On the experimental identification of unstable static equilibria

This paper shows how the presence of unstable equilibrium configurations of elastic continua is reflected in the behaviour of transients induced by large perturbations. A beam that is axially loaded beyond its critical state typically exhibits two buckled stable equilibrium configurations, separated by one or more unstable equilibria. If the beam is then loaded laterally (effectively like a shallow arch) it may snap-through between these states, including the case in which the loading is applied dynamically and of short duration, i.e. an impact. Such impacts, if applied at random locations and of random strength, will generate an ensemble of transient trajectories that explore the phase space. Given sufficient variety, some of these trajectories will possess initial energy that is close to (just less than or just greater than) the energy required to cause snap-through and will have a tendency to slowdown as they pass close to an unstable configuration: a saddle point in a potential energy surface, for example. Although this close-encounter is relatively straightforward in a system characterized by a single degree of freedom, it is more challenging to identify in a higher order or continuous system, especially in a (necessarily) noisy experimental system. This paper will show how the identification of unstable equilibrium configurations can be achieved using transient dynamics.

On damping in the vicinity of critical points

The effect of damping on the behaviour of oscillations in the vicinity of bifurcations of nonlinear dynamical systems is investigated. Here, our primary focus is single degree-of-freedom conservative systems to which a small linear viscous energy dissipation has been added. Oscillators with saddle-node, pitchfork and transcritical bifurcations are shown analytically to exhibit several interesting characteristics in the free decay response near a bifurcation. A simple mechanical oscillator with a transcritical bifurcation is used to experimentally verify the analytical results. A transcritical bifurcation was selected because it may be used to represent generic bifurcation behaviour. It is shown that the damping ratio can be used to predict a change in the stability with respect to changing system parameters.

A heuristic method for identifying chaos from frequency content

The sign of the largest Lyapunov exponent is the fundamental indicator of chaos in a dynamical system. However, although the extraction of Lyapunov exponents can be accomplished with (necessarily noisy) the experimental data, this is still a relatively data-intensive and sensitive endeavor. This paper presents an alternative pragmatic approach to identifying chaos using response frequency characteristics and extending the concept of the spectrogram. The method is shown to work well on both experimental and simulated time series.

Childhood injuries due to falls from apartment balconies and windows

BACKGROUND

Falls from balconies and windows are an important cause of childhood injury. This study investigated the circumstances around such falls and attempted to identify possible measures for their prevention.

POPULATION

Children <15 years living in Dallas County, Texas.

METHODS

Each child treated because of a fall from a building in 1997-99 had information about the injury collected, and a parent was contacted to obtain further information. For apartment related falls, an attempt was made to visit the apartment to measure windows and balcony rails.

RESULTS

Ninety eight children were injured in falls from buildings during the three year period; 39 (40%) were admitted to hospital. Seventy five of the falls (77%) involved apartments, and most occurred around noon or evening meal times. Among apartment falls, 39 (52%) fell from windows, 34 (45%) from balconies, and two (3%) from unknown sites. For more than two thirds of balcony related falls, the child fell from between the balcony rails, all of which were spaced more than 4 inches (10 cm) apart. On-site measurement showed the rails were an average of 7.5 inches (19 cm) apart; all of these apartments were built before 1984. For more than two thirds of window related falls, the window was situated within 2 feet (61 cm) of the floor.

CONCLUSIONS

Two factors are important in falls from apartment windows and balconies: balcony rails more than 4 inches (10 cm) apart, and windows positioned low to the floor. Current building codes do not apply to older apartments, where most of these falls occurred. Nevertheless, these factors may be amenable to environmental modifications that may prevent most of these falls.

Ability of hospitals to care for pediatric emergency patients

CONTEXT

The needs of children in emergency situations differ from those of adults and require special attention, yet there has been no study of the ability of U.S. hospitals to care for emergently or critically ill children.

OBJECTIVE

To estimate the distribution of pediatric services available at U.S. hospitals with emergency departments (EDs).

DESIGN

Self-report survey of 101 hospital EDs.

PARTICIPANTS

Stratified probability sample of all U.S. hospitals operating EDs.

RESULTS

The majority of hospitals that usually admit pediatric patients do not have separate pediatric facilities. Hospitals without a pediatric department, ward, or trauma service usually transfer critically injured pediatric trauma patients; however, nearly 10% of hospitals without pediatric intensive care facilities admit critically injured children to their own facilities. Likewise, 7% of hospitals routinely admit pediatric patients known to require intensive care to their adult intensive care units rather than transferring the patient to a facility with pediatric intensive care facilities. Few hospitals have protocols for obtaining pediatric consultation on pediatric emergencies. Appropriately sized equipment for successful care of infants and children in an emergency situation was more likely to be missing than adult-sized equipment, and significant numbers of hospitals did not have adequate equipment to care for newborn emergencies.

CONCLUSION

Emergent and critical care of infants and children may not be well integrated and regionalized within our health care system, suggesting that there is room for improvement in the quality of care for children encountering emergent illness and trauma.

History and radiographic findings associated with clinically suspected radial head subluxations

OBJECTIVES

To determine: 1) physician practices regarding the use of radiographs for radial head subluxations (RHS), 2) the prevalence of missed fractures in children with a clinical diagnosis of RHS, 3) the relative risk of a fracture with a nonclassic history for mechanism of injury for RHS, and 4) radiographic findings associated with RHS that are difficult to reduce.

METHODS

This study began with a physician survey that addressed the integration of radiographs into the management of RHS. We subsequently conducted a prospective randomized trial with a consecutive sampling of children less than 6 years of age who presented to one of 2 urban pediatric emergency departments and 2 suburban pediatric urgent care centers with a clinical diagnosis of RHS. After informed consent was obtained, reduction was undertaken with a maximum of four attempts (two by hyperpronation and two by supination/flexion), 15 minutes apart. Failure to reduce the RHS resulted in the procurement of a radiograph of the elbow. At the conclusion of the study, all radiographs were evaluated by a radiologist blinded to the diagnosis. Patients receiving radiographs were contacted 2 weeks after discharge for verification of the diagnosis.

RESULTS

Eighty-four percent of 224 physicians returned completed surveys. Fifty-six percent reported using radiographs for failed reduction attempts. In the second phase of the study, 136 patients were enrolled prospectively: 127 were reduced successfully and 9 patients failed attempts at reduction. Of the nine patients receiving radiographs: four had fractures (prevalence of 2.9% with 95% confidence interval (CI) = 0.8-7.4), two had no radiographic findings and normal function on follow up, and three had isolated posterior fat pads on radiograph and normal function on follow-up. The relative risk of a fracture in children with a nonclassic history defined as any mechanism other than "pull" was 1.200 (95% CI = 0.441-3.264); the relative risk was 1.886 (95% CI = 0.680-5.231) when defining a nonclassic history as any mechanism other than "pull" or "fall."

CONCLUSIONS

1) Physicians tend to order radiographs for elbow injuries they initially perceive to be radial head subluxations when attempts at reduction fail. 2) In our study, fractures in children who presented with the classic flexed elbow/pronated wrist position were rare. 3) The relative risk of a fracture in children with a nonclassic history for mechanism of injury was not significant. 4) An isolated finding of a posterior fat pad in a child with RHS that is difficult to reduce was not associated with a fracture in our small sample of children with radiographic findings.

A comparison of supination/flexion to hyperpronation in the reduction of radial head subluxations

OBJECTIVE

To compare supination at the wrist followed by flexion at the elbow (the traditional reduction technique) to hyperpronation at the wrist in the reduction of radial head subluxations (nursemaid's elbow).

MATERIALS AND METHODS

This prospective, randomized study involved a consecutive sampling of children younger than 6 years of age who presented to one of two urban pediatric emergency departments and two suburban pediatric ambulatory care centers with a clinical diagnosis of radial head subluxation. Patients were randomized to undergo reduction by one of the two methods and were followed every 5 minutes for return of elbow function. The initial procedure was repeated if baseline functioning did not return 15 minutes after the initial reduction attempt. Failure of that technique 30 minutes after the initial reduction attempt resulted in a cross-over to the alternate method of reduction. The alternate procedure was repeated if baseline functioning did not return 15 minutes after the alternate procedure was attempted. If the patient failed both techniques, radiography of the elbow was performed.

RESULTS

A total of 90 patients were enrolled in the study. Five patients were removed from further analysis secondary to a final diagnosis of fracture, 84 were reduced successfully, and 1 failed both techniques. Demographic characteristics of each group were similar. Thirty-nine of 41 patients (95%) randomized to hyper-pronation were reduced successfully on the first attempt versus 34 of 44 patients (77%) randomized to supination. Two patients in the hyperpronation group required two attempts versus 10 patients in the supination group. Hyperpronation was more successful; 40 of 41 patients (97.5%) in the hyperpronation group were reduced successfully versus 38 of 44 patients (86%) in the supination group. Of the 6 patients who crossed over from supination to hyperpronation, 5 were reduced on the first attempt and 1 was reduced on the second attempt.

CONCLUSIONS

In the reduction of radial head subluxations, the hyperpronation technique required fewer attempts at reduction compared with supination, was successful more often than supination, and was often successful when supination failed.

Simultaneous outbreaks of Guillain-Barré syndrome and Bell's palsy in Hawaii in 1981

Simultaneous outbreaks of Guillain-Barré syndrome (GBS) and Bell's palsy occurred among residents of Hawaii during the three-month period June through August 1981. The outbreak of GBS (six cases) involved children, while the outbreak of Bell's palsy (22 cases) involved primarily adults. Four of the six patients with GBS had serological evidence of recent infection with cytomegalovirus (CMV) v none of 24 control subjects; no such association with CMV infection could be demonstrated for patients with Bell's palsy. The patients with GBS and Bell's palsy lived in widely scattered areas within the population centers of Hawaii, and no common exposures to specific places or events were identified. Testing for antibodies to respiratory pathogens suggested that a variety of antecedent viral infections, in addition to CMV infection, may have been associated with this unusual simultaneous cluster of illnesses.