Basic HTML Version
Chapter 4 — 4
. .
This diagram shows the relationship between the fall of oxygen and carbon dioxide levels
in the blood with breath-holding. Normally, with breath-holding, (A) the breaking point is
reached before the hypoxic zone is reached. After hyperventilation and breath-holding, (B)
the breaking point is in the hypoxic zone.
b. Hypoxic Blackout due to Hypoxia of Ascent
Most divers will have noticed during a breath-hold dive that the desire to breathe often
decreases with depth. This is probably due to the partial pressure of O
2
in the lungs increasing
as they are compressed. There is a corresponding rise in the partial pressure of O
2
in the blood
which will reduce the hypoxic stimulus to breathing. At depth the diver continues to exercise
and use up his O
2
reserves. As the diver ascends however, the lungs will expand and the partial
pressure of O
2
in them will correspondingly decrease. This produces an abrupt reduction in the
O
2
partial pressure in the blood. It may fall below the threshold and cause unconsciousness
during or immediately after ascent. This phenomenon is termed
hypoxia of ascent
. It may even
occur after a diver has taken the first breath after surfacing, because of the delay between
taking this breath and the replenished oxygen reaching the brain.
Some divers, especially those who are spear fishing, taking photographs or deep breath-hold
diving for other reasons, will not only hyperventilate first, but then dive deep until they feel the
necessity to breath (the break point). These are exposed to the dangers both of hyperventilation
and breath-holding, and also to hypoxia of ascent. Death in such breath-hold divers is common,
as unconsciousness occurs without warning.
BREAK
POINT
Fig 4.2