Sunday, 12 February 2012

Scuba Diving Decompression Sickness: The Bends & Caisson Disease: Predisposing Factors

The risk of developing decompression sickness depends on many factors, most of which are not yet clearly understood. Some of these are discussed below.

Flying After Diving
Because there are fewer molecules of gas at higher altitude, there is less pressure and therefore a lower partial pressure of each gas in the air - which means there is less oxygen available. This hypobaric effect is the opposite of hyperbaric pressure.
This concept is familiar to most people since aircraft flying at high altitudes must have pressurised cabins to support life. All commercial passengers are reminded that in the event of sudden cabin decompression oxygen masks will drop down from above. In normal operation the cabin 'altitude' can be as high as 7,500 - 8,000 feet.
Although reducing the available amount of oxygen to the body can lead to hypoxic conditions, most healthy people will not experience symptoms during a normal commercial flight. The main risk to divers from flying is not the reduction in oxygen partial pressure, but the reduction in atmospheric pressure itself.
The degree of change in a hypobaric atmosphere is less pronounced than in a hyperbaric atmosphere, but pre-existing nitrogen bubbles in the body will grow larger and may exacerbate or even cause DCS. Larger gas spaces, such as a pneumothorax, will also grow.
The length of time required between diving and flying depends on many factors, e.g. depth, duration and number of dives, therefore it is impossible to give a definitive pre-flying period. However, it is recommended that following SCUBA diving an individual should wait at least 12 to 24 hours before flying (Divers Alert Network).
This is one of the main factors in increasing the severity of DCS. In order to off-load the increased burden of inert gas from the body, a good volume of fluid in the circulatory system is needed. If a casualty is dehydrated, gas transport will be inhibited by the reduction in plasma while bubble formation and growth will be encouraged by the relative increase in density of solid matter in the blood.
As bubbles grow and combine to form larger ones, they affect the surrounding tissue making it more permeable to liquid. This results in a loss of fluid from the circulation and ever increasing dehydration. 
Dehydration should be avoided by drinking plenty of water following a dive. Drinking copiously before a dive may allow more inert gas to be absorbed which will increase the risk of DCS if the extra fluid is then lost prior to surfacing.
The effects of alcohol on co-ordination, consciousness and mental reasoning are well documented and have obvious implications in the ability of a diver to carry out the tasks necessary for a safe dive. This reduction in ability also applies to a 'hangover'.
Alcohol will increase the chances of developing nitrogen narcosis due to the depressant effect on the central nervous system. As with any CNS condition, intoxication following a dive will potentially mask some of the symptoms.
Alcohol is also a powerful diuretic that will increase urine output and promote dehydration.

Repetitive diving
Following each dive the body retains some of the inert gas dissolved in the tissues. The correct use of dive tables usually allows enough of the gas to be excreted via the lungs to avoid the formation of bubbles large enough to cause physiological problems.
Repeated dives will increase the risk of DCS if adequate time is not allowed to offload the dissolved gas. For the same reason, the depth and duration of the dive will influence the amount of absorbed gas.
SCUBA diving in warm water will generally result in higher nitrogen loading than in cold water. This is due mostly to the effects of vasodilatation. Becoming cold following a dive may increase urine output and lead to dehydration. In any suspected case of DCS, the casualty must be kept warm and dry.
An increase in adipose tissue leads to an increase in nitrogen loading in the body, since nitrogen is five times more soluble in fat tissue than muscle. Because fat can absorb more gas than muscle, it will take longer for the body to excrete it. It is generally assumed that there is a related increase in risk of decompression sickness.
Obesity also increases the body's oxygen requirements, leading to a likely reduction in physical ability. The long-term implications of obesity are well known. Conditions related to obesity such as diabetes and coronary artery disease are themselves believed to be DCS risk factors.
Fit, healthy individuals are less likely to suffer from most medical conditions than unfit ones but there are possible risks involved with strenuous physical exercise.
It has been suggested that strenuous exercise causes the formation in the body of tiny gas bubbles called 'micronuclei' (Dervay et al, 2002). It takes several hours for these bubbles to disperse.
Age & Gender
There are conflicting studies showing the existence, or lack, of a relationship between a diver's age / gender and the incidence of decompression sickness. A study by the Armstrong Laboratory concluded that there is a three-fold increase in high altitude DCS in men over 42 years old compared to those aged 18-21. The reason for any increased risk in dive related DCS might be due to the distribution and amount of fat in the diver's body. 
Patent Foramen Ovale (PFO)
One of the most significant predisposing risk factors in DCS is a physical abnormality of the heart called Patent Foramen Ovale (PFO) - a small opening between the upper two chambers of the heart (atria).
Every unborn baby has a Foramen Ovale that allows blood already oxygenated by the mother's lungs to by-pass the baby's respiratory system. The opening usually seals up following birth.
If it remains open (patent) then it can cause a 'short circuit' of blood flow allowing bubbles to pass directly from the venous circulation to the brain through the heart, bypassing the filtering effect of the respiratory system.
PFO is present in about 25% of the general population and does not normally cause health problems. However, it affects up to 75% of those with 'unexplained' DCS (Kerut et al, 2001).
Although PFO increases the risk of decompression sickness, individuals with PFO do not necessarily develop symptoms. Also, those who have suffered an 'unexplained' case of DCS do not always have a PFO.
Because bubbles form more easily around physical imperfections, tissue damage (e.g. a ‘bad knee’) can lead to greater risk of bubble formation and DCS. The risk may also be increased due to localised changes in blood perfusion.
Previous DCS
Divers who have had previous occurrences of DCS are far more susceptible to the condition than those who have never had it. This is related to tissue damage caused by previous bubble formation, especially in the Central Nervous System, and pre-existing susceptibilty to the condition.
Some divers, when questioned following an incidence of DCS, reveal that they have possibly suffered ‘sub-clinical’ bends in the past – i.e. the symptoms at the time were not obviously manifested.
Cigarette smoke contains carbon monoxide (CO), which is a poison. This reduces the ability of the red blood cells to carry oxygen. Cigarette smoke also contains nicotine, which acts as a vasoconstrictor and may theoretically increase the risk of DCS due to altered blood perfusion.
Long term problems include chronic obstructive pulmonary disease (COPD) which leads to carbon dioxide (CO2) retention. This has been implicated as a factor in CNS oxygen toxicity. Smokers are also generally less healthy and are more prone to develop circulatory problems. 

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