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Biomedical subjects

P R Johnston

Publications and source records attributed to P R Johnston.

At least 19 recordsLinked to original sources
PubMed2006

The Sony Ericsson WTA Tour 10 year age eligibility and professional development review.

Concerns have long existed over the participation of adolescent athletes in professional sports. In 2004, the Sony Ericsson WTA Tour (WTA Tour) commissioned a Professional Development Advisory Panel (PDAP) to evaluate the WTA Tour's age eligibility rule (AER) and professional development programmes (PDPs) for female tennis players since their inception in 1995. More than 75% of the 628 respondents supported the principles of the AER, and 90% indicated a need for PDPs. Statistical analysis of WTA Tour players' careers found that premature retirements (players leaving the Tour at or before age 21) decreased significantly from 7% before the AER to less than 1% afterward, and median career length increased by 43%. The PDAP recommends that the WTA Tour continues a phased-in, developmentally appropriate AER, enhances the PDPs, and works with other sport governing bodies to coordinate rules and programmes at earlier ages to aid the transition of adolescents into adult sports.

Adolescent↗
PubMed2004

Construction of realistic branched, three-dimensional arteries suitable for computational modelling of flow.

Routinely performed biplane digital coronary angiograms were used to construct a three-dimensional model of the coronary arteries. The technique took the images and automatically picked the centreline and radii in each. By reading the information contained in the DICOM format, the rotation angle between the two images could be ascertained, and the centreline in three dimensions could be determined. Once the centreline and radii had been calculated, a finite volume mesh of the artery was constructed that could be used as input into a fluid dynamics package that solves Navier-Stokes equations. A four-section method was adopted for constructing the mesh, utilising three tubular segments and a small junction section. The tubes were constructed automatically, and the junction procedure was semi-automated, maximising user-control over this region. A structured mesh was used for the tubes, and an unstructured mesh was used to model accurately the irregular shape of the junction. The accuracy of the reconstruction method was established by projection of a mesh onto an independent image of the same artery and the use of a coronary phantom. These tests, along with calibration of the edge detection, established an error of less than 2% in the model.

Coronary Angiography↗
PubMed2002

Recall of health warnings in smokeless tobacco ads.

OBJECTIVE: To determine the effects of health warning characteristics in smokeless tobacco magazine print ads on warning recall, and the implications for current US Federal regulations. DESIGN: Subjects examined two distracter ads and one of nine randomly assigned smokeless tobacco ads varying in health warning presence, size (8 to 18 point font), and contrast (low versus high)-including no health warning. They were then interviewed about ad content using recall and recognition questions. SUBJECTS: A convenience sample of 895 English speaking males aged 16-24 years old who were intercepted at seven shopping malls throughout Massachusetts during May 2000. MAIN OUTCOME MEASURES: Proven aided recall, or recall of a health warning and correct recognition of the warning message among distracters, and false recall. RESULTS: Controlling for covariates such as education, employment/student status, and Hispanic background, proven aided recall increased significantly with font size; doubling size from 10 to 20 point font would increase recall from 63% to 76%. Although not statistically significant, recall was somewhat better for high contrast warnings. Ten per cent of the sample mistakenly recalled the warning where none existed. CONCLUSIONS: As demonstrated by substantially greater recall among ads that included health warnings over ads that had none, health warnings retained their value to consumers despite years of exposure (that can produce false recall). Larger health warnings would enhance recall, and the proposed model can be used to estimate potential recall that affects communication, perceived health risk, and behaviour modification.

Adolescent↗
PubMed2001

The importance of anisotropy in modeling ST segment shift in subendocardial ischaemia.

In this paper, a simple mathematical model of a slab of cardiac tissue is presented in an attempt to better understand the relationship between subendocardial ischaemia and the resulting epicardial potential distributions. The cardiac tissue is represented by the bidomain model where tissue anisotropy and fiber rotation have been incorporated with a view to predicting the epicardial surface potential distribution. The source of electric potential in this steady-state problem is the difference between plateau potentials in normal and ischaemic tissue, where it is assumed that ischaemic tissue has a lower plateau potential. Simulations with tissue anisotropy and no fiber rotation are also considered. Simulations are performed for various thicknesses of the transition region between normal and ischaemic tissue and for various sizes of the ischaemic region. The simulated epicardial potential distributions, based on an anisotropic model of the cardiac tissue, show that there are large potential gradients above the border of the ischaemic region and that there are dips in the potential distribution above the region of ischaemia. It could be concluded from the simulations that it would be possible to predict the region of subendocardial ischaemia from the epicardial potential distribution, a conclusion contrary to observed experimental data. Possible reasons for this discrepancy are discussed. In the interests of mathematical simplicity, isotropic models of the cardiac tissue are also considered, but results from these simulations predict epicardial potential distributions vastly different from experimental observations. A major conclusion from this work is that tissue anisotropy and fiber rotation must be included to obtain meaningful and realistic epicardial potential distributions.

Animals↗
PubMed2000

Selecting the corner in the L-curve approach to Tikhonov regularization.

The performance of two methods for selecting the corner in the L-curve approach to Tikhonov regularization is evaluated via computer simulation. These methods are selecting the corner as the point of maximum curvature in the L-curve, and selecting it as the point where the product of abcissa and ordinate is a minimum. It is shown that both these methods resulted in significantly better regularization parameters than that obtained with an often-used empirical Composite REsidual and Smoothing Operator approach, particularly in conditions where correlated geometry noise exceeds Gaussian measurement noise. It is also shown that the regularization parameter that results with the minimum-product method is identical to that selected with another empirical zero-crossing approach proposed earlier.

Biomedical Engineering↗
PubMed1999

Epicardial ST depression in acute myocardial infarction.

The presence of electrocardiographic ST depression in acute infarction remains controversial and poorly explained. A combined animal and modeling study was performed to evaluate the source of ST changes in acute infarction. In anaesthetized sheep, small infarcts showed uniform ST elevation over the infarction whereas larger infarcts showed marked ST depression over the normal myocardium in addition to the ST elevation. These findings were replicated by bidomain models of the heart. A hollow sphere was used to model a gradually increasing infarct, and this showed that there was a decrease in the ratio of ST elevation to ST depression as the infarct was increased. The current flowing out of the heart must be identical to the current flowing back into the heart. This means that any infarction will produce ST depression as well as ST elevation, the ratio between the two being related to the size of the infarction. Small infarction is associated with a small region of ST elevation and minor ST depression of the remaining myocardium, and as the infarct region increases, the amplitude of the epicardial ST elevation falls and the amplitude of the ST depression increases. Infarction size is proportional to both the height of the ST depression on the epicardium and the strength of the epicardial ST segment dipole.

Animals↗
PubMed1999

An asymptotic estimate for the effective radius of a concentric bipolar electrode.

A concentric bipolar electrode (CBE) (consisting of a central disc and an outer annulus) has been proposed as an approximate method of measuring the surface Laplacian of the body surface electrical potential distribution. The derivation of the surface Laplacian approximation, in terms of the potential difference measured with the bipolar electrode, contained an unspecified parameter which has been dubbed the 'effective radius' of the concentric bipolar electrode. This paper presents an asymptotic analysis to derive an expression for the effective radius in terms of the physical dimensions of the electrode (the radius of the central disc and inner and outer radii of the annulus). Also studied is the way in which the value of the effective radius affects the behaviour of the relative error in the surface Laplacian measurement at various dipole source locations within a conducting medium.

Body Surface Potential Mapping↗
PubMed1998

Comparison of efficacy and ease of handling of salmeterol and terbutaline powder inhalers.

A multicentre, randomised, open, parallel-group study was performed to compare the efficacy, tolerability and ease of handling of salmeterol xinafoate 50 micrograms twice daily via the Diskus inhaler with terbutaline sulphate 500 micrograms four times daily via the Turbuhaler inhaler. Two hundred and sixty-three patients (aged 18-79 years, baseline FEV1 50-90% predicted, mean PEFR 85% of response to salbutamol) were randomised to treatment with salmeterol (n = 136) or terbutaline (n = 127). A statistically significant difference in favour of salmeterol was seen between treatment groups for the primary efficacy variable, mean morning PEFR (difference in adjusted means 25.4 l/min, p < 0.001). Within the groups randomised to each device, ease of handling assessments favoured the Diskus inhaler over the Turbuhaler inhaler. More patients liked the Diskus inhaler than the Turbuhaler inhaler (98% vs 72%, p < 0.001). The Diskus inhaler received better scores than the Turbuhaler inhaler for all features assessed in the device questionnaire.

Adolescent↗
PubMed1997

A new method for regularization parameter determination in the inverse problem of electrocardiography.

Computing the potentials on the heart's epicardial surface from the body surface potentials constitutes one form of the inverse problem of electrocardiography. An often-used approach to overcoming the ill-posed nature of the inverse problem and stabilizing the solution is via zero-order Tikhonov regularization, where the squared norms of both the surface potential residual and the solution are minimized, with a relative weight determined by a so-called regularization parameter. This paper looks at the composite residual and smoothing operator (CRESO) and L-curve methods currently used to determine a suitable value for this regularization parameter, t, and proposes a third method that works just as well and is much simpler to compute. This new zero-crossing method selects t such that the squared norm of the surface potential residual is equal to t times the squared norm of the solution. Its performance was compared with those of the other two methods, using three stimulation protocols of increasing complexity. The first of these protocols involved a concentric spheres model for the heart and torso and three current dipoles placed inside the inner sphere as the source distribution. The second replaced the spheres with realistic epicardial and torso geometries, while keeping the three-dipole source configuration. The final simulation kept the realistic epicardial and torso geometries, but used epicardial potential distributions corresponding to both normal and ectopic activation of the heart as the source model. Inverse solutions were computed in the presence of both geometry noise, involving assumed erroneous shifts in the heart position, and of Gaussian measurement noise added to the torso surface potentials. It was verified that in an idealistic situation, in which correlated geometry noise dominated the uncorrelated Gaussian measurement noise, only the CRESO approach arrived at a value for t. Both L-curve and zero-crossing approaches did not work. Once measurement noise dominated geometry noise, all three approaches resulted in comparable t values. It was also shown, however, that often under low measurement noise conditions none of the three resulted in an optimum solution.

Artifacts↗
PubMed1997

The Laplacian inverse problem of electrocardiography: an eccentric spheres study.

The eccentric spheres model of the heart-torso system is used to study the inverse problem of electrocardiography using measurements of the Laplacian of the body surface potential distribution. Electrical activity is simulated on the six main regions of the inner surface by considering a limited number of current dipoles placed within the inner sphere. The resulting outer surface potential and Laplacian distributions are then calculated in a forward sense. Varying amounts of random noise are added to these distributions before a zero-order Tikhonov regularization scheme is used to recover the inner surface potential distribution. Comparing the calculated and original inner surface distributions indicates that measurements of the outer surface Laplacian can more accurately reconstruct epicardial potentials than measurements of the outer surface potentials. These distributions are more accurate in that the extrema are placed very close to their original positions and are of nearly the same magnitude. Also, multiple extrema and high potential gradients are recovered.

Artifacts↗
PubMed1996

The potential for Laplacian maps to solve the inverse problem of electrocardiography.

This paper presents a method to solve the inverse problem of electrocardiography using the Laplacian of the body surface potentials. The method presented is studied first using trade-off curves from a concentric spheres model representing a heart-torso system. Then a more conventional study is undertaken where a limited number of current dipoles are placed within the inner sphere and noise is added to the resulting potentials and Laplacians on the surface of the outer sphere. The result indicate that measurements of the outer surface Laplacian can more accurately reconstruct epicardial potentials than measurements of the outer surface potentials. The reconstructions are more accurate in that extrema are placed very close to their correct positions and multiple extrema and high potential gradients are recovered. Identical conclusions are observed in the presence of noise and even when the Laplacians are subject to greater noise than the potentials.

Artifacts↗
PubMed1996

A placebo-controlled, crossover comparison of salmeterol and salbutamol in patients with asthma.

We compared the effects of salmeterol (Sm) (50 micrograms twice daily) with that of salbutamol (Sb) (200 micrograms four times daily) and placebo (P) in patients with mild-to-moderate asthma with asthma symptoms and related the effectiveness of these therapies between patients who used concurrent inhaled corticosteroids (ICS) and those who did not. The study was a 12-wk, multicenter, double-blind, placebo-controlled crossover trial with 367 adult asthmatics randomized to each trial medication for 4 wk. Inhaled Sb was provided as rescue medication to all patients throughout the trial. Only 80% of patients, albeit the majority, were receiving maintenance treatment with ICS throughout this trial; this reflects practice current in early 1990. Each study day, patients recorded their morning and evening peak expiratory flows (PEF), assessment of asthma symptoms, and use of rescue medication. Both morning and evening PEF were greater during treatment with Sm than with Sb (mean differences between the treatments of 29.8 and 14.3 L/min, respectively) or P (27.7 and 20.3 L/min, respectively) (p < 0.0001). Sm was also more effective than Sb or P in lowering diurnal variation in PEF and increasing the percentage of symptom-free days and rescue-free days and nights with no sleep disturbance (p < or = 0.0004). Sb was more effective than P in increasing evening PEF and the percentage of symptom-free days (p < 0.05) and rescue-free days (p < 0.0001). The same clinical superiority of Sm compared with Sb and P was observed in those patients using ICS (p < 0.001 for all treatment comparisons), and to a greater extent than in those patients not using ICS (i.e., Sm was more effective than Sb and P in just six of the 20 treatment comparisons; p < 0.05). In conclusion, Sm 50 micrograms twice daily is effective in the management of mild-to-moderate asthma and it further improves asthma control in patients already using ICS.

Administration, Inhalation↗
PubMed1995

The inverse problem of electrocardiology: the performance of inversion techniques as a function of patient anatomy.

Sixteen anatomically correct bodies have been studied to determine the performance of the inversion techniques of zero-order Tikhonov regularization and singular value decomposition. The bodies have varying heart height and diameter, thickness of subcutaneous fat layer, and distance of the heart from the left wall of the chest. Comparisons are made in terms of trade-off curves for noise amplification factor and spread of epicardial potentials. It was found that regularization performs better than singular value decomposition on all bodies; the larger the heart size, the more reliable the results; and for a given heart size, the thinner the subcutaneous fat layer, the more reliable the results. The distance of the heart from left wall of the chest was found to be a less significant factor for a given heart size.

Adipose Tissue↗
PubMed1995

Quality of life in asthma clinical trials: comparison of salmeterol and salbutamol.

Most clinical trials in asthma have focused on outcomes that are primarily of importance to the clinician. Very few have assessed whether patients feel better and can function better in day-to-day activities. The aim of this study was to compare the effects of salmeterol (50 micrograms twice daily), salbutamol (200 micrograms four times a day), and placebo on asthma-specific quality of life and to relate the findings to conventional clinical asthma outcomes. The study was a 12-wk multicenter, double blind, randomized, placebo-controlled crossover trial with each trial medication taken for 4 wk. The subjects were 140 adults with mild to moderate asthma enrolled from 14 respiratory clinics across Canada. Outcome measures were: (1) the Asthma Quality of Life Questionnaire and spirometry at the end of each treatment period; and (2) daily asthma symptoms, morning and evening peak expiratory flow rates (PEFRs), and rescue salbutamol use during the last 14 d of each treatment period. Asthma-specific quality of life, both overall and for the individual domains (activity limitation, symptoms, emotional function, and exposure to environmental stimuli) was better with salmeterol than with placebo (p < 0.0001), and better with salmeterol than with salbutamol (p < 0.001). In both comparisons, differences were not only statistically significant, but most were also clinically important. There were moderate correlations between change in quality of life and change in clinical outcomes.(ABSTRACT TRUNCATED AT 250 WORDS)

Adolescent↗
PubMed1994

Inverse electrocardiographic transformations: dependence on the number of epicardial regions and body surface data points.

The inverse problem of electrocardiography, the computation of epicardial potentials from body surface potentials, is influenced by the desired resolution on the epicardium, the number of recording points on the body surface, and the method of limiting the inversion process. To examine the role of these variables in the computation of the inverse transform, Tikhonov's zero-order regularization and singular value decomposition (SVD) have been used to invert the forward transfer matrix. The inverses have been compared in a data-independent manner using the resolution and the noise amplification as endpoints. Sets of 32, 50, 192, and 384 leads were chosen as sets of body surface data, and 26, 50, 74, and 98 regions were chosen to represent the epicardium. The resolution and noise were both improved by using a greater number of electrodes on the body surface. When 60% of the singular values are retained, the results show a trade-off between noise and resolution, with typical maximal epicardial noise levels of less than 0.5% of maximum epicardial potentials for 26 epicardial regions, 2.5% for 50 epicardial regions, 7.5% for 74 epicardial regions, and 50% for 98 epicardial regions. As the number of epicardial regions is increased, the regularization technique effectively fixes the noise amplification but markedly decreases the resolution, whereas SVD results in an increase in noise and a moderate decrease in resolution. Overall the regularization technique performs slightly better than SVD in the noise-resolution relationship. There is a region at the posterior of the heart that was poorly resolved regardless of the number of regions chosen. The variance of the resolution was such as to suggest the use of variable-size epicardial regions based on the resolution.

Adult↗
PubMed1991

Mathematical modelling of flow through an irregular arterial stenosis.

A mathematical model of flow through an irregular arterial stenosis is developed. The model is two-dimensional and axi-symmetric with the stenosis outline obtained from a three-dimensional casting of a mildly stenosed artery. Agreement between modelled and experimental pressure drops (obtained from an axi-symmetric machined stenosis with the same profile) is excellent. Results are also obtained for a smooth stenosis model, similar to that used for most mathematical modelling studies. This model overestimates the pressure drop across the stenosis, as well as the wall shear stress and separation Reynolds number. Also, the smooth model predicts one instead of three recirculation zones present in the irregular model. The original stenosis is modified to increase the severity from 48 and 87% areal occlusion, while maintaining the same general shape. This has the effect of increasing the pressure drop by an order of magnitude and decreasing the number of recirculation zones to one, with a lower separation Reynolds number.

Arterial Occlusive Diseases↗
PubMed1990

New analgesic assay utilizing trypsin-induced hyperalgesia in the hind limb of the rat.

Subplantar injection of 250 micrograms of trypsin in the rat resulted in a biphasic increase in pain sensitivity (hyperalgesia) with peaks at 10 and 150 min separated by a period of decreased sensitivity to pain (hypoalgesia). Hyperalgesia was assessed by a decrease in response latency to a 3.0-kg force applied to the injected hind limb. Response latencies at 150 min were increased in a dose-dependent manner by pretreatment at 90 min with acetaminophen; phenacetin; the arachidonate cyclooxygenase inhibitors aspirin, indomethacin, and ibuprofen; and the opiate analgesics codeine and morphine. ED50s of 17, 13, 10, 0.48, 1.6, 3.9 and 1.2 mg/kg p.o. were obtained for these drugs, respectively. The hyperalgesia present at 150 min was not affected by pretreatment with antiinflammatory steroids, an antihistaminic, an antiserotonin agent, and an anticholinergic. We recommend measurement of drug-induced increase in response latencies produced 150 min after injection of 250 micrograms of trypsin as the basis for a new sensitive and selective analgesic assay. ED50s obtained in this assay correlate well with doses that are used clinically to produce analgesia. Development of the hypoalgesic component was selectively inhibited by pretreatment with an antiserotonin agent. Additional drug studies indicated that the algesic response to the subplantar injection of trypsin is the resultant of independent, temporally overlapping hyperalgesic and hypoalgesic components.

Animals↗