In vivo evaluation of a closed loop monitoring strategy for induced paralysis.

OBJECTIVE: Reliable closed loop infusion systems for regulating paralysis level can be a great convenience to the anesthesiologists in automating their task. This paper describes the in vivo performance evaluation of a self-tuning controller that is designed to accommodate large variations in patient drug sensitivity, drug action delays and environmental interfering noise. METHODS: The infusion system was evaluated in six adult mongrel dogs. Following the manual induction of paralysis by an anesthesiologist, the controller regulated the infusion of vecuronium to maintain a desired level of paralysis. The integrated EMG response of the hypothenar muscle to a train-of-four stimulation of the ulnar nerve quantified the depth of paralysis. The controller's robustness was tested by contaminating the sensed twitch signal with electrocautery noise and electrode disconnection. RESULTS: The controller reached the initial level of paralysis of 100% in about 4.0 minutes and arrived at the desired level of 90% with an overshoot of 6.38% (+/-6.82). It maintained the desired level of paralysis with a 2.04% (+/-1.20) mean offset at 90% and 0.4% (+/-0.5) mean offset at 80% steady state level, respectively. The mean infusion rate to sustain 90% and 80% paralysis were 2.70 (+/-2.05) and 2.15 (+/-2.57) ((mg/kg)/min), respectively. CONCLUSIONS: The system adapted to a large variation in the sample subject drug sensitivity. It remained stable despite large amplitude disturbances and maintained the paralysis at the desired level following the removal of the disturbances.

Noninvasive blood pressure measurement on the temporal artery using the auscultatory method.

Blood pressures in the temporal artery of five normotensive subjects were recorded using a modified auscultatory setup. The setup comprised a pediatric cuff to occlude the artery and a piezoelectric contact microphone to record the Korotkoff sounds. Both the cuff and microphone were held in their respective positions with an adjustable head band. The recordings were taken under four different conditions: the subject lying supine, the subject sitting at rest, the subject sitting immediately after exercise and the subject moving the head gently. These recordings were compared with readings from the brachial artery, obtained with a commercially available automatic blood pressure measuring device. Korotkoff sounds were analyzed in the time and frequency domain. Results indicate that Korotkoff sounds in the temporal artery are much smaller in amplitude, and do not exhibit the same distinctive phases as those of the brachial artery. Despite these differences, these sounds can be used to detect blood pressures at head level. The accuracy of the readings was within +/- 10%. Successful readings were also obtained with gentle head motions, demonstrating that this setup has the potential to be developed into an ambulatory blood pressure monitoring system.

A novel microporous polyurethane blood conduit: biocompatibility assessment of the UTA arterial prosthesis by an organo-typic culture technique.

An organotypic culture assay has been used to assess the biocompatibility and cytotoxicity of an arterial prosthesis developed at the University of Texas-Arlington (the UTA graft) from a structurally modified polyurethane (PU) elastomer (Tecoflex). The cell culture test was applied to the UTA graft after sterilization by ethylene oxide and by gamma radiation in two separate series. First, small specimens of the prosthesis were incubated for 7 days on a semisolid nutrient medium with their luminal surface in direct contact with endothelium explanted from the aorta of chick embryos. Second, the possibility of cytotoxic contaminants being leached from the polyurethane was assessed by immersing the biomaterial in the liquid culture medium for 5 days at 37 degrees C prior to conducting the organo-typic culture assay on a standard control surface. The structure of the UTA polyurethane prosthesis is porous, but the graft wall is impervious because it contains closed (i.e., noncommunicating) pores. In addition, four other vascular prostheses were included in the study for comparison. They were the Hydrophilic Mitrathane PU graft with a similar impervious, closed pore structure, an experimental Hydrophobic Mitrathane PU graft with a fibrous, open pore structure, and the commercial Impra and Reinforced Goretex expanded PTFE grafts. Following 7 days of cell culture, the biocompatibility and cytotoxicity of the various biomaterials were measured in terms of the area of migrating cells, the density of cells surrounding the explants, and the level of cell adhesion. Comparison of the results against control cultures demonstrated that the UTA graft, along with the other four prostheses, does not release cytotoxic extractables. Microscopic observations of its cultured surface indicated that the UTA graft promotes a high density of cell growth over a limited area, similar to the Hydrophilic Mitrathane graft. This level of biocompatibility is considered inferior to that of the two PTFE and the Hydrophobic Mitrathane prostheses, which promote more extensive cell migration, greater cell adhesion, and cell growth in a continuous single layer.

A novel microporous polyurethane vascular graft: in vivo evaluation of the UTA prosthesis implanted as infra-renal aortic substitute in dogs.

A novel microporous polyurethane blood conduit developed at the University of Texas at Arlington was implanted as an infra-renal substitute in dogs. The prosthesis was fabricated by precipitating a solution of the polymer with dry nitrogen onto a rotating mandrel. The grafts were sterilized either by gamma radiation (series I) or ethylene oxide (series II); they were implanted for the following prescheduled periods: 4, 24, 48 hours, and 1 week (short-term) and 2, 4 weeks, 3 and 6 months (medium-term). The thrombohematological characteristics of each animal were evaluated prior to implantation and confirmed that the index of blood coagulability was normal. In the short-term group, five out of eight grafts were patent and three were partially occluded; four grafts in the medium-term group were patent; one was partially occluded; and three were thrombosed at retrieval. One week after implantation, the prostheses were surrounded by an external capsule, which was present mainly at the two anastomoses. The external capsule covered the entire graft at 3 months. No kinking of the grafts was observed and the presence of a mild yellow stain related to bilirubin uptake was detected at 2 weeks, 1, 3, and 6 months. Histological studies have revealed the formation of a thin internal capsule at both anastomoses, 2 weeks postimplantation, which was not anchored to the graft wall. In the medium-term group, the thrombosed grafts failed to develop an internal capsule, whereas the patent graft exhibited a thick internal capsule made of neocollagenous tissue over the entire graft. This new microporous polyurethane prosthesis did not perform satisfactorily as an infra-renal substitute in dogs and its in vivo stability requires further assessment. Thus, the concept of a polyurethane with closed pores does not achieve what was anticipated.

Infusion line model for the detection of infiltration, extravasation, and other fluid flow faults.

Infiltration or extravasation is the undesirable infusion of fluids and medication into the tissue space surrounding a blood vessel. If left undetected, it may cause severe tissue damage to patients requiring surgical repair or even amputation. While there are numerous patents purporting the automatic detection of infiltration, few seem to be built into current infusion devices, and none are able to distinguish between various faulty fluid flow conditions. The purpose of this study was to demonstrate that the dynamic hydraulic properties of an IV-line can be used to detect infiltration and extravasation as well as other fluid flow faults such as a kinked line or an occluded needle. A pressure step was applied to five IV-lines under five different experimental conditions; the resulting pressure curves were modeled by a third-order system. With a reliability of 92%, the system parameters were used to detect and correctly identify five different fluid flow conditions.

T lymphocyte modification with the UTA microporous polyurethane vascular prosthesis: in vivo studies in rats.

Sequential quantification of blood T cell subsets by immunocytofluorometry was used to investigate the immune response of microporous polyurethane vascular prostheses after intraperitoneal implantation in rats. The experimental prosthesis, as developed by the University of Texas-Arlington group (UTA), and the Mitrathane prosthesis, as developed by Matrix Med., were implanted for 1, 2 and 6 weeks and compared with ePTFE and wounded rats without prostheses (control group). The implants were examined for histopathology by light microscopy. The percentages of CD4-(helper) and CD8-(suppressor) bearing cells of the PTFE group were significantly lower (p less than 0.05) than the control group 1 week post-implantation. The UTA and the Mitrathane grafts exhibited a significant decrease in both T cell subsets at 1 week, and CD4-bearing cells at 2 weeks. At 6 weeks, T cell subsets were similar among all groups. The ratio of CD4/CD8- cells was similar among all groups except for the PTFE group, which was lower than the control group after 1 week. Histological examination of Mitrathane and UTA grafts showed an acute phase of inflammation which lasted at least 2 weeks. Some foreign body giant cells (FBGC) were present 2 weeks post-implantation, and encapsulation was greater than that observed with PTFE grafts. On the other hand, PTFE grafts exhibited a different pattern of inflammation compared to polyurethane grafts. PTFE implants exhibited a moderate chronic inflammatory response for the first week, as shown by the formation of FBGC. At 2 and 6 weeks, the grafts were encapsulated by a thin layer of collagenous tissue and FBGC were still present around the implants, mostly located in contact with the reinforcing mesh.(ABSTRACT TRUNCATED AT 250 WORDS)

Clinical engineering programs in Dallas and Fort Worth hospitals.

The purpose of this paper is to describe current clinical engineering (CE) activities in the Dallas-Fort Worth metropolitan area. The CE directors of the thirteen largest hospitals in this area were interviewed by telephone and questions were asked regarding CE environment, activities, and management. Data are presented on the numbers and types of personnel, the educational background of the CE department directors, the position of the CE department in the management hierarchy, department activities, and management programs. Hospitals ranged in bed count from 380 to 1,509 with 1 to 26 employees. Four of 13 departments operate as profit centers. Departments range from one to four reporting levels below the hospital chief executive officer. Almost every CE department is involved in risk management, but only half measure worker productivity.

Fabrication and characterization of small-diameter vascular prostheses.

We have developed a process to fabricate polyurethane vascular grafts of various dimensions and porosities in our laboratory. A primary feature of the presented fabrication technique is the ability to control surface porosity and roughness, and bulk mechanical properties. The method is based on the spray application of a fine mixture of polymer solution and nitrogen gas bubbles onto a lathe-mounted mandrel. The technique was successfully tested with Tecoflex, a linear segmented aliphatic polyurethane. Other urethane polymers can be used as well. Several polymer coats are applied in a semiautomated process, at the end of which the polymer coating is dried and the tube is slipped off the mandrel. It is the purpose of this paper to describe the fabrication process and present results of the evaluation of grafts. Wall structure was evaluated using scanning electron microscopy and compliance was measured in a specially designed testing apparatus. We developed methods to quantify kink resistance and suture retention capacity of the grafts. These characteristics were correlated with graft fabrication variables: mandrel rpm, horizontal speed of the spray nozzle, gas and polymer solution flow rates. We were able to routinely produce 3-6-mm-ID grafts with 0.5-1.2-mm wall thickness and average bulk pore sizes of 10-250 microns; the wall porosity could be varied between 30 and 70%. Compliance values of the grafts were comparable with corresponding values of carotid and femoral arteries of dogs.

Synthetic vascular graft fabrication by a precipitation-flotation method.

The authors report a new technique for fabricating synthetic vascular grafts. It involves spraying a polymer solution (generated by mixing polymer solution and nitrogen gas in a spray nozzle) onto the surface of a flowing nonsolvent liquid (water): polymer fibers form during precipitation of the spray drops as they travel on the water surface, until picked up by a partially submerged rotating mandrel. Depending on process conditions, these fibers may aggregate to form a continuous layer or remain separated until they are picked up. A number of independent process variables allow control of characteristics of the conduits: gas and polymer solution feed rates, nozzle traverse speed, nonsolvent (water) flow rate, spray-mandrel spacing, and mandrel rpm. The SEM reveals that the graft wall consists of numerous fused polymeric fibers arrayed in both the circumferential and axial directions. The inner surface resembles a mesh of closely spaced fused fibers. The conduits have walls with interconnected pores (water permeabilities between 0.05 to 7.0 ml/min-cm2); nonporous surfaces also can be made. Tensile stress of the grafts at failure (in radial direction) varied between 0.05 to 2.3 MPa, whereas elongation at break ranged between 150 to 600%, depending on the porosity and fabrication conditions. A major advantage of this technique is its ability to produce grafts of a wide variety of fiber sizes and fusion characteristics in an inexpensive, safe, and reliable fashion.

Orthogonal polynomials used as soft tissue energy density functions.

A mathematical method is developed for computing the coefficients of soft tissue energy density polynomials, satisfying certain constraints. The polynomial coefficients are computed in the least squares sense. It is demonstrated that this method: (a) determines up to 30 polynomial coefficients whereas the unmodified least squares method based on Maclaurin power series determines up to nine coefficients; and (b) increases numerical stability and accuracy by several orders of magnitude. All computations are carried out in single precision on a LSI-11/23 laboratory minicomputer. The algorithm is particularly useful for on-line data analysis using small laboratory computers.

Experimental measurements of elastic properties of media and adventitia of bovine carotid arteries.

An experimental procedure is described which has been developed to measure the elastic properties of the outer (adventitial) and inner (medial) layers of excised bovine carotid arteries. The data analysis is based on a two-layered arterial wall model and the theory of large elastic deformations. The energy density functions for both layers are exponentials. The results show that the media and the adventitia are anisotropic; that the media is stiffer, more non-linear, and subjected to higher stresses than commonly assumed; and that both layers are stiffer in the axial direction than in the tangential direction.

Parameter sensitivity analysis and improvement of a two-layer arterial wall model.

The nonlinear two-layer arterial wall model introduced by von Maltzahn, et al. [11] is subjected to a rigorous parameter sensitivity and range of validity analysis. The model is based on the assumption that in large muscular conduit arteries the two mechanically significant layers are media and adventitia. Using curve-fitting techniques, the media is determined to be isotropic and the adventitia to be anisotropic. As a result of the range of validity analysis, the polynomial relationship for the energy density function of the media is changed to an exponential relationship. This leads to new coefficients for the polynomial of the adventitia. All coefficients have specific mechanical meanings. The parameter sensitivity analysis demonstrates convincingly that all model parameters are significantly important.

Stresses and strains in the cone-shaped carotid sinus and their effects on baroreceptor functions.

Two models are introduced to compute the stresses and strains inside the wall of the cone-shaped carotid sinus: a two-layered cylindrical disc model and a two-layered spherical cone model. While they differ in complexity and approach, they both yield very similar stress-strain distributions for various physiological load conditions. These stress-strain distributions are important for strain sensitive baroreceptors in several ways. (a) Both the size and the orientation of the baroreceptors change as strain levels in the wall change. (b) Large stress differences occur at the border between the media and the adventitia which could cause large baroreceptor deformations. (c) Two distinct solutions with different stress-strain fields exist for the cylindrical disc model. A switch from one solution to the other during a normal pressure cycle would also cause large baroreceptor deformations.