Introduction. U.S. Navy Diving Manual (Mar-1970)

Transcription

1 DECOMPRESSION SICKNESS AND OXYGEN TOXICITY IN U.S. NAVY SURFACE-SUPPLIED HE-O2 DIVING Wayne A. Gerth U.S. Navy Experimental Diving Unit 321 Bullfinch Road Panama City, FLORIDA 3247 U.S.A. Introduction The scientific diving community has identified a need to work safely and cost effectively at open-water depths up to 3 fsw. Careful consideration of the risks associated with such diving and the costs of measures to mitigate them - is required to select optimal strategies for meeting this need. Some of this consideration has already been completed, resulting in our focus at this Workshop on surface-supplied heliumoxygen (He-O 2 ) diving. A variety of environmental/physiological hazards govern the types of equipment and procedures and the associated logistical complexity required to conduct this type of diving. Principal among these are inert gas narcosis, which motivates use of helium as the inert gas diluent in diver breathing gases, and decompression sickness (DCS), which is avoided by staged decompression and aggressive exploitation of the physiological oxygen window. Increased exploitation of the oxygen window to reduce DCS risks or decompression times engenders another principal hazard; that of central nervous system (CNS) oxygen toxicity (Tikuisis and Gerth, 23). A brief review of the history of how the risks of DCS and CNS oxygen toxicity have manifest and been managed in U.S. Navy surface-supplied He-O 2 diving from 197 to present illustrates issues that may have to be faced by the scientific diving community as it undertakes similar diving operations. U.S. Navy Diving Manual (Mar-197) New Oxygen Partial Pressure Tables for Helium-Oxygen Decompression were published in the March-197 issue of the U.S. Navy Diving Manual. In the ensuing 14 years, 185 man-dives on these tables to depths of 2 fsw or greater with bottom times of 3 min or more were reported to the Navy Safety Center. These reports indicated that twenty DCS cases occurred in this relatively small number of dives. U.S. Navy Diving Manual Revision 3 (15-May-1991) and Revision 4 (2-Jan-1999) 24

2 The unacceptably high 11% incidence of DCS (2/185) that accompanied use of the 197 Helium-Oxygen Decompression Tables was addressed with extensive revisions of the tables for the 1991 issue of Revision 3 of the Diving Manual. Measures taken to decrease the DCS risks of schedules in the revised tables included: Tables were converted from O 2 partial pressure format to the depth-time format of the Standard Air Decompression tables. In the course of the conversions, schedules were computed with minimum PO 2 (maximum P He ) at each depth. Values 2% lower than operationally allowed were used to compute schedules for dives to depths of 3 fsw or less: 14% O 2 used for dives to depths 2 fsw (operational minimum: 16% O 2 ) 1% O 2 used for dives to depths >2 fsw (operational minima: 12% O 2 for dives to depths of 21 3 fsw; 1% for exceptional exposure dives to depths > 3 fsw) This caused dives to be jumped to more conservative schedules when dived within operational limits. Added gas switch to 4% O 2 at 1 fsw during ascent in dives to depths deeper than 2 fsw. (Switch to 1% O 2 at 5 fsw during ascent retained) Other revisions included: Ascent rate between stops reduced from 6 to 3 fsw/min Reduced bottom PO 2 limit from 1.6 to 1.3 atm The 1991 Revision 3 He-O 2 Decompression Tables appeared unchanged in the January 1999 issue of the U. S. Navy Diving Manual, Revision 4. As in the 14-year period after issue of the 197 He-O 2 Decompression Tables, only a relatively small number of surface-supplied He-O 2 dives was completed by Navy divers after issue of the 1991 tables. From 1995 to 1999, 45 surface-supplied He-O 2 man-dives on the tables were reported to the Navy Safety Center. The numbers of dives in each year using the inwater or surface decompression on oxygen (sur-d-o 2 ) procedures are given in Table 1 (Flynn, 1999). Table 1. Surface-supplied He-O 2 dives completed by year and decompression model. Year Decompression Total In-water sur-d-o Total At least as reported to the Safety Center, the dives were also limited in maximum depth, as shown in Table 2 (Flynn, 1999). Only eight man-dives to depths greater than 25 fsw were reported. Table 2. Surface-supplied He-O 2 dives completed by schedule and bottom time. 25

3 Bottom Time Schedule Depth Group (fsw) (min) (1) 6 21(2) (3) 1(6) * (5) 44(4) * DCS. Note: Numbers in parentheses indicate 6 cases of CNS O 2 toxicity in order of occurrence 26

4 While only a single case of DCS occurred in these dives, six cases of CNS O 2 toxicity occurred. In five of these cases, symptoms proceeded to convulsions, either in the water or immediately on surfacing. U.S. Navy Diving Manual Revision 4, Change A (1-March-21) The incidence and severity of in-water O 2 toxicity events with the 1991 He-O 2 tables was reviewed at a July 1999 meeting in Groton, CT, where it was concluded that the 1991 tables required revision to minimize future risks of such events. At this point, new quantitative analytic tools were in hand to help ensure that prospective revisions would be accompanied by only minimal increases in DCS risk. The first of these was a linearexponential multi-gas (LEM) probabilistic model of DCS incidence and time of occurrence that had been calibrated about a composite data set of 4669 He-O 2 and N 2 -O 2 man-dives. This model, LEMhe8n25, fully described in a later publication (Gerth and Johnson, 22), allowed estimation of the DCS risks of schedules in the original 1991 He-O 2 Decompression Tables (Figure 1), and assessment of the impact on DCS risk of any prospective changes to these schedules. A probabilistic auto-catalytic model of CNS O 2 toxicity (Harabin et al, 1995) was also available for similar analysis of CNS O 2 toxicity risks in original and revised schedules. All analyses were completed with bottom mix gas at the maximum oxygen fraction allowed in the original Revisions 3 and 4 He-O 2 Decompression Tables. Dives to depths of 25 fsw or greater included initial descent to 2 fsw on air followed by a 1 minute stop at this depth for gas switch to bottom mix. Chamber time in sur-d-o 2 procedures included a 5-minute air-breathing break after every 3 min of O 2 breathing, except after the last O 2 -breathing period before surfacing. After consideration of a variety of candidate modifications, a final table set with the following modifications, designated Change A, was selected: Shifts to 6% He/4% O 2 at 1 fsw during ascent from dives deeper than 2 fsw were removed Shifts to 5% He/5% O 2 at 9 fsw or at the first stop shallower than 9 fsw during ascent were added Shifts to 1% O 2 at 5 fsw during ascent were removed; Diver remains on 5/5 He/O 2 to completion of any 4 fsw stop 5 fsw stop time shorter than preceding 6 fsw stop time in any schedule was adjusted to equal the 6 fsw stop time. The 4 fsw stop time was then set equal to the new 5 fsw stop time. In-water decompression stops at 3 and 2 fsw on 1% O 2 were added with stop times determined as follows: - 3 fsw stop time = 3 min + 1/3 of original 4 fsw stop time rounded to next least integer; 27

5 - 2 fsw stop time = 2/3 of original 4 fsw stop time rounded to next larger integer. 28

6 IWD: LEMhe8n25D 2 Rev4 Original 15 P DCS, % Figure 1. LEMhe8n25-estimated DCS risks of surface-supplied He-O 2 in-water decompression schedules in U.S. Navy Diving Manual, Revision 4 (identical to those in U.S. Navy Diving Manual, Revision 3). Each point indicates the risk for one schedule with schedules arranged in order of increasing bottom time in each dive depth group, and with dive depth groups arranged in order of increasing depth from 6 fsw to 3 fsw. Points within each dive depth group are connected with straight lines to help visually resolve the different groups. The last point at the right of the figure indicates the DCS risk of a 3 fsw, 3 min bottom time dive. Estimated DCS risks for exceptional exposures to depths greater than 3 fsw, or for longer bottom times in each dive depth group, are not shown. The impacts of these modifications on measures of pulmonary (Harabin et al, 1987) and CNS O 2 toxicity are illustrated in Figures 2 and 3, respectively. 29

7 IWD: O 2 Toxicity Rev4 ChangeA - Rev4 Original Δ Figure 2. Changes in cumulative unit pulmonary toxic dose (CUPTD) (Harabin et al, 1987) caused by modifications designed to reduce risks of in-water CNS O 2 toxicity in 1999 Revision 4 surface-supplied He-O 2 in-water decompression tables. IWD: O 2 Toxicity Δ Rev4 ChangeA - Rev4 Original Figure 3. Decreases in estimated overall risks of seizure caused by modifications designed to reduce risks of in-water CNS O 2 toxicity in 1999 Revision 4 surfacesupplied He-O 2 in-water decompression tables. 3

8 As shown in Figure 4, the generally decreased risks of O 2 toxicity associated with the modified procedures were purchased with only small increases in estimated DCS risk, except for dives with the longest bottom times to depths of 25 fsw or more. IWD: LEMhe8n25D 4 Rev4 ChangeA - Rev4 Original Δ -1 Figure 4. LEMhe8n25-estimated DCS risk changes caused by modifications designed to reduce risks of in-water CNS O 2 toxicity in 1999 Revision 4 surfacesupplied He-O 2 in-water decompression tables. Surface Decompression on Oxygen (sur-d-o 2 ) The modifications to the in-water decompression procedures also affected in-water portions of decompressions ended with sur-d-o 2. LEMhe8n25-estimated DCS risks of the sur-d-o 2 schedules without the modifications are shown in Figure 5. The schedules for longer deeper dives within operational limits are seen to have substantial estimated DCS risks. Specific modifications to the chamber time requirements for sur-d-o 2 were consequently made to reduce these risks and mitigate adverse impacts of the in-water changes when the sur-d-o 2 option is exercised. These changes included: Split first chamber O 2 -breathing period at 4 fsw into a 15 min period at 5 fsw and a 15 min period at 4 fsw Increase the maximum number of O 2 -breathing periods from 4 to 8: - Periods 2 through 4 at 4 fsw - Periods 5 through 8 at 3 fsw The impact of these changes is illustrated in Figure 6. 31

9 SURD-O 2 : LEMhe8n25D 25 Rev4 Original Figure 5. LEMhe8n25-estimated DCS risks of surface-supplied He-O 2 surface decompression on oxygen (sur-d-o 2 ) schedules in U.S. Navy Diving Manual, Revision 4. SURD-O 2 : LEMhe8n25D Rev4 ChangeA - Rev4 Original Δ Figure 6. LEMhe8n25-estimated DCS risk changes caused by modifications designed to reduce risks of in-water CNS O 2 toxicity in 1999 Revision 4 surfacesupplied He-O 2 sur-d-o 2 tables. 32

10 The decreases in O 2 toxicity risk arising from changes to the in-water decompression procedures propagated into the sur-d-o 2 procedures as well, as shown in Figures 7 and 8. SURD-O 2 : O 2 Toxicity Δ Rev4 ChangeA - Rev4 Original Figure 7. Changes in cumulative unit pulmonary toxic dose (CUPTD) caused by modifications designed to reduce risks of in-water CNS O 2 toxicity in 1999 Revision 4 surface-supplied He-O 2 sur-d-o 2 tables. SURD-O 2 : O 2 Toxicity Rev4 ChangeA - Rev4 Original Δ

11 Figure 8. Decreases in estimated overall risks of seizure caused by modifications designed to reduce risks of in-water CNS O 2 toxicity in 1999 Revision 4 surfacesupplied He-O 2 sur-d-o 2 tables. The modified tables were field tested in surface-supplied He-O 2 dives on the USS Monitor in summer 2. More than 15 dives were completed with sur-d-o 2 procedures to depths of fsw with bottom times up to 4 min. No symptoms or signs of CNS O 2 toxicity and two cases of DCS occurred. The DCS incidence was considered well within acceptable limits, especially in view of the heavy workloads that divers sustained both during and after the dives. The modified tables were published in the U.S. Navy Diving Manual, Revision 4, Change A issued 1-March-21. Summary Documented U.S. Navy operational experience with surface-supplied He-O 2 diving since 197 is relatively limited, and confined largely to dives to depths less than 25 fsw with sur-d-o 2. A series of post hoc changes to decompression tables last computed from first principles by Workman in the mid 196 s have been made to mitigate the risks of DCS and CNS O 2 toxicity in current U.S. Navy surface-supplied He-O 2 diving procedures. However, such risks remain that, though presently acceptable by the U.S. Navy diving community, may not be acceptable to the more conservative scientific diving community. It may be time to develop a new set of decompression tables for surface-supplied He-O 2 diving from first principles with modern probabilistic approaches to decompression. DCS risk will likely remain an issue in surface-supplied He-O 2 diving with operationally acceptable bottom times and in-water decompression, particularly for dives to depths deeper than 25 fsw. References Flynn, E.T Material presented at meeting, Oxygen Toxicity in Surface-Supplied Helium-Oxygen Diving, Naval Submarine Medical Research Center, Groton, CT,4 July Gerth, W.A. and T.M. Johnson. 22. Development and Validation of 1.3 ATA PO 2 -in- He Decompression Tables for the MK 16 MOD 1 UBA. NEDU TR 2-1, Navy Experimental Diving Unit, Panama City, FL, August 22. Harabin, A.L., L.D. Homer, P.K. Weathersby, and E.T. Flynn An analysis of decrements in vital capacity as an index of pulmonary oxygen toxicity. J. Appl. Physiol. 63(3): Harabin, A.L., S.S. Survanshi, and L.D. Homer A Model for Predicting Central System Oxygen Toxicity from Hyperbaric Oxygen Toxicity in Humans. Toxicology and Applied Pharmacology 132,

12 Tikuisis P. and W.A. Gerth. 23. Decompression Theory. In: Brubakk, A.O. and T.S. Neuman, eds. Bennett and Elliott s Physiology and Medicine of Diving. 5 th Edition. W. B. Saunders Co., London. 35