Shallow water bends: fact or fiction?

by Steve Goble

Steve has worked in the Hyperbaric Unit at the Royal Adelaide Hospital since 1985. He was a Royal Navy clearance diver for six years and spent five years in the offshore diving industry as a Life Support Technician.

‘I can’t be bent, Doc, I wasn’t diving deep enough.’ Hyperbaric units around Australia have lost count of the number of times that phrase has been used by customers over the years. Why do we keep hearing this? Because there is still a belief among the diving community that decompression illness (DCI) from shallow exposures is not possible.

Consequently, divers are surprised to discover that the seemingly innocuous dive they had on Saturday has put them in a recompression chamber on Wednesday. During our training (and for some hyperbaric staff that was long ago) we also were taught that ‘you can’t get bent shallower than 30 feet.’ But over the years it became obvious to some of us that there was a steady number of patients who had done exactly that. A call from a colleague who noticed the same trend, but was having trouble convincing some diving groups, prompted me to try and find out how frequently shallow water DCI occurs. Actually the conversation went more like ‘Great idea Steve, I’m really busy, why don’t you do it?’ So, having been volunteered, I set to the task.

First, what depth is shallow?

10msw was chosen as the cut-off point. All Australian hospital-based hyperbaric units were contacted and asked to provide details of patients treated for DCI whose only exposures, immediately prior to presentation, had been to 10msw or less. Information requested was age, gender, dive profiles, presenting symptoms and initial treatment. Information was received from Townsville General Hospital, Prince of Wales Hospital in Sydney, the Alfred hospital in Melbourne and the Royal Adelaide Hospital.

Any profile which suggested the possibility of a CAGE (cerebral arterial gas embolism) was discarded; that is, divers who remarked on rushed, free, or very rapid ascents, and those with very short bottom times in the order of five minutes.

A total of 50 divers were found whose profiles fitted the criteria. Of these 50 divers, 15 had only had one exposure to depth with only one ascent.

It is possible some of those 15 single exposure divers may have had some underlying predisposing factor, such as a PFO (patent foramen ovale). This is a heart condition in which a hole in the dividing wall between two of the heart’s chambers fails to close after birth. This may sometimes allow some blood to bypass the lungs. If a dive has produced bubbles but no symptoms, normally these bubbles are filtered by the lungs. In a person with a PFO, some of these bubbles can directly enter the arterial circulation causing symptoms.

How great is the risk?

In Australia there were 350 cases of DCI treated during the 1996-97 financial year. At least 35 shallow water DCIs occurred during that time. As the other three hyperbaric units also treat shallow water DCI, it means that at least ten percent of divers treated for DCI in Australia were diving to 10msw or shallower. This figure makes it fair to conclude that DCI from shallow exposures is a very real problem, which is more common than generally realised.

Shallowest dives.

The shallowest depth for a single dive was three metres; unfortunately the bottom time is unknown. One diver dived to 4.6 metres for 64 minutes, followed by a dive to three metres for 31 minutes. He presented with limb pain 48 hours later. Another diver’s three metre exposure was the second in a series of four dives. This diver dived to five metres for 30 minutes with four ascents, three metres for 30 minutes, eight metres for 20 minutes and 10 metres for 28 minutes, the last three dives having one ascent each. Unfortunately the surface intervals are unknown. However, having progressively deeper exposures for the last three dives is a known risk factor. The shallowest exposure implicated in a multiple series was 1.7 metres. This diver dived to 7.6 metres for 50 minutes with four ascents, seven metres for 35 minutes with four ascents and 1.7 metres for 210 minutes with more than ten ascents.

Other profiles.

Among the more extreme profiles were two scientific divers who did 19 dives to 10msw for a total bottom time of four hours. There were also two divers who had greater than 20 ascents in a four metre dive. One diver had four nine-metre exposures over three days, and noticed symptoms of a fuzzy head and generalised tingling six minutes after his last dive. He required one USN6 treatment (about which more follows below) and seven 90 minute followup treatments before he could be discharged from care. However, it could be unfair to include him, as two years before this incident he had a serious CAGE and was warned not to dive again.

Presenting symptoms.

Presenting signs and symptoms were difficult to determine accurately due to differing methods of data collection. However, limb pain showed as the most common presenting symptom, reported by 36 divers. Paraesthesia (pins and needles) was reported by at least ten divers and headache, fatigue, lightheadedness and weakness were the next most common problems. There were six cases reported as neurological, and five reported as constitutional (meaning general unwellness). In general the signs and symptoms were milder than those for divers who had been to deeper depths.

Treatment profiles.

Most divers (approximately 95%) were treated initially using US Navy treatment table 6. This is a four hour and 45 minute table, a fairly standard choice for a diver’s first treatment. In short, the chamber is compressed to 18msw and the patient is placed on oxygen for 20 minutes followed by a five minute air break. This cycle is repeated two more times, then the chamber is decompressed to 9msw over 30 minutes with the patient on oxygen. Once at 9msw the patient gets a 15 minute air break. This is followed by 60 minutes on oxygen, 15 minutes off, 60 minutes on, then decompression to the surface over 30 minutes with the patient and attendant breathing oxygen.

Two divers’ symptoms were considered mild enough for them to be treated with US Navy treatment table 5, a shorter two hour and 15 minute treatment table; they did not require followup treatment. Most patients require a number of followup treatments. These are usually carried out at a treatment depth of 18msw for one hour. A diver generally needs three or four treatments.

Risk factors. Looking through the few profiles noted above, some obvious risk factors can be noted. Analysis of all the dive profiles in the series revealed a worrying trend. It was noted in cases that were not single exposures that a number of risk factors were common. 29 divers had multiple exposures (more than two dives in one day, one ascent per dive). Six divers did only one dive, but had multiple ascents. There were seven divers who did more than one dive and more than one ascent per dive, and four divers who had only dived in a swimming pool but had done numerous ascents. Apart from multiple dives and multiple ascents, it was noted that some divers also undertook their multiple exposures by going progressively deeper. I may be old-fashioned, but when I learned to dive that was a cardinal sin. It may be that more care would have been used for deeper dives.

Who is most at risk? Of the 50 divers, ten were instructing at the time and six were students under instruction. Due to the number of dives and ascents undertaken in some forms of instructional diving, instructors can be more at risk than the average diver. There was not enough data to determine if there was an age or gender bias.

What can we do about it?

While we can say that ten percent of divers with DCI were only shallow diving, we do not know how many shallow dives are performed in the diver population as a whole. It may be that the number of shallow dives far outnumbers the deeper dives. This would make the incidence very small.

It was not possible to discover whether any of the divers had further investigation to rule out PFO or any other predisposing medical condition. What can be gathered from the data is that exposure between five and ten metres is not as trouble-free as once thought. Of the 50 cases studied, 43 were diving between five metres and ten metres. Most of these people’s profiles showed risk factors – multiple ascents, multiple dives, extended bottom time, and progressively deeper depths – which they may have deliberately avoided for deeper exposures. Basically, it looks as though divers do not take shallow diving as seriously as they take deeper diving.

Until predisposing medical factors can be ruled out, these dives need to be treated with as much care and respect as any other dive. Recognised risk factors, especially multiple ascents, still need to be avoided. The attitude of ‘I can’t be bent, I was too shallow’ needs to be laid to rest.

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