Altitude and Decompression Sickness



Decompression sickness (DCS) may develop after a dive when very small bubbles grow as nitrogen diffuses into them from surrounding supersaturated tissue. If the bubbles remain small enough, the diver never knows they are there, and another successful dive goes into the logbook. DCS occurs if the bubbles become too large, are too numerous or are located at sensitive sites. The chances of developing DCS are normally low, but they increase with extensive bubble growth. DAN® has observed DCS incidence rates ranging from two DCS cases in 50,000 dives (one case per 25,000 dives) for Caribbean liveaboard diving to 28 DCS cases in 16,887 dives (one case per 600 dives) for cold-water wreck dives.
Flying After Diving
What would happen if the liveaboard or wreck divers flew home at an 8,000-foot cabin altitude immediately after diving? Per Boyle's Law, any bubbles would immediately expand by one-third, and additional bubble growth would occur as the dissolved nitrogen diffused into the bubbles from the surrounding tissue. The risk of developing DCS in this situation is why divers are advised to wait at sea level after diving before they fly. For recreational divers, the recommended minimum preflight surface intervals are 12 hours following a single no-decompression dive and 18 hours after multiple dives or multiple days of diving. More flexible guidelines are available in the U.S. Navy Diving Manual. Many dive computers predict and display when flying is deemed safe, although the methods used vary among computers. NASA has flying-after-diving procedures that use oxygen breathing during the preflight surface interval so astronauts can fly soon after underwater extravehicular activity (EVA) training in the Neutral Buoyancy Lab at the Johnson Space Center (Pollock and Fitzpatrick 2004).
Diving at Altitude
Diving at altitude is more complicated. Because of the lower atmospheric pressure, the relative difference between the atmospheric pressure and the pressure underwater is increased. Thus, the impact of diving to any given depth is greater than it would be at sea level. For this reason altitude dives have shorter no-decompression times.

Because of the lower atmospheric pressure, the pressure difference
between the surface and depth is greater for altitude dives than dives to
the same depth from sea level.
While it is possible to compute separate tables for each altitude, testing such tables would be time consuming and expensive. A more common practice has been to apply empirical corrections to existing sea-level dive tables, such as the U.S. Navy tables. The most frequently used correction methods were described by E.R. Cross (1967, 1970).

To employ the "Cross corrections," multiply the planned depth of the altitude dive by the pressure at sea level divided by the pressure at altitude. This "equivalent depth" will be deeper than the actual depth to compensate for the higher altitude. It can then be used in place of the actual depth with a normal dive table. For example, in the U.S. Navy tables, if the actual depth is 60 feet and the altitude is 8,000 feet, the equivalent depth is 81.1 feet. The dive can then be planned with normal dive tables using an 82-foot depth. If there are to be decompression stops, the stop depths must be corrected as well.

To simplify this process, the U.S. Navy Diving Manual (2008) provides a correction table in which the equivalent depth in feet can be looked up for table depths in 10-foot depth increments and 1,000-foot altitude increments. Below altitudes of 300 feet, no adjustments to the standard sea-level tables are needed. The highest altitude in the U.S. Navy tables is 10,000 feet.

Many dive computers have absolute pressure transducers and can be rezeroed for the surface pressure at altitude. Since depth gauges are really pressure gauges, there is no need to convert from feet of freshwater (ffw) in an altitude lake when using tables in feet of seawater (fsw), but bear in mind that the actual depth below the surface in ffw will be greater than the depth in fsw displayed on the gauge. Most pneumatic depth gauges cannot be reset to zero at altitude and are not recommended for altitude diving. If used, a correction of 1 foot per 1,000 feet of altitude should be added to pneumatic depth gauge readings.
Wait to Equilibrate
Another factor that must be considered is the equilibration of a diver's tissues with the atmospheric nitrogen pressure at altitude. For example, if a diver with a tissue nitrogen tension of 0.79 atmosphere (atm) at sea level ascends immediately to an altitude of 8,000 feet, the inspired nitrogen partial pressure would be 0.58 atm (0.79 x 0.74). If the diver waits 12 hours before diving, the excess tissue nitrogen will have washed out, and the diver's tissues will have equilibrated with the atmospheric nitrogen at 0.58 atm. However, should a dive be made before equilibration is complete, the excess tissue nitrogen behaves like residual nitrogen after a repetitive dive and is assigned a repetitive group (RG). The RG is converted to residual nitrogen time (RNT) for the upcoming dive according to a specified table. By the U.S. Navy tables, for example, a diver who ascends directly to 8,000 feet is in RG "G" upon arrival. If an immediate dive to 60 feet was planned, 40 minutes of RNT must be added to the actual dive time. If the diver waited for 6-8 hours before diving, the RNT would be only 8 minutes.
Extreme Altitude
Most altitude diving is conducted below 8,000 feet, altitudes generally well tolerated by healthy individuals. For higher altitudes, however, acclimatization to the reduced oxygen partial pressure may be an issue. Acute mountain sickness (AMS) can develop with sudden exposure to high altitude. Some of the highest altitude dives reported were made at 12,500 feet in Lake Titicaca on the border of Peru and Bolivia, initially in 1968 by Jacques Cousteau and later by others.
Safety Planning
Diving at altitude requires careful implementation of altitude-correction factors, an adequate supply of oxygen for first aid and clear plans for emergency evacuation to a medical facility with a hyperbaric chamber.
Supersaturation, Bubble Growth and Decompression Sickness
All body tissues contain dissolved oxygen, carbon dioxide and nitrogen. In a person who is equilibrated with air at sea level, the dissolved nitrogen partial pressure is about 79 percent of the one atmosphere of pressure, or 0.79 atmosphere (atm). If we fly in a commercial airliner at a cabin altitude of 8,000 feet (the maximum cruising cabin altitude according to Federal Aviation Administration regulations), the barometric pressure decreases from 1 atm at sea level to 0.74 atm.

Under these conditions, a state of supersaturation is said to exist in our tissues where the nitrogen tissue tension (0.79 atm) exceeds the barometric pressure (0.74 atm) by 0.05 atm (0.79–0.74). Most of the nitrogen from supersaturated tissues is carriedto the lungs in the circulation (that's good), but some may diffuse into small bubbles known as gas nuclei (that's potentially bad). These bubbles don't grow large enough to cause DCS below a threshold altitude of about 16,000-18,000 feet.

If a person was suddenly taken from sea level to a pressure equivalent of 30,000 feet altitude (0.29 atm, the pressure in an astronaut's space suit), the supersaturation would be much larger, 0.50 atm (0.79–0.29), and massive bubble growth would occur. Under these conditions DCS would be almost inevitable. Astronauts avoid this problem by breathing 100 percent oxygen before space walks (extravehicular activity, or EVA) to wash out much of their tissue nitrogen and reduce the supersaturation.
References
  1. Cross ER. "Tecnifacts, from a master diver." Skin Diver 1967; 16(12): 60.

  2. Cross ER. "Tecnifacts — High altitude decompression." Skin Diver 1970; 19(11): 17.

  3. Pollock NW, Fitzpatrick DT. "NASA flying after diving procedures." In: Sheffield P, Vann RD (eds). DAN Flying After Recreational Diving Workshop Proceedings. Durham, N.C.: Divers Alert Network, 2004: 59-64. (Available for download at www.DAN.org.)

  4. U.S. Navy Diving Manual, Volume 2, Revision 6. NAVSEA 0910-LP-106-0957. Naval Sea Systems Command: Washington, D.C., 2008: Volume 9. (Available for download at www.supsalv.org/00c3_publications.asp.)

© Alert Diver — Summer 2011