Post descent collapse. This may occur up to four hours after the primary… — Page 55, Lesson 87

I want to walk you through a specific complication of decompression sickness called post-descent collapse. This is a delayed, dangerous event that can happen up to four hours after the primary symptoms of DCS have appeared. The mechanism here is that nitrogen bubbles in the body have combined—they've clumped together—and therefore have not gone back into solution in the blood or tissues. These larger bubbles then reach the heart, which can cause a sudden, severe cardiovascular event. That's why it's called a collapse: the heart's function can be compromised by these gas bubbles. Now, the key way to avoid decompression sickness altogether is preoxygenation before exposure to high altitudes. This means breathing 100% oxygen for a period before ascent, which reduces the body's store of nitrogen as much as possible. Less nitrogen in the tissues means less gas available to form bubbles when pressure drops. Let's move to what happens if DCS symptoms appear in flight—whether in a passenger or a crew member. The pilot's immediate action is to commence an immediate descent to a level at which the symptoms are relieved. The aircraft should then land as soon as possible. While descending and after landing, the sufferer should be kept warm and rested, and put onto a 100% oxygen supply. Even if the patient appears to have recovered on landing, urgent medical assistance must be sought—this is non-negotiable, because the symptoms can recur or worsen. Now, a very important practical consideration: flying and diving. DCS is rare in normal flight, but the incidence is greatly increased for individuals who have been diving using compressed air shortly before a flight. Here's the physics: the pressure that a 30-foot column of sea water exerts is the same as that exerted by the atmosphere at sea level—that is, 760 mm Hg. So a person at a depth of 30 feet is exposed to a pressure of 2 atmospheres: one atmosphere from the air above the water, and the other from the water itself. In scuba diving, air under pressure is used, and this increases the amount of nitrogen dissolved in the body. On subsequent ascent—whether from a dive or in an aircraft—this nitrogen may come out of solution, giving rise to DCS. The following rules must be strictly observed by both crew and passengers. Failure to adhere results in incidents each year where individuals develop DCS in flight at altitudes as low as 6,000 feet. Here are the rules, stated exactly as they appear: DO NOT FLY WITHIN 12 HOURS OF SWIMMING USING COMPRESSED AIR AND AVOID FLYING FOR 24 HOURS IF A DEPTH OF 30 FEET HAS BEEN EXCEEDED. So, to summarise: a shallow dive to 30 feet or less requires a 12-hour wait before flying. If you exceed 30 feet depth, the waiting period extends to 24 hours. These are hard limits, not recommendations.