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PubMed1990

Casson fluid model for pulsatile flow of blood under periodic body acceleration.

Pulsatile flow of a Casson fluid under the influence of a periodic body acceleration has been studied in this paper. An implicit finite difference numerical procedure has been used to analyze the flow. Applicability of this method has been checked by comparing the obtained results with the analytical solution for Newtonian flow and explicit scheme solution. The agreement between the implicit and explicit scheme solutions and the analytical solution is good (error less than 1%). Flow variables have been computed at three locations in cardiovascular system (wide (femoral) and narrow (arteriole and coronary) tubes). Effects of yield stress, tube radius and pressure gradient combined, body acceleration amplitude and frequency etc., on flow have been studied. The following observations have been made: (i) Initial transient time It changes with yield stress in narrow tubes are insignificant, whereas in wide tubes It decreases with yield stress; (ii) The axial velocity and fluid acceleration variations with yield stress are uniform (changes only quantitatively, profiles shape remain same) in narrow tubes, whereas in wide tubes these variations are non-uniform (profiles change qualitatively as well as quantitatively); (iii) Yield stress effects on wall shear amplitude are insignificant in narrow tubes (congruent to 0.3% in arteriole and congruent to 6% in femoral); and (iv) For Newtonian fluid, mean flow rate does not change with body acceleration amplitude a0 and frequency fb but it increases (decreases) with a0(fb) for Casson fluid.

Acceleration↗
PubMed1986

[Shielding design and detection of neutrons from medical and industrial electron accelerators--simple method of design calculation for neutron shielding].

The neutron leakage from medical and industrial electron accelerators has become an important problem and its detection and shielding is being performed in their facilities. This study provides a new simple method of design calculation for neutron shielding of those electron accelerator facilities by dividing into the following five categories; neutron dose distribution in the accelerator room, neutron attenuation through the wall and the door in the accelerator room, neutron and secondary photon dose distributions in the maze, neutron and secondary photon attenuation through the door at the end of the maze, neutron leakage outside the facility-skyshine.

Electrons↗
PubMed1999

Origins of the short latency vestibular evoked potentials (VsEPs) to linear acceleration impulses.

OBJECTIVE: To verify the vestibular origin of the short latency (t < 12.5 msec) vestibular evoked potentials (VsEPs) in response to linear acceleration impulses (L-VsEPs) and to differentiate between the contributions of the otolith organs and the semi-circular canals (SCCs) to their initiation. DESIGN AND METHODS: L-VsEPs (stimulus intensity, 3 g; rise time, 1.0 to 1.5 msec) were recorded in fat sand rats (Psammomys obesus) before and after unilateral labyrinthectomy, plugging of the SCCs in the remaining ear, and bilateral labyrinthectomy. Auditory nerve brainstem evoked responses (ABRs) and VsEPs to angular acceleration impulses (A-VsEPs) were also recorded. Wave amplitudes and latencies were statistically analyzed (MANOVA, repeated t-tests). RESULTS: In the intact animal, the linear VsEPs consisted of 5 to 6 waves, several mV in amplitude, with short latencies. The latency of the first wave was 2.0 msec. These waves were abolished after bilateral labyrinthectomy. Before and after plugging of the SCCs, linear acceleration VsEP wave latencies did not change, although amplitudes were slightly reduced. Similar results were obtained with respect to ABRs recorded from the same ear. Angular acceleration VsEPs were abolished after SCC plugging. CONCLUSIONS: These and other results confirm that the linear VsEPs are compound action potentials of the vestibular pathway, the first wave is the response of the vestibular nerve, and they are initiated mainly in the otolith organs.

Animals↗
PubMed1999

The effect of head orientation on the vestibular evoked potentials to linear acceleration impulses in rats.

OBJECTIVE: To investigate the influence of linear acceleration impulses delivered when the head is held in different static head orientations, on the first wave of the short latency vestibular evoked potential (VsEP). The first wave is the compound action potential of the primary vestibular neurons synchronously activated. BACKGROUND: It has been shown previously that the VsEP elicited in response to linear acceleration is initiated mainly in the otolith organs. These organs are responsive to both dynamic and static linear forces, including gravity. METHODS: VsEPs to linear acceleration stimuli (4g) were recorded when the rats head was oriented so that a) the plane of the utricular macula was aligned with the plane of the stimulus, b) in supine position and c) with the head pitched up and down in various angles with respect to gravity (stimulus-head spatial relation remained constant) as compared to a reference position. RESULTS: With the stimulus aligned with the plane of the utricular macula, the amplitude of the first wave of the L-VsEPs was significantly larger than in the reference position. In the supine position, the amplitude of the first wave was significantly larger and the latency was significantly shorter. The amplitude of the first VsEP wave tended to be larger in the "head up" orientations as compared to the "head down" orientations (not statistically significant). CONCLUSIONS: These results demonstrate the influence of head position and gravity on the VsEPs to linear acceleration impulses, which is in accordance with their otolithic origin.

Acceleration↗
PubMedDate not supplied

[Informativeness of pulse bloodfilling of auricular helical vessels for evaluation of human tolerance to plusGz acceleration].

The possiblity of using photoelectric plethysmography as an objective method to assess the human tolerance to accelerations was examined. The tolerance of 40 young healthy male test subjects to plus GZ accelerations of 3, 5, 6 and 7 g with an increase gradient of 0.4 g/sec and a plateau time of 30 sec was measured. The test subjects induced an arbitrary tension of the abdominal and leg muscles in order to better tolerate accelerations. When the tolerance reached its limit (as judging by optic disorders), the auricular pulse amplitude decreased 4-fold in 84% test subjects. The threshold value of the amplitude can be used as an informative indicator of a group and individual tolerance to plus GZ accelerations. A portable photoelectric plethysmograph has been designed for operational medical control of the health state of pilots inflight.

Acceleration↗
PubMed1975

Re-evaluation of a tilt-back seat as a means of increasing acceleration tolerance.

Relaxed tolerance was determined on seven subjects exposed to rapid onset (RO; 1 G/s) and gradual onset (GO; 1 G/10 s) acceleration at seat back angles of 13 degrees, 30 degrees, 45 degrees, 55 degrees, 65 degrees, and 75 degrees from the vertical. There was no significant difference between relaxed tolerance at the control angle of 13 degrees and tolerance at 30 degrees. However, at 45 degrees there was a significant 0.5 G increase in tolerance compared to control. Thereafter, tolerance continued to increase in an exponential manner to 8 G at 75 degrees, an increase over control of 100%. As relaxed tolerance increased with increasing back angle, peak heart rate during acceleration significantly decreased. In addition, four subjects were instrumented with an esophageal balloon for the measurement of intrathoracic pressure, which was equated as work during the M-1 straining maneuver. The amount of thoracic pressure necessary to maintain a preselected visual field declined as the back angle was increased from 13 degrees to 45 degrees to 65 degrees at the same acceleration level. The increase in relaxed tolerance along with the decrease in heart rate and the decrease in esophageal pressure at the greater back angles all demonstrate the acceleration protection provided by the tilt-back seat.

Acceleration↗
PubMed2001

60Cobalt vs. linear accelerator in the treatment of locally advanced cervix carcinoma: a comparison of survival and recurrence patterns.

OBJECTIVE: To compare the survival and recurrence patterns of patients with locally advanced cervical carcinoma treated with 60cobalt radiotherapy units and linear accelerators. METHODS: Two hundred and forty-eight patients with cervical carcinoma stages IIB-IVA who were treated with primary irradiation between the years 1985 and 1988 comprised the study group. The median survival of patients treated with 60cobalt units and linear accelerators was calculated using the method of Kaplan and Meier and compared using the log-rank test. Recurrence patterns were compared using chi-square analysis; p < .05 was considered significant for all tests. RESULTS: One hundred and ninety-five patients were treated with 60cobalt units (Group 1) and 53 patients were treated with a linear accelerator (Group 2). Group 1 and 2 were similar with regard to mean age and weight, stage distribution, and mean dose to point A. The rate of recurrence was comparable between Group 1 and 2 (65.6% vs. 64.2%) and no significant difference was found in overall survival between the groups (20 months vs. 21 months. p = 81). There was a trend toward increasing pelvic recurrence in Group 1 (50.8%) compared to Group 2 (35.8%, p = .08). CONCLUSIONS: 60Cobalt units and linear accelerators offer comparable rates of overall survival in patients with locally advanced cervix carcinoma.

Cobalt Radioisotopes↗
PubMed2000

The human linear vestibulo-ocular reflex to transient accelerations: visual modulation of suppression and enhancement.

Visual modulation of the linear vestibulo-ocular reflex (LVOR) was investigated in 4 normal subjects exposed to translational interaural transient accelerations of 0.08 g and 0.17 g. Binocular eye movements were recorded with the scleral search-coil technique. LVOR modulation with target proximity was studied using earth-fixed targets at distances of 30 and 60 cm (LVOR-E). LVOR suppression (LVOR-S) was provoked by similar targets which were head-fixed. For both conditions, linear acceleration evoked compensatory slow-phases whose velocities were, from onset, enhanced in proportion to acceleration and target proximity. At 80 ms after motion onset, i.e. before visually-guided eye movements could aid target fixation, gains (eye velocity/relative target velocity) during LVOR-E averaged 0.32 (S.D. 0.07) over all combinations of accelerations and target distances. At this time, eye velocities for LVOR-S were on average 33% lower than for LVOR-E (1.8 degrees/s vs. 2.7 degrees/s). During LVOR-S, a marked suppression of eye movements appeared at 102 ms (S.D. 10 ms). We conclude that mechanisms other than pursuit can be used to attenuate the LVOR at < 80 ms but their effect is weak. The marked suppression observed around 100 ms might be due to an early visual effect on vestibular pathways or by some independent voluntary control mechanism.

Acceleration↗
PubMed2001

Compensation for the effects of head acceleration on jaw movement in speech.

Recent studies have demonstrated the ability of subjects to adjust the control of limb movements to counteract the effects of self-generated loads. The degree to which subjects change control signals to compensate for these loads is a reflection of the extent to which forces affecting movement are represented in motion planning. Here, we have used empirical and modeling studies to examine whether the nervous system compensates for loads acting on the jaw during speech production. As subjects walk, loads to the jaw vary with the direction and magnitude of head acceleration. We investigated the patterns of jaw motion resulting from these loads both in locomotion alone and when locomotion was combined with speech production. In locomotion alone, jaw movements were shown to vary systematically in direction and magnitude in relation to the acceleration of the head. In contrast, when locomotion was combined with speech, variation in jaw position during both consonant and vowel production was substantially reduced. Overall, we have demonstrated that the magnitude of load associated with head acceleration during locomotion is sufficient to produce a systematic change in the position of the jaw. The absence of variation in jaw position during locomotion with speech is thus consistent with the idea that in speech, the control of jaw motion is adjusted in a predictive manner to offset the effects of head acceleration.

Acceleration↗
PubMed1993

A mathematical model of the cardiovascular response to +Gz acceleration.

The cardiovascular system is the limiting factor for human tolerance to positive Gz (head-to-foot) acceleration induced during maneuvers of fighter aircrafts. Safe handling of modern fighter aircrafts with higher acceleration capabilities require the use of countermeasures such as a G-suit or breathing at a positive pressure. A better understanding of the mechanisms involved in the cardiovascular response to +Gz acceleration would help improve the design and application of protective measures. This paper presents a simple mathematical model of the cardiovascular system which incorporates arterial and cardiopulmonary baroreflexes, left ventricular-peripheral circulation interaction and decreased venous return. This model is capable of reproducing observed overall cardiovascular response to +Gz acceleration.

Acceleration↗
PubMed1997

Cerebral blood flow velocity and cranial fluid volume decrease during +Gz acceleration.

Cerebral blood flow (CBF) velocity and cranial fluid volume, which is defined as the total volume of intra- and extracranial fluid, were measured using transcranial Doppler ultrasonography and rheoencephalography, respectively, in humans during graded increase of +Gz acceleration (onset rate: 0.1 G/s) without straining maneuvers. Gz acceleration was terminated when subjects' vision decreased to an angle of less than or equal to 60 degrees, which was defined as the physiological end point. In five subjects, mean CBF velocity decreased 48% from a baseline value of 59.4 +/- 11.2 cm/s to 31.0 +/- 5.6 cm/s (p<0.01) with initial loss of peripheral vision at 5.7 +/- 0.9 Gz. On the other hand, systolic CBF velocity did not change significantly during increasing +Gz acceleration. Cranial impedance, which is proportional to loss of cranial fluid volume, increased by 2.0 +/- 0.8% above the baseline value at the physiological end point (p<0.05). Both the decrease of CBF velocity and the increase of cranial impedance correlated significantly with Gz. These results suggest that +Gz acceleration without straining maneuvers decreases CBF velocity to half normal and probably causes a caudal fluid shift from both intra- and extracranial tissues.

Acceleration↗
PubMed1998

High sustained +Gz acceleration: physiological adaptation to high-G tolerance.

Since the early 1940s, a significant volume of research has been conducted in an effort to describe the impact of acute exposures to high-G acceleration on cardiovascular mechanisms responsible to maintaining cerebral perfusion and conscious in high performance aircraft pilots during aerial combat maneuvers. The value of understanding hemodynamic characteristics that underlie G-induced loss of consciousness has been instrumental in the evolution of optimal technology development (e.g., G-suits, positive pressure breathing, COMBAT EDGE, etc.) and pilot training (e.g., anti-G straining maneuvers). Although the emphasis of research has been placed on the development of protection against acute high +Gz acceleration effects, recent observations suggest that adaptation of cardiovascular mechanism associated with blood pressure regulation may contribute to a protective 'G-training' effect. Regular training at high G enhances G tolerance in humans, rats, guinea pigs, and dogs while prolonged layoff from exposure in high G profiles (G-layoff) can result in reduced G endurance. It seems probable that adaptations in physiological functions following chronically-repeated high G exposure (G training) or G-layoff could have significant impacts on performance during sustained high-G acceleration since protective technology such as G-suits and anit-G straining maneuvers are applied consistently during these periods of training. The purpose of this paper is to present a review of new data from three experiments that support the notion that repeated exposure on a regular basis to high sustained +Gz acceleration induces significant physiological adaptations which are associated with improved blood pressure regulation and subsequent protection of cerebral perfusion during orthostatic challenges.

Acceleration↗
PubMed1999

Evaluation of cerebral stresses under acceleration taking into account the lateral ventricles.

In certain flight configurations, fighter pilots are exposed to high Gz acceleration that may induce inflight loss of consciousness (LOC). That LOC is usually preceded by visual prodromes as greyout and blackout. The pathophysiological cause of these phenomena is used to be related to the effects of accelerations on the vascular system (Burton, 1988; Whinnery, 1990). However technological advances have created aircraft generating high accelerations with rapid onset rates (1-6 Gs-1). The symptomatology of inflight LOC has changed and prodromes no longer appear. Pilots also reported a lacunar amnesia of the LOC. In order to evaluate the potentially adverse effect of acceleration on the brain tissue, it was important to study its mechanical behavior under hypergravity. An approximation of the cerebral stresses was obtained by coupling an 'ex vivo' experiment (Guillaume et al., 1997) with a numerical simulation. Firstly, the calculations have been realized considering the brain as homogeneous. Secondly, the cerebral ventricles have been individualized. The results of these two approaches were compared.

Acceleration↗
PubMed1975

Cardiopulmonary Effects of Combined Exercise and +Gz Acceleration.

Acceleration (+Gz) handicaps gas exchange in man by creating a ventilation/perfusion (VA/Q) mismatch in the lung. At 1 G, leg exercise makes VA/Q more nearly homogeneous, primarily because peripheral pumping reduces venous pooling and improves pulmonary capillary filling. Four men were studied at rest and pedaling 0, 300 and 600 kpm/min at +1, 2, and 3 Gz. Acceleration raised the resting Vo2, VE, VE/Vo2 and HR but lowered O2 pulse and PetCO2. Since acceleration increased the cost of moving the legs (unloaded pedaling), all work data were considered as a function of VO2. Low levels of exercise returned all variables toward their control (1 G) values. At higher workloads, the combination of G with exercise caused divergence from control measurements, probably also reducing maximal work capacity. Cardiogenic oscillation amplitude increased with G and decreased with work, evidence that exercise improved VA/Q homogeneity at all G-levels. Where man must undergo prolonged exposure to moderate acceleration, use of the peripheral pump might offer some advantages over the conventional anti-G suit.

Acceleration↗
PubMedDate not supplied

[Effectiveness of a helium-oxygen gas mixture in transverse accelerations].

Healthy male test subjects, aged 20 to 45, were exposed to acceleration of +6 Gx for 1200 sec and of +10 Gx for 60 sec. Three gradients of the centrifuge acceleration were used in the latter case. In every experimental run the test subjects were exposed to accelerations while breathing air or a helium-oxygen atmosphere. The positive effect of the helium-oxygen atmosphere was seen in an increase of the respiration intensity and in an elevation of pulmonary ventilation and gas exchange. This may be associated with a decline of the aerodynamic resistance to breathing. The positive effect of the helium-oxygen atmosphere was more pronounced during accelerations of +4 to 8 Gx and depended on the total exposure time.

Acceleration↗
PubMed2004

Eye movements of goldfish depending on the relationship between acceleration vector and otolith organ.

Prior experiments demonstrated that the acceleration deviating 15 degrees to the right (left) from the longitudinal body axis could not produce vertical eye movement in left (right) eye of the goldfish. From these results, we expected that vertical eye movement of goldfish for the acceleration perpendicular to the longitudinal body axis might different between right and left eye. However, in this experiments, there were no clear difference in magnitude of vertical eye movements for the acceleration shifted 15 degrees around the left-right body axis. On the other hand, the response of right eye was larger than that of left eye for the acceleration applied from left to right of the body. These results suggest that stimulation from medial to lateral and lateral to medial of the otolith organ has different effect on vertical eye movement in each eye of goldfish.

Acceleration↗
PubMed2005

Human ocular torsion and perceived roll responses to linear acceleration.

We investigated if human ocular torsion (OT) and perceived roll (PR) are elicited in response to either dynamic interaural linear acceleration or dynamic roll tilt of the gravito-inertial force (GIF). We expanded a previous study [26] that measured only OT across a limited frequency-range (from 0.35 Hz to 1 Hz) by simultaneously measuring OT and PR at three very low (0.01, 0.02 and 0.05 Hz) and one high (1 Hz) frequencies. Three experimental conditions were investigated: (1) Y-Upright with acceleration along the interaural (Y) axis while upright, (2) Y-Supine with acceleration along the Y-axis while supine, and (3) Z-RED with acceleration along the rostro-caudal Z) axis with right-ear-down (RED). OT was measured by video-oculography, while PR was measured by use of a somatosensory bar. OT and PR were qualitatively different. Large OT responses were measured for Y-Upright and Y-Supine, while large perceived roll responses were observed for Y-Upright and Z-RED. OT for Z-RED was small, and PR for Y-Supine was absent. In conclusion, OT and PR appear governed by qualitatively different neural mechanisms. OT appears mostly influenced by central low-pass filtering of interaural graviceptor cues, while PR appears mostly influenced by roll tilt of the GIF.

Acceleration↗