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Biomedical subjects

K B Stokes

Publications and source records attributed to K B Stokes.

At least 19 recordsLinked to original sources
PubMed1992

Septal His-Purkinje ventricular pacing in canines: a new endocardial electrode approach.

Ventricular activation sequences and cardiac performance are influenced by pacing sites. Stimulation of or close to the specialized atrioventricular (AV) conduction system optimizes paced ventricular function compared to alternative epi- or endocardial muscle conduction sites. This study reports a new endocardial electrode implant approach to approximate septal His-Purkinje ventricular pacing. Five 6-month-old beagles were used. A custom, platinum-iridium, exposed helical screw electrode (Medtronic, Inc.), 4.5-mm long, with a 17.8-mm2 surface area, was designed with a polyurethane covered 4 filar MP35N nickle conductor lead. An 8 French sheath (USCI, Inc.) was modified as introducer to permit simultaneous implant intracardiac pressure and electrogram recordings. Following a thoracotomy, the introducer was inserted through the right atrial appendage and advanced to record optimal His-bundle electrogram while maintaining atrial pressure along the septal tricuspid valve annulus. After electrode implant, ECG demonstrated narrow paced QRS morphology. Mean implant values showed sensed R wave 6.3 mV, slew rate 0.65 V/sec, pacing impedance 319 ohms, and threshold 0.9 V/3.3 mA at 0.5-msec output. Necropsy showed implant above the tricuspid annulus with electrode extension into and contained within the proximal ventricular septum. This study demonstrates that an endocardial septal approach to His-Purkinje ventricular pacing to optimize paced ventricular function is feasible with a new electrode design and precise septal implant technique. Alternative introducer designs may permit transvenous application of this approach.

Animals↗
PubMed1991

Unusual varieties of diaphragmatic herniae.

Unusual varieties of diaphragmatic herniae can be classified into two major groups, congenital and acquired. The late-presenting Bochdalek herniae often present difficulties in diagnosis which may lead to inappropriate treatment. The prime example is the herniated stomach, which is mistaken for a tension pneumothorax. Strangulation is a rare, but an important, complication of Bochdalek herniae. A number of techniques for closure of large diaphragmatic defects are described with recommendation of those procedures which can be performed rapidly and effectively in a critically ill infant. The literature concerning eventration is confusing due to different definitions of the condition by different authors. It may be difficult to distinguish preoperatively between this condition and congenital diaphragmatic hernia with a sac. Such distinction is often not important as the decision for intervention is based on evaluation of clinical and radiological considerations. The majority of Morgagni herniae are asymptomatic and only rarely does strangulation supervene. There is a small group of infants with Morgagni hernias who present in early infancy with respiratory symptoms. Paralysis of the diaphragm due to phrenic nerve palsy recovers spontaneously in the majority of patients. The selective use of diaphragmatic plication for this condition is widely accepted, but the decision and appropriate timing for surgical intervention is often difficult. The results of surgery are very good both in the early postoperative period and also on long-term follow-up. The diagnosis of traumatic diaphragmatic hernia is often overlooked in the presence of other major injuries. The danger of strangulation of contents of this hernia is ever present and repair should be undertaken without delay once the diagnosis is made.

Diagnosis, Differential↗
PubMed1991

The mythology of threshold variations as a function of electrode surface area.

It has been established that the chronic thresholds of cardiac pacing leads vary as a function of the (spherical) electrode's radius or (geometric) surface area and the thickness of fibrotic encapsulation. Where the radius of the electrode is equal to the thickness of the fibrous capsule (about 0.7 to 1 mm for polished surfaces), threshold should be at a minimum. Where the radius of the electrode is larger or smaller than the thickness of the fibrous capsule, then thresholds should increase since the electric field strength required to stimulate decreases as the square of the distance between the electrode's surface and stimulatable tissue. In addition, it has become (incorrectly) accepted that small electrodes do not sense well. About 8-mm electrodes, therefore, became the "standard" surface area, providing the best tradeoffs between pacing and sensing. Analysis of 18 years of canine data in our laboratory, however, suggest that these relationships may be overemphasized for the surface areas of clinical interest. In fact, new small porous and steroid-eluting electrodes do not have high thresholds, are efficient, and their sensing is excellent.

Animals↗
PubMed1990

Growth after gut resection for Crohn's disease.

Seventeen children underwent 19 bowel resections as part of their management of Crohn's disease. Thirteen children had evidence of retardation of linear growth preoperatively, of whom 12 exhibited catch up growth, crossing at least one centile band, after resection. Seven of the 12 showed early signs of puberty at the time of resection and one child was regarded as being fully pubertal. All children were symptomatic before their surgery; at one year 11 were asymptomatic and three others were substantially improved. Surgical resection of localised Crohn's disease, followed by adequate nutritional support has lead to catch up growth and a prolonged symptom free period in most of our patients and this was not limited by their age or stage of puberty.

Adolescent↗
PubMed1988

Infantile myofibromatosis causing biliary and pancreatic obstruction: a case report.

A case of obstructive jaundice caused by a solitary myofibroma in an 18-week-old infant is presented. Preoperative investigations demonstrated total biliary and partial pancreatic obstruction, although no obstructing lesion was identified radiologically. At operation a 3-cm solitary myofibroma was found in the head of the pancreas. Because of the morbidity associated with resection at this site, a bypass procedure was performed in the hope that the lesion will regress.

Cholestasis↗
PubMed1988

Paired comparisons of steroid-eluting and nonsteroid endocardial pacemaker leads in dogs: electrical performance and morphologic alterations.

The effects of steroid elution from endocardial pacemaker electrodes on electrical performance and the thickness and cellularity of the reactive fibrous connective tissue formed around the stimulating electrode (peri-electrode tissue) were determined. Comparison was made with a nonsteroid electrode implanted in the same cardiac chamber (right ventricle) in each of six dogs for 6 weeks. Paired Students' t-tests showed that steroid-eluting leads had significantly (P less than .05): (1) lower voltage stimulation thresholds (as determined in sequential measurements made on the conscious animals during the experiment and on the anesthetized dogs at termination of the study); (2) less fibrous connective tissue formation around the electrode surfaces; and (3) fewer cells per unit area of peri-electrode fibrous connective tissue. There were also fewer (P less than .10) mast cells in the reactive connective tissue surrounding steroid-eluting leads. The thinner reactive connective tissue surrounding the steroid-eluting electrodes was correlated with lower voltage stimulation thresholds (r = 0.7, P less than .01). This is consistent with the hypothesis that the effect of the peri-electrode connective tissue is to increase the virtual surface area of the electrode, decreasing current density in adjacent stimulatable tissue. The relatively fewer total cells and mast cells in the peri-electrode connective tissue of the steroid-eluting electrodes suggest that the observed differences in fibrous connective tissue thickness, and therefore voltage stimulation threshold, may be related to a relatively decreased population of inflammatory cells due to the anti-inflammatory properties of the steroid.

Animals↗
PubMed1988

A new low threshold platinized epicardial pacing electrode: comparative evaluation in immature canines.

A prospective comparison of pacing and sensing capabilities between the conventional Medtronic Model 4951 platinum-iridium epicardial pacing electrode and a new modified "platinized" version of the same electrode was performed in immature canines to determine if the new electrode design improves pacing in the immature myocardium. The conventional electrode was modified by electroplating platinum black particles onto the surface to increase the effective or true microscopic surface area, yet essentially maintain the same overall geometric electrode size. Both epicardial electrodes were inserted into the right ventricular myocardium with the lead pad sutured to the epicardium, and externalized to the scruff in five puppies (age 3 months). An additional left ventricular lead was implanted to permit chronic pacing following epicardially-induced atrioventricular block. Acute and chronic sensing and pacing capabilities of each externalized electrode were performed at implant and weekly up to 4 months. Histologic examination of each electrode implant site was performed at the end of the study period. At implant, both electrodes exhibited comparable values for sensed R waves, lead impedances, and pacing thresholds. During the study, the platinized electrode exhibited lower pacing thresholds. Analysis of all postimplant data demonstrated this threshold difference to be significantly lower (P less than .01) for the platinized version. Lead impedance and sensing capabilities remained comparable between the two designs. Histologic study demonstrated less fibrotic infiltration at the platinized electrode site. This preliminary evaluation indicates that for the duration of the postimplant study period, the platinized epicardial electrode design was associated with significantly lower thresholds and less fibrosis as a function of time compared to the conventional smooth electrode surface design. The new platinized electrode limits exit block in the developing immature myocardium and permits safe pacing at lower pulse widths and voltages to increase battery life.

Animals↗
PubMed1988

Preliminary studies on a new steroid eluting epicardial electrode.

Acute and chronic canine atrial and ventricular thresholds and sensing were compared for a steroid eluting epicardial plaque electrode, a similar steroid free plaque electrode and two standard leads. The steroid eluting electrode had low, stable thresbolds in atrium and ventricle compared to the steroid free plaque and standard leads. This new electrode also provided improved atrial sensing. Thus, the concept of a steroid eluting epicardial electrode shows promise for further evaluation.

Animals↗
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↗
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↗