On the "adiabatic approximation" for design of control laws for linear, time-varying systems

Control laws are often designed for linear time-varying processes by solving the algebraic Riccati equation for the optimum control law at each instant of time. Such designs may be called "adiabatic approximations." Although they are not optimum, they can result in closed loop systems which perform well. The stability of systems designed using the adiabatic approximation can be assessed by the "second method of Lyapunov." Stability is assured if a readily computed test matrixF, which depends on the rate of change of the parameters of the system, is negative-definite.     

Luxury perfusion following anterior ischemic optic neuropathy

To study the role of adenosine in sleep regulation, the adenosine A1 receptor agonist N6-cyclopentyladenosine (CPA) and the antagonist caffeine were administered to rats. Intraperitoneal (i.p.) CPA 1 mg/kg but not 0.1 mg/kg, suppressed rapid-eye-movement (REM) sleep and enhanced electroencephalographic (EEG) slow-wave activity (power density 0.75-4.0 Hz) in non-REM sleep. The latter effect was remarkably similar to the response to 6-h sleep deprivation. The effects persisted when CPA-induced hypothermia was prevented. Caffeine (10 and 15 mg/kg i.p.) elicited a dose-dependent increase in waking followed by a prolonged increase of slow-wave activity in non-REM sleep. The combination of caffeine (15 mg/kg) and sleep deprivation caused less increase in slow-wave activity than sleep deprivation alone, indicating that caffeine may reduce the buildup of sleep pressure during waking. The results are consistent with the involvement of adenosine in the regulation of non-REM sleep.     

Antibiotic pharmacodynamics in cerebrospinal fluid

The CSF half-lives of lipophilic agents, such as quinolones, are similar to those in serum and peak concentrations in CSF are achieved relatively quickly. In contrast, the pharmacokinetics of hydrophilic agents (beta-lactams and vancomycin) in CSF often differ from those in serum. In particular, the half-lives of these agents in CSF tend to be extended, and the time to achieve peak concentrations in CSF is delayed. Hydrophilic antibiotics, such as beta-lactams, penetrate poorly through the BBB, but CSF penetration is significantly increased in the presence of inflammation. In contrast, lipophilic antibiotics, such as quinolones, enter the CSF more efficiently and their penetration is not inflammation dependent. The pharmacodynamic properties of antibiotics in CSF are generally similar to those in other body sites; beta-lactam agents and vancomycin are time-dependent, whereas the quinolones and aminoglycosides are concentration-dependent. However, a notable difference from infections in other sites is that quinolones have a short PAE in CSF and need to continually exceed the MBC for maximal effectiveness. Thus, in CSF, quinolones demonstrate features of both concentration-dependency and time-dependency, evidence that the AUC/MBC is an important predictor of effectiveness. With the exception of quinolones, many antibiotics appear to have prolonged sub-MIC effects and longer half-lives in CSF than in serum, suggesting that dosing intervals longer than those used traditionally would be effective in meningitis. However, this requires clinical verification.     

Development and validation of a Monte Carlo simulation of photon transport in an Anger camera

The geometric component of the point spread function (PSF) of a gamma camera collimator can be determined analytically, and the penetration component can be calculated readily by numerical ray-tracing. A Monte Carlo simulation of photon transport which includes collimator scatter is developed. The simulation was implemented with an array processor which propagates up to 1024 photons in parallel, allowing accurate estimates of the total radial PSF in less than a day. The simulation was tested by imaging monoenergetic point sources of Tc-99m, Cr-51, and Sr-85 (140, 320, and 514 keV, respectively) on a General Electric Star Cam with low-energy, general-purpose, and medium-energy collimators. Comparisons of measured and simulated PSFs demonstrate the validity of the model and the significance of collimator scatter in the degradation of image quality.     

Base cations and micronutrients in forest soils along three clear-cut chronosequences in the northeastern United States

Determining effects of clear-cutting on base cations and micronutrients is essential for ensuring the sustainability of forestry for biofuels and wood products. The objective of this study was to quantify long-term changes in forest floor and mineral soil base cations (Ca, Mg, and K) and micronutrient (Mn, Zn, and Cu) concentrations and pools following clear-cutting in forests aged 1–120 years. We studied forest soils along three clear-cut chronosequences located in the Adirondack Ecological Center in Newcomb, NY, Bartlett Experimental Forest in Bartlett, NH, and Harvard Forest in Petersham, MA. We utilized a strong-acid extraction to quantify base cations and micronutrient concentrations and pools, which may better assess nutrients over the chronosequences than the conventional exchangeable extraction. Generalized linear mixed-effect models (GLMMs) show forest floor and mineral soil Ca, Mg, Mn, and Cu concentrations and pools decreased with increasing forest age across the three study areas. Potassium and Zn concentrations and pools were not significantly different with stand age and neither did soil C and N pools and pH using GLMMs. We calculated that 32–67% of the Ca pool decrease can be attributed to uptake by regenerating vegetation but only 0.02–9% of Mg, Mn, and Cu after harvest. Thus, leaching was likely to the dominant loss process for Mg, Mn, and Cu following clear-cutting. Our results suggest nutrient pools decreased for over a century following clear-cutting, but it is unclear if this will impact plant growth.