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Biomedical subjects

Stephen W Davies

Publications and source records attributed to Stephen W Davies.

3 recordsLinked to original sources

Natural history of limbs with arterial insufficiency and chronic ulceration treated without revascularization.

OBJECTIVES: The natural history of limbs affected by ischemic ulceration is poorly understood. In this report, we describe the outcome of limbs with stable chronic leg ulcers and arterial insufficiency that were treated with wound-healing techniques in patients who were not candidates for revascularization. METHODS: A prospectively maintained database of limb ulcers treated at a comprehensive wound center was used to identify patients with arterial insufficiency, defined as an ankle-brachial index (ABI) <0.7 or a toe pressure <50 mm Hg. Patients were treated without revascularization when medical comorbidity or anatomic considerations did not allow revascularization with acceptable risk. Ulcers were treated with a protocol emphasizing pressure relief, débridement, infection control, and moist wound healing. Risk factors analyzed for their affect on healing and amputation risk included age, gender, diabetes mellitus, chronic renal insufficiency (serum creatinine > 2.5 mg/dL), severity of ischemia measured by ABI or toe pressure, wound grade, wound size, and wound location. RESULTS: Between January 1999 and March 2005, 142 patients with 169 limbs having arterial insufficiency and full-thickness ulceration were treated without revascularization. Mean patient age was 70.8 +/- 4.5. Diabetes mellitus was present in 70.4% of limbs and chronic renal insufficiency in 27.8%. Toe amputations or other foot-sparing procedures were performed in 28% of limbs. Overall, limb loss occurred in 37 patients. By life-table analysis, 19% of limbs required amputation < or =6 months of initial treatment and 23% at 12 months. Complete wound closure was achieved in 25% by 6 months and in 52% by 12 months. Statistical analysis showed a correlation between ABI and the risk of limb loss. In patients with an ABI <0.5, 28% and 34% of limbs experienced limb loss at 6 and 12 months, respectively, compared with 10% and 15% of limbs in patients with an ABI >0.5 (P = .01). The only risk factor associated with wound closure was initial wound size (P < .005). CONCLUSIONS: Limb salvage can be achieved in most patients with arterial insufficiency and uncomplicated chronic nonhealing limb ulcers using a program of wound management without revascularization. Healing proceeds slowly, however, requiring more than a year in many cases. Patients with an ABI <0.5 are more likely to require amputation. Interventions designed to improve outcomes in critical limb ischemia should stratify outcomes based on hemodynamic data and should include a comparative control group given the natural history of ischemic ulcers treated in a dedicated wound program.

Adult↗

DNA microarray stochastic model.

A stochastic model of the DNA microarray image pixels is presented. The model includes spot pixel intensity distribution, interpixel correlations and the intensity distribution of background noise. The data is indicative of a small exponential additive noise process and a larger Gaussian fluctuation that scales with spot intensity. Correlations are observed among pixels in the spot and between test and control images. The correlated fluctuations may be attributed to variations across each spot in the amount of DNA placed on the spot during the array fabrication process. The model may be used in gene expression estimation algorithm development, both to test new algorithms through simulation and to develop optimum algorithms. The model should also be easily adapted to new array based technologies in proteomics.

Computer Simulation↗

A genetic circuit amplifier: design and simulation.

A genetic circuit amplifier is designed using an electronic inverting amplifier as a starting point. Two simulation methods are used to analyze circuit performance in terms of the impulse and sinusoidal responses of electrical engineering. The first method is an exact stochastic simulation based on a kinetic model of the circuit. The second method incorporates statistical thermodynamic analysis. The simulations are used to analyze amplifier performance in response to classical systems analysis stimuli: impulses and sine waves. Degradation reactions, analogous to leakage off circuit capacitors, are found to have considerable impact on circuit response. For the nonlinear gain element used in our exemplary circuit, the selection of bias level based on controlling protein degradation rate plays an important role in determining circuit behavior. A parameter without electronic analog, the circuit plasmid copy number, is crucial to circuit operation. These simulations suggest that the copy number must be less than 50 for desired circuit operation.

Amplifiers, Electronic↗