Biomedical subjects

R D Vann

Publications and source records attributed to R D Vann.

25 records · Page 2Linked to original sources

PubMed1987

A likelihood analysis of experiments to test altitude decompression protocols for shuttle operations.

The principle of maximum likelihood and the method of linear regression both are used to fit mathematical models to experimental data, but likelihood can be applied to binary data such as the outcome of a decompression, whereas linear regression cannot. Maximum likelihood was applied to 548 individual altitude exposures from 30 experimental pressure profiles tested by NASA and the Air Force. Twelve decompression models were studied including modified Haldane models and models which assume that stationary bubbles cause Type I decompression sickness. The data was best represented by a model in which a bubble in tissue is surrounded by a diffusion barrier, but this representation was statistically indistinguishable from a single tissue Haldane model with a halftime of 508 min. By providing a quantitative measure of the agreement between theory and data, the principle of maximum likelihood offers an opportunity for improving the understanding of decompression mechanisms and for developing safer and faster decompression procedures.

Altitude↗

PubMed1985

Air and nitrox saturation decompression: a report of 4 schedules and 77 subjects.

Seventy-seven subjects were decompressed from air or nitrogen-oxygen (nitrox) saturation exposures at 18.3 to 40.2 meters sea water (msw) [60 to 132 feet sea water (fsw)] using four different decompression schedules. A h schedule for decompression from an air saturation-excursion profile at 18.3 msw (60 fsw) resulted in pain-only decompression sickness (DCS) symptoms in 2 of 23 subjects. A 32 and 35 h schedule from a different air saturation profile at 19.8 and 22.9 msw (65 and 75 fsw), respectively, resulted in DCS symptoms in 1 of 24 subjects. A third and fourth schedule for air or nitrox saturation at 40.2 msw (132 fsw) resulted in DCS symptoms in 3 of 12 and 1 of 18, respectively. No serious (type II) symptoms were observed as a result of any of the decompressions. All DCS cases consisted of knee pain occurring either in the last 3 msw of the decompression or shortly after surfacing. Doppler ultrasound monitoring revealed venous gas emboli (VGE) in several subjects, but generally only shallow to 6.1 msw (20 fsw). Results demonstrate an overall DCS incidence of 9%, and all cases were pain-only and localized to the knee. The third schedule (U.S. Navy heliox saturation decompression schedule) seems to produce a higher incidence of DCS than the other schedules when used in air or nitrox exposures. Differentiation between the schedules designed for nitrox was impossible due to the limited number of subjects in each and the variable nature of the exposures.

Adult↗

PubMed1984

A theoretical method for selecting space craft and space suit atmospheres.

A theoretical method for selecting space craft and space suit atmospheres assumes that gas bubbles cause decompression sickness and that the risk increases when a critical bubble volume is exceeded. The method is consistent with empirical decompression exposures for humans under conditions of nitrogen equilibrium between the lungs and tissues. Space station atmospheres are selected so that flight crews may decompress immediately from sea level to station pressure without preoxygenation. Bubbles form as a result of this decompression but are less than the critical volume. The bubbles are absorbed during an equilibration period after which immediate transition to suit pressure is possible. Exercise after decompression and incomplete nitrogen equilibrium are shown to increase bubble size, and limit the usefulness of one previously tested stage decompression procedure for the Shuttle. The method might be helpful for evaluating decompression procedures before testing.

Decompression Sickness↗

PubMed1984

Decompression induced nitrogen elimination.

A method for measuring nitrogen elimination after air diving has been developed in which a subject breathes air instead of oxygen or helium-oxygen. Accuracy is improved with this method because only nitrogen absorbed during the dive is eliminated. Nitrogen stored in the lungs and tissues at sea level is unaffected. Measurements were made with a closed-circuit breathing apparatus using a spirometer as a counterlung. The oxygen partial pressure in the apparatus was controlled at 0.209 +/- 0.003 atm. The spirometer volume was recorded periodically with the subject holding his breath at functional residual capacity. Increases in spirometer volume were used to define a nitrogen elimination curve. Elimination measurements were made after resting and exercising dives to 60, 100, and 130 fsw (2.8, 4.0, and 4.9 atm) at the U.S. Navy no-decompression exposure limits. Exercise during a dive increased the volume of nitrogen eliminated after the dive, but results for both resting and exercising divers were variable. Possible causes of this variability include bubble formation and changes in blood flow.

Adult↗

PubMed1984

Decompression from a deep nitrogen/oxygen saturation dive--a case report.

Ten divers participated in a 4.5 d nitrogen/oxygen saturation dive to 165 fsw. There were daily 2 h excursions to 61 msw (200 fsw). The divers breathed air during the excursions and 0.51 bar (0.5 atm) oxygen in nitrogen at 50.3 msw (165 fsw). The final decompression began 6 h after the last excursion. The oxygen partial pressure was 0.51 bar (0.5 atm) from 50.3 to 13.7 msw (165 to 45 fsw), and air was used from 13.7 msw (45 fsw) to the surface. By 6.1 msw (20 fsw), four divers had developed decompression sickness. A fifth diver developed decompression sickness during a commercial air flight 68 h after surfacing. Comparison of ascent rates for this dive and for air or nitrogen/oxygen saturation dives reported in the literature suggests that deeper dives require slower rates of ascent. Dives shallower than 30.5 msw (100 fsw) had a mean ascent rate of 1 msw/h (3.2 fsw/h) and 14 decompression incidents in 107 man-exposures. Dives deeper than 30.5 msw (100 fsw) had a mean rate of 0.76 msw/h (2.5 fsw/h) and 14 incidents in 45 man-exposures.

Adaptation, Physiological↗

PubMed1980

Evidence for gas nuclei in decompressed rats.

In vitro and in vivo studies have shown that pressure treatment before decompression reduces bubble formation. This has been interpreted as evidence that bubbles originate from pre-existing gas nuclei. The present experiments were conducted to determine if pressure treatment during air or liquid breathing before a 2-h air dive would reduce the decompression sickness (DCS) indicence for rats. Control dives, preceded by liquid breathing with 17 and 24 rats to 135- and 150-fsw had DCS incidences of 35% and 29%. Liquid-breathing pressure treatment at 1347 fsw before 135 fsw dive (11 rats) or at 1796 fsw before the 150-fsw dive (24 rats) reduced the incidences to 0% and 8%. In the air-breathing experiments, a 240-fsw control dive with 200 rats gave an 83% incidence. Pressure treatment at 600 fsw (195 rats) or 100 fsw (153 rats) reduced the DCS incidence to 74% and 64%. These experiments suggest that the bubbles responsible for DCS in the rat originate from gas nuclei.

Air↗

PubMed1975

Bubble growth and mechanical properties of tissue in decompression.

A survey of decompression literature leads to the conclusion that when tissue is subjected to gaseous supersaturation, pre-existing gas micronuclei grow into the gas bubbles which are routinely observed in decompression studies. These micronuclei may originate from mechanically induced tribonucleation or cavitation within joints. A new tissue model for decompression sickness based upon failure theory in rubber is proposed. The model shows theoretically that pre-existing sea-level nuclei can be stabilized at depth by elastic forces in tissue. These same elastic forces restrain the growth of nuclei when supersaturation occurs. Mechanical stress will lower the gaseous supersaturation required for growth of nuclei. Gaseous supersaturation, mechanical stress, and the elastic properties of various tissues interact to produce unbounded bubble growth leading to tissue lesions when combined gaseous and mechanical supersaturation exceeds a threshold value. The recommendation is made that the high levels of supersaturation generally used for the decompression of men be reduced.

Animals↗