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Thursday, March 31, 2011

COPD patients may feel dyspnea in flight

If you have COPD you may have noticed that your breathing is a bit more difficult when you are flying. And this poses an additional challenge for you that people with "normal" lungs don't have to worry about.

In fact, a new study by Norwegian scientists revealed that COPD patients flying were over six times more likely to experience hypoxia (low oxygen in the blood) and feel dyspnea (feeling of shortness of breath) while flying.

One of the main reasons is because those with COPD already carry less oxygen in their arteries, which are the vessels in the body that carry freshly oxygenated blood to tissues. When the lung are chronically obstructed, less oxygen is able to get to the lungs, and this results in less blood to the arteries.

While under the pressure of the cabin of an airplane, the amount of oxygen in the air is already less than on the ground. For example, the fraction of oxygen inspired (FiO2) at ocean level is 21%. Yet the higher you go the lower the percentage of oxygen that's available to breath.

For example, at the top of a mountain, or in the cabin of an airplane, the FiO2 may be as low as 19%. This might cause dyspnea in a person with normal lungs. Yet in a person with COPD, who already has less oxygen in the lungs and arteries, this will almost certainly cause a feeling of shortness of breath.

Consider the following facts according to Egan's Fundamentals of Respiratory Care:

The air we inhale consists of 21% oxygen. In the medical arena we refer to this 21 percent as the fractional concentration of oxygen, otherwise known as the fraction of inspired oxygen (FiO2).

Thus, to compute the partial pressure of oxygen (PO2) you multiply the FiO2 by the total pressure of the atmosphere. A normal atmospheric pressure is 760 mm Hg at sea level.

Or, the formula would look like this:
  • PO2 = 0.21 * 760 = 160 mm Hg
So the normal PO2 of oxygen we inhale is 160 mm Hg
Now, according to Egans, "At a typical cruising altitude of 30,000 feet, the barometric pressure outside the airplane cabin is about 226 mm Hg. Thus, the partial pressure of the inspired oxygen (PO2) would be calculated as such:
  • PO2 = 0.21 * 226 = 47 mm Hg
"Thus, should the cabin depressurize, travelers inside would be exposed to this low PO2, most people would become unconscious within seconds, and will eventually die of lack of oxygen (anoxia)."
To fix this problem, the passengers would wear oxygen masks that supply 70% oxygen. This new formula would be calculated as such:
  • PO2 = 0.75 * 226 = 158 mm Hg
This would supply enough oxygen to sustain life.
Still, as you go higher, the PO2 will decrease the higher you go because the barometric pressure changes. So COPD patients who already have a low PO2 inside their arteries are going to feel the effect of a lower atmoshperic PO2 as compared to someone with normal lungs and a normal PO2.
Consider that oxygen travels the path of least resistance. By this, the normal PO2 in the atmoshpere is 160, the normal PO2 in the arteries is 104, and the normal PO2 in the veins is 40. So oxygen travels easily from the air, to the lungs and arteries, and then from the tissues to the veins.
However, with COPD the PO2 in your arteries might be 60 instead of 104. So if the PO2 in the atmosphere is low, the PO2 in your arteries will be even lower. Hence, you may feel dyspneic before other passengers would.
It should be said here that the percent of oxygen in the air inhaled (whether it be 21% or 75%) does not determine how the oxygen works in the body. How the oxygen works is determined by the partial pressure of oxygen inhaled (PO2).
So, the lower the PO2 inspired the more dyspnea a person will feel. This is why mountain climbers and pilots sometimes carry extra oxygen with them. By increasing the FiO2, we can increase the PO2.
The barometric pressure
So if you are flying, and you have COPD, you may want to discuss the the airline the possibility that you might require oxygen in flight. If you already have oxygen at home, then you definitely want to either take it with you, or work with the airline to make sure oxygen is available to you in flight.



TOTWTYTR said...

This doesn't surprise me one bit. I used to work on the primary ALS unit for an area that included an international airport. We got an awful lot of calls for chest pain or dyspnea from incoming flights. In many cases, the symptoms had resolved by the time we saw the patient. My working theory (I need a grant for a study) was that the lower cabin pressures and decreased level of oxygen in the cabin at flight altitude could cause chest pain and dyspnea in patients with cardiac disease.

This study would seem to support that conclusion.

truthbeknown said...

"For example, at the top of a mountain, or in the cabin of an airplane, the FiO2 may be as low as 19%."

This is actually untrue. It is 21% whether at sea level or the top of a mountain (*or in a plane)
It is the decrease in the barometric pressure not the decrease in FIO2 that causes the lower oxygen partial pressure.