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Biomedical subjects

K B Stokes

Publications and source records attributed to K B Stokes.

34 records · Page 2Linked to original sources
PubMed1988

Factors and interactions affecting the performance of polyurethane elastomers in medical devices.

Polyurethanes offer the greatest versatility in compositions and properties of any family of polymers. For implantable medical devices, a few specific elastomeric polyurethane compositions have demonstrated a combination of toughness, durability, biocompatibility and biostability not achieved by any other available material. Because of the complex behavior of implantable polyurethanes in the body environment, designers and fabricators of polyurethane-containing devices must pay particular attention to the choice of composition and design of components. Subsequent treatment during qualification, fabrication, sterilization, storage, implantation, in vivo operation and explantation also determine the performance and provide the means for assessing the efficacy of the polyurethane in the implanted device.

Biocompatible Materials↗
PubMed1988

Polyether polyurethanes: biostable or not?

Certain polyether polyurethanes have been shown to be biostable in long-term implant studies. Others retain good bulk properties, but have been shown to develop cracks on their tissue contacting surfaces. Two cracking mechanisms have been identified, in vivo stress cracking and metal ion oxidation. Stress cracking is the result of an interaction between the in vivo mammalian environment and residual stress (strain) in the implanted polymer. Mild autooxidation can be initiated by stress cracking. More extensive autooxidation can be initiated and propagated by corrosion of metallic device components, especially the corrosion products of cobalt. Both mechanisms are controllable, thus, do not necessarily preclude the use of polyether polyurethanes in implantable devices.

Biocompatible Materials↗
PubMed1987

Recurrent intussusception: barium or surgery?

A review was conducted of children with intussusception admitted to the Royal Children's Hospital over a 16 year period: of 630 episodes of intussusception, 28 represented recurrences (4.4%): 10 of these occurred in children aged 2 years or over. Duration of symptoms and the incidence of an incorrect initial diagnosis were markedly reduced with recurrence. The clinical presentation was similar to the first episode apart from rectal bleeding which was less common, reflecting the shorter history. Sixteen barium enemas were performed, of which 10 were successful. There were 18 operations with eight resections; seven for localized lesions and one for failure of manual reduction in a patient with an inverted Meckel's diverticulum. Barium reduction for recurrence should not be attempted in children over 2 years of age in whom no laparotomy was performed for their first episode, for most second recurrences, or in multiple polyposis.

Barium Sulfate↗
PubMed1987

Proglumide exhibits delta opioid agonist properties.

Recently, it was reported that proglumide, a cholecystokinin (CCK) antagonist, potentiates the analgetic effects of morphine and endogenous opioid peptides and reverses morphine tolerance by antagonizing the CCK system in the central nervous system of the rat. In order to provide additional insight into the mode of action of this agent, we assessed the effect of proglumide in the isolated guinea pig ileum and the mouse, rat and rabbit vas deferens. Furthermore, we studied the in vitro binding affinity of this substance to mouse brain synaptosomes. Our results show that proglumide inhibits, dose dependently, the electrically stimulated twitches in the mouse vas deferens and guinea pig ileum, but not in the rat or rabbit vas deferens. The inhibitory action of proglumide on the mouse vas deferens, but not on the guinea pig ileum, is antagonized by naloxone and by the selective delta-antagonist, ICI 174,864, in a competitive fashion. Other CCK antagonists were found to be devoid of such activity on the mouse vas deferens. In vitro binding studies showed that proglumide displaces D-ala-D-[leucine]5-enkephalin (DADLE), a delta agonist, but not ethylketocyclazocine (EKC), a preferentially selective kappa agonist. The effect of proglumide appeared to be elicited presynaptically since it did not alter the norepinephrine-induced contractions of the mouse vas deferens. Our results suggest that proglumide might exert its opiate-like effects by activation of delta-opioid receptors.

Animals↗
PubMed1986

Ten-year experience with implanted polyurethane lead insulation.

Polyurethane leads have been implanted in humans since 1977. Because of cases of stress cracking found in 1981, changes in the manufacturing process were made. Subsequent performance was excellent. There remains concern regarding the ability to predict accurately long-term performance. However, the advent of reliable, statistically accurate actuarial data coupled with accelerated tests predictive of human performance has restored confidence in the newer polyurethane lead technology.

Electrodes, Implanted↗
PubMed1984

Esophageal anastomosis--an experimental model to study the anastomotic lumen and the influence of a transanastomotic tube.

The influence of an indwelling transanastomotic silicon tube on healing esophageal anastomosis in puppies was studied by making resin casts of the reconstructed esophagus. These casts demonstrated stenosis but no difference in the group with a transanastomotic tube was seen. Stenotic index of the puppies varied from 0.7 to 0.97 (no stenosis = 1). An important observation made during the study was the presence of shelfing of scar tissue on the posterior esophageal wall. This was the site of mucosal ulceration, granuloma formation, and fibrous scarring where interrupted 0000 silk knots had been tied intraluminally. The anterior wall had a linear scar, where the knots were tied extraluminally. Extraluminal knots would be part of the technique to achieve a minimal amount of scar tissue at the site of the esophageal anastomosis.

Animals↗
PubMed1983

Future generation pacemakers.

Interchangeability between pacing modes will be the key feature in the pacemaker of the 1980s. The usefulness of the various modes will be enhanced by the array of pacing parameter options made available within each mode. Noninvasive adjustments of rates, AV intervals, signal detection sensitivities, and both atrial and ventricular stimulating energies will allow for necessary fine-tuning and ease of trouble-shooting throughout the life of the pacemaker system. Battery energy will be conserved by trimming output pulse characteristics to safe but reasonable levels. On the other hand, should myocardial thresholds increase, flexibility to reprogram pulse width and/or amplitude will avoid the need for reimplanting new electrodes or pulse generator. The physician will be able to evaluate underlying rhythm mechanisms and even perform single electrophysiologic studies by using special programmable features. Advanced telemetry features will facilitate patient followup and help identify pacing problems. Various telemetry options will include information on programmed and actual performance parameters, and on endocardial signals from both the atria and ventricles. Future generation pacemakers will be a collection and refinement of present technologies into single pacing systems. It is not overly optimistic to envision within this decade a small, long-life, easily implanted, sophisticated pacemaker that is capable of several modes of operation for treatment of both bradycardias and tachycardias, and is noninvasively adjustable to various and changing patient conditions.

Arrhythmias, Cardiac↗
PubMed1978

Esophageal atresia with tracheoesophageal fistula: end-to-end versus end-to-side repair.

Despite early enthusiasm by some authors for the end-to-side repair of esophageal atresia with tracheoesophageal fistula, many have returned to the end-to-end technique. The present study compares the results of these two operative procedures. A retrospective analysis was made of 52 consecutive cases in which primary repair was performed. The patients were divided according to the three preoperative risk groups descibed by Waterston. The mortality for the end-to-end and end-to-side repairs was similar in each of the three risk groups. Similarly, there was no significant differnences in the incidence of anastomotic leak or recanalization of the fistula. However, the rate of anastomotic stricture in the end-to-end group was significantly higher (p less than 0.001) than in the end-to-side group. A description of the end-to-side technique is given, and its advantages are outlined.

Esophageal Atresia↗
PubMedDate not supplied

Fulminant hepatic failure with intractable ascites due to an echovirus 11 infection successfully managed with a peritoneo-venous (LeVeen) shunt.

A case is described of an infant who at 5 days of age developed an echovirus 11 infection with hepatic failure and intractable ascites, refractory to medical treatment. The ascites was successfully controlled with a peritoneo-venous (Le Veen) shunt inserted at 6 weeks of age. The shunt was subsequently removed when the infant was 4 months of age with no recurrence of ascites. At long-term follow-up he is growing and developing normally. This case shows that an adult Le Veen shunt can be successfully modified for use in the newborn with worthwhile long-term survival.

Ascites↗
PubMedDate not supplied

Biostability considerations for implantable polyurethanes.

Polyurethanes have become the most valuable implantable elastomers for uses requiring toughness, durability, biocompatibility and biostability. They are inherently stable in the body environment. However, physical and chemical changes may be effected by conditions of processing, fabrication, use or interactions with other device components. Most prominent modes of polyurethane degradation include mineralization, environmental stress-cracking and oxidation. While the mechanisms of these forms of degradation are not fully understood, an awareness of their causes and effects can lead to procedures that provide all of the long-term functionality required for the sophisticated polyurethane-based devices of today and tomorrow.

Animals↗