Time of Useful Consciousness
Time of useful consciousness (TUC) is defined as the amount of time an individual is able to perform flying duties efficiently in an environment of inadequate
oxygensupply.cite book |title=Fundamentals Of Aerospace Medicine: Translating Research Into Clinical Applications, 3rd Rev Ed. |last=Dehart |first=R. L. |coauthors=J. R. Davis |year=2002 |publisher=Lippincott Williams And Wilkins |location=United States |isbn=9780781728980 |pages=720 ] It is the period of time from the interruption of the oxygen supply or exposure to an oxygen-poor environment to the time when useful function is lost, and the individual is no longer capable of taking proper corrective and protective action. It is not the time to total unconsciousness. The TUC has also been called Effective Performance Time (EPT). At the higher altitudes, the TUC becomes very short; considering this danger, the emphasis is on prevention rather than cure.
Medical analysis and variations
There are many individual variations of hypoxia, even within the same person. Generally,
old agetends to reduce the efficiency of the pulmonary system, and can cause the onset of hypoxia symptoms sooner. [cite journal |author=Yoneda I, Tomoda M, Tokumaru O, Sato T, Watanabe Y |title=Time of useful consciousness determination in aircrew members with reference to prior altitude chamber experience and age |journal=Aviat Space Environ Med |volume=71 |issue=1 |pages=72–6 |year=2000 |month=January |pmid=10632134 |doi= |url= |accessdate= ] Smoking drastically reduces oxygen intake efficiency, and can have the effect of reducing tolerance by 1,000-2,000 meters. [cite journal |author=Yoneda I, Watanabe Y |title=Comparisons of altitude tolerance and hypoxia symptoms between nonsmokers and habitual smokers |journal=Aviat Space Environ Med |volume=68 |issue=9 |pages=807–11 |year=1997 |month=September |pmid=9293349 |doi= |url= |accessdate= ] Hypoxia can be simulated in an altitude chamber. This can be useful for identifying individual symptoms of hypoxia, along with rough estimates of the altitude that causes problems for each person. Identifying symptoms is often helpful for self-diagnosis in order to realize when altitude should be reduced. Although the times in the table below are often called average TUCs, an average failure is meaningless to a person who has a shorter TUC.
The table below reflects various altitudes with the corresponding average TUC. [cite web|url=http://www.theairlinepilots.com/medical/decompressionandhypoxia.htm|title=Cabin Decompression and Hypoxia|author=Mark Wolff|date=2006-01-06|accessdate=2008-09-01|publisher= [http://www.theairlinepilots.com theairlinepilots.com] ] These times have been established from observations over a period of years and are for an individual at rest. Any exercise will reduce the time considerably. For example, usually upon exposure to hypoxia at FL 250, an average individual has a TUC of 3 to 5 minutes. The same individual, after performing 10 deep knee bends, will have a TUC in the range of 1 to 1.5 minutes.
A rapid decompression can reduce the TUC by up to 50 percent caused by the forced exhalation of the lungs during decompression and the extremely rapid rate of ascent.
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