General equilibrium of an ecosystem.

Ecosystems and economies are inextricably linked: ecosystem models and economic models are not linked. Consequently, using either type of model to design policies for preserving ecosystems or improving economic performance omits important information. Improved policies would follow from a model that links the systems and accounts for the mutual feedbacks by recognizing how key ecosystem variables influence key economic variables, and vice versa. Because general equilibrium economic models already are widely used for policy making, the approach used here is to develop a general equilibrium ecosystem model which captures salient biological functions and which can be integrated with extant economic models. In the ecosystem model, each organism is assumed to be a net energy maximizer that must exert energy to capture biomass from other organisms. The exerted energies are the "prices" that are paid to biomass, and each organism takes the prices as signals over which it has no control. The maximization problem yields the organism's demand for and supply of biomass to other organisms as functions of the prices. The demands and supplies for each biomass are aggregated over all organisms in each species which establishes biomass markets wherein biomass prices are determined. A short-run equilibrium is established when all organisms are maximizing and demand equals supply in every biomass market. If a species exhibits positive (negative) net energy in equilibrium, its population increases (decreases) and a new equilibrium follows. The demand and supply forces in the biomass markets drive each species toward zero stored energy and a long-run equilibrium. Population adjustments are not based on typical Lotka-Volterra differential equations in which one entire population adjusts to another entire population thereby masking organism behavior; instead, individual organism behavior is central to population adjustments. Numerical simulations use a marine food web in Alaska to illustrate the model and to show several simultaneous predator/prey relationships, prey switching by the top predator, and energy flows through the web.

Comparison of laparoscopic cholecystectomy versus elective open cholecystectomy.

Laparoscopic cholecystectomy has essentially replaced open cholecystectomy as the procedure of choice for gallbladder disease. This rapid shift to laparoscopic cholecystectomy, however, has resulted more from marketing forces than from prospective clinical trials. To evaluate the safety and efficacy of laparoscopic cholecystectomy, the first 486 laparoscopic cholecystectomies at two institutions were studied. These results were then compared to the results of the last 6 months of elective open cholecystectomy cases prior to the introduction of laparoscopic surgery. The age, sex, height, and weight were similar in both groups. The mean operative time was 78.8 +/- 1.8 min for laparoscopic cholecystectomy and 62.7 +/- 2.6 min for open cholecystectomy (p < 0.01). The mean time for tolerating a regular diet was 1.23 +/- 0.04 days in the laparoscopic group versus 2.44 +/- 0.07 days in the open group (p < 0.01). Laparoscopic cholecystectomy patients required only oral pain medications by a mean of 1.22 +/- 0.03 days postoperatively compared to 2.55 +/- 0.07 days postoperatively for those undergoing open cholecystectomy (p < 0.01). The mean length of hospitalization was 1.58 +/- 0.07 days for laparoscopic patients and 3.55 +/- 0.11 days for open patients (p < 0.01). Thirty-one patients undergoing laparoscopic cholecystectomy were converted to open cholecystectomy (6.4%). The most common reasons for conversion to open cholecystectomy were acute inflammation, adhesions, and bleeding. For the laparoscopic patients, the morbidity rate was 8.4% and the mortality rate 0.2% (1 death). In the open cholecystectomy group the morbidity rate was 8.0% and there were no deaths. The most troublesome complication in laparoscopic cholecystectomies continues to be bile leaks and bile duct injuries.(ABSTRACT TRUNCATED AT 250 WORDS)

Risks and benefits of laparoscopic cholecystectomy in the community hospital setting.

Laparoscopic cholecystectomy has rapidly become more popular than open cholecystectomy. To further evaluate the safety and efficacy of laparoscopic cholecystectomy in the community setting, the first 190 patients to undergo the procedure in Saginaw, Michigan were studied. There were 159 females and 31 males. The mean age was 47.8 years. All patients had symptoms consistent with biliary tract disease and most had gallstones proven by preoperative ultrasound. The mean operative time was 84 minutes but decreased from 161 minutes the first month of the study to 74 minutes by the seventh month. Eighty-seven percent of patients were tolerating a regular diet by postoperative day 1. Ninety-six percent of patients were requiring only oral pain medications by postoperative day 1. Seventy percent of patients were discharged by postoperative day one while 91% were discharged by postoperative day 2. Six patients were converted to open cholecystectomy due to acute inflammation, significant bleeding or extensive adhesions. There were no deaths and the morbidity rate was 9.5%. The most significant complication was postoperative bile leak which occurred in two patients. Patients returned to work a mean of 16.1 days following surgery and to their normal daily activities at home a mean of 12.9 days postoperatively. This study of the first 190 laparoscopic cholecystectomies in Saginaw affirms the safety and feasibility of this procedure in the community setting in selected patients.

Mechanism and management of persistent withdrawal occlusion.

An unresolved complication of the use of totally implantable central venous access ports (e.g., Mediport, Infusa-port) is persistent withdrawal occlusion (PWO), i.e. the unimpeded capacity for infusion of fluids accompanied by the inability to withdraw blood. This study demonstrates the mechanism of persistent withdrawal occlusion and describes a method for resolving this complication. Of 42 cancer patients with totally implantable central venous access ports, 8 (19%) patients developed 11 episodes of PWO. Venograms demonstrated a sheath around the catheter beginning at the catheter entrance to the central vein and extending 1-5 cm beyond the catheter tip. Each episode of PWO was treated with 250,000 units of urokinase dissolved in 150cc D5/W infused through the port over 90 minutes. Venograms were obtained immediately after each urokinase infusion. Follow-up ranged from 13-130 days. After urokinase infusion the venogram showed no change in the sheath in 1 episode of PWO and complete dissolution of the sheath in 10 episodes of PWO. PWO recurred once in one patient and twice in another patient. PWO resolved only in the 10 episodes in which sheath dissolution was demonstrated. Urokinase infusion, as described, is effective in resolving persistent withdrawal occlusion. The method is repeatable and safe. That resolution of PWO by urokinase infusion was accompanied by dissolution of the sheath suggests that the sheath is composed primarily of fibrin and that flap action of the sheath is the mechanism causing PWO.