Nonlinear Sigma Model Analysis of the AFM Phase Transition of the Kondo Lattice

We have studied the antiferromagnetic quantum phase transition of a 2D Kondo-Heisenberg square lattice using the non-linear sigma model. A renormalization group analysis of the competing Kondo -- RKKY interaction was carried out to 1-loop order in the $\epsilon$ expansion, and a new quantum critical point is found, dominated by Kondo fluctuations. In addition, the spin-wave velocity scales logarithmically near the new QCP, i.e breakdown of hydrodynamic behavior. The results allow us to propose a new phase diagram near the AFM fixed point of this 2D Kondo lattice model.Comment: 4 pages, 4 figure

Effects of local cooling on skin perfusion response to pressure: implications to pressure ulcer prevention

Pressure ulcers have long been an important healthcare issue in both acute and long-term care settings. Temperature is one of the extrinsic causative factors for this multi-factorial disease not yet fully explored. Previous animal studies revealed that skin cooling reduced the severity of ulceration compared to non-cooling. Cooling is also used widely in plastic surgery and organ transplants for tissue preservation. However, the underlying protective mechanism of local cooling remains unclear. Our study's objective was to measure the effect of cooling on tissue's response to pressure using skin perfusion response on human subjects. Reactive hyperemia is a normal protective physiological response occurring after vessel occlusion. Laser Doppler flowmetrey was used to measure cutaneous perfusion. We hypothesized that local cooling would reduce a rigid indenter induced post-ischemic reactive hyperemic response. Ten young healthy non-smokers were recruited into the study. A repeated measures design was used where all subjects were subjected to pressure with cooling to 25°C and pressure without cooling test sessions. Each test session contained five levels of pressure control: light contact (10 minutes), 60 mmHg (30 minutes), light contact (20 minutes), 150 mmHg (3 minutes), light contact (10 minutes). The cooling intervention was performed during the period of 60mmHg contact pressure. Our results showed a significantly attenuated peak perfusion response after 60mmHg (p=0.019) but not after 150mmHg (p=0.241) of pressure for the cooling session compared to the non-cooling. This study suggests that local cooling may protect skin from the harmful effects of prolonged pressure in this young healthy population. The study protocol would be modified to investigate populations at risk of pressure ulcers

Effectiveness of local cooling on enhancing tissue ischemia tolerance in people with spinal cord injury

People with spinal cord injury (SCI) are at risk of pressure ulcer development due to impaired mobility, sensation or changes in tissue properties. Increased skin temperature is one of the least explored risk factors for pressure ulcers. Since people with SCI also encounter thermoregulation deficits, investigation of the effectiveness of local skin cooling in this population is particularly important. Three groups of subjects were recruited: 1) 14 subjects with SCI at T6 and above, 2) 8 subjects with SCI below T6, and 3) 14 healthy controls. Reactive hyperemic response was the main study outcome and was measured after three different combinations of stimuli: 1) pressure only, 2) pressure with fast cooling (-4°C/min) and 3) pressure with slow cooling (-0.33°C/min). Spectral density of the skin blood flow (SBF) was used to investigate the underlying microcirculatory control mechanisms. Five of the subjects did not have reactive hyperemia in all test sessions and were excluded from statistical analysis. In the control group, the normalized peak SBF and perfusion area were close to significantly greater in pressure only as compared to fast cooling (p=0.023 and p=0.023, respectively) and slow cooling (p=0.033 and p=0.016, respectively). Although this phenomenon was not significant when analyzing subjects with SCI alone, significant changes were observed in the signal attributed to the metabolic control mechanism and were observed in this population with pressure only (p=0.019) and pressure with slow cooling (p=0.041). Since the reactive hyperemic response is mediated by different control mechanisms, the less obvious changes in reactive hyperemia in people with SCI may be due to alterations in microcirculation after injury. Results from this study suggest that local skin cooling is beneficial to ischemic tissue by decreasing the metabolic demand, and this is generally consistent with previous animal studies and our pilot study. Findings from this study also suggest that investigating time domain parameters and time-dependent spectral analysis of the SBF signal is helpful in understanding circulatory control in people with different levels of neurological deficits. This study contributes toward justification for the development of support surfaces with microclimate controls to enhance tissue integrity