A “NORMAL” DIVE THAT RESULTED IN DECOMPRESSION SICKNESS
These days, dive computers are widely used by divers to monitor their profiles, but is complete reliance on such technology advisable?
We were on the third day of a trip that involved two dives a day at an island resort. This was the first dive in the morning. It was a normal dive: We spent less than a minute at 30 metres, and the swim-through exit was at 29 metres. I spent about 10 minutes at 24 metres, then another 15 minutes around 18 metres. I made a slow ascent with a three-minute safety stop at five metres. After surfacing, I experienced: • intense bilateral shoulder pain; • arm to fingertip pain; and • tingling and numbness in both
my legs and toes.
Fortunately, there was oxygen on the dive boat, so oxygen first aid commenced fast and I was able to keep breathing oxygen on the drive to the hospital. While at the hospital, my symptoms improved as I continued breathing oxygen.
During the hospital visit that day,
I had some improvement on oxygen but had recurrent neurologic symptoms that required six hours per day of hyperbaric treatment for the following two days. I returned home after 72 hours and had two more 2-hour treatments in the hyperbaric chamber in my home state to see if the residual tingling/numbness in my toes and fingers would abate. At the end of treatment, 99 percent of my symptoms had resolved.
Two decades ago, the maximum allowable total dive time at a 30-metre depth would have been around 20 minutes if the dive was planned using tables, as it commonly was. This dive profile may even have been permitted using multilevel planning techniques, but today, dive computers do the work for us and such a profile is considered “normal”. The risk of decompression sickness is thought to be low among recreational divers because the majority of dives come nowhere near the no-decompression limits. After two previous days of repetitive diving, this diver suffered an injury, which was diagnosed as a serious case of decompression sickness, even though he didn’t violate the dive computer’s limits.
John Lippmann, DAN Asia-Pacific’s Founder, has said*: With the advent of dive computers and decompression software, diving has become much more flexible. Without the inherent restrictions of the dive tables, dives tend to be far longer and more frequent, and surface intervals far shorter. The discovery of more wrecks, often in deeper water, as well as other deeper dive sites has also encouraged many of us to dive deeper.
Most divers these days use dive computers and appear to be reasonably confident they will avoid decompression illness (DCI) as long as they dive within the limits of these devices. The reality is that the vast majority of the divers we deal with that are diagnosed with DCI have been using dive computers and have been diving within the limits advised by their computers.
Dive computer algorithms are based on decompression models, which are generalisations of what might be occurring within a diver’s body. These models have inherent inaccuracies and cannot cater for the broad individual variation between divers and the type of dives undertaken. The more these models are pushed to their extremes, the greater the potential for inaccuracies.
Deeper dives, longer dives, short surface intervals and repetitive diving, especially multiple repetitive diving, can all lead to higher inert gas loads, greater bubble formation and consequently less accurate decompression calculations. When these factors are combined, as they often are in current day diving scenarios, we can be inviting problems.
In another incident, a diver suffered severe cerebral DCI while diving at
Truk Lagoon. Looking through his dive profiles, I was somewhat unsurprised with the outcome. From a dive safety perspective, several concerns arose: • The diver conducted three dives each day for four consecutive days.
The deepest dive for the whole trip was completed on the last day at 54 metres.
The average depth of all the dives undertaken was 30 metres. The length of the dives ranged from 45 to 69 minutes, with an overall average of 52 minutes of total underwater time.
The length of the daytime surface intervals ranged from 1 hour 20 minutes to 2 hours 35 minutes, with an average of 1 hour 53 minutes.
These dives are quite typical of what many divers complete at Truk Lagoon (and some other dive destinations). Divers need to be aware that deeper diving is associated with a higher risk of DCI and the resulting DCI is more likely to affect the nervous system. Very long dives also carry a higher risk of DCI.
In the reported case, it was prudent of the diver to make a safety stop, but of particular note is that there was oxygen on-board the boat, and the diver continued breathing oxygen on the way to the hospital. After multiple hyperbaric treatments, the diver was 99 percent recovered, which is good news, though it should be pointed out that bilateral symptoms affecting all four limbs are not typical of decompression sickness. Regardless, this case serves as a reminder that our dive computer may well keep recalculating our allowable limits, but that does not mean we should dive to those limits. If 100,000 divers dive to the limits then, even though they did not “break the rules”, by probability alone some will get the bends.
OPPOSITE PAGE Dive computer algorithms are based on generalised decompression models which do not cater for variations between individuals RIGHT Today, dive tables have largely been replaced by dive computers, which allow for more flexible dive profiles