Mock System Research Articles

Direct-current injury from external pacemaker results in tissue electrolysis

In two patients undergoing open heart operations, electrochemical bums developed at the sites of connection to an external pacing system. Investigation revealed that failure of the pacing generator caused a small, continuous, direct current to pass through the patients, resulting in electrolysis at the sites of contact with the pacing and grounding wires. This electrolytic reaction was recreated in a mock pacing system and resulted in tissue injury and disintegration of the pacing wire. Guidelines to help recognize and prevent this complication are presented.

Superiority of the University of Wisconsin solution over simple crystalloid for extended heart preservation

To compare the effects of the University of Wisconsin solution with those of an extracellular crystalloid solution, Krebs-Ringer bicarbonate, as cardiac preservation media, we studied 35 adult dogs in an isolated heart preparation. Four groups of seven hearts were preserved in University of Wisconsin solution for 6 or 12 hours or in Krebs-Ringer bicarbonate solution for 6 or 12 hours. An additional group of seven hearts with no ischemia was used for a control group. In the four preservation groups, hearts were arrested by electrolyte solution (Normosol with potassium chloride, 20 mEq/L, added, 4 degrees C), flushed with 200 ml of the preservation solution, and then stored in the same solution at 1 degree to 2 degrees C. The hearts were mounted on an isolated heart preparation equipped with a computer-controlled servo-pump system that used a mock arterial system to modulate the aortic input impedance presented to the left ventricle. Left ventricular pressure-volume loops were measured on-line for 2 hours of reperfusion with autologous warm oxygenated blood. Elastance was derived from the end-systolic pressure-volume relationship, and diastolic compliance was derived from the end-diastolic pressure-volume relationship. The total left ventricular performance was assessed by the preload recruitable stroke work area, the slope, and its x-intercept, all of which derived from the stroke work (pressure-volume area)-end-diastolic volume relationship. Extended global ischemia had more deleterious effects on the end-diastolic than the end-systolic pressure-volume relationship. In confirmation with other studies, elastance did not accurately reflect the level of ventricular contractile dysfunction because of the significant amount of diastolic dysfunction. The preservation of myocardial systolic and diastolic functions, as demonstrated by the preload recruitable stroke work area and diastolic compliance, was better in the University of Wisconsin solution groups than in the Krebs-Ringer bicarbonate solution groups after 6 and 12 hours of preservation. In addition, 6 hours of preservation with University of Wisconsin solution maintained normal systolic and diastolic functions as compared with those of the control group. Preservation with University of Wisconsin solution prevented any myocardial edema formation; by contrast, this was significantly increased after 12 hours in Krebs-Ringer bicarbonate solution. Groups preserved with University of Wisconsin solution had less reperfusion injury as evidenced by the release of coronary sinus creatine kinase during reperfusion; they also had improved oxygen use during reperfusion.(ABSTRACT TRUNCATED AT 400 WORDS)

A newly designed flow-regulating device in shunt therapy of hydrocephalus.

A new flow-regulating device for shunting in hydrocephalus was designed to limit an excess flow by a pressure-head between the cerebral ventricle and the abdominal cavity in a standing position. The device has two diaphragm valves connected to each other by two lines: one is a shunting line to transfer cerebrospinal fluid and the other is a control line filled with a control fluid with a higher density than cerebrospinal fluid. The performance of the model device with a natural rubber sheet diaphragm was tested in a mock system, using glycerol for the control fluid and water for the transfer fluid in vitro. Results show that device decreases the excess flow when the pressure-head between two valves exceeds 35 cm.

Ureteral stents — materials and new forms

Three different types of polymers are currently used for self-retained ureteral stents: thermoplastic materials such as polyurethanes, and thermoset elastomers such as silicone and hydrogels. Polyurethane stents are easy to form and have high drainage capacity, whereas silicone shows the best biocompatibility but a lower drainage efficacy than the former. A mock urinary system consisting of a “collecting system” and a 9-F tube was used to evaluate the flow characteristics of various double-pigtail stents in cases of urinary obstruction. For simulation of an unobstructed urinary system a human urogenital system was used. “Inner flow” polyurethane stents showed the best drainage as compared with “inner flow” silicone and “outer flow” ESWL stents in an obstructed ureter, whereas ESWL stents maintained the best flow in an unobstructed ureter or in respect to conventional stents with obstructed sideports.

Bacterial/Fungal Growth in a Combined Parenteral Nutrition Solution

Appropriately mixed, compatible solutions of glucose, amino acids and lipid have recently become available for clinical use. While a single hyperalimentation solution has several advantages over the conventional two-bottle technique, its effect on infusion-related septicemia is unknown. An in vitro, mock infusion system identical to that used in our new-born intensive care unit was set up to assess the relative growth rates of three microorganisms in several parenteral nutrition mixtures. Growth of Staphylococcus epidermidis, Escherichia coli and Candida albicans was measured in seven different alimentation solutions, including two combined solutions. Generally, microbial growth was the same or decreased in combined solutions as compared to fat alone although considerably greater than that observed in nonlipid containing solutions. In addition, the ability of these organisms to pass in-line terminal filters of pore size 0.22 and 1.2 microns was assessed.

Effects of atherosclerotic lesions on ultrasonic beam and CW Doppler signals.

Doppler ultrasound has been shown to be a useful clinical tool in the noninvasive detection of vascular stenoses, occlusion, or plaques. The presence of atherosclerotic lesions in blood vessel walls can distort the Doppler ultrasonic signal, producing inverted or biphasic tracings. This observation, in fact, has been used frequently as a criterion for the diagnosis of plaque formation and calcification of these lesions. To define the source of this distortion, we have examined the effects of various types of atherosclerotic lesions on the transmitted ultrasonic beam generated by a continuous wave (CW) Doppler ultrasonic probe. These results are compared to the Doppler flow tracings obtained by a CW ultrasonic Doppler flowmeter from blood vessels with overlaying lesions in a mock flow system. It is concluded that severe distortion in the form of beam refraction or diffraction can be produced by calcified lesions, resulting in biphasic or inverted flow signals.

Pseudomonas maltophilia Bacteremia in Children Undergoing Open Heart Surgery

Pseudomonas maltophilia was isolated from intraoperative blood cultures in eight of 13 children undergoing open heart surgery during a five-week period. Antibiograms were identical and included resistance to prophylactic antibiotics. The source of the outbreak was traced to contamination of both the calibration device used on the pressure monitoring system and the sensor surface of transducers used in this system. In a mock system, calibration with a contaminated device resulted in recovery of the organism from transducer dome fluid. Dye studies confirmed the integrity of the transducer membrane but demonstrated reflux of dome fluid into the monitoring line fluid. A case-control study revealed no patient- or surgery-related factors predisposing to P maltophilia bacteremia and no excess of morbidity or mortality in patients as a result of bacteremia. The outbreak was confined to patients undergoing open heart surgery and was terminated abruptly by sterilization of transducers and revision of the calibration device.