Thank you very much for the wonderful and informative article and interview of Dr. Anna Hicks by Annette O’Neil (“Thin Air—Busting Lingering Myths About Hypoxia,” May 2019 Parachutist). It is indeed very important to inform our fellow skydivers about the risks of hypoxia.
An additional factor influencing the risk of hypoxia is the time spent at a specific altitude. However, most aeromedical studies show the degree of hypoxia as a steady-state result after a certain time of exposure to a specific reduced oxygen level. That is the logical and only useful view as far as pilots are concerned, because they intend to travel for certain times and distances. Skydivers, however, reach a certain altitude and usually jump right away, starting a rapid descent toward more oxygen. This reduces the hypoxia risk before exit since tissue-oxygen levels (luckily) do not sink low enough quickly enough as to reach a steady state. The message is to avoid a prolonged stay at high altitude (i.e., avoid a go-around or second jump run).
The tingling sensation in the fingers and toes (dysaesthesia) is not a result of hypoxia but is instead caused by too-intensive breathing (hyperventilation), as happens rather frequently to skydivers who are too strongly occupied with their oxygen bottles on the way up. This causes the loss of carbon dioxide (hypocapnia), which changes the pH (blood acidity) and therefore reduces blood calcium (which moves toward the tissues and, among other things, causes the increased pulse rate). This tingling does not get any better by breathing more oxygen. The uninformed skydiver will only increase the problem and the symptoms.
To avoid this (common!) problem of hyperventilation, during the 1987 formation skydiving world record attempts from 20,000 feet MSL, every two or three jumpers used the same onboard oxygen source. They inhaled deeply before passing on the oxygen mask, then held their breath until they received the oxygen mask again. This procedure kept the participants safe from hypoxia, as well as from hyperventilation and dysaesthesia. The only hypoxic and hyperventilation symptoms that did occur were among those jumpers who were not participating in the record attempts and therefore exited the aircraft on another jump run, which took many more minutes. This confirms the time factor as described above. These non-record participants also had a tendency to use one oxygen mask each, as there were enough left unused after the exit of the record participants.
Dr. Johan van Stappen | Aeromedical Examiner and USPA Lifetime Member #6290