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At least 1,513 records · Page 84Linked to original sources
PubMed1981

Multiple hyperbaric exposures during pregnancy in sheep.

Eleven sheep were subjected to hyperbaric comparable to 165 feet of sea water 31 times between the one hundred twelfth and one hundred thirty-seventh days of pregnancy. During 13 dives the maternal and fetal circulations were monitored for bubble formation during decompression. Bubbles were detected by external doppler probes in eight of 12 ewes, but in none of the fetuses. Nine ewes were delivered of normal lambs at term. In one, twin fetuses died during an abnormal labor. The pregnancy of another was terminated by cesarean section after decompression to look for bubbles in the fetal circulation.

Animals↗
PubMed1983

Lack of harmful effects from simulated dives in pregnant sheep.

The purpose of the study was to determine if a near-maximum exposure to air at increased atmospheric pressure causes gross fetal malformations, decreased birth weight, or death when administered to pregnant sheep during peak development of the embryo. Twenty-eight timed-pregnant sheep were alternately assigned to a series of 25- or 30-minute exposures at either 4.6 atmospheres absolute or surface pressure between days 12 and 40 of gestation. About day 130 of pregnancy, 11 experimental and eight control fetuses were recovered, weighed, measured, fixed, and examined for defects. No major structural malformations were present. One minor variation, an undescended testis, occurred in a treated fetus. The results indicate that a series of short, marginally tolerated "dives" by pregnant sheep during peak development does not affect fetal health or survival.

Animals↗
PubMed1992

Numerical phase algorithm for decompression computers and application.

Present generation decompression computers employ a simplified algorithm, limiting dissolved gas build-up in tissue and blood according to a method proposed by Haldane 80 years ago. Such a model works well for single dives, but is usually liberal and theoretically incomplete for multiple exposures within 24 hr spans. Using the critical phase hypothesis in a bubble model, we have extended the classical model of Haldane to multi-exposures. This model is discussed, and a decomputer algorithm described for multi-diving. The focus is permissible bubble excess, not just dissolved gas per se, with phase constraints affecting all tissues, fast and slow, and requiring a systematic lowering of repetitive tissue tensions. Deep repetitive and shallow multi-day exposures are impacted most by the procedure. Within nucleation theory deeper-than-first dives are also treated. A set of multi-diving fractions, xi, accounting for micronuclei excitation and regeneration, reduced bubble elimination in repetitive activity, and coupled effects on tissue tension, are proposed, with xi representing a set of multiplicative factors (less than one) applied to critical tissue tensions for multi-exposures. These factors affect repetitive activity over short time spans, deeper-than-previous and continuous multi-day activities, compared to standard computer software, and are easily encoded into existing decompression meters, potentially extending their range and flexibility over exposure regimes.

Algorithms↗
PubMed1987

Computational decompression models.

Early computational models for decompression are based on supersaturation assumptions for dissolved gases. Such models, and our understanding of decompression biophysics, have been extended in the past 20 years by analyses of phase separation of gases. Generally termed thermodynamic decompression (or phase equilibration), these studies postulate a continuous exchange of inert gas between tissues and nucleation sites (gas micropockets), consistent with many commonplace phenomena. Postulates lead to decompression schedules and transfer mechanisms that differ from their earlier predecessors. The precise physical and computational bases supporting both viewpoints are described and contrasted.

Decompression↗
PubMed1990

Modeling dissolved and free phase gas dynamics under decompression.

Dissolved and free gases do not behave the same way in tissue under pressure, and their interaction is complex. Differences are highlighted, particularly with respect to time scales, gradients and transport. Impacts of free phases on diving are described, contrasting increased off-gassing pressures, slower ascent rates, safety stops and reduced repetitive exposures as consistent practical measures within Haldane models (limited supersaturation) which can be played off against buildup of dissolved gas. Simple computations illustrate the points.

Algorithms↗
PubMed1990

Reduced gradient bubble model.

An approach to decompression modeling, the reduced gradient bubble model (RGBM), is developed from the critical phase hypothesis. The phase limit is introduced, extended, and applied within bubble-nucleation theory proposed by Yount. Much is different in the RGBM algorithm, on both theoretical and applied sides, with a focus on permissible bubble excesses rather than just dissolved gas buildup, something of a departure from traditional models. Overall, the approach is conservative, with changes in parameter settings affording flexibility. Marginal profiles permitted by tables and meters are restricted by the bubble algorithm. Highlighted features of the conservative algorithm include: (1) reduced no-stop time limits from the varying-permeability model (VPM); (2) short safety stops (or shallow swimming ascents) in the 10-20 feet of sea water (fsw) zone; (3) ascent and descent rates of 60 fsw/min, or slower; (4) restricted repetitive exposures, particularly beyond 100 fsw, based on reduced permissible bubble excess; (5) restricted spike (shallow-to-deep) exposures based on excitation of additional micronuclei; (6) restricted multi-day activity based on regeneration of micronuclei; (7) consistent treatment of altitude diving within model framework; (8) algorithm linked to bubble-nucleation theory and experiment. Coupled to medical reports about the long term effects of breathing pressurized gases and shortcomings in dissolved gas models, conservative modeling seems prudent.

Algorithms↗
PubMed1975

Absorption of in vivo inert gas bubbles.

The resolution rates of gas bubbles in perfused tissue are examined using a mathematical model which is consistent with transient-state and steady-state data from large gas pocket experiments. The tissue surrounding the bubble is assumed to have an infinite number of infinitesimally small capillaries (distributed sink). The time for complete disappearance of a bubble depends on initial bubble size. blood perfusion rate of the tissue, physical characteristics of the inert gas and the fraction (1-Pa/Pg), where Pa and Pg are inert gas partial pressures in the surrounding tissue and inside the bubble, respectively. Calculated time for disappearance of a nitrogen bubble of 1 mm radius in an O2-breathing man is 100-250 min, depending on blood perfusion. Breathing of air increases the time of persistence of the bubble about 10-fold. The "transient state", when gas is dissolving in the immediate surroundings of the bubble, is most influential when there is no perfusion and the partial pressure gradient gas volume is the largest. However, our major conclusion is that the initial transient state has little effect on total lifetime of the bubble in any case.

Decompression Sickness↗
PubMed1980

Cardiopulmonary consequences of decompression stress.

Clinically undetectable venous gas emboli (VGE) can be routinely demonstrated in both animals and men undergoing clinically uneventful decompression from hyperbaric environments. The effect of VGE on gas exchange and pulmonary hemodynamics in this setting is unclear. We therefore have studied the cardiopulmonary effects of clinically uneventful decompression on unanesthetized sheep. Sheep were instrumented with a femoral arterial catheter, Swan-Ganz catheter, chronic tracheostomy, and were loosely restrained in the prone position. Following baseline measurements of pulmonary artery pressure (pPA), mean pulmonary artery wedge pressure (pPAw), cardiac output (CO), arterial and mixed venous blood gases, Doppler ultrasonic bubble detection, and V/Q scans using 133Xe, the animals were exposed to: (a) a 15-min exposure to 6.03 ATA (n = 8), (b) a 17.5-min exposure to 6.03 ATA, or (c) a 15-min control period at 1 ATA (n = 7). All measured parameters remained unchanged in the control group. VGE were detected in all animals exposed to pressure. In both exposure groups CO fell 20% (P < 0.05) and pulmonary vascular resistance (PVR) rose 60% (P < 0.05) compared to the control group. This rise in PVR was significantly in excess of that predicted for the observed fall in CO. Analysis of pPA and pPAw suggested neither right nor left ventricular failure. No significant abnormalities of gas exchange were detected. V/Q scans remained unchanged in all animals. We conclude that CO and PVR are altered by clinically uneventful decompression stress.

Animals↗
PubMed1977

Spectral analysis of Doppler ultrasonic decompression data.

Several aspects of spectral analysis of bubble transients in Doppler ultrasonic decompression data are discussed. The computation of energy density spectra, using fast Fourier transform techniques for analyzing bubble transients, is described. Spectral analysis of data from probes implanted within animals, using a conventional Fourier analyzer, provided good visual indications of bubble events and interesting changes in spectral structure. A new transient spectral analysis technique that is suitable for quantitative real-time monitoring of small decompression bubbles is described. In a feasibility study using data from an implanted probe, an increase of 900% in bubble signal/noise ratio was observed.

Animals↗