Decompression Tables for Diving at Altitude
Condition: Decompression Sickness; Acute Mountain Sickness
MeSH terms: Altitude Sickness; Decompression Sickness
Study type: Interventional

Summary: 
      The aims of this proposal are to test current USN procedures for adjusting decompression
      procedures during air diving at 8,000 and 10,000 ft altitude and to provide a decompression
      algorithm for no-stop dives to 100 feet of sea water (fsw) at 10,000 and 12,000 ft altitude
      using enriched O2 (PO2=1.3 ATM). Additionally, the experiments will determine whether a
      period of hyperbaric hyperoxia, such as would be experienced during a dive at altitude,
      reverses altitude acclimatization, resulting in a return of acute mountain sickness (AMS)
      symptoms.
    

Detailed description: 
      Three different types of experiments will be performed:

      (A). Testing of Cross corrections breathing air at 8,000 (0.743 ATA) and 10,000 ft (0.688
      ATA). Subjects will be decompressed in a hypobaric chamber to one of the altitudes above. A
      no-stop dive to 60 fsw will be tested at each altitude (2.82 ATA at sea level, 2.56 ATA at
      8,000 ft, 2.51 ATA at 10,000 ft). Using the Cross corrections, the virtual depth for both
      altitudes is 90 fsw, for which the no-stop time is 30 minutes. During the dive the subject
      will be immersed/submersed at rest/mild exercise in 28°C water. Ascent rate will be 30
      fsw/min. Upon surfacing the diver will be monitored for 12 hours for symptoms of
      decompression sickness and transthoracic echocardiography (rest and leg/arm motion) to
      examine for venous gas embolism (VGE) at 5, 15, 30, 60 and 120 minutes after surfacing. After
      that, measurements will be continued until no bubbles are detected.

      (B). Testing of a no-stop dive to 100 fsw at 10,000 ft and 12,000 ft breathing 35% O2. For
      this series of experiments all subjects will remain at altitude for 48 hours before diving,
      in order to allow for resolution of AMS symptoms. Appropriate depth-time profiles have been
      assessed by calculating the equivalent sea level air depth for each of these dives (PN2
      values 2.42 and 2.36 ATM, respectively). These yield equivalent air depths of 68 and 66 fsw,
      respectively. Cross corrected virtual depths would therefore be 99 and 104 fsw, yielding
      no-stop times of 25 and 20 minutes, respectively. The Cross correction method will be used in
      this instance because, although largely untested, it is presently in use by the Navy. As with
      the air dives described above, the more provocative dives (12,000 ft) will be completed
      before the ones at 10,000 ft, which will increase the confidence for the bottom time used at
      the lower altitude.

      (C). Testing to determine whether a high PO2 dive would reverse altitude acclimatization and
      re-establish susceptibility to AMS. Subjects will ascend in the chamber to 15,000 ft in a
      graded fashion over 12 hours (15,000 ft has been chosen due to extensive Duke experience with
      this altitude). Then, after 48 hours they will simulate a hyperoxic dive by breathing 100% O2
      for 120 minutes at 1.3 ATA. This will simulate, for example, a 2 hour dive at 12,000 ft to 65
      fsw breathing 50% O2. The diver will then return to 15,000 ft and remain at that altitude for
      24 hours to allow for AMS symptoms to recur (if indeed they do). Lake Louise AMS scores will
      be collected every 8 hours. AMS symptoms will be treated with acetaminophen, NSAIDs and
      anti-emetics as needed. Subjects will be assessed clinically every 12 hours for high altitude
      pulmonary edema (HAPE) and high altitude cerebral edema (HACE). Occurrence of either HAPE or
      HACE will require that the subject be returned immediately to 1 ATA and treated
      appropriately.
    

Intervention name(s): Pressure; Breathing Gas
Intervention type(s): Other; Other
Intervention description(s): Subjects undergo altitude exposure, diving exposure, and return to altitude. Altitudes: 8,000, 10,000, 12,000, or 15,000 feet Dives: 60 or 100 feet of sea water; Subjects will breathe air, 35% Oxygen, Balance Nitrogen, or 100% Oxygen during the diving exposure
Intervention model: Sequential Assignment
Intervention model description: This design is a two factor, incomplete block design - three altitudes (8,000, 10,000, and 12,000 feet) and 2 breathing gases (air, 35% O2-N2). The investigators expect to see 0 events in the combinations tested, calculated by the Bayesian Method. If the rate of events is larger than expected, the investigators will use logistic regression to assess if the main effect Air (Air vs. 35% O2-N2) or altitude (8,000, 10,000 or 12,000 feet) or the interaction of these 2 factors imparts relative increased risk. The investigators will set a Type-I error rate of 0.05 (two-tailed).In the presence of 'zero cells', the investigators will either (1) collapse cells into reasonable groups, or (2) add a very small set of constants to the frequency and assess the impact on the effect sizes and tests of significance.1 In the absence of an interaction effect, main effects will be reported.
Masking: None (Open Label)
Primary purpose: Prevention