Observation of columnar microstructure in step-graded Si1-xGex/Si films using high-resolution X-ray microdiffraction.

Columnar microstructure in step-graded Si(1-x)Ge(x)/Si(001) structures with low threading dislocation densities has been determined using high angular resolution (approximately 0.005 degrees ) x-ray microdiffraction. X-ray rocking curves of a 3-microm-thick strain-relaxed Si(0.83)Ge(0.17) film show many sharp peaks and can be simulated with a model having a set of Gaussians having narrow angular widths (0.013 degrees -0.02 degrees ) and local ranges of tilt angles varying from 0.05 degrees to 0.2 degrees. These peaks correspond to individual tilted rectangular columnar micrograins having similar (001) lattice spacings and average areas of 0.8 to 2.0 microm(2).

Cardiovascular responses to positive pressure breathing using the tactical life support system.

The improved protection afforded by the Tactical Life Support System (TLSS) vs. other partial pressure ensembles has not been reported with respect to the cardiovascular effects of positive pressure breathing (PPB). Nine seated subjects wearing TLSS were exposed to 30, 50, and 70 mm Hg PPB (breathing air) with four times this pressure in the G-suit. Experiments were conducted at ground-level in order, separated by 4 min rest and preceded by a 1-min control period. Stroke volume and cardiac output (SV, CO) and indexes (SI, CI) were determined by impedance cardiography. Mean arterial pressure (MAP) was directly related to PPB level, increasing by 23%, 32%, and 47% for each PPB level, respectively (p less than 0.01). HR, SV, and CO were unaffected after 4 min of 30, 50, and 70 mm Hg PPB. The results indicate that cardiovascular function decay is less severe than that reported using other PPB ensembles at similar PPB levels. Improved protection is most likely due to the greater pressurization of the G-suit and the 45% greater bladder volume in the leg bladders, leading to restored venous return and SV.

Decrement in postural control during mild hypobaric hypoxia.

The effects of mild hypoxia on the postural control system of 39 naive subjects were examined by measuring the postural sway with a Kistler force platform, at ground level and at one of four simulated altitudes: 1,521 m (5,000 ft), 2,438 m (8,000 ft), 3,048 m (10,000 ft), or 3,658 m (12,000 ft). The total sway increased above the ground level controls for the 1,521 m, 2,438 m, and 3,048 m exposures (p less than or equal to 0.005) as did the sway at the lowest measurable frequency (p less than or equal to 0.002), but no change in sway was seen in those subjects exposed to 3,658 m as compared to ground level values. Significant interaction between altitude and exposure was observed at p less than or equal to 0.04, reflecting the definite effect at the lower altitudes and the lack of an effect at the higher altitude. The multiple comparison test indicated no difference between the responses at 1,524 m, 2,438 m, and 3,048 m. Both arterial oxygen saturation, SaO2, and the end-tidal oxygen partial pressure, PetO2, decreased in relation to the test altitudes with a statistically significant interaction between altitude and PetO2 (p less than or equal to 0.02), and SaO2 (p less than or equal to 0.005). There was no significant interaction between altitude and end-tidal carbon dioxide partial pressure (p = 0.4853). The postural control mechanisms, as an intergrative functional unit, are very sensitive to acute mild hypoxia. Arguments are advanced to indicate that intervention of compensatory mechanisms at higher altitudes may explain the recovery of postural stability at 3,658 m.

Postural stability during slow-onset and rapid-onset hypoxia.

The standing steadiness and postural tremor of seven male volunteers were examined during slow and rapid induction of hypoxia. Spectrum analysis of position coordinates and tremor data generated from a quartz multicomponent force measuring platform and a tremor transducer showed that postural tremor (pt) and anteroposterior sway (ay) increased significantly during hypoxia. Rate of onset of hypoxia, however, did not affect the development of the increased tremor, whereas rapid-onset hypoxia consistently induced a greater anteroposterior sway than slow-onset hypoxia. Lateral sway (ax) was not affected by either slow- or rapid-onset hypoxia; however, it was significantly greater when eyes were closed compared to when they were opened. Anteroposterior sway (ay) was also significantly greater when visual cues were eliminated during both slow- and rapid-onset hypoxia. Heart rate (HR) and cardiac index (CI) of subjects increased significantly during both conditions of hypoxia.