Equivalent air depth: fact or fiction.

In mixed-gas diving theory, the equivalent air depth (EAD) concept suggests that oxygen does not contribute to the total tissue gas tension and can therefore be disregarded in calculations of the decompression process. The validity of this assumption has been experimentally tested by exposing 365 rats to various partial pressures of oxygen for various lengths of time. If the EAD assumption is correct, under a constant exposure pressure each incremental change in the oxygen partial pressure would produce a corresponding incremental change in pressure reduction tolerance. Results of this study suggest that the EAD concept does not adequately describe the decompression advantages obtained from breathing elevated oxygen partial pressures. The authors suggest that the effects of breathing oxygen vary in a nonlinear fashion across the range from anoxia to oxygen toxicity, and that a simple inert gas replacement concept is no longer tenable.

Visual recognition thresholds in a compressed air environment.

An experiment was performed to determine the effects of different high-pressure air environments on human binocular visual recognition time as a function of stimulus size and type. Eight adult male volunteers were randomly exposed to high-pressure air environments in hyperbaric test chambers instrumented for visual studies. Analysis of variance for a three-factor repeated-measures design revealed significant main effects for the variables of stimulus size and pressure, indicating that recognition time (RT) increases as a function of decreasing stimulus size and increased pressure. A significant interaction was also observed between the independent variables of pressure and stimulus type with stimulus type having the greatest effect at low pressure. These results are discussed for their applicability to the design of underwater equipment, visual displays, and the occupational safety of underwater workers.

Use of oxygen for optimizing decompression.

For over 70 years, decompression has been facilitated by the use of elevated oxygen partial pressures. Oxygen has been administered even though little is known about the proper dosage or the way in which this benefit is derived. The historical literature indicates that there is an envelope or narrow range of oxygen partial pressures that can be used. If the oxygen is too low, the incidence of decompression sickness increases; if the oxygen is too high, oxygen poisoning becomes a problem. The present study was designed to explore this oxygen envelope and to define the relationships between oxygen partial pressure, exposure time, and pressure, and to delineate their effects on pressure-reduction limits. To define the ED50 (the effective dose that produced signs of decompression sickness in 50% of the animals), we exposed 820 female albino rats to 42 experimental conditions. Results suggest that the optimum oxygen level and the size of the oxygen envelope both depend on the ambient hydrostatic pressure and the exposure time. For short "shallow" exposures, the optimum oxygen level is high and the oxygen envelope is large; for long "deep" exposures, the optimum oxygen level is reduced and the envelope is restricted.

Species differences in decompression.

In an effort to bring together the diverse laboratory-animal decompression studies, a literature review and statistical evaluation were undertaken. Although 22 different species that had been used in decompression studies were identified, systematic data were available for only 7 of these species: man, goat, dog, guinea-pig, rat, hamster, and mouse. Mathematical functions using physiological data on these seven species were developed to estimate 1) saturation time (the time for the body to equilibrate after an increase in hydrostatic pressure), and 2) no-decompression saturation-exposure limits (the maximum saturation-exposure pressure from which an abrupt return to 1 ATA can be tolerated). Data from man, rat, and mouse were used to develop physiological relationships for two additional decompression variables: change in pressure-reduction limits associated with increased exposure pressure and time to onset of decompression symptoms. Finally, data on rats for two other decompression variables, gas elimination time and optimum decompression stop time, are discussed in the hope that this will stimulate additional animal laboratory research in other mammalians. The general functional relationships developed in this paper provide a preliminary and rough means for extrapolating among species the decompression results obtained during animal laboratory experiments.

Pressure reduction limits for rats subjected to various time/pressure exposures.

The impact of the combined effects of exposure time and hydrostatic pressure on pressure reduction is explored in this study. In Phase I of the study, excursion dives were made to 10, 20, and 30 ATA for 5, 10, 20, 40, or 80 min. In Phase II, the animals were saturated at 1.3, 10, or 20 ATA for 60 min; each saturation exposure was followed by a 10-atm excursion dive of either 1, 5, 10, 20, or 40 min. The chamber gas mixture during all pressure exposures was 0.51 ATA oxygen, 0.79 ATA nitrogen, and the remainder helium. The subjects were 655 rats; during each pressure exposure 5 rats were exercised in a rotating cage. After each exposure, the rats were abruptly decompressed to a lesser pressure for observation and tabulation of the decompression sickness incidence. Results suggest that neither the starting saturation pressure nor the differential excursion pressure alters the time required for an animal to reach equilibrium with the surrounding environment. Pressure-reduction values, however, vary with both the exposure pressure and exposure time. These results will have a direct impact on the formulation of future decompression models.

Decompression sickness during saturation dives.

Available Navy saturation diving data were analyzed for an evaluation of the therapeutic adequacy of decompression sickness treatment procedures and for delineation of precipitant factors in the etiology and treatment of decompression sickness during saturation dives. None of the cases of decompression sickness recorded during saturation dives involved more than musculoskeletal or joint pain, and in 96% of the cases the joint pain was confined to the diver's knees. In 89% of the cases symptoms appeared while the divers were still under pressure. The subsequent recompression treatment of these cases resulted in full relief in only 35% of the cases; the remaining 65% completed the therapy and subsequent decompression with residual pain which diminished over a period of weeks. The adequacy of the recompression appears to be inversely proportional to the depth of reported onset of symptoms and the time required to obtain even partial relief is directly related to the magnitude of the recompression ratio used. Four explanations are suggested for the limited recompression therapy common in saturation diving: increase in musculoskeletal pain with recompression, peer pressure to avoid extension of the chamber confinement, lack of severe neurological symptoms, and the tremendous depths required to obtain a reasonable recompression ratio. The author further suggests that future treatment procedures will require a departure from the accepted concept of radically decreasing the volume of inert gas bubbles by increasing pressure.

Pressure-reduction limits for rats following steady-state exposures between 6 and 60 ATA.

The role of pressure reduction in the formation and growth of bubbles is universally recognized and its significance in decompression theory has been accepted. Yet the allowable limits of pressure reduction for man and animal are uncertain. This study sought to evaluate the pressure-reduction limits for rats following steady-state exposures at pressures greater than 1 atm. To define the relationship, 350 albino rats were exposed to 1 of 12 specified pressure levels between 6 and 60 ATA and then abruptly decompressed to a preselected reduced pressure level for observation. The pressure-reduction levels were selected to determine for each saturation-exposure level an ED-50 (i.e. the effective dose that will produce decompression sickness in 50% of the animals). The results demonstrate three consistent findings: (1) there is a linear relationship (r = .99) between the magnitude of a safe pressure reduction and the saturation exposures between 6 and 43 ATA; (2) at pressures greater than 43 ATA, there is a qualitative change in the decompression sickness symptoms and a reduction in the precision of the mathematical relationship (r = .44); and (3) the magnitude of the pressure change required to increase the incidence of decompression sickness from 10% to 90% is directly related to the magnitude of the exposure pressure. The implications of these results for deep operational diving are discussed.

An audiometric survey of Navy divers.

The pure tone audiograms of a diverse group of U.S. Navy divers were examined across four major variables: (1) number of years of Navy diving experience, (2) previous noise exposure history, (3) previous history of barotrauma, and (4) type of equipment used, i.e. scuba and helmet. The results obtained suggest that these variables had only minimal effects on auditory sensitivity and that when the hearing of these divers was compared to a normal population of nondivers, no significant differences were detected. A comparison of these findings with those in the existing diving literature was then made.

Intentional tremor on a helium-oxygen chamber dive to 49.5 ATA.

Tremor is a well-recognized manifestation of the high pressure nervous syndrome (HPNS). As such, its measurement and analysis during deep hyperbaric exposures can be an important index of central nervous system integrity. During the U.S. Navy's experimental chamber dive to a depth equivalent to 1600 fsw (49.5 ATA), objective measures of intentional tremor were obtained at several depths. Six subjects were pressurized in 6 days to 49.5 ATA. After spending 7 days at this pressure, they were decompressed in 19 days to the surface. Measures of intentional tremor were obtained predive and at pressure levels of 13.1, 31.3, 49.5, 40.4, and 31.3 ATA using the Naval Medical Research Institute Mark 3 Mod 1 tremor device. Each subject's microtremor was measured while he produced a force of 50 grams and 500 grams against a finger force transducer. Unlike previous studies of HPNS tremor, special attention was given to amplitude rather than frequency analysis. All subjects displayed a marked increase in tremor that interfered with fine motor performance at depths greater than 1000 fsw. A statistically significant increase in signal frequency was also observed.

Relationship between saturation exposure pressure and subsequent decompression sickness in mice.

Despite the fact that the pressure reduction is acknowledged to be the single most cogent factor in producing decompression sickness, little has been done to define accurately the allowable limits beyond 2 ATA. This study provides some theoretical guidelines for future manned dives related to this problem. There were 324 albino mice used to define the relationship between saturation exposure pressure and the safe abrupt pressure reduction. The results from both the helium-oxygen and nitrogen-oxygen exposures support the idea of a linear, depth-dependent relationship between the saturation depth and the allowable pressure reduction. Support is presented for the use of a modified decompression ratio P1/P2 (P1 equals saturation pressure and P2 equals pressure following decompression) to account for the observed incidence of decompression sickness. An attempt is made, using the existing human data to relate this empirical relationship to the operational dive setting.