surfaces hidden within bulk tissue: a simple technique to control the position and orientation of dry fracture planes.

A simple, reliable method for the exposure of specific structures hidden within bulk tissue for viewing by SEM is described. The method employs dry fracture after critical-point drying and differs from other dry fracture methods in that the fracture plane is 'engineered' by the experimenter, thus overcoming the tendency for natural planes of weakness within the specimen exclusively to define the fracture plane. The technique retains the simplicity of the very high-quality in situ cellular relationships normally associated with random dry fracture. Attention is given to novel means of circumventing the artefacts that are normally a problem with dry fracture techniques.

Are beta 1 integrins involved in Xenopus gastrulation?

The molecular basis of vertebrate gastrulation is poorly understood. Work on urodele amphibians has implicated beta 1-containing integrins, but the limited information available for Xenopus indicates otherwise: peptides containing the RGD sequence do not inhibit gastrulation and induction of cell spreading in presumptive ectodermal cells by activin is not accompanied by an increase in synthesis of integrin beta 1. Here we report that beta 1-containing integrins are, nevertheless, the principal fibronectin receptors in the Xenopus gastrula, although their cell surface levels are low. Antibodies recognizing the external domain of the molecule can, unlike peptides containing the RGD site, block gastrulation when introduced into the blastocoel. These results allow us to propose a model to explain the role of integrin beta 1 in Xenopus gastrulation.

Mesoderm-inducing factors and the control of gastrulation.

One of the reasons that we know so little about the control of vertebrate gastrulation is that there are very few systems available in which the process can be studied in vitro. In this paper, we suggest that one suitable system might be provided by the use of mesoderm-inducing factors. In amphibian embryos such as Xenopus laevis, gastrulation is driven by cells of the mesoderm, and the mesoderm itself arises through an inductive interaction in which cells of the vegetal hemisphere of the embryo emit a signal which acts on overlying equatorial cells. Several factors have recently been discovered that modify the pattern of mesodermal differentiation or induce mesoderm from presumptive ectoderm. Some of these mesoderm-inducing factors will also elicit gastrulation movements, which provides a powerful model system for the study of gastrulation, because a population of cells that would not normally undertake the process can be induced to do so. In this paper, we use mesoderm-inducing factors to attempt to answer four questions. How do cells know when to gastrulate? How do cells know what kind of gastrulation movement to undertake? What is the cellular basis of gastrulation? What is the molecular basis of gastrulation?

Triphasic behavioral response of motor units to submaximal fatiguing exercise.

We have measured the firing rate and amplitude of 4551 motor unit action potentials (MUAPs) recorded with concentric needle electrodes from the brachial biceps muscles of 10 healthy young adults before, during, and after 45 minutes of intermittent isometric exercise at 20% of maximum voluntary contraction (MVC), using an automatic method for decomposition of electromyographic activity (ADEMG). During and after exercise, MUAPs derived from contractions of 30% MVC showed progressive increase in mean firing rate (P less than or equal to .01) and amplitude (P less than or equal to .05). The firing rate increase preceded the rise in mean amplitude, and was evident prior to the development of fatigue, defined as reduction of MVC. Analysis of individual potentials revealed that the increase in firing rate and in amplitude reflected different MUAP subpopulations. A short-term (less than 1 minute) reduction in MUAP firing rates (P less than or equal to .05) was also observed at the onset of each test contraction. These findings suggest that motor units exhibit a triphasic behavioral response to prolonged submaximal exercise: (1) short-term decline and stabilization of onset firing rates, followed by (2) gradual and progressive increase in firing rates and firing variability, and then by (3) recruitment of additional (larger) motor units. The (2) and (3) components presumably compensate for loss of force-generating capacity in the exercising muscle, and give rise jointly to the well-known increase in total surface EMG which accompanies muscle fatigue.

Motor unit firing rates and firing rate variability in the detection of neuromuscular disorders.

We have used automatic decomposition electromyography (ADEMG) to study 41 muscles in 29 patients with well-defined peripheral and central motor disorders. In motor neuron diseases motor unit action potentials (MUAPs) showed increased amplitudes, firing rates and firing variability. Relatively large MUAPs sometimes were not identified by the computer program if they lacked sufficient high-frequency signal content, or were too variable in shape. In myopathies the MUAPs showed reduced amplitudes, durations and turns, and sometimes dramatic increases in firing rates. Also, the mean number of MUAPs per recording site was often increased, indicating excessive recruitment. In polymyositis (the best studied myopathy) the nature and magnitude of the MUAP shape and firing abnormalities were usually similar at different levels of contractile force, suggesting that motor units are affected without regard to recruitment order. In upper motor neuron paresis (multiple sclerosis), the shape properties of the MUAPs were normal, but mean firing rates were reduced, and firing variability increased. These findings confirm many of the traditional criteria for distinguishing neurogenic from myopathic disease electrophysiologically at the level of the individual MUAP. In addition, they demonstrate the potential diagnostic sensitivity of MUAP firing rate measurements for detecting neuromuscular dysfunction, and for differentiating between some cases of central and peripheral paresis, but not for distinguishing peripheral neurogenic from myopathic weakness, since firing rates tend to increase in both. Increased firing rate variability may be a marker of central or peripheral neurogenic weakness.

Properties of motor unit action potentials recorded with concentric and monopolar needle electrodes: ADEMG analysis.

We compared the configurational and firing properties of 7270 motor unit action potentials (MUAPs) recorded with either concentric (CNE) or monopolar (MNE) needle electrodes from the brachial biceps and anterior tibial muscles of 10 healthy young adults (mean age 27 +/- 4.5 years) using automatic decomposition electromyography (ADEMG). In both muscles, mean MUAP amplitude, rise rate, and number of turns were significantly greater when recorded with MNE (paired t-test, P less than 0.001 in each case). Similar findings were observed at all three tested levels of isometric contractile force: threshold, 10% of maximum voluntary contraction (MVC), and 30% MVC. In contrast, there was no significant difference between electrode types on measurements of mean MUAP duration or firing rate (P greater than 0.05 in each case). These findings indicate that it is acceptable to generalize normative data on MUAP duration and firing rate from one electrode type to another, but that measures of MUAP amplitude and complexity require independent normative databases.

Influence of contractile force on properties of motor unit action potentials: ADEMG analysis.

We have used automatic decomposition electromyography (ADEMG) to measure the configurational and firing properties of 13,206 motor unit action potentials (MUAPs) in the brachial biceps, brachial triceps and anterior tibial muscles of 30 healthy adults (22 men, 8 women; mean age 48.6 +/- 16.9 years, range 20-76) at three levels of isometric contractile force: threshold, 10% of maximum voluntary contraction (MVC), and 30% MVC. In all muscles, the increment in contractile force from threshold to 10% MVC was associated with a significant (P less than 0.05, paired t-test) increase in mean MUAP firing rate and number of turns per MUAP. The increment from 10% to 30% MVC led to highly significant (P less than 0.005) increase in mean firing rate, number of turns, amplitude and rise rate. Each force increment was associated with an increase in the number of simultaneously-active MUAPs per recording site; and with a significant decrease in mean MUAP duration in all muscles, due to noise-dependency of the duration measurement. Quantitatively, the changes in MUAP properties with force were comparable to or exceeded the effects of age, gender differences, or intermuscular variability. Test-retest measurements 2 years apart in a subgroup of young adults showed good correspondence of mean MUAP properties with force standardization. These results demonstrate that contractile force is a major determinant of MUAP shape and behavior properties, and so must be precisely measured or controlled in clinical EMG studies.

Age effects on properties of motor unit action potentials: ADEMG analysis.

We have measured the configurational and firing properties of 13,206 motor unit action potentials (MUAPs) from the brachial biceps, brachial triceps, and anterior tibial muscles in 10 young (20-40 years), 10 middle-aged (40-60 years), and 10 elderly (60-80 years) normal individuals, using an automatic method for decomposition of the electromyographic (EMG) interference pattern (ADEMG). Recording were made during stable isometric contractions at threshold, 10%, and 30% of maximum voluntary contraction using standard concentric needle electrodes. At supra-threshold forces, an average of 5.9 simultaneously active MUAPs were identified at each recording site. Mean amplitudes, durations, and numbers of turns all increased linearly with age in both low-threshold and high-threshold MUAPs (p less than 0.01), suggesting an ongoing process of progressive denervation and compensatory reinnervation. Mean MUAP firing rates decreased with age (p = 0.01) when force was measured proportionately, but not when measured absolutely. In a subgroup of 12 age-matched gender pairs, men had larger mean MUAP amplitudes, rise rates, and numbers of turns (p less than 0.05), probably reflecting larger muscle fiber diameters. These findings amplify previous observations from traditional analysis of lowest-threshold single MUAPs; establish a base of normative adult data for ADEMG; and further validate the clinical applicability of rapid, automatic EMG decomposition.

Respiratory teaching of patients: how effective is it?

In order to evaluate the effectiveness of a structured respiratory teaching programme, clients with chronic obstructive pulmonary disease who were hospitalized in a 13-month period both prior to and following the implementation of the programme were compared on several parameters to assess changes in their ability to manage their disabilities. The study demonstrated that clients who participated in the teaching sessions had fewer hospital admissions, shorter lengths of hospital stay, and longer stays in the community. A sub-group of clients who were or had been in at-risk occupations was identified as representing the 'revolving door' client with shorter hospital and community stays. The teaching programme is described as well as specific elements that can be used for a shorter teaching experience to respond to time constraints when necessary.

Visual electrical evoked potentials: evaluation of ocular injuries.

Clinical neurophysiologists may be asked to participate in the evaluation of patients with injured eyes. We describe a method for eliciting evoked potentials of cerebral origin using electrical pulse stimuli delivered to the globe of the eye through a contact lens electrode mounted on the cornea. These visual electrical evoked potentials (VEEPs) are contrasted with conventional flash visual evoked potentials in normal subjects and in 19 eyes of 17 patients with severe ocular damage, mostly recent trauma. The findings suggest that the site of transcorneal electrical excitation is not the photoreceptors, but more likely one of the nerve cell layers of the retina. VEEP recordings offer a way to circumvent the opacification of the ocular media by blood, which may otherwise hamper the evaluation of retinal function in the injured eye. Preserved VEEP response does not necessarily predict the capacity to recover visual function. Absent VEEP response is an unfavorable prognostic sign, which may be considered in arriving at a decision about enucleation.

Peptide characterization of secretin preparations.

The present study was designed to examine and compare the peptide composition and relative immunochemical purity of GIH and Boots secretin preparations. Gastrointestinal peptides were measured by radioimmunoassay using antibodies to secretin, gastrin, immunoreactive cholecystokinin, vasoactive intestinal peptide, gastric inhibitory peptide, and somatostatin. Boots secretin was found to contain substantial quantities of gastrin, immunoreactive cholecystokinin, vasoactive intestinal peptide, gastric inhibitory peptide, and somatostatin. In contrast, GIH secretin contained only a very small amount of vasoactive intestinal peptide. GIH also contained approximately three to four times more secretin per unit as did Boots secretin. Intravenous infusion of Boots, but not GIH, secretin in seven healthy volunteers produced significant increases in venous plasma of all peptides. Results of these studies indicate that Boots secretin contains large and variable quantities of gastrointestinal peptides other than secretin and that the contents of both secretin and the other peptides vary among different lots. Because the quantity of these peptides is sufficient to increase significantly their blood levels and consequent biological effects, it is concluded that GIH is preferable to Boots secretin in the clinical evaluation of patients with suspected chronic pancreatitis or gastrinoma.

Evoked potentials in hysteria and malingering.

Evoked potentials are often helpful in confirming the nonorganic nature of sensory symptoms in hysteria and malingering. In a retrospective analysis, such cases were found to comprise approximately 1% of referrals to a university hospital laboratory. However, the diagnostic usefulness of electrophysiologic tests is limited in some clinical settings: normal responses may be encountered in subjects with certain organic deficits, and "abnormal" responses can sometimes be produced voluntarily by normal subjects. In malingering--as opposed to hysteria--the role of the technologist in monitoring patient compliance with the test procedure is particularly important. Like other laboratory investigations, evoked potential findings must be interpreted within the context of each clinical situation, and with full appreciation of the sensitivities and specificities of the tests.