Failure risk management: adaptive performance control and mission abort decisions.

The failure behavior of safety-critical systems typically depends on the system performance level, which offers opportunities to control system failure risk through dynamic performance adjustment. Moreover, mission abort serves as an intuitive way to mitigate safety hazards during mission execution. Our study focuses on systems that execute successive missions with random durations. To balance mission completion probability and system failure risk, we examine two decision problems: when to abort missions and how to select the performance level prior to mission abort. Our objective is to maximize the expected revenue through dynamic performance control and mission abort (PCMA) decisions. We consider condition-based PCMA decisions and formulate the joint optimization problem into a Markov decision process. We establish the monotonicity and concavity of the value function. Based on this insight, we show that optimizing the mission abort policy requires a series of control limits. In addition, we provide conditions under which the performance control policies are monotone. For comparative purposes, we analytically evaluate the performances of some heuristic policies. Finally, we present a case study involving unmanned aerial vehicles executing power line inspections. The results indicate the superiority of our proposed risk control policies in enhancing operational performance for safety-critical systems. Dynamic performance adjustment and mission abort decisions provide opportunities to reduce the failure risk and increase operational rewards of safety-critical systems.

Mission risk control via joint optimization of sampling and abort decisions.

Information-driven mission abort is an effective way to control the failure risk of safety-critical systems during mission executions. We investigate the optimal sampling and mission abort decisions of partially observable safety-critical systems, where the underlying system health state can only be revealed by sampling. In contrast to previous studies, we employ partial health information to jointly determine: (a) whether to execute sampling, and (b) when to abort the mission in a dynamic manner, so as to minimize the expected total cost incurred by sampling, mission failure, and system malfunction. Dynamic sampling and mission abort policies are devised following the belief state, whose optimization model is cast into the framework of a partially observable Markov decision process. Some structural insights with regard to the value function, control limit selection, and optimality existence are presented. The performance of the proposed sampling and abort policy is tested by numerical experiments, which are proved to outperform other heuristic abort policies in mission loss control.

Joint optimization of mission abort and protective device selection policies for multistate systems.

For safety-critical systems such as aircrafts and submarines, mission abort is commonly deployed to enhance system survivability at the cost of reducing mission success probability. In addition to mission abort, protective device can also mitigate the failure risk of safety-critical systems by reducing the magnitude of external shocks. Considering the effect of protective device on system failure behavior, this article proposes a condition-based mission abort policy where a mission is terminated and rescue procedure starts immediately if the state of system is worse than a control limit. Based on the developed mission abort policy, mission reliability and system survivability are evaluated to analyze the risk of mission failure and system failure. The optimal mission abort threshold balancing the tradeoff between mission reliability and system survivability is investigated. Furthermore, the joint optimization of mission abort and protective device selection policies is explored by simultaneously optimizing the defensive factor and abort threshold. A numerical example on a hydraulic system is presented to illustrate the applicability of the proposed policies.

Distribution characteristics of cells in splenomegaly due to hepatitis B-related cirrhotic portal hypertension and their clinical importance.

Objectives To investigate peripheral cytopenia in patients with splenomegaly due to hepatitis B-related cirrhotic portal hypertension (HBRCPH) by comparing blood cell counts from enlarged spleens with peripheral blood. Methods This prospective study involved patients undergoing splenectomy at the Nangfang Hospital from June 2013 to December 2015. Blood cell counts from peripheral blood were compared with those from splenic blood taken during splenectomies. Results Clinical data were available from 30 patients. White blood cell (WBC), red blood cell (RBC) and platelet counts were statistically significantly lower in peripheral blood compared with splenic blood. After splenectomy, peripheral blood cell counts increased significantly compared with pre-operative levels. Platelet and WBC counts in the lower spleen were significantly higher than those in the porta lienis (middle segment) and upper spleen. Conclusions In patients with splenomegaly due to HBRCPH, the counts of three blood cell lineages were significantly higher in the spleen than in peripheral blood. Splenectomy can aid the return of peripheral blood cell counts to normal levels. The most significant retention of platelets and WBCs occurred in the lower spleen which may be useful information for surgeons performing partial splenectomies.

Clinical analysis of pericardial devascularization by preserving vagus trunks in 42 patients with portal hypertension.

Pericardial devascularization (PD) is less of a burden and provides better bleeding control to patients compared to shunt procedures, and so has been widely used in portal hypertension in China. However, because the vagus trunks are interrupted during surgery, patients easily suffer from postoperative stomach adynamia. Based on our understanding from autopsy of the path of vagus trunks along the distal oesophagus, we designed the operative procedure of PD by preserving vagus trunks (PDPVT) to treat portal hypertension. Between May 1991 and January 2003, patients with portal hypertension were treated surgically using PDPVT (n = 42), single PD (n = 32), or PD with pyloroplasty (PD+PP; n = 16). Operative mortality was 2.4% in the PDPVT group and 6.3% in both the PD and PD+PP groups (p > 0.05). The postoperative rebleeding rate was 9.5% in the PDPVT group and 12.5% in both the PD and PD+PP groups (p > 0.05). There were no differences in operative time and estimated blood loss between the three groups (p > 0.05). The recovery time for gastroenteric function was shorter with PDPVT (mean, 3.5 days) than with PD (mean, 5.7 days) and PD+PP (mean, 4.2 days; p < 0.02). Incidences of early satiety and enterogastric reflux were significantly lower in the PDPVT group (both 4.8%) than in the PD group (46.9% and 18.8%) and PD+PP group (12.5% and 100%; p < 0.005). Incidences of retention of gastric juice, diarrhoea and late gallstones were 12.5%, 15.6% and 7.1%, respectively, in the PD group, and 12.5%, 18.8% and 6.3%, respectively, in the PD+PP group, but none of these were observed in the PDPVT group. Since it preserves vagus trunks, PDPVT can maintain normal stomach dynamics and physiological function of hepatobiliary and gut systems better than PD and PD+PP, thus reducing incidences of postoperative complications. Thus, PDPVT is superior to PD and PD+PP in the treatment of portal hypertension.