Seminar - oxygen usage in sport skydiving

[billvon 2,991]

Below is the text supplement for a talk I gave at PIA this year. PM me if you would like the entire thing. There's a powerpoint presentation and several bits of supporting material.

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Supplemental Oxygen Usage in Sport Skydiving
Bill von Novak
PIA 2007

Text supplement to presentation


Most skydivers know that oxygen is an important component of the air we breathe, and that it drops off the higher up we go. Supplemental oxygen usage becomes common somewhere above 14,000 MSL (although the exact altitude varies based on the dropzone/jumper/boogie) and some jumpers have made jumps at altitudes that require bailout oxygen (i.e. 30,000 foot skydives.) Jumpers have always pushed these limits, with jumpers using no bailout oxygen up to 26,000 feet, and using no oxygen at all for jumps up to 17,000 feet. As jumpers push these limits, it becomes more and more important to understand the details behind oxygen usage in sport skydiving.

Oxygen makes up 21% of our atmosphere at sea level. Most of the rest is nitrogen, at 78%. Nitrogen is a mostly inert gas; it does not do much for us (or for most plants and animals) but does help reduce the intensity of fires and provides protection from solar radiation. It can also pose problems for us because it goes into solution (and comes out of solution) in our blood relatively slowly. SCUBA divers can have problems if they ascend faster than the nitrogen in their blood can escape. When this happens, the nitrogen can come out of solution in their blood, and the bubbles can collect in their tissues causing decompression sickness (DCS.) The same thing can happen to skydivers during rapid ascents.

The remainder of the atmosphere consists of trace gases including carbon dioxide (CO2.) Carbon dioxide is a waste product in humans, and we exhale it regularly. Since carbon dioxide builds up fast when we exercise and when we breathe in enclosed spaces, CO2 concentrations are the primary way our body knows to speed up breathing rate. When CO2 concentrations in our blood become high, we breathe harder. This works well at sea level, but at higher altitudes we can have normal levels of CO2 even when we're not getting enough oxygen. This is a problem at higher altitudes, since our body doesn't have a good "feel" for when we are not getting enough oxygen.

Fortunately, our bodies have several compensatory mechanisms that help us deal with lower oxygen concentrations. The primary one is hemoglobin. Hemoglobin binds to oxygen very strongly, and does not release it until the outside oxygen concentrations drop significantly (as they do in our tissues.) This works well up about 20,000 feet. At that level, the oxygen concentration drops low enough that hemoglobin starts releasing any extra oxygen in the lungs, and does not pick up much new oxygen for distribution to the tissues. Climbers often call altitudes above these "the death zone" because their hemoglobin can compensate for lower O2 concentrations only to a certain point - and after that point, it tends to fail dramatically. Within a few thousand feet, climbers see a change from adequate oxygenation to serious hypoxia.

Another compensatory mechanism has to do with our circulatory systems. As mentioned previously, most of our "sense" of not having enough air to breathe comes from too much CO2, not from too little oxygen. We do, however, have backup oxygen sensors in our carotid arteries that can sense low O2 levels. When they sense dropping O2 levels, our bodies increase our pulse rate and respiration rates to help compensate. Often, since these do not work as well as our CO2 sensors, our breathing becomes erratic; we may stop breathing completely for a few moments, then breathe deeply for a while, then stop again. This sort of breathing is called Cheyne-Stokes respiration and is seen often in climbers.

In addition, people can become acclimated to higher altitudes by increasing the amount of hemoglobin in their blood and shifting its "affinity curve" - in other words, shifting the point at which it starts to pick up oxygen so that lower concentrations of oxygen are still transported well. Someone living in Denver will generally be better adapted to higher altitudes than someone who lives on Long Island. This adaptation takes some time (days) so it's not generally seen in skydivers who go to high altitudes for only minutes at a time.

All this means that humans are pretty good at dealing with lower concentrations of oxygen up to a certain point. Beyond this point, our ability to function drops off very rapidly. Thus there are three levels of hypoxia. In the indifferent level (from approximately sea level to 6000 feet) our body simply doesn't notice the slightly lower levels of oxygen. In the compensatory level, from about 6000 to 15,000 feet, our body does a good job of compensating for the lower levels of oxygen. In the disturbance level (above 15,000 feet) we start to notice the effects of decreasing oxygen levels.

Different people respond to hypoxia in different ways. Most people notice some confusion in their thinking, difficulty in remembering, and general "slowness." Some people begin to get tunnel vision, or begin to lose color vision, or hear ringing in their ears. Since everyone perceives hypoxia differently, learning how you respond to lower oxygen levels can be critical in anticipating your impairment. This can be done by simply not using oxygen on higher altitude skydives, but a much safer and more effective method is to take a "chamber ride," a test where several people are taken to higher altitudes in a sea-level pressure chamber. Often, the operator will plan exercises that let people determine how impaired they are at a given altitude. This experience can be invaluable in getting a feel for how each jumper reacts to hypoxia.

There are a few 'external' ways of determining whether or not someone is hypoxic. Some people's lips will turn bluish as the oxygen saturation of their blood begins to decrease. A very accurate method of determining blood oxygenation is the use of a pulse oximeter. This is a small device that clamps onto a finger or an earlobe and measures the oxygen saturation of a jumper's blood. From tests that I have run, jumpers may see a range similar to this:

Sealevel 99%
7K: 95%
10k: 90%
14K: 80-86%
14K active: 78-80%

Most people start to feel hypoxia below about 80% O2 saturation. This agrees with the common guideline that most jumps below about 14,000 feet do not need supplemental oxygen. Jumpers who exit above this altitude may benefit from supplemental oxygen.

How much you will benefit depends upon several factors, including overall fitness, any local acclimatization based on the elevation a jumper normally lives at, whether the jumper smokes, what activity level the jumper sees before exit etc. A common problem some jumpers have at altitudes near 15,000 MSL is hypoxia when they are front float on a bigway - such an exit position requires a lot of energy to be expended for 20-30 seconds before exit, and this energy can draw saturation levels down.

There are several ways to improve oxygen saturation levels without the use of supplemental oxygen. The #1 method is to remain calm and inactive before exiting the airplane. This lowers peripheral oxygen demand and allows more oxygen to remain in your blood. A second way is to breathe deeply and slowly. Normally, an average person exchanges 70% of their lung volume. This is no problem at sea level, but on jump run at 12,500 feet MSL this can result in increasing the "effective altitude" to 15,000 feet. Breathing deeply can increase this exchange to 90% and eliminate most of this problem.

Both the FAA and the USPA discuss use of oxygen for higher altitude skydiving. FAA and USPA rules require everyone to use oxygen above 15,000 feet; USPA recommends O2 if jumpers spend more than 10 minutes above 10,000 feet, and the FAA requires O2 to be used above 12,500 feet if there for more than 30 minutes.

Why is time an issue? A side effect of how we transport oxygen is that we have some "buffer time" while our hemoglobin still contains some residual oxygen that it stored at lower altitudes. This allows us to function for some time above the limit we would ordinarily become hypoxic. This time is called the "time of useful consciousness." This can vary, but one table of times looks like this:

18,000 feet – 20 min
22,000 feet – 10 min
25,000 feet – 3 min
28,000 feet – 2 min
30,000 feet – 1 min

This means that at 25,000 feet, if you are on enough oxygen to make your ppO2 the same as sea level, you can go off oxygen and be functional for up to 3 minutes. Note that this does NOT mean that you have 3 minutes of useful consciousness after exit if you are not using supplemental oxygen. As skydivers, we often rely on these tables to do high altitude jumps. During the 400-way, for example, jumpers went off oxygen at the 10 second mark and dove out of the plane at exit altitudes up to 26,000 feet. After approximately 90 seconds they were at 10,000 feet, where hypoxia is no longer an issue. As long as the jumpers could get to that altitude sooner than 3 minutes, their hemoglobin could store enough oxygen to keep them going - provided they are not overly exerting themselves.

These limits are starting to be pushed, though. With record attempts going as high as 26,000 feet, with some jumpers having to muscle their way into floater positions, and with the usual miscommunications, hypoxia is becoming more and more of a concern.

There are several ways that supplemental oxygen can be supplied in the aircraft. The most common method is the constant-pressure metered orifice. In this system, a plenum (usually a thick pipe) is supplied with constant-pressure oxygen from a tank. A pressure regulator controls the pressure in the plenum. Metered orifice taps (a fancy way of saying "identically sized holes") then allow some oxygen to escape. As long as the pressure in the pipe remains constant, and there isn't much backpressure on the hoses themselves (i.e. they are not kinked) this can work as a simple way to distribute oxygen. Often one tap, usually the pilot's, has a simple flow indicator on the hose. This gives a visual indication that that hose is seeing sufficient flow, usually one or two liters per minute. Generally, the rule of thumb for oxygen flow is one liter per minute for every 10,000 feet - so 20,000 feet would require 2 liters per minute.

Some common problems/risks with this system:

Oxygen is, as always, a significant fire hazard. Oxygen itself does not burn, but makes any fire much, much worse. Even aluminum will burn in a pressurized oxygen environment. It is imperative that the oxygen can be turned off rapidly in case of an in-flight fire or emergency landing.

Such a system can be difficult to set correctly without a flow indicator. A system that is blowing perceptible amounts of oxygen out the end of the hose is usually set to deliver far too much oxygen. A flow rate of 2lpm (good to 20,000 feet) is not very strong. Too much oxygen is not a direct health hazard, but can cause dehydration and drying of mucus membranes - and oxygen also accelerates bacterial growth. The combination can cause sickness to spread rapidly.

Small plenum pipes can lead to pressure drops along their length. This leads to too-low flow rates at the end of the pipe even if closer hoses have acceptable flow.

Another system involves the use of constant-flow regulators. These are better for aviation usage, since they guarantee a flow rate of X liters per minute. A skydiver going to 25,000 feet can set the flow rate to 2.5 liters per minute and be assured that they are getting sufficient oxygen. These regulators require one regulator per jumper and are somewhat expensive.

A third kind of system uses an altitude-compensated regulator. These need no adjustment at all. They start delivering oxygen at around 10,000 feet and increase the flow as the user gains altitude. These are the most foolproof delivery systems around, and also the most expensive.

Any system, though, can be rendered ineffective by incorrect usage at the user end of the system. There are several delivery systems in common usage in skydiving:

The nasal cannula is a common and easy-to-use method of delivering oxygen. A loop of vinyl tubing loops around the head, and the tension of the loop holds two soft vinyl tubes in the user's nostrils. This is an OK method of oxygen delivery, but can be less effective if the user is talking a lot or breathing through his mouth. A similar system was integrated into helmets during the Arizona 300-ways in 2002.

A mask is a somewhat better method, since it encompasses both the nose and the mouth. They can be bulky and difficult to "talk through" though.

A third method that skydivers have evolved somewhat independently is the oral tube. The user just sticks the tube in his mouth and breathes normally. Since our throats normally leave the passages between our lungs, nose and mouth open, this method of delivery can work as long as the user keeps his mouth closed around the tube. This has the benefit that some of the "dead space" in the respiratory space is eliminated; the user does not have to 'clear' the expired air in his sinuses before he receives new oxygenated air. This reduces the amount of oxygen needed. One drawback is that since the gas flow in the mouth and upper throat is one-way, drying of the nose and throat is an issue. This can be mitigated by sucking on hard candy (a popular solution) or by keeping flow rates reasonably low. Often alcohol wipes are used periodically on the tube to prevent transmission/buildup of bacteria.

A fourth method is the helmet method of delivery. The jumper just sticks the tube somewhere inside the helmet to get oxygen delivered to the airspace inside. This has one advantage - there is a small amount of oxygen retained after the oxygen tube is removed, and this can allow the user some additional oxygenation after the tube is removed (i.e. during exit.) This method has the following disadvantages:

1. It's efficacy is determined by how well the helmet seals. A poorly sealed helmet can allow most of the oxygen to escape unused.

2. It requires the user to nose-breathe, and nose-breathing in a closed helmet can lead to fogging before exit.

All these supplemental systems operate under the same assumption - that oxygen usage in the aircraft will extend the jumper's time of useful consciousness so that they can begin the skydive and reach a breathable altitude before they become hypoxic. As mentioned previously, though, we are beginning to push those limits. At 26,000 feet, a calm jumper has about 3 minutes of useful consciousness, but an active jumper has closer to 1.5 minutes. This is getting very close to being too short a time.

One simple method is to stay on O2 until the last possible moment. When using the helmet or oral O2 methods, it's possible to keep oxygen on until exit, at which point the hose is pulled out of the jumper's helmet/mouth by the motion of the exit. This can result in lost hoses, and it can also be a problem for floaters - hoses long enough to accommodate floaters will end up trailing out the door. It also, at best, adds a few seconds to the time of useful consciousness, which results in an additional few hundred feet of useful altitude.

Another possibility is bailout oxygen. Military HALO systems use 100% oxygen systems, but at altitudes near 25,000 feet, supplemental oxygen is sufficient to avoid hypoxia in freefall. There are several systems available, some of which fit inside a jumpsuit.

An example system is shown in the accompanying presentation. The bottle contains 36 liters of oxygen, sufficient for 18 minutes of usage at 20,000 feet. In ordinary usage a jumper would remain on aircraft oxygen until a few minutes before exit, then switch to the bailout system. This system uses a constant flow regulator making it easier to set flow rates.

This system was tested on several high altitude jumps (18,000 MSL) at Perris Valley, CA and worked well. It allowed floating for extended periods of time without worrying about hypoxia. Oral tube delivery worked well, since the tube can exit under the lip of a typical full-face helmet and enter the jumpsuit collar with minimal external exposure/snag hazard.

Another system that showed some promise was the E-OX system proposed several years back. This system used small (CO2-cartridge-sized) disposable oxygen canisters that screwed into a receptacle on a helmet. A simple orifice regulated flow, and provided a few minutes of oxygen. Unfortunately, it is not easy to ship pressurized canisters of oxygen in the US, and this approach was abandoned.

An additional issue that may rear its head as skydivers go to higher altitudes (and get there faster) is decompression sickness (DCS). Most SCUBA divers are familiar with DCS as a possible result of ascending too quickly from too deep beneath the water; the rapid decrease in pressure can result in nitrogen coming out of solution in the bloodstream, and the resulting bubbles can accumulate in joints and other tissues, causing pain and numbness. A similar problem can happen when aviators ascend rapidly. Indeed, many military pilots who plan on climbing rapidly to altitude in unpressurized aircraft will prebreathe oxygen to flush the nitrogen out of their bloodstream before the ascent.

Skydivers do not ordinarily worry about DCS, both because they typically do not ascend to very high altitudes and because when they do they are at those altitudes for a short time. During a rapid ascent to 26,000 feet, though, this may well be an issue - the US Air Force sets 25,000 feet as the altitude at which DCS becomes an issue.

Prebreathing is sometimes used to combat DCS in high altitude sport skydiving. Prebreathing a mixture of gas with no nitrogen can flush nitrogen from a jumper's blood, reducing the risk of DCS. Superior Jumps, an outfit that provides sport jumps to 30,000 feet with military breathing equipment, uses 100% oxygen prebreathing before going to their jump altitude at 30,000 feet. This may not be practical for sport skydivers, since 100% oxygen must be provided for up to 30 minutes before the jump, and that means a sealed mask and large reserves of oxygen.

All systems have limits, and in terms of bailout oxygen, an important limit is reached at 42,000 feet. Beyond this altitude, even 100% oxygen (i.e. high flow rate mask, such as is used in HALO systems) is insufficient to keep the partial pressure of oxygen above about 2psi. To go beyond these altitudes, some military aircraft have positive-pressure breathing systems, where a differential pressure of up to 1PSI is maintained in the mask. This increases O2 pressure in the lungs, but makes it very difficult to breathe out. Such systems are beyond the scope of sport skydiving.

The final limit occurs at 65,000 feet. At that altitude, water boils at 98 degrees F. Since most of the human body is water, that poses some obvious problems. Beyond these altitudes, pressure suits are required.

Sport skydivers are beginning to push the limits of high altitude skydiving in several ways - in terms of decompression limits, oxygenation limits and time-of-useful-consciousness limits. As we go higher and higher to achieve formation records and for additional freefall time, more attention must be paid to the equipment that we rely on to let us breathe at these extreme altitudes. We must also take into account failure modes and backups, since we are now reaching altitudes were equipment failures become more than an inconvenience.

[yjumpinoz 0]

I don't always agree with you in the SC, but you are right on the money on O2. Your research very well could prevent a very tragic accident in the future. Thanks for sharing that with everyone....

[dragon2 2]

We definately need a new gameplan for the camera jumpers, as they are more active during the jump and get less oxygen before exit...

ciel bleu,
Saskia

[billvon 2,991]

>We definately need a new gameplan for the camera jumpers, as
> they are more active during the jump and get less oxygen before
> exit...

Yep. I was discussing the possibility of building bailout systems for some "critical" people (camerapeople, spotters, floaters) with a few people at PIA. The problem is that most manual systems take some expertise, and hypoxic people lack expertise. The really slick fully automatic systems eliminate that problem but cost thousands.

[Orange1 0]

Quote

A very accurate method of determining blood oxygenation is the use of a pulse oximeter. This is a small device that clamps onto a finger or an earlobe and measures the oxygen saturation of a jumper's blood. From tests that I have run, jumpers may see a range similar to this:

Sealevel 99%
7K: 95%
10k: 90%
14K: 80-86%
14K active: 78-80%

Bill, great article.

Some perspective on the above numbers from a completely different situation: my daughter was hospitalised recently with an infection complicated by asthma. They wouldn't let her go home until her O2 sat was 95+ (ie equivalent to an altitude we would see as low). To be honest those numbers wouldn't have meant much to me without some idea of what it means "in practice". So, a very different situation, but I thought the perspective might be useful.

Skydiving: wasting fossil fuels just for fun.