Inverse relationship between heat stable enterotoxin-b induced fluid accumulation and adherence of F4ac-positive enterotoxigenic Escherichia coli in ligated jejunal loops of F4ab/ac fimbria receptor-positive swine.

Heat-labile enterotoxin (LT) produced by enterotoxigenic Escherichia coli (ETEC) increases bacterial adherence to porcine enterocytes in vitro and enhances small intestinal colonization in swine. Heat-stable enterotoxin-b (STb) is not known to affect colonization; however, through an induction of net fluid accumulation it might reduce bacterial adherence. The relationship between fluid accumulation and bacterial adherence in jejunal loops inoculated with ETEC strains that produce LT, STb, both, or neither toxin was studied. Ligated jejunal loops were constructed in weaned Yorkshire pigs in two independent experiments (Exp. 1, n=5, 8-week-old; Exp. 2, n=6, 6-8-week-old). Each pig was inoculated with six F4ac(+)E. coli strains: (1) LT(+), STb(+) parent (WAM2317); (2) STb(-) (ΔestB) mutant (MUN297); (3) MUN297 complemented with STb (MUN298); (4) LT(-) STb(-) (ΔeltAB ΔestB) mutant (MUN300); (5) MUN300 complemented with LT (MUN301); and (6) 1836-2 (non-enterotoxigenic, wild-type). Pigs were confirmed to be K88 (F4)ab/ac receptor-positive in Exp. 2 by testing for intestinal mucin-type glycoproteins and inferred to be receptor-positive in both Exp. 1 and 2 based on histopathologic evidence of bacterial adherence. Strains that produced STb induced marked fluid accumulation with the response (ml/cm) to WAM2317 and MUN298 significantly greater than that to the other strains (P<0.0001). Conversely, bacterial adherence scores based on immunohistochemistry and CFU/g of washed mucosa were both lowest in the strains that expressed STb and highest in those that did not. For the two experiments combined, the Pearson correlation coefficient (R) between fluid volume (ml/cm) and log CFU per gram was -0.57021 (P<0.0001); R(2)=0.3521 (n=197). These results support the hypothesis that enterotoxin-induced fluid accumulation flushes progeny organisms into the lumen of the bowel, thereby increasing the likelihood of fecal shedding and transmission of the pathogen to new hosts.

Development of a flexible inflow cannula with titanium inflow tip for the NEDO biventricular assist device.

A newly designed flexible inflow cannula has been developed for a biventricular assist device (BVAD). The inflow tip was designed for long-term ventricle drainage. Considering the anatomic differences between the right and left ventricles, a flexible inflow cannula, as a mass production model with a titanium tip, was developed and investigated in chronic BVAD animal experiments using the NEDO permanently implantable centrifugal blood pump. These flexible inflow cannulae consist of flexible plastic tubing with wire supported, titanium left and right tips. The right titanium inflow tip was designed with inner and outer stoppers to be inserted into the right ventricle wall and a beak shape to avoid sucking. The left tip was designed to be inserted into the left ventricle cavity. Five chronic BVAD bovine studies were performed to investigate the new beak shaped titanium tip. In these studies, the new beak shaped titanium tip and cannula prevented stenosis by a proliferation of pseudoneointimal, and supplied adequate blood flow to the pump without sucking. This newly designed inflow cannula with the beak shaped titanium tip was successfully implanted for 90 days.

Centrifugal blood pump with a hydraulically-levitated impeller for a permanently implantable biventricular assist device.

A permanently implantable biventricular assist device (BVAD) system has been developed with a centrifugal pump which is activated by a hydraulically-levitated impeller. The pump impeller floats hydraulically into the top contact position; this position prevents thrombus formation by creating a washout effect at the bottom bearing area, a common stagnant region. The pump was subjected to in vitro studies using a pulsatile mock circulation loop to confirm the impeller's top contact position and the swinging motion produced by the pulsation. Eleven in vivo BVAD studies confirmed that this swinging motion eliminated blood clot formation. Twenty-one pumps im-planted for up to three months did not reveal any thrombosis in the pumps or downstream organs. One exception was a right pump which was exposed to severe low flow due to the kinking of the outflow graft by the accidental pulling of the flow meter cable. Three ninety-day BVAD studies were achieved without thrombus formation.

Development of silicone rubber hollow fiber membrane oxygenator for ECMO.

Silicone rubber hollow fiber membrane produces an ideal gas exchange for long-term ECMO due to nonporous characteristics. The extracapillary type silicone rubber ECMO oxygenator having an ultrathin hollow fiber membrane was developed for pediatric application. The test modules were compared to conventional silicone coil-type ECMO modules. In vitro experiments demonstrated a higher O2 and CO2 transfer rate, lower blood flow resistance, and less hemolysis than the conventional silicone coil-type modules. This oxygenator was combined with the Gyro C1E3 centrifugal pump, and three ex vivo experiments were conducted to simulate pediatric V-A ECMO condition. Four day and 6 day experiments were conducted in cases 1 and 2, respectively. Case 3 was a long-term experiment up to 2 weeks. No plasma leakage and stable gas performances were achieved. The plasma free hemoglobin was maintained within a normal range. This compact pump-oxygenator system in conjunction with the Gyro C1E3 centrifugal pump has potential for a hybrid total ECMO system.

Antithrombogenicity of the Gyro permanently implantable pump with the RPM dynamic suspension system for the impeller.

In 1995, a group at Baylor College of Medicine started to develop the NEDO biventricular assist device (BVAD) using two Gyro permanently implantable (PI) centrifugal pumps. This pump consists of a sealless pump housing and an impeller supported with a double pivot bearing. In May 2001, an RPM dynamic suspension system (RPM-DS) for the impeller was developed to improve durability and antithrombogenicity without a complex magnetic suspension system. From March 2000 to March 2002, eight BVAD bovine experimental studies were performed for more than 1 month. Two pumps were implanted in two cases without the RPM-DS (group A) and in six cases with the RPM-DS (group B). In group A, the survival period was 45 and 50 days. The primary reason for termination was an increase in the requiring power, which was related to deposition of white thrombus on the bottom bearing. In group B, the survival period was 37, 48, 51, 60, 80, and 90 days. The reasons for termination were not related to thrombus formation. No thrombus was observed in the pumps except for one right pump. In that experiment, the thrombus formation may have occurred when that pump had a low flow rate at a level of 1 L/min for 6 hr. These studies demonstrate the apparent antithrombogenic effect of RPM-DS. The NEDO BVAD is ready to move into a 3-month preclinical system evaluation.

In vivo evaluation of the NEDO biventricular assist device with an RPM dynamic impeller suspension system.

Since 1995, the Baylor College of Medicine group has been developing the NEDO Gyro permanent implantable (PI) pump. The Gyro PI pump has achieved outstanding results up to 284 days with no thrombus formation during the left ventricular assist device (LVAD) animal experiments. However, in biventricular assist device (BVAD) animal experiments, thrombus formation did occur. An in vitro experiment showed the reason for thrombus formation was caused by the missed magnetic balance between the impeller and the actuator. On the basis of this result, the revolutions per minute (RPM) impeller suspension system was developed. Six long-term animal studies were performed in bovine models. Survival periods were 90, 80, 60, 51, 48, and 37 days, respectively. No thrombus was observed in the pumps with the exception of one right pump. In that experiment, the thrombus formation may have occurred when the pump had a low flow because of outflow kinking. In this article, the antithrombogenic effect of this RPM impeller suspension system will be discussed.

Assessing the calf pulmonary function during a long-term biventricular assist device study with a centrifugal blood pump.

Pulmonary congestion due to inappropriate pump flow management is one major problem necessary to avoid during long-term biventricular assist device (BVAD) implantation. Our objective was to assess the effects of pulmonary arterial flow rate and flow rates of both (right and left) bypass pumps. Six healthy calves, which had been implanted with a BVAD system, were selected for this retrospective study. Pulmonary artery flows, both pump flow rates, oxygen saturation of the arterial blood, and pulmonary arterial pressures were assessed as parameters of pulmonary function as was routine clinical evaluation of respiratory rate and character and chest auscultation. The average pulmonary artery flow rate (PAF), systolic pressure of pulmonary artery (sPAP), and oxygen saturation were 148.8 ml/kg per min, 35.1 mm Hg, and 95.3%, respectively. Pulmonary dysfunction occurred in one case, in which the mean PAF, sPAP, and oxygen saturation were 169 ml/kg per min, 66.1 mm Hg, and 90.9%, respectively. The ratio for the right/left pump flow rate (R/L ratio) for the case having pulmonary dysfunction was 1.57 even though the ratio for the other cases was less than 1. Maintaining an R/L ratio less than 1 and/or PAF less than 160 ml/kg per min and PAP less than 50 mm Hg is recommended as the initial conditions to target to avoid pulmonary dysfunction during a BVAD implantation with a beating heart condition.

Extracorporeal membrane oxygenator compatible with centrifugal blood pumps.

Coil-type silicone membrane oxygenators can only be used with roller blood pumps due to the resistance from the high blood flow. Therefore, during extracorporeal membrane oxygenation (ECMO) treatment, the combination of a roller pump and an oxygenator with a high blood flow resistance will induce severe hemolysis, which is a serious problem. A silicone rubber, hollow fiber membrane oxygenator that has a low blood flow resistance was developed and evaluated with centrifugal pumps. During in vitro tests, sufficient gas transfer was demonstrated with a blood flow less than 3 L/min. Blood flow resistance was 18 mm Hg at 1 L/min blood flow. This oxygenator module was combined with the Gyro C1E3 (Kyocera, Japan), and veno-arterial ECMO was established on a Dexter strain calf. An ex vivo experiment was performed for 3 days with stable gas performance and low blood flow resistance. The combination of this oxygenator and centrifugal pump may be advantageous to enhance biocompatibility and have less blood trauma characteristics.