Comparing digraph and Petri net approaches to deadlock avoidance in FMS.

Flexible manufacturing systems (FMSs) are modern production facilities with easy adaptability to variable production plans and goals. These systems may exhibit deadlock situations occurring when a circular wait arises because each piece in a set requires a resource currently held by another job in the same set. Several authors have proposed different policies to control resource allocation in order to avoid deadlock problems. These approaches are mainly based on some formal models of manufacturing systems, such as Petri nets (PNs), directed graphs, etc. Since they describe various peculiarities of the FMS operation in a modular and systematic way, PNs are the most extensively used tool to model such systems. On the other hand, digraphs are more synthetic than PNs because their vertices are just the system resources. So, digraphs describe the interactions between jobs and resources only, while neglecting other details on the system operation. The aim of this paper is to show the tight connections between the two approaches to the deadlock problem, by proposing a unitary framework that links graph-theoretic and PN models and results. In this context, we establish a direct correspondence between the structural elements of the PN (empty siphons) and those of the digraphs (maximal-weight zero-outdegree strong components) characterizing a deadlock occurrence. The paper also shows that the avoidance policies derived from digraphs can be implemented by controlled PNs.

Vasoactive intestinal polypeptide (VIP) secretion and refractory diarrhea in patients with AIDS or AIDS-related complex (ARC).

Elevated plasma levels of vasoactive intestinal polypeptide (VIP) (as assessed by a radio-immunoassay), were found in 7/11 patients with AIDS or AIDS-related Complex (ARC), evaluated because of prolonged intractable diarrhea with either an infectious (6 cases) or a non-infectious (5 cases) etiology. Six subjects have been treated with the somatostatin analogue octreotide, which gave both a favourable clinical response and a significant reduction in plasma VIP concentrations. Evaluation of plasma VIP levels may provide a pathophysiological basis for explaining the efficacy of octreotide therapy in HIV-infected patients suffering from both infectious and non-infectious refractory diarrhea.

Elevated plasma levels of vasoactive intestinal peptide in AIDS patients with refractory idiopathic diarrhoea. Effects of treatment with octreotide.

OBJECTIVE: To evaluate plasma levels of vasoactive intestinal peptide (VIP) in AIDS patients with refractory idiopathic diarrhoea, and to assess the role of treatment with octreotide. PATIENTS: Three AIDS patients were evaluated for severe watery diarrhoea of 2-6 months' duration, which was complicated by weight loss, weakness, and fluid and electrolyte abnormalities. They had not shown a significant response to several regimens of empirical antimicrobial chemotherapy, or symptomatic antidiarrhoeal treatment. METHODS: A complete diagnostic examination, including repeated microbiological evaluation and radiological, ultrasonographic, endoscopic and histological examination, was performed. Plasma levels of VIP were determined by radioimmunoassay and compared with concentrations in a group of healthy subjects. RESULTS: Since no clinically significant results were obtained from standard diagnostic evaluation and empirical therapeutical attempts, idiopathic refractory diarrhoea was diagnosed. Plasma concentrations of VIP were moderately elevated in all three subjects examined, with levels of 11.5, 17.5 and 9.5 pmol/l (values < 8.8 pmol/l in the control group). One patient received 50-100 micrograms octreotide three times daily subcutaneously for 6 months, resulting in complete resolution of diarrhoea and significant improvement in body weight and quality of life, together with a reduction in VIP concentration to within normal values. CONCLUSIONS: Although the somatostatin analogue octreotide has been used successfully in the management of both infectious and non-infectious AIDS-related diarrhoea, gastrointestinal neuroendocrine function and circulating humoral mediators of diarrhoea have not hitherto been investigated extensively in HIV-infected subjects. Our data on the association of idiopathic secretory diarrhoea and elevated plasma VIP levels provide a possible pathophysiological rationale for identifying AIDS patients whose refractory diarrhoea may be more responsive to octreotide treatment.