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Medical investigation of suspected hypoxic events.

We continue to emphasize aeromedical issues and the role they play as causal factors in the majority of our mishaps. While fatigue and spatial disorientation have been identified as the top two aeromedical factors, hypoxia can be fatal but is not always easily identified. Hypoxia is a self-reported physiologic event in most cases, and it's involvement in a mishap can challenge the most skilled investigator.


We do a very thorough job of investigating mishaps that involve fatalities. But, when it comes to physiologic events such as hypoxia, we sometimes drop the ball, particularly when the events don't result in a mishap or a fatality. Here are several medical considerations when investigating hypoxic events:

Encourage all aircrew to promptly report any suspected physiologic events. Events involving suspected problems such as oxygen-system contamination are time-critical, because the clinical evidence may be extremely short-lived. Make sure the involved aircraft and gear are "downed." It's easy to release the aircraft and gear if you determine further investigation isn't needed, but you may lose critical evidence if aircraft are reused before you get to them. Besides, anyone who uses the aircraft or gear may be at risk. Have engineering investigations (EIs) performed on all suspect equipment.

Flight surgeons, you must perform at least a focused history and physical examination on affected aircrew. Pay particular attention to the neurological and cognitive exams, but don't omit the cardiovascular and pulmonary systems, vital signs, and drug/supplement use history. Obtain a 12-lead electrocardiogram (ECG) and pulse oximetry. A chest X-ray is indicated if you suspect atelectasis or other lung disorders. Collect urine and venous blood samples from affected aircrew. Order a complete blood count (CBC) at the absolute minimum, but consider toxicological studies as well. Arterial blood gas (ABG), carbon-monoxide level (smokers) and acetylene tests may be indicated in some cases.

"Why acetylene?" The Naval Safety Center released a message in August 2005 when it became apparent that contamination from OBOGS units might be seen as a cause of hypoxia incidents. The following guidance was included: "A remote possibility of contamination by gases such as acetylene, carbon monoxide, and carbon dioxide can cause additional hypoxia risks not previously identified, particularly in OBOGS-equipped aircraft. To better understand these possible contamination concerns, the following additional actions are requested by Naval Safety Center in all hypoxia cases to rule out contamination as a cause factor. Submit a red top blood specimen to AFIP with a DD-1323 (toxicological examination request and report form) and in the section incident/hypoxia block mark "hypoxia episode" screen for "acetylene." AFIP toxicology lab will continue to routinely screen all blood samples for carbon monoxide."

In cases where there aren't other obvious causes for hypoxia and a contaminated OBOGS system may be at fault, consider submitting the additional labs above.

The value of reporting physiological events cannot be overstated, and it's required by NATOPS 3750.6. Increasing awareness and improving training to counter the effects of events such as hypoxia are key to overcoming them. Share your experiences, report these events, and improve your awareness of physiologic events.

Contact the Naval Safety Center Aeromedical Division at 757 444-3520 ext. 7268 (DSN 564) for additional assistance.
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Title Annotation:The Initial Approach Fix
Geographic Code:1USA
Date:Jul 1, 2010
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