PubMed Health⌕ Search

Biomedical subjects

L Hrebien

Publications and source records attributed to L Hrebien.

6 recordsLinked to original sources
PubMed1993

A mathematical model of G time-tolerance.

We propose a model to explain experimentally observed effects of Gz onset rates and levels on the time of occurrence of loss of vision and/or consciousness. The model is based on the existence of two generally accepted parameters: a G limit beyond which cerebral perfusion ceases, and a buffer time between loss of perfusion and loss of function. When applied to ramp onset G profiles, the model predicts a generally hyperbolic locus of endpoints, similar to the well-known Stoll curve, except for the dip. The advantage of the model is its applicability to any G onset profile. Data from the literature support the assumptions of the model and its results, including the absence of the dip in the locus for a ramp onset. The results call into question some concepts used to design G avoidance inflight strategies and the usefulness of some experimental centrifuge methods. The model may enable an increase in the accuracy of predictions of the time of visual or cerebral loss of function under various G profiles.

Adaptation, Physiological↗
PubMed1988

Current and emerging technology in G-LOC detection: pulse wave delay for +Gz tolerance assessment.

We have found that pulse wave delay increases linearly with +Gz experienced by conscious subjects and that G-tolerance limits, as measured using conventional light bars, occur repeatedly at the same pulse wave delays or delta delay. When protective modalities such as anti-G suits or supinating seats are used, the delta delays increase at a slower rate as a function of +Gz. G-tolerance thresholds occur at higher +Gz levels with protection but the delta pulse wave delays reach the same value for all tolerance levels. This parameter can be used to warn expert systems of the approach of G-LOC during actual flight and/or provides an objective measure of G protection provided by new or modified anti-G equipment. Therefore, this tool can be used in the research setting to evaluate the efficacy of G-protective equipment in an objective manner.

Cognition Disorders↗
PubMed1987

Reflectance photoplethysmography as an adjunct to assessment of gravitational acceleration tolerance: preliminary findings.

We have examined the feasibility of using reflectance photoplethysmography to assess Gz acceleration tolerance. Reflectance plethysmograms recorded using a sensor placed on the region of the superficial temporal artery were analysed along with the mean value and the pulsatile component of the Doppler velocity recorded from the opposite temporal artery. The photoplethysmogram signal and pulsatile and mean Doppler velocities were examined as predictors of impending peripheral light loss (PLL) during the experiments. Photoplethysmography correctly predicted a large percentage of the PLL runs (80.5%) and non-PLL runs (98.3%). Mean Doppler velocity predicted a higher percentage of PLL runs (88.1%), but with an unacceptably low rate of non-PLL runs (77.2%). The pulsatile Doppler velocity yielded only 50.7% correct prediction of PLL runs. The results of this preliminary study indicate that, with an improved design of the sensor and the electronics, it may be possible to use reflectance photoplethysmography in acceleration tolerance experiments as a reliable predictor of impending peripheral light loss.

Acceleration↗
PubMed1985

Factors affecting human tolerance to sustained acceleration.

Linear increases in G tolerance with increases in anti-G suit (AGS) bladder inflation pressure occurred when relaxed subjects were exposed to acceleration plateaus while riding a centrifuge and were either seated upright, (15 degrees seat back angle or SBA) or supine (60 degrees SBA). Supine G tolerance with AGS bladder inflation decreased as G onset time was increased by factors of two and four. Changing the mode of operation of a new servo-controlled anti-G valve regulating AGS bladder pressure had no effect on G tolerance nor on AGS comfort scores. Comfort was unaffected by G onset time and reduced with high AGS bladder pressures. Results support the hypotheses that G protection provided by simultaneously applied anti-G techniques is additive and that the simple hydrostatic model of the circulatory system can adequately account for AGS bladder pressure changes required for G protection when body position is changed.

Acceleration↗