The Bends

(Decompression Sickness)

Whilst the human body is able to respond to immersion in a number of different ways,  it is clear that we are poorly adapted to diving in comparison to many mammals. There is one particular problem which can lead to effects varying from joint pain, to rashes, paralysis and even death: this is the well-documented phenomenon of decompression sickness (DCS), or, commonly, “the bends.”

So (apart from being the title of a hit album for Radiohead) what is the bends and how does it occur? What are its effects? And, of particular interest here, why don’t diving mammals seem to suffer from it?

What is “the bends”?

When DCS was first described, the term “the bends” was used specifically to describe joint pain. Other names were used to describe increasing severities of DCS, such as the “chokes” for problems related to breathing, and the “staggers” when the nervous system had been affected. The symptoms of DCS arise from inert gases which are dissolved under high pressure coming out of a solution into bubbles within the body on depressurisation. However, not all bubbles formed in the body result in DCS.

The amount of gas dissolved in a liquid at a particular pressure is described by Henry’s Law. This states as pressure over a liquid increases, the amount of gas dissolved in that liquid increases (and vice versa). This explains why there is a risk of DCS as divers re-surface, since the as the the diver experiences decreases, the amount of gas dissolved in the blood decreases and bubbles begin to form.

“Out-gassing” is the process by which gases normally come out of solution in the lungs during re-surfacing. However, if re-surfacing takes place too quickly for out-gassing to occur only in the lungs, this process can take place inside the bloodstream or the tissues of the body and venous bubbles form. Nitrogen is the most common gas in air, but any inert gas which is breathed at high pressure can lead to DCS.

DCS was originally described over 100 years ago and, although it is relatively uncommon, due to its potential severity, much research has been conducted into its prevention.

The deeper you dive into the ocean, the greater the surrounding pressure becomes. DCS occurs most often when a diver has breathed gas which is at a higher pressure than the surface pressure. Certain factors increase the probability of DCS developing:

1. The depth and duration of a dive – the deeper or longer the dive, the more gas is absorbed into body tissues under pressure, in higher concentrations than normal

2. The speed of an ascent and the recovery time before the next dive– when the speed of ascent from depth is too fast, and the amount of time between dives is too short, absorbed gases cannot be offloaded safely through the lungs, and these gases therefore come out of solution and form “micro bubbles” in the blood.

3. Individual factors – the older someone is, the worse the condition of the heart, the higher the fat content, greater levels of dehydration, the consumption of alcohol and any previous joint injuries are all factors which increase the chances of DCS developing.

The list of potential problems humans face when it comes to diving is extensive. For more information, further details can be found by following the link entitled “symptoms of decrompression sickness in humans” on the right.

How do diving mammals manage to avoid decompression sickness?

The major difference between diving mammals and scuba-divers, who are the most likely sufferers of decompression sickness (DCS), is that scuba divers breathe pressurised gas whilst diving, whereas diving mammals are do not. As a result, breath-holders do not necessarily have to overcome the same set of problems when surfacing as scuba divers do.

Harbour seals (and other such breath-hold diving mammals) have many adaptations helping them avoid symptoms of decompression sickness.

While marine mammals perform single deep and long dives without apparent decompression sickness symptoms, more remarkable still are the extensive foraging bouts of many diving mammals and birds. Such dive behaviour should result in tissue accumulation of nitrogen, increasing the risk of decompression sickness. However, despite performing such bouts of repeated and long dives interspersed with short intervals, diving mammals have rarely been reported to suffer from decompression sickness during natural dives. It is believed that physiological adaptations help to reduce nitrogen concentrations and risk of DCS.

In animal physiology textbooks, termination of gas exchange is routinely cited as the primary mechanism by marine mammals employ to protect themselves against DCS.

Alveolar Collapse and Pulmonary Shunt

Back in 1940, when our man Per Scholander published his original work on diving mammals, he noted that they had a compressible ribcage and stiff upper airways. He suggested that increasing pressure at depth would compress the lungs and force air into the upper airways, which would reduce gaseous exchange during a dive leading to a pulmonary shunt (when lungs are adequately supplied with blood, but the blood is not supplied with air) and therefore prevent nitrogen uptake during breath-hold dives.

Compression of the respiratory system will on the one hand tend to increase the alveolar-capillary partial pressure gradient, and therefore increase the rate of gaseous exhange. On the other hand, further compression will reduce the gas exchange surface area and increase the diffusion distance. As a result, compression will initially promote diffusion and inert gas uptake, but as the depth of the dive increases the developing pulmonary shunt will reduce uptake.

Other defence strategies

Few alternative explanations have been put forward to explain how marine mammals avoid elevated inert gas uptake during breath-hold diving. Kooyman (1973) summarized most of these in a review on the respiratory adaptations in marine mammals. Possible physiological adaptations include:

(1) Increased tissue and blood nitrogen solubility (i.e. mammals living with elevated blood and tissue levels)

(2) Utilisation of a nitrogen-absorbing tissue

(3) Changes in cardiac output and varying blood flow distribution as part of the dive-response

(4) Behavioural adaptations

The nitrogen absorbing tissue could be fat tissue, which has a nitrogen-absorption rate which is 5 times higher than lean tissue. Most diving mammals have large amounts of fat below the skin which serves to reduce heat loss and act as an energy reservoir during extended periods without food. The five-fold higher nitrogen solubility, combined with the reduced cardiac output and re-distribution of blood flow that represents the dive response, results in researchers suggesting adipose tissue could play a role in reducing risk of DCS for diving mammals and birds.

The dive response being part of a mammal’s defence strategy against decompression sickness makes intuitive sense. It has been shown that bradycardia in the descent and bottom phase of a dive, coupled with a reduced ascent rate and pre-surface tachycardia (an increase in heart rate), has reduced the concentration of nitrogen in venous blood by up to 45% (see study by Falmahn et al., 2006). The circulatory system is responsible for both removing carbion dioxide and supplying oxygen. A balance needs to be found: blood flow distribution among the tissues is a trade-off between the need to exchange metabolic gases and the need to reduce the risks of decompression sickness. Thus, the concentration of nitrogen in the blood at the end of a dive or an extended bout is a function of the need to supply oxygen to, and remove carbon dioxide from, central organs while reducing uptake of nitrogen at the same time. The extent to which changes in blood flow are used as a means to reduce high levels of nitrogen from the blood without ischemic injury is currently unknown.

It has been shown that some behavioural adaptations may reduce inert gas compression as well. For example, as previously discussed, Falmahn showed that in some species a reduction in ascent rate prior to surfacing can reduce the burden of inert gas by up to 45%. It has long been suggested that seals which exhale prior to submersion do so in order to reduce the depth at which lungs collapse and gaseous exchange ceases. Fur seals have been recorded exhaling during their ascent from depth to the surface, possibly as a preventative measure against bubble formation, prolonging pulmonary shunt and therefore preventing gaseous exchange and shallow-water blackout.

… All of which may help prevent excessive inert gas uptake in addition to pulmonary shunt and alveolar collapse.

Are there any occurrences of DCS symptoms in marine mammals?

There have been several recent occurrences of diving mammals seemingly suffering from some form of decompression sickness-like symptoms. Mass stranding events, such as the one pictured below, have led to concern, on occasion, that the diving behaviour of whales is being disrupted. It could be that diving behaviour of mammals is important in reducing risk of decompression sickness.

The 194 pilot whales and half a dozen bottlenose dolphins stranded on Naracoopa Beach on Tasmania state's King Island

Historically, it was widely believed that diving mammals were immune to DCS. However, it is little-known that, as far back as 1940, Scholander reported two possible cases of decompression sickness in a fin whale and hooded seal respectively during a single dive. So, even back then with less advanced technology, Scholander suspected cases of DCS amongst certain diving mammals.

Mass stranding events of beaked whales and dolphins investigated by Jepsom and his colleagues in 2003 correlated with US naval exercises using high-frequency sonar. It was suggested that the sonar activity may have led to disturbances in the natural dive behaviour of these species, resulting in dive profiles that caused bubble formation.

More recently, in 2004, detailed inspection of sperm whale carcases revealed signs of osteonecrosis. This is a condition where the mammal’s bones have become damaged due to a poor blood supply, which, it has been postulated, could have been a result of some form of DCS.

Clearly, much care needs to be taken with use of technology and patterns of diving behaviour, and in particular and changes in this behaviour, should be monitored where possible in order to avoid any potential mass extinctions of diving species which may otherwise be avoidable.

Do human breath-hold divers suffer from DCS?

Human breath-hold divers do not breathe pressurized gas and the only inert gas added is the nitrogen that remains in the lungs from the inhalation before immersion. The issue of whether decompression sickness (DCS) occurs in human breath-hold divers has been debated since the 1960s, with a growing number of cases reported with symptoms resembling those of scuba divers. In 1965, a scientist named Lanphier stated that “decompression sickness is virtually impossible for the skin diver because he cannot submerge deep enough or remain long enough to take up a troublesome amount of nitrogen.’’ Although this statement has long been accepted as true for single or occasional breath-hold dives, it does not account for dives that are made repeatedly, to great depth, and at very short intervals.

Several incidents of decompression sickness have been reported after shallow repeated dives, notably in people that regularly dive for fish or to collect pearls. For example, reports of accidents in pearl divers from the Tuamotu Islands presented the classic signs of decompression sickness.

The reports include the ‘‘Taravana’’ syndrome, first described in pearl divers by Cross in 1962 and Bagnis in 1968 as a diving syndrome seen in working Tuamotu Island natives diving in the Takatopo Lagoon. ‘‘Taravana’’ has been translated as ‘‘to fall crazily’’ and is assumed to correspond to decompression sickness in these divers.

Collaborated findings seem to suggest, the greater the depth, number, and frequency of repeated dives, and the shorter the surface intervals, the greater the risk of decompression sickness.

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24 04 2016
Deranged Physiology of Aviation – EmergencyPedia

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