Elevated altitudes produce unique challenges for the diver. The reduced atmospheric pressure at the surface of any mountain lake affects the divers’ depth gauges, as does fresh water, which is less dense than sea water. When the diver ascends from depth, the rate of change as the ambient pressure drops is far greater than when ascending from a dive at sea level. These factors need to be compensated for, otherwise dives considered relatively safe in the sea, might generate copious bubbles of inert gas within a diver’s bodily tissues, causing Decompression Sickness (DCS), popularly known as “the bends”. The bends may range from a mild skin rash, through increasing severity to paralysis and death. It appears that “altitude bends” are a modern disease, meaning we probably have much more to learn yet before we fully understand the mechanisms involved.
As a diver descends, the pressure surrounding the diver increases. This increase does not affect divers wearing rigid “atmospheric” suits but, for the majority of us who wear flexible exposure protection, we compensate for the increased pressure by increasing the pressure of the gas we breath. Ignoring minor variations due to the weather, at sea-level the ambient air pressure approximates one atmosphere of pressure, at a depth of 33’ in the sea the pressure should be two atmospheres, and another atmosphere of pressure is added for each additional 33’ of depth. Thanks to the development of the SCUBA regulator by Emile Gagnan and Jacques Cousteau, when a diver breathes compressed gas at depth then the gas is delivered at a pressure equivalent to the surrounding pressure. In this way the diver does not have to “suck” his gas from a much lower pressure down to a higher pressure, (and this is why we cannot simply use a long snorkel or hose). The pressure is “regulated” by the SCUBA unit, which senses what the ambient pressure is.
Inhaling gas at increased pressure solves one problem (of delivering gas to the lungs), but as the blood transports this gas around the body the diver’s tissues naturally move towards equilibrium with the new ambient pressure by absorbing the gas. When the diver later ascends to a much lower pressure, such as at the surface, these tissues now have a greater pressure of gas dissolved within them than the surrounding air pressure, and this gas moves towards equilibrium once again, this time by leaving the tissues. It is generally accepted that the rate of this move towards equilibrium, that is, the size of the difference between the tissue pressure and the ambient pressure, is largely responsible for the generation of bubbles within a diver’s tissues. The principle is similar to opening a can of soda: if you open the can suddenly then the soda will fizz, due to the sudden difference between the dissolved pressure and the ambient pressure. If you open the can slowly then the soda will not fizz as much, because the change is more gradual. If you have flown in a commercial jet, which usually has a much lower ambient air pressure in the cabin than on the ground, then did you notice that your soda was unusually fizzy? That would have probably been due to the even greater difference between the dissolved gas pressure in the soda (usually around 1.5 atmospheres) and the ambient pressure in the cabin. This is equivalent to one of the main concerns of a diver at high altitude: the increased difference between the pressure of the gas dissolved within his tissues after a dive and the (much lower) ambient pressure at the surface of the mountain lake. These increased differences first become a cause for concern at altitudes of just 1000’ or higher.
Popularity of diving at altitude
There are many reasons people dive at high altitude: searching for particular objects such as WWII aircraft, training when the sea is inhospitable or too distant to be practical, for scientific research, even just for the plain fun of it. Currently, there at least 30 dive businesses above 4500’ advertising in Johannesburg, 53 advertising in Colorado, the University of California conducts scientific diver training in Lake Tahoe, at an altitude of 6,200ft, and the Bolivian Navy maintains a school of diving on the shores of Tiquina, at 12,500ft.
For some, the challenge of diving at altitude is the purpose. In 1968 a team led by Jacques Cousteau established the record for altitude diving in Lake Titicaca, at an altitude of 12,500ft. In the 1980’s an American team made a series of dives in the South American Andes, at 19, 450ft.
Methods of compensation
Dive tables are a tabular matrix of depths and times that relate to post-dive estimates of the resulting pressures within a range of theoretical tissues. If a diver stays too deep for too long then his tissues will have so much pressure within them that he will not be able to safely ascend to the surface. He will need to “decompress” on the way up or too many bubbles will form. Of course, remembering the can of soda analogy: it is not just the amount of gas in the tissues that needs to be limited, it is the rate of change when the ambient pressure drops that is the second key factor to account for. The faster the rate of change then the lower the limits (shorter times and/or shallower depths). Therefore, each table is designed with a maximum rate of ascent in mind and this rate of ascent is dependent upon altitude. Modern divers rely on personal dive computers to generate real-time limits and these computers utilize a governing algorithm to estimate how many minutes might be allowably left at whatever depth the diver is at. These algorithms, as with the algorithms used to generate dive tables, vary between dive computer manufacturers. Not only do the algorithms differ, dive computers differ in other ways too, such as in the frequency a diver’s time limits are computed. One model may estimate the remaining allowable time once every second whereas another model may estimate the remaining allowable time every ten seconds. Other safety mechanisms differ between models too, such as ascent-rate alarms, which emit a regular beep if the maximum ascent rate, (allowed by the individual dive computer’s algorithm), is exceeded. Many dive computers utilize a variable ascent rate too, allowing faster ascents at deeper depths, requiring the diver to slow his ascent nearer the surface, as the rate of change increases exponentially. The debate between proponents of the constant ascent rate, originally recommended by a scientist called Hill, and the variable ascent rate, originally recommended by Haldane, is known as the “Hill vs. Haldane controversy” (Marroni, 2002).
Of course, remember that the underlying causes of decompression sickness are still unproven. The evidence is convincingly supportive but the scientifically proven link remains elusive. We think we understand the mechanisms of bubble generation and the causes of decompression sickness but many of the assumptions used to predict our limits are based on empirical trial-and-error, where limits have been predicted and then revised downwards after in-water use. Accordingly, there is a variety of algorithms in use today that rely on different physiological and physical assumptions about human tissues, bubbles and gas kinetic theory. For recreational dives in the sea these various algorithms usually result in similar predictions of time limits for each depth, give or take a small proportion of the total allowable time. For example, most dive computers and tables allow a diver to make his first dive of the day to 30m for between 16-25 minutes, (most allow around 20 minutes). Some then assume the inert gas is washed out more quickly during a surface interval between dives, and others impose higher time penalties for dives made when divers already have residual gas left over from previous dives. The upshot of all this is that algorithms vary in many ways, and the ways they compensate for dives at high altitude also vary.
Possibly the most common method of adapting tables for use at high altitude is to convert the maximum depth a diver is planning to reach into an “equivalent sea dive” depth (Paulev & Zubieta-Calleja Jr, 2007), which is a way of reducing the time allowed by using the time limit from a deeper depth. This method is known as the “Haldane method” (Hennessy, 1977), later referred to by the US Navy as the “Cross Correction”, after E.R. Cross promoted the method in 1967 and again in 1970 (Egi & Brubank, 1995). The higher the altitude, the more a diver adds to his planned actual depth when searching for his limit. For example, a diver may be planning to go to 18m depth. To find his limit he will look at the 18m time limit at sea level, the 21m limit at 5000ft and the 27m limit at 10,000ft altitude (Bell & Borgwardt, 1976). There are a number of theoretical ways to adapt sea-level dive tables for use at altitude, and even more ways being utilized by personal dive computers. In one recent study (Buzzacott & Ruehle, 2009) the order of a series of dive computers when ranked according to how conservative they were at sea-level was reversed at 10,000 feet, so that the most conservative at sea-level became the most generous at altitude, and the most generous at sea-level became the most conservative at altitude.
Recreational diving at altitude carries risks that are additional to diving at sea level. Additional training is required by recreational divers. For decompression diving the jury is still out concerning which method is best for adapting existing decompression schedules for use at altitude. Accordingly, any team planning significant exposure to decompression stress at altitude are well advised to consult a diving physiologist with experience in altitude diving. Furthermore, all divers should accept that whatever dive schedule is adopted, the assumptions underpinning that model may be untested or unproven and that many decompression dives at high altitude could even be considered experimental in nature. Some tables, for example, have been tested in water up to a certain altitude and remain unproven beyond that height (Boni, Schibli, Nussberger, & Buhlmann, 1976). To minimize risk of the bends additional prophylactic measures should be taken when possible, such as engaging in a suitable pre-dive exercise regime, the introduction of additional oxygen into the breathing mix, removal of inert gas from the breathing mix, warmth during decompression to promote peripheral circulation, an ascent-rate rate reference such as a weighted line or suspended trapeze, a horizontal pose to have the natural buoyancy of the lungs promote maximal surface area for gas exchange, and immediate post-dive assistance to reduce diver workload.
Diving at altitude can be a lot of fun, a challenge, and there are many worthy reasons to dive in mountain lakes. Take care though – diving at altitude is a lot less forgiving if you get it wrong. A simple matter like a stuck buoyancy jacket inflator button might bring you up quickly and you’d be more likely to get away with it in the sea than you will in the mountains. Add complications like having to cross a mountain pass to get to the hospital and a relatively minor bend could turn really nasty very quickly, and no-one wants to end-up paralyzed from the neck down.