Dacron graft as replacement to dissected aorta: A three-dimensional fluid-structure-interaction analysis.

Aortic dissection (AD) is a serious medical condition characterized by a tear in the intima, the inner layer of the aortic walls. In such occurrence, blood is being diverted to the media (middle) layer and may result in patient death if not quickly attended. In the case where the diseased portion of the aorta needs to be replaced, one common surgical technique is to use a graft made of Dacron, a synthetic fabric. We investigate the response of a composite human aortic segment-Dacron graft structure subjected to blood flow using the three-dimensional fluid-structure-interaction (FSI) capability in Abaqus. We obtain stress and strain profiles in each of the three layers of the aortic walls as well as in the Dacron graft. Results are compared when elastic and hyperelastic models are used and when isotropy vs. anisotropy is assumed. The more complex case (hyperelastic-anisotropy) is represented by the Holzapfel-Gasser-Ogden (HGO) model which also accounts for the orientation of the fibers present in the tissues. The fluid flow is taken as Newtonian, incompressible, pulsatile and turbulent. The simulation show that for all the cases, the von Mises stress distribution at aorta-Dacron interface is well below the ultimate strength of the aorta. No significant change in radial displacement at the interface of the two materials due to blood flow is observed. Computation cost is also addressed and results show that the hyperelastic-anisotropic model takes about three times longer to run than the elastic isotropic case. Trade-off between accuracy and computational cost has to be weighted.

Liver transplantation for hepatitis B in the United States.

AIM: To evaluate the impact of hepatitis B virus (HBV) on US health care system, we reviewed the Organ Procurement and Transplantation (OPTN, formerly UNOS) HBV database. METHOD: We reviewed records of liver transplantations (LTx) performed in the United States listed for the diagnoses of HBV between 1993 and mid-October 2004. Both acute as well as chronic cases were included. Coinfection with hepatitis C virus was excluded from study. The specific states selected for review were chosen from those areas that are receiving large numbers of new immigrants from high HBV endemic areas (ie, Texas, Pennsylvania, California, New York, and Florida). One-, 3-, and 5-year patient survival rates for both cadaveric and living related donors were analyzed. Survival rates were obtained from OPTN database as Kaplan-Meyer survival test. RESULTS: Between 1993 and mid-October 2004, 53,312 LTx had been performed nationwide. Of these, 2314 (4.34%) were performed for the diagnosis of HBV; 1816 cases (78%) were due to chronic HBV infection (45 of them were living donor LTx) and 498 cases (22%) were due to HBV-induced acute liver failure (seven of them were living donor LTx). Three- and 5-year survival rates of chronic HBV-related LTx patients were better than acute HBV-related and overall LTx patients. CONCLUSION: HBV is generally considered to have a minor health significance by many community gastroenterologists. With growing immigration from overseas, it may eventually have a higher impact on LTx. Therefore, it is crucial to further educate gastroenterologists and primary care physicians caring for this specific group of patients.

An experimental model to study thermogenesis in the neonatal surgical patient.

The nonshivering thermogenic response of the newborn to cold exposure and the potentially detrimental effects of the resultant hypermetabolic state have been documented by numerous animal and clinical studies. It is standard practice to protect the newborn infant from cold stress by controlling environmental temperature and preventing heat loss. Although these measures are well accepted, little attention has been paid to the effect of surgical factors, such as operative trauma, anesthetic agents, and muscle relaxants on nonshivering thermogenesis. The purpose of this investigation is to develop an experimental model to study the effect of surgical factors on the neonatal thermogenic response. Newborn rabbits were chosen as the experimental subject because these animals rely solely on nonshivering thermogenesis. One-hundred-eleven animals were studied. In the newborn rabbit, there was an inverse relationship between age and the percentage of brown fat. Body weight doubled in the first week of life, while the quantity of brown fat fell from a maximum of 2.9% of body weight in the first day of life to 1.56% at 7 days of age. It was possible to maintain the newborn rabbit in a warm environment for periods of up to 2 hours without changes in blood flow to the brown fat or oxygen consumption. The thermogenic response to cold did not appear to be altered in terms of oxygen consumption when the animals were lightly anesthetized with pentobarbital or paralyzed with metocurine and locally anesthetized. The rise in oxygen consumption was similar in awake, intact animals and those that received pentobarbital or metocurine and xylocaine.(ABSTRACT TRUNCATED AT 250 WORDS)

Anesthesia blocks nonshivering thermogenesis in the neonatal rabbit.

Nonshivering thermogenesis (NST) is a normal physiological response of the neonate to cold exposure, characterized by increased blood flow to metabolically active brown fat stores. It is standard practice during neonatal surgery to warm the ambient environment in order to avoid consuming vital energy stores. While NST has been well-studied in the neonate, the response during anesthesia and paralysis has not been fully characterized. Rabbit pups (aged 1 to 7 days) were randomized into several groups. The experimental groups consisted of animals mechanically ventilated and administered either metocurine, pancuronium, curare, fentanyl, nitrous oxide (N2O), or halothane. The controls were spontaneously breathing animals. Oxygen consumption (VO2), an index of metabolic activity, was measured at thermoneutrality (39 degrees C) and after cold exposure (25 degrees C). Control and metocurine animals had a significant increase in VO2 in response to cold exposure. The increase in VO2 was not noted in animals that received curare, pancuronium, fentanyl, N2O, or halothane. To test the effect of anesthetic withdrawal during cold exposure on VO2, additional series of animals were studied. One group received continuous halothane throughout the period of cold exposure; the other had cessation of the halothane during cold exposure. Both groups were rewarmed subsequently. The animals that had withdrawal of halothane during cold exposure had a marked and significant increase in VO2 compared with the control group (continuous halothane). VO2 returned to near-baseline levels upon rewarming. The authors conclude that many commonly used anesthetic and paralyzing agents inhibit the thermogenic response to cold exposure. However, cessation of anesthesia (halothane) in a cold environment results in a marked increase in metabolic activity.

Uncommon complications of human fascioliasis in Alexandria.

Fascioliasis is becoming more frequently discovered among citizens of Alexandria. The main presenting clinical manifestations including hepatic pain, colics, fever, anorexia, discomfort with meals and hepatomegaly. We select here 4 cases with unusual presentation from patient's record of the last year: one case developed liver abscess due to F. hepatica infection. Two cases were encountered during cholecystectomy in patients suffering from cholecystitis with cholelithiasis; in one of them the gall bladder had ruptured and the patient developed an abscess in the liver related to the gall bladder bed. In both cases F. hepatica worms were found in the bile duct. The fourth case presented with acalcular cholecystitis with empyema of the gall bladder.

Intestinal transplantation in composite visceral grafts or alone.

Under FK 506-based immunosuppression, the entire cadaver small bowel except for a few proximal and distal centimeters was translated to 17 randomly matched patients, of whom two had antigraft cytotoxic antibodies (positive cross-match). Eight patients received the intestine only, eight had intestine in continuity with the liver, and one received a full multivisceral graft that included the liver, stomach, and pancreas. One liver-intestine recipient died after an intestinal anastomotic leak, sepsis, and graft-versus-host disease. The other 16 patients are alive after 1 to 23 months, in one case after chronic rejection, graft removal, and retransplantation. Twelve of the patients have been liberated from total parenteral nutrition, including all whose transplantation was 2 months or longer ago. The grafts have supported good nutrition, and in children, have allowed growth and weight gain. Management of these patients has been difficult and often complicated, but the end result has been satisfactory in most cases, justifying further clinical trials. The convalescence of the eight patients receiving intestine only has been faster and more trouble free than after liver-intestine or multivisceral transplantation, with no greater difficulty in the control of rejection.