OBJECTIVES:
1.  State 5 guidelines for reducing the risk of DCS at altitude.
2.  Explain the number one (critical) factor to consider when diving at altitude.
3.  State 3 normal diving factors that change with altitude diving and explain them.
4.  Calculate the end of dive letter group for a repetitive dive at altitude.

 Using the picture below, brainstorm at least 10 things that you see in the picture that might need to be considered when diving at altitude.
 

The main reason for having dive tables is to keep our bodies from becoming saturated with nitrogen.  Saturation can occur if a diver dives TOO DEEP, stays TOO LONG, or ascends TOO QUICKLY (or a combination of the three).  To reduce the risk of DCS general diving practices should include:  limiting depth, limiting bottom time, increasing the surface interval time, ascending at a rate that does not exceed 30 feet per minute, and completing a safety stop at 15 feet for 3 minutes. 

DECOMPRESSION:

 At present your body is in equilibrium with the air around you. That includes those gases that make up the surrounding air.  Nitrogen is an inert gas—our bodies do not metabolize it at the surface, we simply ingas and outgas nitrogen in the same partial pressures as the surrounding air.  In other words because, at sea level, the partial pressure of nitrogen is .79 atmosphere (or 11.6 psi), our bodies also contain a partial pressure of nitrogen that is .79 (or 11.6 psi).  When we breathe air that has a higher partial pressure of nitrogen, such as the case as when we scuba dive, then our bodies, once again, attempt to become in equilibrium with the new partial pressure.  For instance at 33 feet the partial pressure of nitrogen doubles to 1.58 atmospheres (or 23.2 psi).  This is twice the partial pressure of nitrogen in your body at the beginning of the dive. Your body will retain nitrogen at this point in an attempt to reach 1.58 atm (23.2 psi).  This is known as ingassing. 

1.  What is the partial pressure of oxygen at the surface? (use 21% of the atmosphere)
2.  What is the partial pressure of oxygen at 33 feet of sea water?

 Ascending from a dive creates the opposite response.  For example, if you dived to 33 feet until your body once again was in equilibrium with the air you were breathing, then your body would contain a partial pressure of nitrogen of 1.58 atmospheres (or 23.2 psi).  Upon surfacing, you would be breathing air that contained a partial pressure of nitrogen that was .79 atmosphere (or 11.6 psi)—half as much.  You would begin outgassing nitrogen until, once again, you became in equilibrium with the partial pressure of nitrogen.

 The problem begins with the ingassing of the nitrogen, but is only manifested as a hazard when the outgassing begins.  The body is made up of numerous types of tissues:  Brain, muscle, organ, heart, fat, etc.  Not all tissues absorb and release nitrogen at the same rate.  Instead of determining what rate each tissue uptakes and releases nitrogen, Haldane created a model of tissue compartments to represent the rates that certain tissues deal with dissolved gases.  This model encompasses most of the various tissues and has been revised several times since the initial model was introduced.  According to the model, tissue times are measured in half-lifes.  That is a half-life is the time that it takes a certain tissue to become half saturated, or for half of the gas to be eliminated.  For example, a tissue with a half-life of 20 minutes will outgas half of its gas in 20 minutes.  In the next 20 minutes half of the remaining gas will be eliminated, and half of the remaining gas will be eliminated in the next 20 minutes, and so on, until equilibrium is once again reached.

3.  Using a 40 minute tissue as a model, how much gas remains in a saturated tissue after 120 minutes?

 These various tissues are responsible for the determination of the maximum dive time limits that are set on most dive tables.  (Although tables vary, Haldane’s model is responsible for most of the dive tables that are used today, not all, but most.)  Most computers also utilize the tissue compartment method of determining dive limits.  The only difference here is that dive computers take into account the outgassing that goes on when a dive ascends to a shallower depth.

 Decompression sickness occurs when the tissues (or a specific tissue) contains so much nitrogen that it cannot escape the tissue fast enough on ascent.  The nitrogen is trapped in the tissue, yet due to the decrease in pressure, the nitrogen is forced out of solution and into the formation of bubbles causing decompression sickness. 

4.  Explain how a soda can or bottle can be used to explain the concept of decompression sickness.  (The soda is your tissue or blood and the carbon dioxide bubbles are the nitrogen bubbles in your system.)

According to Haldane’s model the critical ratio for bubble formation—or at least for the perception of decompression sickness—is 2:1.  What this means is that the tissue can withstand a two fold increase in gas pressure as a result in ascending to a shallower depth.  For example, your tissues can safely saturate at 33 feet, giving your tissues twice as much nitrogen as normal.  You can then safely return to the surface and allow that tissue to outgas—at the surface you have a two fold increase in tissue pressure—hence the no decompression limit for single dives at 33 feet (according the the US Navy).  An example that does not fit the 2:1 ratio is a trip to 66 feet.  In this example the tissue could saturate to three times the surface pressure.  Upon return to the surface, the tissue would in fact have three times more gas pressure than normal.  Decompression sickness would be the likely result.  It is important to remember that all of this information and these examples have been calculated at sea level, 14.7 psi, one atmosphere of pressure.

ALTITUDE:

 Altitude diving presents one major problem with the decompression theory discussed above:  The atmospheric pressure at altitude is less than the atmospheric pressure at sea level (where dive tables are designed to be used).  As an example, the atmospheric pressure at 18,000 feet is half the atmospheric pressure of that at sea level (.5 atm, or 7.35 psi, as opposed to 1 atm, or 14.7 psi).  The bottom line is that a doubling of this pressure does not occur at 33 feet , as it does at sea level, it occurs at 11 feet.  Diving at 33 feet (at 18,000 feet) actually provides 22.05 psi (a three fold increase in pressure)—not 29.4 psi.  Now, most of us will never dive at 18,000 feet, but for an extreme example, it does show how diving at altitude changes the way that we should be thinking about depth/time relationships.

 What does this reduced atmospheric pressure mean to you and to your diving techniques?  Well, it means a few modifications and predive calculations, to say the least.  To begin with there is a difference now between actual depth and what is known as “theoretical depthâ€� (see the bottom of the page for a complete table).  Actual depth is the depth that you are at in the water column, while “theoretical depthâ€� is the depth that corresponds to standard dive tables and no decompression limits at sea level.  In the above example at 18,000 feet, diving down to 11 feet is the actual depth, but the “theoretical depthâ€� is 33 feet—a doubling of the atmosphere pressure according to the dive tables.  Capillary gauges show “theoretical depthâ€� when used shallower than 60 feet, and may be interpreted as no decompression depth on the first dive, but other gauges may need to be adjusted to account for altitude.

5.  Using an altitude conversion table and 3000', at what depth (actual) does the atmospheric pressure double? (Hint:  find 3000', find the theoretical depth that corresponds to a doubling of atmospheric pressure at sea level, follow it across to actual depth.)
6.  Using an altitude conversion table and 8000', at what depth (actual) does the atmospheric pressure triple? (Hint:  use the procedure above at 8000' with a tripling of pressure.)

 However, should a capillary gauge not be used or the gauge being used not calibrated for the correct altitude being dived, then a correction factor must be used to determine where to enter the dive tables.  This modification is a calculation of “theoretical depth.â€�  It is the ratio between the ambient pressure at sea level and the ambient pressure at a specific altitude (each altitude, therefore, has its own correction factor).  To figure the correction factor simply divide the ambient pressure at sea level (in feet of saltwater or psi) by the ambient pressure at altitude (in the same unit).  This number can then be multiplied by the actual depth of the dive to determine the theoretical depth at that altitude.

7.  What is the theoretical depth for a dive to 50 feet at an elevation of 6000'?
8.  What is the actual depth of a dive at 5000' using a theoretical depth of 72 feet?

 In addition to calculating between actual and “theoretical depths,â€� old tables recommend that ascent rates be modified and should be slower at altitude, too.  In fact, a correction factor of 3.5% can be incorporated for each 1000 feet of elevation above sea level.  This relates to the old ascent rate of 60 feet per minute, but is not necessary when using the Navy's new ascent rate of 30 feet per minute.  (The slower ascent rate encompasses all of the calculated ascent rates up to 10,000 feet of elevation.)  This slower ascent rate allows additional time to outgas, and is designed to take into account the reduced ambient pressure at the surface.

Before diving the altitude of the dive must be taken into consideration in a different way.  Driving to altitude is like ascending from a dive (there is a reduction in atmospheric pressure/water pressure), therefore, one of two things must happen prior to diving.  Divers should acclimate to the altitude prior to diving or they begin their dive using a letter group.  The letter group can be determined by using one letter per 1000 feet of elevation.  For example, a dive at 4000 feet would have the diver begin with a letter group of "D."

9.  What is the beginning letter group for a dive at 8000 feet?

 Leaving the dive site is another factor to consider.  Should your journey home take you over mountains or just to higher elevations, then additional factors should be considered.  Increasing elevation after diving is a kin to flying after diving, and extreme caution should be taken.  Increasing elevation is a further reduction in atmospheric pressure.  This reduction, just like ascending during a dive and causes a change in equilibrium of nitrogen in your tissues.  If the elevation is high enough, Haldanes critical ratio could be surpassed and decompression sickness could occur.  If you are unsure of the procedure to follow, then follow the flying after diving rules:  wait 12 hours after a single, non repetitive dive; wait 24 hours after a repetitive dive; and wait 48 hours after a decompression dive.  Obviously if the elevation change is lower upon your exit from altitude diving, then no problem should exist.

 Two additional factors should be considered when altitude diving.  Altitude diving creates a change from salt water to fresh water and an inherent change in buoyancy conditions.  According to Archimene’s principle, because fresh water is less dense, it weighs less, and because it weighs less, objects tend to sink (are less buoyant) more in fresh water.  What this equates to is that you will require less weight on your weight belt in fresh water—barring the changing of any equipment.  As a general rule, fresh water will decrease buoyancy by 2.5% of the total diver's weight.  Be sure to recheck your buoyancy prior to diving in fresh water.

10.  If the total weight of a diver is 230 pounds, how much weight must be removed?

 A reverse situation occurs with your wet suit, however.  Because of the decrease in atmospheric pressure, your wet suit will provide you more buoyancy—the air pockets will expand at altitude.  The approximate increase in buoyancy is .2% per 1000 feet in elevation.  This effect, however, is not enough to offset the effects of needing less weight in fresh water.  Again, it is recommended that you recheck your buoyancy before diving in fresh water at altitude.

11.  If the same diver is diving at 6000 feet, how much weight must be added to compensate for altitude?

Using both the NAUI dive tables and an altitude conversion table complete the following dive scenario:

12.  If the altitude of the dive is 5000 feet, what is the beginning letter group?

13.  Determine the end of dive letter group after completing one dive to 50 feet for 20 minutes, then resting for 2 hours before diving 40 feet for 35 minutes.

14.  How much weight should "YOU" dive with?  (Include how much you use in the ocean.)

15.  How long should you wait to travel through a pass that reaches 6000 feet?  What alternative to waiting to return home do you have?