We have studied the oxidation of SiGe alloys of different compositions (between 25 and 75 at.% Ge). All of the oxidations were performed at 900 °C in wet atmosphere on 7500-Å-thick films grown by molecular beam epitaxy. Below 50 at.% Ge, the oxidation remains similar to what has been described previously, i.e., initially, the rate is enhanced by the presence of Ge, the oxide formed is pure SiO2 and a Ge pileup forms at the SiO2/Si-Ge interface. In these relatively thick films, we propose that after extended oxidation, the decrease of Si concentration at the interface slows down oxidation rates enough so that eventually, the oxide thickness for the SiGe alloys ends up smaller than that of pure Si. For alloys containing above 50 at.% Ge, a markedly different behavior is found: A two-layer oxide is formed, consisting of a mixed (Si,Ge) O2 layer near the surface, and a pure SiO2 layer underneath. The rates of oxidation in this case are even faster, since both Ge and Si are being oxidized. The general behavior is explained in terms of the balance of Si and Ge diffusion fluxes, to and from the interface, needed to sustain oxidation.
Patients recovering from acute surgical stress often excrete increased 17-OH corticosteroids with no change in 17-ketosteroids. The explanation for these findings is unclear. In order to investigate possible divergence between cortisol and adrenal androgen metabolism in acute stress, repeated morning cortisol and dehydroepiandrosterone (DHA) measurements were made in patients undergoing ACTH stimulation 48 to 96 hours preoperatively, followed by determinations before and during major surgery, also performed in the morning. Cortisol and DHA are largely metabolized by the liver, so liver blood flow under a constant general anesthetic regimen known not to affect cortisol metabolism was monitored by pre- and intraoperative indocyanine green dye clearance. Results indicated no difference between the cortisol and DHA stimulation resulting from two hours of ACTH stimulation or major surgery, and a small (14.4%) decline in hepatic blood flow during general anesthesia. However, while DHA concentrations remained constant immediately preceding surgery, cortisol concentrations increased by 61% (P less than 0.05). Previous studies have also demonstrated increased concentrations of cortisol before surgical procedures, presumably due to psychological stress. However, this is the first demonstration of a dissociation between concentrations of cortisol and an adrenal androgen due to psychological stress.
A 15-year experience with 92 subcutaneous arterial bypass grafts for lower extremity revascularization has been reviewed. Fifty-nine AF and 33 FF bypass operations were performed on 89 patients whose average age was 66 years. The overall five-year survival was 33% compared to an expected survival of 80%. 88% of the AF, and 76% of the FF operations were performed for limb salvage, bypass of an aortic aneurysm, or replacement of an infected aortic graft. The remainder were performed for intermittent claudication on patients who were too ill to withstand an intra-abdominal operation. 75% of the patients with AF grafts and 64% of those with FF grafts experienced complete relief of lower extremity ischemia, including all of the patients with claudication. Graft patency was analyzed by the life table method. In the FF series, 74% of the grafts remained patent for one year; 73% for two years; 66% for three years; and 53% for four years. A 50% incidence of thrombosis occurred at the end of two years in the AF group. The patency rate of the AF grafts was also studied with regard to the type of graft material employed: a 50% incidence of thrombosis was reached at 36 months with knitted Dacron; at 18 months with weave-knit Dacron; and at 9 months with woven Dacron. THESE DATA INDICATE THAT: (1) contrary to our previous report, weave-grafts provide adequate blood flow to the lower extremities but do not remain patent as long as more conventional types of reconstruction; (2) subcutaneous grafts should be performed only when an intra-abdominal procedure is contraindicated or life expectancy is limited.
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