If you've been around Aeromedical Transport for any length of time, you’ve probably heard a lot of talk about Flight Physiology and the impact of the Gas Laws:
Boyle’s, Charle’s, Gay-Lussac’s, Henry’s, Graham’s, Dalton’s, and the Ideal Gas Law.
While it’s important to understand each of these, and the effect each one has on your patient during different phases of flight, the two you MUST understand inside and out for you Aeromedical Exam (FP-C or CFRN) are Boyle’s Law and Dalton’s Law.
Boyle’s Law says…
At a constant temperature, the volume of a gas is inversely proportional to it’s pressure.
P1 x V1 = P2 x V2
(this formula is a difficult for me to grasp, but I include to help others)
To remember Boyle’s Law, think “B” for Balloon. What happens to a balloon as it rises higher in the sky? It expands.
As the balloon ascends, the outside air pressure drops. According to Boyle’s Law, as the pressure drops, the volume of air inside the balloon increases. Because the balloon is capable of stretching, the expanding volume of gas inside the balloon causes the container (the balloon) to expand too.
Dalton’s Law says…
The total pressure of a mixture is equal to the sum of the partial pressures of each gas in the mixture
P = P1 + P2 + P3…
The partial pressure of a gas in a mixture is equal to it’s percentage within that gas X the pressure of that gas.
Sorry, but I don’t have a good way to remember Dalton’s Law. If you know of one, please share with the group.
The most important example of Dalton’s Law is the partial pressure oxygen in a volume of air.
We know that at Sea Level the Standard Atmospheric Pressure is 760 torr, and that Air is comprised of 21% oxygen, 78% nitrogen, and 1% other gases.
If we apply Dalton’s Law to normal air at standard sea level pressure than…
The partial pressure of oxygen in air (P1) = 0.21 X 760 torr.
P1, or the partial pressure of oxygen, then equals 160 torr at sea level.
So how do these effect my patient?
Boyle’s Law Effects:
ETT / Foley / Blakemore / Other Cuffs – Expansion of Cuff Can Cause Tissue Necrosis
Watch Your Cuff Pressures
Air Left Inside an IV Bag – Expansion of Air Causes Infusion Rates to Increase as Altitude Increase (Unpressurized Cabin)
Remove ALL Air From IV Bag Before Flight
Air Inside Pleur-Evac – If Not Vented Can Cause Re-Collapse of Lung
Vent or Place to Suction
Air Trapped Inside Body Cavity – Air Expansions Can Cause Problems
Small Pneumothorax can Tension
Pneumocephalus (Air Trapped in the Skull) Can Cause Neurological Damage
Air Trapped Inside Newly Applied Cast Can Expand and Damage Cast
Cast Should be Cut Along Lateral and Medial Sides and Wrapped with Elastic Bandage to Allow for Expansion
Explosive Decompression at High Altitudes can Result in Oxygen Actively Leaving Blood Causing Rapid Hypoxia and Loss of Consciousness due the partial pressure of oxygen being higher inside the blood compared to the partial pressure of oxygen in the atmospheric air.
Crew Members May Require Oxygen At Higher Altitudes in Unpressurized Cabins (See Effects of Altitude and Zones of Hypoxia).
Patients May Require Supplemental Oxygen Just Because You’re Flying At Altitude.
Patients Who Already Receiving Supplemental Oxygen to Maintain Normal Oxygenation WILL Require Higher FiO2 During Flight.
I underlined the last point above because I consistently hear that this concept is covered on both Aeromedical Exams (FP-C and CFRN).
To determine the exact amount of oxygen a patient needs to maintain a constant level of blood oxygenation at a higher elevation requires a bit of math….., but there is a easy method to solve their oxygenation problem.
First the Easy Method… Ready?
JUST TURN UP THE FIO2 A LITTLE BIT!
Now I’ll explain the math and give the rational behind my simple answer above. But before I get into this, there’s a few things we need to know about changes in air pressure and converting one atmospheric pressure to another.
First, air pressured can be expressed in several ways: inHg, mmHg, torr, but the most useful for our purpose is mmHg or torr.
Second, to convert from inHg (inches of Mercery) to mmHg (a more useful number) you multiple inHg by 25.4.
Third, as you ascend, the atmospheric pressured drops by about 25 mmHg / 1000ft at lower altitudes and closer to 20 mmHg/ 1000ft at higher altitudes. (Click here).
(These are approximations, but are close enough for what we’re doing. I will generally just us 25mmHg/1000ft to ensure my calculations are at least what the patient needs).
Forth, you need to know the formula for determining the required FiO2, so here it is…
(Starting FiO2 x Starting Air Pressure) / Air Pressure at Altitude = FiO2 Requirements at Altitude
So Now What?!?
Air Pressure is most often reported in either inches of Hg (i.e. 29.92 – Standard Air Pressure at Sea Level) or mm Hg (i.e. 760mm Hg Standard Air Pressure at Sea Level, This is the same as 760 torr)
As I said before, to convert from inHg to mmHg, which is more useful for our calculations, multiple inches of Hg by 25.4.
Example: 29.92 in Hg x 25.4 = 760 mm Hg (Standard Air Pressure at Sea Level). See how their the same?
Here’s a picture of the altimeter in the aircraft I fly in. Notice the pressure is in the lower left is in inHg.
(You’ll also notice the altitude is 7400ft. That’s the altitude of the base I fly out of so we’re already starting quite high)
Using the formula above we get…
30.21 X 25.4 = 767 MMHG OR TORR
767 mmHg is the Atmospheric Pressure at ground level. This is important to know because it gives us a starting point for determining the patients required FiO2 once we get to altitude during the flight.
Hint! If you’re flying in an unpressurized cabin, figure out the air pressure once you land at the sending facility or scene and and write it down in mm Hg.
Now, lets take, for example, a patient who’s vented on 35% FiO2 that we take from the hospital near my base. During our typical flight our max altitude is around 9000ft. That’s a gain of about 2000ft.
If I wanted to titrate my patients FiO2 to an FiO2 necessary for them to maintain a constant oxygenation level before leaving the referring facility, I could take the information I already know…
Air pressure at the sending facility = 767 torr
The patients current FiO2 demands = .35
And the anticipated air pressure once we reach cruise altitude of 9000ft = 717 mmHg
…and calculate what FiO2 my patient will require once we reach altitude.
What Do You Notice?
Right! From 35% to 37%. Not a big difference. That’s why I said earlier, just turn it up a bit.
Now I will caution you, that’s not always the case. Let’s look at another EXTREME example.
Say you pick up a patient from sea level (We’ll use standard atmospheric pressure to keep it easy), and during your flight you’ll have to cross a high ridge where your pilot will have to climb to 10,000 ft MSL.
What can you expect you’re patients FiO2 requirements to be if their requiring 35% FiO2 when you pick them up?
.35 X 760 / 510 = 52%
That’s a 17% increase. Still not that much more.
For test purpose, you’ll likely get a questions that will ask you what FiO2 your patient will need if you’re going from one altitude to another. If you remember that a few thousand foot altitude change requires a few additional LPM of O2, or a few more percentages of FiO2, you’ll be fine.
Unless you’re making HUGE changes in altitude, you’re patient shouldn’t need more than a few more liters per minute.
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