Acceleration atelectasis

Flying Safety, March, 2002 by William Bary, Lawrence Spinetta

Oxygen can be a pilot's best friend when experiencing any number of inflight emergencies, including smoke in the cockpit and sudden cabin depressurization. Using oxygen for long periods of time, however, can lead to postflight symptoms, including cough, shortness of breath, chest pain and postflight oxygen-absorption ear block. Under some conditions, prolonged oxygen use can lead to the development of a condition known as acceleration atelectasis. Breathing 100% oxygen for an extended period, coupled with repeated high-G maneuvers while wearing a suit, may cause breathing difficulties and temporary lung injury. It's a relatively uncommon occurrence with minimal long-term concerns, but bears mention in light of a recent physiologic occurrence here at Tyndall.

The Merck Manual of diagnosis and therapy (http://www.merck.com/pubs/mmanual) defines atelectasis as a "shrunken, airless state affecting all or part of a lung." The most common symptoms include shortness of breath, rapid breathing, cough and chest pain. Although usually not incapacitating, military aircrew experiencing temporary lung injury from acceleration atelectasis could certainly have difficulty completing their required flight duties. Single-seat fighter pilots are at the most risk. F-15C, F-16 and A10 pilots fly high-G profiles on most training sorties. Fortunately, acceleration atlectasis doesn't happen frequently.

Tactical Navy and Marine Corps jets are configured to deliver 100% oxygen (95% oxygen and 5% argon for on-board oxygen generating systems [OBOGS]) at all times. They fly with 100% oxygen for protection against rapid decompression and to ensure a closed oxygen system in case of water entry (e.g., the catapult doesn't work properly on a carrier launch and they're thrown into the ocean). Navy and Marine aviators rarely experience anything more than mild symptoms. Navy Lieutenant Will Gotten, a former F18 pilot and exchange F-is pilot, referred to his experience with atelectasis as the "G cough."

Here's the Tyndall experience: On a recent student sortie, a pilot flew a G-intensive mission with 100% oxygen selected. After landing, the pilot experienced "chest discomfort" and was unable to take deep breaths. After finding it difficult to climb stairs, the pilot notified the flight surgeon. A chest X-ray and exam revealed some of the tissue at the bases of the pilot's lungs had collapsed, decreasing the amount of surface area for gas exchange.

Young healthy lungs can tolerate a certain amount of this, and most pilots under similar conditions probably experience a small degree of it on occasion without symptoms. Under normal conditions (breathing atmospheric air), the alveoli (air sacs) within the lungs contain only 20% oxygen. The mixed composition of atmospheric air helps keep the alveoli inflated. hemoglobin in the blood has among affinity for oxygen and pulls it quickly from the air sacs, leaving behind residual components of air nitrogen, carbon dioxide, etc.). When a pilot selects 100% oxygen, more oxygen is absorbed into the bloodstream and less of the atmospheric air is available to keep the alveoli inflated. Repeated, sustained high-G maneuvers induce enough downward force on the lungs to exert an additive effect on the process. Simultaneously, the inflated C-suit exerts an upward force on the lung, further compressing it. The result is lung tissue that collapses much like a sponge and remains so until "normal" air is restored and deep bre athing reopens the airways.

For the Tyndall pilot, a follow-up chest X-ray later in the week showed resolution of the atelectasis, but he was DNIF for a few days while his symptoms resolved.

In addition to prolonged use of high concentrations of oxygen and increased accelerative forces, the following can reasonably be considered risk factors or atelectasis:

* Smoking

* Illness that has lowered resistance or weakened the aviator

* Lung disease, including emphysema and bronchiectasis

* Use of drugs that depress alertness or consciousness, such as sedatives, barbiturates, tranquilizers and alcohol

To combat the risk of acceleration atlectasis, aircrew should quit smoking, resist the urge to fly when sick and limit alcohol use the night prior to a sortie.

Flying with Combat Edge (CE) also may help prevent atelectasis. Positive pulmonary pressure from pressure breathing for G (PBG) during high-C maneuvering tends to keep your alveoli open. Additionally, CE pressure breathing won't automatically give you 100% oxygen. With NORM (not 100% oxygen) selected, the CE regulator is very efficient and continues to aspirate ambient air during PBG to a maximum of 80% oxygen-rich. In other words, Combat Edge doesn't saturate your alveoli with oxygen. This, combined with the positive pressure, helps prevent your air sacs from collapsing.

Although 100% oxygen is available, pilots may want to sensibly limit their 100% oxygen consumption when not needed. If extended 100% oxygen is required or desired, consider limiting high-G maneuvers. As always, consult a flight surgeon if you experience breathing difficulties during or after a flight.