The introduction of cannulas into aviation, first allowed by the FAA in 1982, proved a boon for pilots using supplemental oxygen and for those selling the systems. The comfort of the cannulas helped make oxygen systems more popular, especially for passengers who were reticent about donning an uncomfortable mask. This translated into more low-altitude flights than some pilots might have preferred.
A few years later along came conserving cannulas that dramatically reduced the expense of using supplemental oxygen, as well as lowering the hassle factor even more because less frequent oxygen fills are required. Once again, supplemental oxygen use soared.
However, questions about cannula effectiveness and FAA requirements to use a mask above 18,000 feet have always generated lots of questions, some of which well try to answer here.
For purposes of this article, well consider only conventional oxygen systems for light aircraft, not pressure-breathing systems that are used above 30,000 feet. Those can only be used with a special mask.
What Regs Say
The regulations on cannulas are clear and are found in FAR 23.1447. If certification for operation above 18,000 feet (MSL) is requested, each oxygen dispensing unit must cover the nose and mouth of the user. In other words, use of a mask is required above 18,000 feet. Even below 18,000 feet, the pilot must have a mask available, even when using a cannula.
In establishing this 18,000-foot maximum for cannula use, the FAA took a conservative stance. For many individuals, it may be perfectly safe to use a cannula well into the flight levels, for others it may not be.
Problems arise when breathing efficiency is compromised as a result of age, health problems or temporary circumstances. Then its all too easy for hypoxia to occur if using a cannula. Of course, these same types of deficiencies can cause hypoxia at well below 18,000 feet for some who might use a cannula, or even a mask.
The introduction of pulse oximeters that directly measure your blood oxygen saturation level has taken the vagueness out of the situation (see Aviation Consumer, September 2001). Its now easy to see whether or not youre getting adequate oxygen, regardless of the delivery device.
With this information, many pilots tell us they regularly fly much higher than 18,000 feet using a cannula, disregarding the FAAs rules. It could be argued that its safer than not using an oximeter and relying upon the FAAs arbitrary limits that dont account for a wide range of variables.
Flow Monitoring
However, theres also the question of flow monitoring. With no automatic alarms available on any of these devices used in aviation, oxygen levels must be continually monitored, a procedure that isnt always followed, or even possible on some systems.
One of the most common problems associated with cannula use at higher altitudes is that of mouth breathing. While breathing through your nose using a cannula may provide adequate oxygenation, if you suddenly start breathing through your mouth, blood oxygen levels can drop precipitously. Moreover, breathing rate and depth can also affect oxygenation levels, both of which can be affected by outside influences.
The typical mask provided as standard with all conventional supplemental oxygen systems is a basic non-rebreather style (with bag attached) constructed of clear, soft pliable plastic. Its available in both adult and child sizes.
The oxygen line is attached to the lower portion of the mask with a plastic fitting. A single adjustable elastic strap holds the mask to your face. Some have a soft aluminum strip over the nose bridge that can be used to adjust the fit to your face for comfort and to reduce leakage.
These masks have a typical price in the $7 range. Theyre really designed as a semi-disposable medical item, but most pilots use them for years with minimal problems, provided they keep them clean and out of the sun. Ultraviolet from sunlight, as well as heat, will shorten their usable life.
Its always a good idea to keep some alcohol swabs available to clean and sterilize the mask before and after use, something thats even more important if the mask is being used by different persons.
The FAA says that masks of any kind cannot be safely used with a beard, but we have found they work adequately, with no oxygen deficit, as measured by an oximeter, at altitudes up to 24,000 feet, assuming a person of normal physiology.
In other countries, where beards are common due to the culture, masks of all types have been used without problems, even during altitude chamber rides where problems would be most noticeable to observers.
Its virtually impossible to carry on a conversation or speak into a microphone with one of these masks on your face. An easy and inexpensive solution has long been to simply cut a small slit or a pair of crossed slits through the side of the mask and slip your headsets mic through it.
The amount of oxygen that leaks past the boom is negligible and in our tests, users were as intelligible as with the best purpose-built masks with integral microphones. Drinking a beverage or eating requires the mask be removed, or at least lifted up away from the mouth, awkward at best and a poor idea at higher altitude.
Cannulas
A conventional cannula is little more than a tube into the nose for all practical purposes. They cost approximately $6 to $7 each. A loop from the supply line goes over the ears with a pair of short tubes that fit into the nostrils. It primarily offers the advantage of comfort over a mask, assuming you dont mind a short tube stuck up your nose.
You can use any microphone as you normally would, and you can eat and drink unimpeded. There are no savings in oxygen; the same flow rate specified for use with a mask, one liter per minute per 10,000 feet of altitude, must be used. Because the flow rate is relatively high, this can have an adverse effect on the nasal and airway linings-dryness, mainly-a problem also found with masks.
Conserving cannulas originally were developed for those who, for medical reasons, were wedded to an oxygen cylinder. Aside from cost, the biggest disadvantage was lack of mobility for any length of time.
Small portable cylinders would only suffice for brief outings, typically no more than a couple of hours. The conserving cannula is about four times more efficient by re-circulating your own breath.
How It Works
The conserving cannula is based on the premise that nearly three quarters of your normal respiration time is spent either exhaling or not breathing at all. And not even all the air inhaled is actually used.
With typical constant flow systems fitted with a mask or regular cannula, more than 75 percent of the oxygen delivered is wasted. A conserving cannula incorporates a reservoir that collects a portion of the exhaled air from the initial exhalation phase, which, when using supplemental oxygen, is almost entirely oxygen.
The bladder is inflated as you exhale and at the start of the inhalation cycle its collapsed and delivers its charge. The flow rate is adjusted to compensate for this increase in initial oxygen delivery during the portion of the cycle that delivers the most oxygen to the lungs. If you examine a flowmeter with ranges for both conserving cannula and mask, youll see the difference between the two scales. Conserving cannulas come in two designs, both priced in the $30 range. The most commonly used incorporates the reservoir into a flesh colored, wide pliable plastic container that rests immediately below the nostrils, and is commonly referred to as a mustache style for its resemblance to one.
Its worn in the same manner as a conventional cannula. You can drink or eat while wearing it, although drinking from a cup can be a challenge. Because theres less oxygen flowing constantly, theres less tendency for irritation of the nasal passages and airway. For many pilots and passengers, this has proved a significant advantage.
The so-called pendant style conserving cannula operates the same way, but is different in design. The reservoir is contained in a round clear plastic pendant that lies on your chest. Considerably heavier tubing, compared with the other cannulas, loops up under the nose with the requisite pair of tubes into the nostrils.
Its held in place via ear loops of soft plastic. From its appearance, youd think it would be far less comfortable than the mustache style, but it proved to be acceptably comfortable to wear in our tests.
Without the bulky reservoir beneath the nose, its less noticeable, if that matters, and its less of a bother to drink from a cup while wearing it. Donning is more complicated, but not much trouble once youve set it up initially. For the occasional passenger, it would be more trouble than the mustache style, in our estimation.
Our tests with oximeters revealed no measurable performance difference between the two cannula styles. In our experience, most prefer the mustache design, but then most havent tried the alternative. These cannulas can last for years if kept clean and out of the sunlight.
Worth mentioning is one other oxygen-conserving strategy: Mountain High Equipment and Supply Companys unique EDS pulse demand systems, which use electronic sensing to minimize oxygen use, have been tested by UND Aerospace in their altitude chamber to 36,000 feet using the standard non-conserving cannula with adequate oxygenation of participants in the test.
We know from our own flight tests of these units that they worked fine for us to 24,000 feet and we have reports from users in the field who regularly fly to 30,000 feet with the EDS units. However, these units are prone to many of the same potential failings as a conventional system using cannulas.
The apnea alarm provides a small margin of protection for someone who unconsciously switches to mouth breathing, but the alarm itself is too weak to be relied upon. Chamber testing aside, we dont think flying above 25,000 feet with cannulas is a good idea. Even with a mask, constant oximeter monitoring, a back-up oxygen bottle, a co-pilot and altitude chamber training are good things to have.
Conclusion
For conventional system users, conserving cannulas are the way to go. Infrequent passengers will likely find the mustache style easiest to don, but for regular users, it simply depends upon which feels more comfortable.
Given the relatively minimal cost, if what you are using now isnt comfortable, it may be worth trying the alternative. Theres no question that using cannulas above 18,000 feet is safe for many, if not most pilots.
However, we hesitate to recommend it unconditionally without the availability of a pulse oximeter with an alarm to constantly monitor your oxygenation level. Unfortunately, at the moment, none are available for aviation use.
We wouldnt hesitate to use cannulas for passengers up to the mid-20s, provided that they are able to regularly monitor themselves with an oximeter. We believe that each occupant should have a mask available in case of problems, not just the pilot.
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-by Douglas S. Ritter
Douglas S. Ritter is an Aviation Consumer contributing editor and editor/publisher of www.equipped.org.
