Engineering topics

Shi, X.

Publications and source records attributed to Shi, X..

80 records · Page 5

NASA NTRS1997

Interaction of central venous pressure, intramuscular pressure, and carotid baroreflex function

Seven healthy volunteer men participated in an experiment involving lower body positive pressure (LBPP) of 30 Torr and acute volume expansions of 5-6% (VE-I) and 9-10% (VE-II) of their total blood volume (TBV) to differentiate the effect of increased intramuscular pressure and central venous pressure (CVP) on the maximal gain (Gmax) of the carotid baroreflex. During each experimental condition, the heart rate (HR), mean arterial pressure (MAP; intraradial artery or Finapres), and CVP (at the 3rd-4th intercostal space) were monitored continuously. Gmax was derived from the logistic modeling of the HR and MAP responses to ramped changes in carotid sinus transmural pressure using a protocol of pulsatile changes in neck chamber pressure from +40 to -65 Torr. The increase in CVP during +30-Torr LBPP was 1.5 mmHg (P < 0.05) and was similar to that observed during VE-I (1.7 mmHg, P > 0.05). The Gmax of the carotid baroreflex of HR and MAP was significantly decreased during LBPP by -0.145 +/- 0.039 beats x min(-1) x mmHg(-1) (38%) and -0.071 +/- 0.013 mmHg/mmHg (25%), respectively; however, VE-I did not affect Gmax. During VE-II, CVP was significantly greater than that elicited by LBPP, and the Gmax of the carotid baroreflex of the HR and MAP responses was significantly reduced. We conclude that carotid baroreflex responsiveness was selectively inhibited by increasing intramuscular pressure, possibly resulting in an activation of the intramuscular mechanoreceptors during LBPP. Furthermore, it would appear that the inhibition of the carotid baroreflex, via cardiopulmonary baroreceptor loading (increased CVP), occurred when a threshold pressure (CVP) was achieved.

NASA Discipline Cardiopulmonary↗

NASA NTRS1997

Baroreflex regulation of blood pressure during dynamic exercise

From the work of Potts et al. Papelier et al. and Shi et al. it is readily apparent that the arterial (aortic and carotid) baroreflexes are reset to function at the prevailing ABP of exercise. The blood pressure of exercise is the result of the hemodynamic (cardiac output and TPR) responses, which appear to be regulated by two redundant neural control systems, "Central Command" and the "exercise pressor reflex". Central Command is a feed-forward neural control system that operates in parallel with the neural regulation of the locomotor system and appears to establish the hemodynamic response to exercise. Within the central nervous system it appears that the HLR may be the operational site for Central Command. Specific neural sites within the HLR have been demonstrated in animals to be active during exercise. With the advent of positron emission tomography (PET) and single-photon emission computed tomography (SPECT), the anatomical areas of the human brain related to Central Command are being mapped. It also appears that the Nucleus Tractus Solitarius and the ventrolateral medulla may serve as an integrating site as they receive neural information from the working muscles via the group III/IV muscle afferents as well as from higher brain centers. This anatomical site within the CNS is now the focus of many investigations in which arterial baroreflex function, Central Command and the "exercise pressor reflex" appear to demonstrate inhibitory or facilitatory interaction. The concept of whether Central Command is the prime mover in the resetting of the arterial baroreceptors to function at the exercising ABP or whether the resetting is an integration of the "exercise pressor reflex" information with that of Central Command is now under intense investigation. However, it would be justified to conclude, from the data of Bevegard and Shepherd, Dicarlo and Bishop, Potts et al., and Papelier et al. that the act of exercise results in the resetting of the arterial baroreflex. In addition, if, as we have proposed, the cardiopulmonary baroreceptors primarily monitors and reflexly regulates cardiac filling volume, it would seem from the data of Mack et al. and Potts et al. that the cardiopulmonary baroreceptor is also reset at the beginning of exercise. Therefore, investigations of the neural mechanisms of regulation involving Central Command and cardiopulmonary afferents, similar to those being undertaken for the arterial baroreflex, need to be established.

NASA Discipline Cardiopulmonary↗

NASA NTRS1996

Diminished forearm vasomotor response to central hypervolemic loading in aerobically fit individuals

The aim of this study was to test the hypothesis that cardiopulmonary baroreflex control of forearm vascular resistance (FVR) during central hypervolemic loading was less sensitive in exercise trained high fit individuals (HF) compared to untrained average fit individuals (AF). Eight AF (age: 24 +/- 1 yr and weight: 78.9 +/- 1.7 kg) and eight HF (22 +/- 1 yr 79.5 +/- 2.4 kg) voluntarily participated in the investigation. Maximal aerobic power (determined on a treadmill), plasma volume and blood volume (Evans blue dilution method) were significantly greater in the HF than AF (60.8 +/- 0.7 vs. 41.2 +/- 1.9 ml.kg-1.min-1, 3.96 +/- 0.17 vs 3.36 +/- 0.08 1, and 6.33 +/- 0.23 vs 5.28 +/- 0.13 1). Baseline heart rate (HR), central venous pressure (CVP), mean arterial pressure (MAP, measured by an intraradial catheter or a Finapres finger cuff), forearm blood flow (FBF, plethysmography), and FVR, calculated from the ratio (MAP-CVP)/FBF, were not different between the HF and the AF. Lower body negative pressure (LBNP, -5, -10, -15, and -20 torr) and passive leg elevation (LE, 50 cm) combined with lower body positive pressure (LBPP, +5, +10, and +20 torr) were utilized to elicit central hypovolemia and hypervolemia, respectively. Range of CVP (from LBNP to LE+LBPP) was similar in the AF (from -3.9 to +1.9 mm Hg) and HF (from -4.0 to +2.2 mm Hg). However, FVR/CVP was significantly less in the HF (-1.8 +/- 0.1 unit.mm Hg-1) than AF (-34 +/- 0.1 unit.mm Hg-1). The FVR decrease in response to increase in CVP was significantly diminished in the HF (-1.46 +/- 0.45 unit.mm Hg-1) compared to the AF (-4.40 +/- 0.97 unit.mm Hg-1), and during LBNP induced unloading the FVR/CVP of the HF (-2.01 +/- 0.49 unit.mm Hg-1) was less (P < 0.08) than the AF (-3.28 +/- 0.69 unit.mm Hg-1). We concluded that the cardiopulmonary baroreceptor mediated FVR reflex response was significantly less sensitive to changes in CVP in individuals who practice exercise training.

NASA Discipline Number 93-10↗

NASA NTRS1993

A Type 2 supernovae constraint on neutrino(sub e) - neutrino(sub s) mixing

The role of a resonant nu(e) - nu(s) oscillation is discussed in the event of a supernova explosion. It is concluded that a significant nu(e) - nu(s) mixing may hinder the ability of the supernova to explode. It may also cool the proto-neutron star too quickly with respect to the observed cooling time of several seconds. The constraints on the nu(e) - nu(s) mixing parameters based on the above arguments are calculated.

Shi, X.↗

NASA NTRS1993

A diminished aortic-cardiac reflex during hypotension in aerobically fit young men

We compared the aortic-cardiac baroreflex sensitivity in eight average fit (AF: VO2max = 44.7 +/- 1.3 ml.kg-1 x min-1) and seven high fit (HF: VO2max = 64.1 +/- 1.7 ml.min-1 x kg-1) healthy young men during hypotension elicited by steady state sodium nitroprusside (SN) infusion. During SN mean arterial pressure (MAP) was similarly decreased in AF (-12.6 +/- 1.0 mm Hg) and HF (-12.1 +/- 1.1 mm Hg). However, the increases in heart rate (HR) were less (P < 0.023) in HF (15 +/- 3 bpm) than AF (25 +/- 1 bpm). When sustained neck suction (NS, -22 +/- 1 torr in AF and -20 +/- 1 torr in HF, P > 0.05) was applied to counteract the decreased carotid sinus transmural pressure during SN, thereby isolating the aortic baroreceptors, the increased HR remained less (P < 0.021) in HF (8 +/- 2 bpm) than AF (16 +/- 2 bpm). During both SN infusion and SN+NS, the calculated gains (i.e., delta HR/delta MAP) were significantly greater in AF (2.1 +/- 0.3 and 1.3 +/- 0.2 bpm.mm Hg-1) than HF (1.2 +/- 0.2 and 0.6 +/- 0.2 bpm.mm Hg-1). However, the estimated carotid-cardiac baroreflex sensitivity (i.e., the gain difference between the stage SN and SN + NS) was not different between AF (0.7 +/- 0.2 bpm.mm Hg-1) and HF (0.6 +/- 0.1 bpm.mm Hg-1). These data indicated that the aortic-cardiac baroreflex sensitivity during hypotension was significantly diminished with endurance exercise training.

NASA Discipline Number 14-10↗

NASA NTRS1992

Monte Carlo exploration of Mikheyev-Smirnov-Wolfenstein solutions to the solar neutrino problem

The paper explores the impact of astrophysical uncertainties on the Mikheyev-Smirnov-Wolfenstein (MSW) solution by calculating the allowed MSW solutions for 1000 different solar models with a Monte Carlo selection of solar model input parameters, assuming a full three-family MSW mixing. Applications are made to the chlorine, gallium, Kamiokande, and Borexino experiments. The initial GALLEX result limits the mixing parameters to the upper diagonal and the vertical regions of the MSW triangle. The expected event rates in the Borexino experiment are also calculated, assuming the MSW solutions implied by GALLEX.

Shi, X.↗

NASA NTRS1991

Solar neutrinos and the MSW effect for three-neutrino mixing

Researchers considered three-neutrino Mikheyev-Smirnov-Wolfenstein (MSW) mixing, assuming m sub 3 is much greater than m sub 2 is greater than m sub 1 as expected from theoretical consideration if neutrinos have mass. They calculated the corresponding mixing parameter space allowed by the Cl-37 and Kamiokande 2 experiments. They also calculated the expected depletion for the Ga-71 experiment. They explored a range of theoretical uncertainty due to possible astrophysical effects by varying the B-8 neutrino flux and redoing the MSW mixing calculation.

Shi, X.↗

NASA NTRS1985

Numerical simulation of vortex breakdown

The breakdown of an isolated axisymmetric vortex embedded in an unbounded uniform flow is examined by numerical integration of the complete Navier-Stokes equations for unsteady axisymmetric flow. Results show that if the vortex strength is small, the solution approaches a steady flow and the vortex is stable. If the strength is large enough, the solution remains unsteady and a recirculating zone will appear near the axis, its form and internal structure resembling those of the axisymmetric breakdown bubbles with multi-cells observed by Faler and Leibovich (1978). For apppropriate combinations of flow parameters, the flow reveals quasi-periodicity. Parallel calculations with the quasi-cylindrical approximation indicate that so far as predicting of breakdown is concerned, its results coincide quite well with the results mentioned above. Both show that the vortex breakdown has little concern with the Reynolds number or with the critical classification of the upstream flow, at least for the lower range of Reynolds numbers.

Shi, X.↗