Adduction induces large optic nerve head deformations in subjects with normal-tension glaucoma

PURPOSE

To assess intraocular pressure (IOP)-induced and gaze-induced optic nerve head (ONH) strains in subjects with high-tension glaucoma (HTG) and normal-tension glaucoma (NTG).

DESIGN

Clinic-based cross-sectional study.

METHODS

The ONH from one eye of 228 subjects (114 subjects with HTG (pre-treatment IOP≥21 mm Hg) and 114 with NTG (pre-treatment IOP<21 mm Hg)) was imaged with optical coherence tomography (OCT) under the following conditions: (1) OCT primary gaze, (2) 20° adduction from OCT primary gaze, (3) 20° abduction from OCT primary gaze and (4) OCT primary gaze with acute IOP elevation (to approximately 33 mm Hg). We then performed digital volume correlation analysis to quantify IOP-induced and gaze-induced ONH tissue deformations and strains.

RESULTS

Across all subjects, adduction generated high effective strain (4.4%±2.3%) in the LC tissue with no significant difference (p>0.05) with those induced by IOP elevation (4.5%±2.4%); while abduction generated significantly lower (p=0.01) effective strain (3.1%±1.9%). The lamina cribrosa (LC) of HTG subjects exhibited significantly higher effective strain than those of NTG subjects under IOP elevation (HTG: 4.6%±1.7% vs NTG: 4.1%±1.5%, p<0.05). Conversely, the LC of NTG subjects exhibited significantly higher effective strain than those of HTG subjects under adduction (NTG: 4.9%±1.9% vs HTG: 4.0%±1.4%, p<0.05).

CONCLUSION

We found that NTG subjects experienced higher strains due to adduction than HTG subjects, while HTG subjects experienced higher strain due to IOP elevation than NTG subjects-and that these differences were most pronounced in the LC tissue.

Differing Associations between Optic Nerve Head Strains and Visual Field Loss in Patients with Normal- and High-Tension Glaucoma

PURPOSE

To study the associations between optic nerve head (ONH) strains under intraocular pressure (IOP) elevation with retinal sensitivity in patients with glaucoma.

DESIGN

Clinic-based cross-sectional study.

PARTICIPANTS

Two hundred twenty-nine patients with primary open-angle glaucoma (subdivided into 115 patients with high-tension glaucoma [HTG] and 114 patients with normal-tension glaucoma [NTG]).

METHODS

For 1 eye of each patient, we imaged the ONH using spectral-domain OCT under the following conditions: (1) primary gaze and (2) primary gaze with acute IOP elevation (to approximately 35 mmHg) achieved through ophthalmodynamometry. A 3-dimensional strain-mapping algorithm was applied to quantify IOP-induced ONH tissue strain (i.e., deformation) in each ONH. Strains in the prelaminar tissue (PLT), the retina, the choroid, the sclera, and the lamina cribrosa (LC) were associated (using linear regression) with measures of retinal sensitivity from the 24-2 Humphrey visual field test (Carl Zeiss Meditec). This was performed globally, then locally according to a previously published regionalization scheme.

MAIN OUTCOME MEASURES

Associations between ONH strains and values of retinal sensitivity from visual field testing.

RESULTS

For patients with HTG, we found (1) significant negative linear associations between ONH strains and retinal sensitivity (P < 0.001; on average, a 1% increase in ONH strains corresponded to a decrease in retinal sensitivity of 1.1 decibels [dB]), (2) that high-strain regions colocalized with anatomically mapped regions of high visual field loss, and (3) that the strongest negative associations were observed in the superior region and in the PLT. In contrast, for patients with NTG, no significant associations between strains and retinal sensitivity were observed except in the superotemporal region of the LC.

CONCLUSIONS

We found significant negative associations between IOP-induced ONH strains and retinal sensitivity in a relatively large glaucoma cohort. Specifically, patients with HTG who experienced higher ONH strains were more likely to exhibit lower retinal sensitivities. Interestingly, this trend in general was less pronounced in patients with NTG, which could suggest a distinct pathophysiologic relationship between the two glaucoma subtypes.

Antral recirculation in the stomach during gastric mixing

We investigate flow in the stomach during gastric mixing using a numerical simulation with an anatomically realistic geometry and free-surface flow modeling. Because of momentum differences between greater and lesser curvatures during peristaltic contractions, time-averaged recirculation is generated in the antrum, with retropulsive flow away from the pylorus and compensation flow along the greater curvature toward the pylorus. Gastric content in the distal stomach is continuously transported to the distal antrum by the forward flow of antral recirculation, and it is then mixed by the backward retropulsive flow. Hence, the content inside the antral recirculation is well mixed independently of initial location, whereas the content outside the recirculation is poorly mixed. Free-surface modeling enables us to analyze the effects of posture on gastric mixing. In the upright, prone, and right lateral positions, most of the antrum is filled with content, and the content is well mixed by antral recirculation. In contrast, in the supine and left lateral positions, most of the content is located outside antral recirculation, which results in poor mixing. The curved, twisted shape of the stomach substantially supports gastric mixing in fluid mechanical terms.

Practical application of CellML 1.1: The integration of new mechanisms into a human ventricular myocyte model

CellML 1.1 was released as a formal specification in February 2006 with the first release of a complete implementation of the CellML API following in December. The combination of these two developments paves the way for a powerful new paradigm in mathematically modeling cardiac cellular electrophysiology. In this article we explore the practical application of this paradigm using the example of integrating new mechanisms into a well known model of human ventricular myocyte electrophysiology. Through practical application of the CellML 1.1 paradigm we demonstrate the advantages inherent in such an approach and contrast them to more traditional methods of model description, exchange, and publication. This work has also provided the impetus for some recent developments in regard to CellML metadata specifications. The development of the tools and techniques used in this work has helped define some guidelines that should prove useful in future developments in this field. By following these guidelines model authors can increase the usability of their work by other scientists. This work presents the first attempt to utilize annotated CellML models to present not only the underlying mathematical models but also specify the numerical simulations and graphical outputs in an interchangeable, machine readable format. By doing so, all simulations are able to be duplicated by anyone with access to a capable tool. Similarly, identical graphical representations of the numerical simulation results can be produced.

Interactive reference descriptions of cellular electrophysiology models

With the advent and popularization of technologies such as CellML and SBML it has become relatively easy to share and exchange mathematical models of biological systems. However, as such technology evolves it is inevitable that not all tools will correctly interpret the full range of models able to be described. It is also necessary to completely describe the mathematical models allowing model users to appropriately use each model. To address these issues we have previously developed methods and tools based on community defined standards which allow for the specification of a reference description of a mathematical model. Such reference descriptions provide a comprehensive description of both the mathematical model and the outcomes expected when performing specific numerical simulations using the model. Thus providing for quantitative validation of tool interpretation of a given model as well as informing the model user as to all aspects of the model definition. Further development of this work has led to the conclusion that presenting such reference descriptions in a user friendly and interactive manner provides a powerful tool for the dissemination and understanding of the mathematical model. Furthermore, such a tool is seen as an invaluable teaching aid in the education of biomedical engineers and scientists allowing for anything from a superficial to a very comprehensive interaction with the mathematical model.

Cardiac electromechanics and the forward/inverse problems of electrocardiology

The mechanical motion of the heart plays a role in determining the waveforms observed in an ECG. This study is designed to ascertain, from a theoretical perspective, the influence of this motion. This is achieved through an analysis of a detailed forward model including a full bidomain description and a strongly coupled model of cardiac electromechanics. Simulations were run on identical problems with and without the inclusion of mechanical deformation and the results were analyzed with a view towards the inverse problem of electrocardiology. Initial results have shown the QRS complex to be largely invariant under deformation, but significant changes in T wave morphology have been observed. Further analysis has revealed that it is the effect of the cell-level mechanics on repolarization that is primarily responsible for these changes as opposed to the tissue deformation.

Partial urethral resection in the surgical treatment of vulvar cancer does not have a significant impact on urinary continence. A confirmation of an authority-based opinion

Partial resection of the urethra is sometimes necessary in the surgical treatment of locally advanced vulvar cancer. In this study, the frequency of urinary incontinence after partial urethral resection was compared with that of patients who were treated without partial resection of the urethra. Eighteen patients with vulvar cancer encroaching or infiltrating the urethra, treated by a radical vulvectomy and partial urethrectomy, were compared with 17 patients treated by vulvectomy without partial removal of the urethra. Data on urinary incontinence pre- and postoperatively from both groups were retrospectively collected from the patient files. A questionnaire on urinary incontinence was sent to a subset of patients from both groups in order to get information on the current micturation pattern. In four out of 18 patients (22%) with a partial urethrectomy, incontinence was reported, versus two out of 17 patients (12%) in the control group (P= 0.860). Eight patients in the study group and 12 in the control group are currently alive, and all responded to the questionnaire. Two (25%) in the study group and three (25%) in the control group reported to have current symptoms of urinary incontinence. This retrospective study shows that partial resection of 1–1.5 cm of the distal urethra in addition to a radical local excision for vulvar cancer does not result in a significant increase in the frequency of urinary incontinence, compared with vulvar cancer patients without partial urethrectomy.

Upper airway obstruction caused by ingestion of concentrated acetic acid

We report a case of rapidly progressive severe upper airway obstruction in a small child caused by accidental ingestion of 80% acetic acid. Emergency cricothyrotomy was necessary after both endotracheal intubation and bag-valve-mask ventilation were not possible. Although intubation was eventually achieved, a tracheostomy was necessary. Toxin spilled over the anterior chest and abdomen caused third degree skin burns which required grafting. Mild liver dysfunction was observed. Complete recovery occurred.

Modelling gastrointestinal bioelectric activity

The development of an anatomically realistic biophysically based model of the human gastrointestinal (GI) tract is presented. A major objective of this work is to develop a modelling framework that can be used to integrate the physiological, anatomical and medical knowledge of the GI system. The anatomical model was developed by fitting derivative continuous meshes to digitised data taken from images of the visible man. Structural information, including fibre distributions of the smooth muscle layers and the arrangement of the networks of interstitial cells of Cajal, were incorporated using published information. A continuum modelling framework was used to simulate electrical activity from the single cell to the whole organ and body. Also computed was the external magnetic field generated from the GI electrical activity. The set of governing equations were solved using a combination of numerical techniques. Activity at the (continuum) cell level was solved using a high-resolution trilinear finite element procedure that had been defined from the previously fitted C1 continuous anatomical mesh. Multiple dipolar sources were created from the excitation waves which were embedded within a coupled C1 continuous torso model to produce both the cutaneous electrical field and the external magnetic field. Initial simulations were performed using a simplified geometry to test the implementation of the numerical solution procedure. The numerical procedures were shown to rapidly converge with mesh refinement. In the process of this testing, errors in a long standing analytic solution were identified and are corrected in Appendix B. Results of single cell activity were compared to published results illustrating that the key features of the slow wave activity were successfully replicated. Simulations using a two-dimensional slice through the gastric wall produced slow wave activity that agreed with the known frequency and propagation characteristics. Three-dimensional simulations were also performed using the full stomach mesh and results illustrated the slow wave propagation throughout the stomach musculature.

Epidemiology and prevention of unexpected in-hospital deaths

INTRODUCTION

The medical literature has only documented the importace of adverse events in terms of patient outcomes and cost to the health service over the last decade. This review focuses on the incidence, risk factors and prevention on unexpected in-hospital deaths.

DISCUSSION

The incidence of unexpected in-hospital deaths in the two major retrospective studies was 0.5% and 0.8% of all hospital admissions. A prospective study of surgical patients documented an incidence of nearly 7.1%. The risk factors that have been associated with hospital death include: increasing age; non-elective operative procedure; negligence; human error and unstable bedside observations in the hours prior to death. Although a number of intreventions have been suggested to prevent unexpected in-hospital deaths, very few studies have been performed to determine effectiveness. In a single institution historical control study, the introduction of a Medical Emergency Team was associated with 50% reduction in the casemix adjusted incidence of in-hospital cardiac arrest calls.

CONCLUSION

There are a number of studies to suggest that in-hospital deaths are both predictable and preventable. More work is required to determine effective strategies to manage this problem.

A deformable finite element derived finite difference method for cardiac activation problems

We present a finite element (FE) derived finite difference (FD) technique for solving cardiac activation problems over deforming geometries using a bidomain framework. The geometry of the solution domain is defined by a FE mesh and over these FEs a high resolution FD mesh is generated. The difference points are located at regular intervals in the normalized material space within each of the FEs. The bidomain equations are then transformed to the embedded FD mesh which provides a solution space that is both regular and orthogonal. The solution points move in physical space with any deformation of the solution domain, but the equations are set up in such a way that the solution is invariant as it is constructed in material space. The derivation of this new solution technique is presented along with a series of examples that demonstrate the accuracy of this bidomain framework.

Torso coupling techniques for the forward problem of electrocardiography

The calculation of body surface potentials from a known cardiac source is traditionally formulated in terms of a two step process. The first step involves the generation of some form of equivalent cardiac source (typically dipole based) at a resolution significantly lower than that of a continuum cell. The second then places that source into a volume conductor within which the potential fields are calculated. This approach does not properly capture the feedback between the torso and the extracellular potential field. Presented here are the details of two new methods which calculate continuous potential fields throughout the torso that are the direct result of cardiac cellular electrical activity. These new methods are termed the Boundary Iteration Method and the Direct Assembly Method. While these two methods return essentially identical answers, there is a definite tradeoff between computational speed and memory overhead with the direct assembly method proving faster but requiring significantly more memory. Examples are given that demonstrate the convergence and accuracy of these methods in an idealized coupled torso system. These techniques are then applied to an anatomically based model of a slice through a human male torso.

Bilateral frontal haemorrhages associated with continuous spinal analgesia

We report a case of intracerebral haemorrhages associated with continuous spinal analgesia. Continuous spinal analgesia is frequently employed for postoperative analgesia in high-risk patients in our institution. The analgesia is administered via a 20 gauge catheter passed through an 18 gauge Tuohy needle (Portex). A 71-year-old man with severe respiratory impairment had an intrathecal catheter placed for postoperative analgesia. He had a difficult postoperative course, including wound dehiscence, and died from respiratory failure some five weeks postoperatively. On day nine postoperatively he had two tonic-clonic seizures and was subsequently found to have developed bilateral frontal intracerebral haemorrhages. There was no previous history of seizures. Although several confounding variables exist, the most likely explanation for the intracerebral event appears to be an association with the dural puncture and intrathecal catheter Possible mechanisms and risk factors are discussed.

Extracorporeal membrane oxygenation in pregnancy

We describe the use of extracorporeal membrane oxygenation in pregnancy. There were no major complications, and the outcome was successful for mother and baby.

An analysis of excess mortality not predicted to occur by APACHE III in an Australian level III intensive care unit

The APACHE III derived standardized mortality ratio has been suggested as a statistic to measure intensive care unit (ICU) effectiveness. From 1991 data collected on 519 consecutive admissions to the Royal Adelaide Hospital ICU a standardized mortality ratio of 1.25 was calculated. Of the 174 deaths only 95 had a prediction of death greater than 0.5. As part of a quality assurance study we undertook a retrospective case note audit to try to identify factors that were associated with the low mortality prediction (< 0.5) in hospital deaths. Firstly we analysed the patient population that died to determine the factors that were different between patients who had a mortality prediction of greater than 0.5 versus those who had a mortality prediction of less than 0.5. Next we analysed the patient population with a mortality prediction of less than 0.5 and compared actual survivors with patients who died in hospital. Amongst low mortality prediction patients admitted to the Royal Adelaide Hospital ICU we identified age, a history of acute myocardial infarction, presentation to ICU after a cardiac arrest or with an elevated creatinine and the development of acute renal failure and septicaemia during the ICU admission as being associated with in-hospital mortality. We also documented that late hospital deaths on the ward after ICU discharge occurred more frequently with low predicted hospital mortality ICU patients. Factors other than the APACHE III score may be associated with hospital deaths of ICU patients.

Carbon dioxide embolism following diagnostic hysteroscopy

A 50-year-old woman ASA 2 underwent carbon dioxide hysteroscopy under general anaesthesia. Monitoring showed a sudden and rapid fall in end-tidal carbon dioxide followed by oxygen desaturation. She became pulseless and cyanosed. Resuscitation with oxygen, intravenous adrenaline and head-down tilt restored her to haemodynamic stability. Hyperbaric therapy was also administered as air embolism could not be excluded.

Legionnaires' disease outbreak in south western Sydney, 1992. Clinical aspects

OBJECTIVES

To document the clinical, laboratory and radiological features of patients with Legionella pneumophila serogroup 1 pneumonia during an outbreak, and probe for any relationship between clinical or laboratory features and outcome.

DESIGN AND SETTING

Prospective identification of patients with Legionnaires' disease in an outbreak from 15-26 April 1992 in the South Western Sydney Area Health Service, centred on the Fairfield area.

PATIENTS

Twenty-six patients (22 men, four women) were confirmed to have the disease, based on the presence of clinical features of pneumonia, with L. pneumophila serogroup 1 isolated on culture, or evidence of seroconversion.

RESULTS

Seventeen patients (65.4%) were culture-positive for L. pneumophila serogroup 1 and nine were diagnosed on serological criteria. A direct fluorescent antibody (DFA) test of sputum performed well as a rapid diagnostic method. Twenty-three patients (89%) presented with hyponatraemia, 14 (54%) with renal impairment and nine of 19 (47%) with elevated serum creatinine phosphokinase levels. Overall mortality was 23% (71% for patients requiring mechanical ventilation). Eleven of 119 patients (10.2%) who did not have Legionnaires' disease showed serological evidence of previous exposure.

CONCLUSIONS

The duration of symptoms and severity of biochemical abnormalities at presentation were not related to outcome. The sputum DFA test is useful for rapid diagnosis during outbreaks.

Intensive care unit resource utilisation

The cost-effectiveness of the Intensive Care Unit after three decades of development has yet to be demonstrated. Accurate ICU resource allocation is limited by our inability to measure cost-effectiveness. Measurement tools have been developed and refined that will give a prediction of in-hospital mortality of groups of critically ill patients. However, these measures will not predict with certainty individual patient outcomes, and take no account of quality of life. Methodology to examine long-term outcome and quality of life after intensive care is still in its infancy. Measurement of ICU cost is limited by a lack of cost-accounting models that not only reflect true cost but that are clinically applicable.