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Biomedical subjects

T David

Publications and source records attributed to T David.

At least 55 records · Page 3Linked to original sources
PubMed1997

Time-dependent stress and displacement of the eye wall tissue of the human eye.

Myopia or short sightedness, is the most important predisposing factor to retinal detachment. The relative risk of detachment rises with increasing myopia. The model characterizes that because the severity of myopia increases with the axial length (antero-posterior diameter) of the eyeball, the relative risk of retinal detachment rises with increasing eye size. We present a mathematical model of the time-dependent shear stress force that occurs in the thin eye wall shell supporting the vitreous humour inside the eye globe during the acceleration and deceleration phases of saccadic eye movement. Results show that the shear force increases as the thickness of the eye wall decreases. It is common for myopes to have thinner eye wall tissue than emmetropes. In addition, if account is taken of the increased force required to provide normal saccadic movement of myopic (larger) eyes, then the shear force is up to seven times greater than that experienced for emmetropes.

Biomechanical Phenomena↗
PubMed1997

Three-dimensional study of the effect of two leaflet opening angles on the time-dependent flow through a bileaflet mechanical heart valve.

A three-dimensional (3-D), time-dependent computational fluid dynamics (CFD) model was used to investigate the effect of leaflet opening angle on the flow through a fully open bileaflet heart valve up to peak systole. A laminar flow model of a Newtonian fluid was used, and the peak systolic. Reynolds number was 1500, based on the aortic radius and the average velocity at peak systole. This resulted in a Reynolds number of 5800, based on the aortic radius and the local maximum velocity. The flow fields through and downstream of the bileaflet valves were complex, with strong time-dependent 3-D vortices being found in planes parallel and perpendicular to the leaflets. The parametric study of the effect of leaflet opening angle showed that, as the leaflet opening angle increased from 78 degrees to 85 degrees, the flow downstream of the valve leaflets became more centralized, and the wake downstream of the leaflet decreased in size. However, as the opening angle increased from 78 degrees to 85 degrees, the maximum shear rate and the maximum velocity increased, suggesting that the design of the central orifice geometry was also an important consideration.

Biophysical Phenomena↗
PubMed1997

[Mooren's ulcer following combined extracapsular crystalline lens extraction and trabeculectomy].

We report the case of a 70-year-old patient who presented with Mooren's ulcer in her right eye following extracapsular cataract extraction combined with trabeculectomy. She was successfully treated by a sclerocorneal graft combined with a large conjunctival resection. Clinical and histological features, and therapeutic outcomes are discussed and compared to previously published data.

Aged↗
PubMed1996

Results of disc prosthesis after a minimum follow-up period of 2 years.

STUDY DESIGN: This study analyzed retrospectively 46 patients undergoing artificial disc replacement. OBJECTIVE: To assess the clinical and radiographic outcomes of patients who had Charitè SB III disc prosthesis. SUMMARY OF BACKGROUND DATA: Results of disc prosthesis have been reported only at very short-term follow-up periods; preoperative diagnosis and criteria used to evaluate the clinical outcomes were not reported. METHODS: Forty-six patients who had had a disc prosthesis were evaluated clinically and radiographically at least 2 years after surgery. Preoperative diagnosis included disc degeneration in 22 patients and failed disc excision in 24 patients. Disc prosthesis was implanted at a single vertebral level in 36 patients and at two levels in 10 patients. Follow-up evaluation was performed after an average of 3.2 years (range, 2-5 years). RESULTS: Sixty-three percent of patients reported satisfactory results. The success rate was 69% in patients who underwent isolated disc replacement and 77% in those with no previous back surgeries. Seven patients who had unsatisfactory results underwent posterolateral fusion without removing the artificial disc. Two patients underwent removal of the prosthesis. No failure of the implants or loosenings or wear of the polyethylene core were found. Vertebral motion averaged 9 degrees at the operated levels and 16 degrees at the adjacent levels. CONCLUSION: A wrong surgical indication, rather than failure of the prosthesis, appears to be the main cause of unsatisfactory results of disc replacement at medium-term evaluations. Prospective and longer term studies are needed to establish whether disc prosthesis may offer advantages compared with spinal fusion.

Adult↗
PubMed1996

A three-dimensional, time-dependent analysis of flow through a bileaflet mechanical heart valve: comparison of experimental and numerical results.

The flow through a bileaflet mechanical heart valve during the first half of systole was predicted using computational fluid dynamics (CFD). A three-dimensional model of the geometry of the ventricle, valve, sinus and aorta was developed. Flow through the valve was assumed to be Newtonian and laminar. The peak systolic Reynolds number was 1500 based on the aortic radius and the mean aortic velocity. Flow visualisation and laser Doppler anemometry (LDA) experiments were performed and the results were compared to the CFD model. Good agreement between the LDA measurements and CFD predictions was found in the jets through the major orifices of the valve. The global flow fields predicted by the CFD showed reasonable agreement with the flow visualisation. A starting vortex was shed from the valve leaflets of the CarboMedics valve and the prototype valve. As systole progressed the two major orifice jets were directed towards the aortic wall and a weaker central jet was seen in both the experimental and CFD models. Large vortices were present on either side of the central orifice jet in the sinus area of both models. The three-dimensional time-dependent CFD model was considered to give a reasonable indication of the dominant flow patterns downstream of the bileaflet heart valve and has the potential to be an extremely useful tool to analyse the different designs of existing and future bileaflet valves.

Aorta↗
PubMed1996

The integrated design of mechanical bi-leaflet prosthetic heart valves.

A flow model utilizing an irrotational, inviscid algorithm of vortex-ring elements simulating the leaflets and source/sink elements simulating the aortic root coupled with a boundary layer model has been developed to model the internal flow phenomena of bi-leaflet mechanical heart valves implanted in the aortic root. The inviscid representation evaluates the aerodynamic lift, the induced drag, the pitching moment and flow velocity along the leaflet surface thus providing data for evaluating the boundary-layer thickness, the shear stress and flow separation point by the boundary layer theory. Full integration with the geometry enables immediate updates of the flow solution when changes in geometry have been made. It is shown that the effects of the internal flow domain model are necessary in the correct evaluation of lift and drag for subsequent dynamic analysis. The environment presented provides for the ability to produce significant and immediate design changes so that crucial decisions may be made whilst still within the software design loop. New designs are shown along with data for the improved flow model.

Algorithms↗
PubMed1996

Dynamic analysis and geometry models for the design of bi-leaflet prosthetic mechanical heart valves.

This paper provides new geometry definitions for the axi-symmetric stiffening (or sewing) ring, as part of a bi-leaflet prosthetic heart valve, and presents a dynamic behaviour analysis of the leaflet. An optimal stiffening ring geometry may be constructed by considering the point of flow separation on the stiffening ring (measured in the downstream coordinate), the effective orifice area of the stiffening ring and the associated dynamic behaviour of the leaflet. The dynamic model is accomplished by utilizing a second-order rotating system to simulate the opening and closing characteristics of the leaflet. The moments due to the aerodynamic loads are evaluated from an irrotational inviscid flow model, coupled with boundary layer theory, modelling the internal flow phenomena of the bi-leaflet heart valve implanted in the aortic root. It has been shown that this internal flow model provides the correct evaluation of lift and induced drag and the subsequent dynamic characteristics.

Algorithms↗
PubMed1996

Morbidity outcome in early versus conventional tracheal extubation after coronary artery bypass grafting: a prospective randomized controlled trial.

INTRODUCTION: We undertook a prospective, randomized, controlled clinical trial to evaluate morbidity outcomes and safety of a modified anesthetic technique to provide shorter sedation and early extubation (1 to 6 hours) than those of the conventional anesthetic protocol used for prolonged sedation and extubation (12 to 22 hours) in patients after coronary artery bypass grafting. METHODS: One hundred twenty patients undergoing elective coronary artery bypass grafting were prospectively assigned randomly to either an early extubation group (n = 60; 15 micrograms.kg-1 fentanyl and 2 to 6 mg.kg-1.hour-1 propofol and isoflurane) or to a conventional extubation group (n = 60; 50 micrograms.kg-1 fentanyl and 0.1 mg.kg-1 midazolam and isoflurane). Cardiac morbidity (postoperative myocardial ischemia, postoperative myocardial infarction, and perioperative sympathoadrenal stress response), respiratory morbidity (postextubation apnea, alveolar-arterial oxygen gradient, pulmonary shunting, oxygen consumption, atelectasis, and reintubation), hemodynamic values and vasoactive medication requirements, intraoperative awareness, postoperative cognitive function, 30 day mortality, and intensive care unit and hospital lengths of stay were compared between the two groups. RESULTS: Fifty-one of the 60 patients in each group (85%) were extubated within the defined time period. Postoperative extubation time and intensive care unit and hospital lengths of stay were significantly shorter in the early group. At 48 hours after operation, there were no significant differences between the two groups in myocardial ischemia incidences, ischemia burdens, or creatine kinase isoenzyme MB levels. Four patients in the conventional group, but not in the early group, had postoperative myocardial infaction. The extubation anesthetics used were effective in suppressing the perioperative plasma catecholamine stress response in both groups. Postextubation apnea characteristics were similar between the groups. Intrapulmonary shunt fraction improved significantly in the early group at 4 hours after extubation. The incidences and degree of atelectasis did not differ significantly between the two groups. The incidences of treated postoperative complications were comparable between the two groups, but three patients in the conventional group died as a result of stroke or postoperative myocardial infarction. CONCLUSION: Early extubation after coronary artery bypass grafting is safe and does not increase perioperative morbidity. There is an improvement in postextubation intrapulmonary shunt fraction and a reduction in intensive care unit and hospital lengths of stay.

Aged↗
PubMed1996

Early tracheal extubation after coronary artery bypass graft surgery reduces costs and improves resource use. A prospective, randomized, controlled trial.

BACKGROUND: Economics has caused the trend of early tracheal extubation after cardiac surgery, yet no prospective randomized study has directly validated that early tracheal extubation anesthetic management decreases costs when compared with late extubation after cardiac surgery. METHODS: This prospective, randomized, controlled clinical trial was designed to evaluate the cost savings of early (1-6 h) versus late tracheal extubation (12-22 h) in patients after coronary artery bypass graft (CABG) surgery. The total cost for the services provided for each patient was determined for both the early and late groups from hospital admission to discharge home. All costs applicable to each of the services were classified into direct variables, direct fixed costs, and overhead (an indirect cost). Physician fees and heart catheterization costs were included. The total service cost was the sum of unit workload and overhead costs. RESULTS: One hundred patients having elective CABG who were younger than 75 yr were studied. Including all complications, early extubation (n = 50) significantly reduced cardiovascular intensive care unit (CVICU) costs by 53% (P < 0.026) and the total CABG surgery cost by 25% (P < 0.019) when compared with late extubation (n = 50). Forty-one patients (82%) in each group were tracheally extubated within the defined period. In the early extubation group, the actual departmental cost savings in CVICU nursing and supplies was 23% (P < 0.005), in ward nursing and supplies was 11% (P < 0.05), and in respiratory therapy was 12% (P < 0.05). The total cost savings per patient having CABG was 9% (P < 0.001). Further cost savings using discharge criteria were 51% for CVICU nursing and supplies (P < 0.001), 9% for ward nursing and supplies (P < 0.05), and 29% for respiratory therapy (P < 0.001), for a total cost savings per patient of 13% (P < 0.001). Early extubation also reduced elective case cancellations (P < 0.002) without any increase in the number of postoperative complications and readmissions. CONCLUSIONS: Early tracheal extubation anesthetic management reduces total costs per CABG surgery by 25%, predominantly in nursing and in CVICU costs. Early extubation reduces CVICU and hospital length of stay but does not increase the rate or costs of complications when compared with patients in the late extubation group. It shifts the high CVICU costs to the lower ward costs. Early extubation also improves resource use after cardiac surgery when compared with late extubation.

Aged↗
PubMed1996

The influence of open leaflet geometry on the haemodynamic flow characteristics of polyurethane trileaflet artificial heart valves.

In vitro velocity data were obtained downstream of two versions of the Leeds polyurethane trileaflet heart valve in a simulated pulsatile flow regime using laser Doppler velocimetry. The main difference between the two valves studied was the manufacturing method used to create the valves. The film-fabricated valve was constructed from solvent-cast sheets of polyurethane, thermally formed into the correct leaflet geometry. The dip-cast valve used a stainless steel mould which was dipped into a polyurethane solution to produce the valve leaflets. Significant differences were visible between the fully open leaflet shape of each valve. The distribution of mean axial velocity and Reynolds normal stress (RNS) was shown to be dependent on the shape of the fully open valve orifice. For the film-fabricated valves, flow recirculation and high values of RNS were present downstream of the frame posts. The maximum value of RNS obtained downstream of the film-fabricated valve at peak systole was 147 N/m2. Results for the dip-cast valve showed a more uniform distribution of mean axial velocity and RNS resulting from the more circular central orifice produced by the dip-cast leaflets. The maximum value of RNS obtained downstream of the dip-cast valve at peak systole was 109 N/m2. These results demonstrate the effect of the open valve geometry on the flow characteristics downstream of trileaflet valves and that minor changes to the open leaflet geometry can significantly affect the flow characteristics and the possibility of flow-related blood damage occurring in vivo.

Aortic Valve↗
PubMed1996

[Minimally invasive myocardial revascularization. Initial experience with myocardial revascularization without extracorporeal circulation].

The authors present an account on the first phase of application of minimally invasive techniques of revascularization of the heart into clinical practice at their department. Since December 1994 to the end of January 1995 in selected 15 patients from a total of 240 patients with IHD 16 arterial and 8 venous grafts were sutured on the beating heart from median sternotomy. None of the patients died and none need reoperation. All patients are free from anginous complaints after operation and their clinical condition is satisfactory. Revascularization of the heart by means of arterial and venous grafts on the beating heart from median sternotomy is a technically reproducible method which enables us to elaborate further minimally invasive procedures for the treatment of IHD. The presented paper is a preliminary communication without claims of statistical validity of the achieved results.

Aged↗
PubMed1995

Corneal wound healing modulation using basic fibroblast growth factor after excimer laser photorefractive keratectomy.

Photorefractive keratectomy with the 193-nm excimer laser is one of the most promising innovations in refractive surgery. However, routine clinical application is hindered by two main obstacles, i.e., postoperative subepithelial opacity and possible regression of the refractive result. Because we have previously demonstrated beneficial effects of basic fibroblast growth factor (bFGF) on corneal epithelial healing in different in vivo models, we investigated the influence on both epithelial and stromal healing of topical bFGF after excimer laser keratomileusis in a rabbit model. After 7-mm circular deepithelialization, the eyes of 24 New Zealand White rabbits received identical deep stromal laser ablations (depth 50 microns, 6 D, diameter 5 mm) and were randomly assigned to one of four treatment groups [control (phosphate-buffered saline, PBS), bFGF 10 micrograms/application, dexamethasone 0.1%, bFGF + dexamethasone, n = 12 eyes/group]. All treatments were administered four times daily--bFGF until complete epithelial healing, dexamethasone and PBS until 3 months postsurgery. Wound surface regression on time was determined by means of computer-assisted image analysis of fluorescein-stained corneas. Corneal opacity was observed biomicroscopically and graded using a previously established scoring system. After bFGF application for 2-3 days only, a highly significant acceleration in epithelial wound healing speed was found compared with the rate of the other three treatment groups (p < 0.001). A combined therapy (bFGF + steroid) had no effect on the healing rate. Compared with control values, mean scores for subepithelial haze in the other groups were found to be nearly 50% lower during the first 2 postoperative months.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1995

Defining potential targets for immunotherapy in Burkholderia cepacia infection.

Burkholderia cepacia has become a serious source of infection in patients with cystic fibrosis. Antibiotic therapy is difficult as the bacteria are intrinsically resistant to most antibiotics. The present study compared the antibody response by immunoblot of 50 negative control sera, 22 patients with cystic fibrosis and no evidence of B. cepacia, 9 clinically well patients with cystic fibrosis colonised by B. cepacia and 5 patients with cystic fibrosis and deteriorating or fatal B. cepacia infection. Nineteen antigenic bands varying in apparent molecular weights from 19 to 170 kDa were identified. Two bands at 19 and 21 kDa were only present when the organism was grown in an iron-deficient medium. The band at 30 kDa was identified as a porin and the possession of IgG antibody carried a statistically significant (P = 0.00003) better prognosis. This antigen was thus a potential target for immunotherapy.

Antibodies, Bacterial↗
PubMed1995

Determination of the curvatures and bending strains in open trileaflet heart valves.

The leaflets of trileaflet artificial heart valves manufactured from polyurethane, gluteraldehyde-treated porcine aortic valves and pericardial tissue are subject to cyclic stresses and strains which can reduce the lifetime of the implanted valves through leaflet calcification and fatigue failure. A detailed knowledge of the stress state within a valve leaflet throughout a cardiac cycle is desirable in order to improve the geometry of the valve leaflets and ultimately improve the valve performance. An experimental method to evaluate the radius of curvature at the free edge of the open valve leaflet is presented. The technique has been applied to polyurethane trileaflet heart valves manufactured within the authors' laboratory and to commercially available bioprosthetic valves in the fully open position under steady and pulsatile flow conditions. Simple bending theory has been applied to the polyurethane valves to calculate bending stresses and strains at the free leaflet edge based on the measured curvature. The results showed that in the fully open position the highest curvatures occurred at the commissural regions for all the valves analysed. Additional areas of high curvature were present along the free leaflet edge. Average curvatures as high as 0.85 mm-1 were observed at the leaflet commissures for the polyurethane valves with a resultant bending stress of 0.72 MPa. The porcine bioprosthetic valves showed average curvatures as high as 2.5 mm-1 which also occurred at the leaflet commissures. The results of the study have been compared to values of stress obtained from numerical analysis of closed polyurethane valve leaflets reported in the literature.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗