December 8, 2016

Trapped gas (part II): Decompression sickness


Trapped gas (part II): Decompression sickness


Last month I talked about trapped gas in regards to sinus, teeth, ears, and GI discomfort and how to deal with those problems. This month I’ll finish the discussion by touching on the problem of decompression sickness (DCS).

Historically, aviators have referred to DCS as “aviator’s bends” or simply “the bends.” While these common terms are not technically correct, they at least do some justice to common symptoms of the malady.

DCS is actually a condition that manifests from exposure to low barometric pressure (read “high altitude”) in which nitrogen that is normally dissolved in the tissues comes out of solution and form bubbles in the body. While this condition is more regularly associated with diving, it has cropped up in other activities that involve large changes in pressure. In our case, we’re concerned with altitude-induced decompression sickness, but there are some cautions for pilots who are also divers.

To explain the mechanism by which dissolved nitrogen comes out of physical solution think of how a carbonated beverage works. When you pop the lid, the higher pressure inside the bottle equalizes with the lower pressure outside, and the soda fizzes. That fizz is the dissolved gas (carbon dioxide) being exposed to lower pressure and coming out of solution from the liquid.

Likewise, under pressure, and sealevel pressure is adequate, we all have nitrogen dissolved in our tissues. If the outside air pressure is reduced enough, the nitrogen wants to come out of the tissues. When it does, if large enough bubbles of coalescing nitrogen form, physiological problems can occur.

Normally these problems occur when the nitrogen comes out of solution too quickly. Among the more common flight circumstances that increase the likelihood of these problems are fast climbs to high altitude, and rapid decompression while flying at an already high altitude.

DCS causes a wide variety of symptoms, some specific and some not so specific. When we say that a symptom is not so specific, we’re indicating that its one that is common to DCS, but also common to a multitude of other non-DCS related problems. The danger of experiencing non-specific symptoms is that a pilot might think they have a relatively benign problem, but in reality may have an entirely more serious problem.

In categorizing possible DCS symptoms, the type of DCS is the main consideration. Since nitrogen bubble formation is possible in many locations of the body, the types of DCS refer to particular affected areas of the body. The most common type of DCS, according to the Civil Aeromedical Institute of the Federal Aviation Administration in the U.S., as well as most other aviation physiology sources, is the “bends.”

The bends categorizes DCS symptoms of mainly joint pain in the larger joints of the body. This is normally a deep pain the ranges a mild dull ache to sharp and unbearable. Any kind of motion of the joint generally aggravates the pain. The bends most frequently occurs at altitude, but can also occur during descent or even well after the flight is completed.

A less common form of DCS is categorized as “neurologic.” Accounting for far fewer cases, neurologic DCS occurs when the central nervous system is impacted by bubbles. With this type of DCS, the brain, spinal cord, and peripheral nerves are susceptible. The most commonly reported symptom is headache; although this is a non-specific symptom. Specific symptoms range from visual abnormalities to seizures, vertigo, burning or stinging of the lower chest and back, numbness, and muscle weakness; and a slew of others!

Roughly equal is occurrence to neurologic DCS is what is called the “skin bends.” As the name implies, the symptoms can be expected to be associated with skin accordingly. Itching in the upper torso, and especially around the ears, face and neck is possible. As well, the sensation of insects crawling on the skin (termed “the creeps”) is common. Mottled or splotchy appearance around the upper torso area, also with itching, is also possible.

Even less common than neurologic or skin DCS is a type called “the chokes.” The chokes affects the lungs and will be felt as a deep burning pain under the sternum (breast bone). The pain is made worse by breathing and so leads to shortness of breath.

Also common with the chokes is a dry unproductive cough. This later symptom is not by itself specific to DCS however. Prolonged breathing of 100% oxygen can lead to “false chokes,” which may be due to the dehydrating effect of breathing aviator’s oxygen.

Factors that make a pilot more susceptible to DCS mainly include exposure to high altitude, rate of ascent, previous DCS injury, age and body fat, hydration level, and workload during flight. Certainly a combination of these factors can improve the chances of a DCS event, and as such many factors may be hard to avoid, so prevention is key.

Denitrogenation is considered to be the gold standard when the risk for DCS is high. This process involves prebreathing 100% oxygen for around 30 minutes if the time above FL250 would be limited to no more than 10 minutes. Otherwise, longer denitrogenation is recommended.

If preoxygenation of the body on the ground is not possible, then breathing supplemental oxygen while at higher altitudes may be helpful. Scuba divers need to take special precautions prior to flight since breathing under pressure while diving increases the dissolved nitrogen in body tissues. General recommendations for divers are to wait at least 12 hours prior to flying after a dive, and 24 hours if a controlled ascent was required. Failure to abide by these rules can lead to DCS at altitudes as low as 5,000 MSL. Also, some sources recommend that a pilot wait to go diving at least 12 hours after flying to reduce the possibility of diver’s bends during ascent from the dive.

If DCS should occur during flight, the first step is to identify the type of DCS being experienced. Since it’s likely to be “the bends,” the affected area (joints) should be immobilized if possible. An emergency should be declared and descent begun. If supplemental oxygen is available, this is the time to use it, regardless of current altitude.

Once on the ground, seek competent medical assistance. In this case, competent means medical personnel with experience in aviator’s DCS, although any doctor that knows how to treat any form of DCS is also a good place to start. Keep in mind that particularly bad cases of DCS may require use of a hyperbaric chamber, and travel may be required to get to one!

These chambers are used to increase the pressure beyond normal sea-level pressure in order to reduce the size of the bubbles, after which a slow decrease in pressure and supplemental oxygen will help to denitrogenate the body.

So now when you hear the phrase “trapped gas” you’ll understand a good deal more about the many serious problems that can occur as a result of higher altitude flight. We know from the accident and incident reports that pilots suffering from the various sorts of trapped gas afflictions are, at a minimum, seriously distracted from the task of flying the airplane.

Remember that trapped gas is not limited to the intestines; it can be found in the ears, sinuses, teeth, and also manifest itself in the joints when nitrogen bubbles accumulates. Heeding the advice in both this, and last month’s article, will go a long way towards keeping you out of trapped gas troubles!

This month’s Pilot’s Primer is written by Donald Anders Talleur, an Assistant Chief Flight Instructor and Researcher at the University of Illinois, Institute of Aviation. He has been flying since 1984 and in addition to flight instructing since 1990, has worked on numerous research contracts for the FAA, Air Force, Navy, NASA, and Army. He has authored or co-authored over 200 aviation related papers and articles and has an M.S. degree in Engineering Psychology, specializing in Aviation Human Factors.