“Altitude Related Hypoxia”
By:
While studying for the Flight Instructor Rotorcraft Helicopter (FHR) exam, one of the questions posed to the candidate was the following; (Let’s see if you not only get it right, but understand)
During a climb to 18,000 feet, the percentage of oxygen in the atmosphere:
- Remains the same.
- Increases.
- Decreases.
The Aeronautical Information Manual, paragraph
This is the textbook answer and rationale that the Federal Aviation Administration (FAA) poses to our fellow Pilots; however, as a Flight Paramedic and national lecturer on the topic for the last 10 years, I wanted to expound on this topic a little more in depth.
Although I am also a Commercial rated Helicopter Pilot and Instrument Instructor (IGI), the majority of my experience has focused entirely on the air medical side of the operation, managing and working with several Part 135, CAMTS Accredited (Commission of Accreditation of Air Medical Programs) HEMS programs.
The theory of the question above can be explained by a gas law called
Now, why is this important to us, as Pilots? “I am the PIC, I will let the medical folks worry about this…I do not fly high enough to worry about this…”
This gas law explains why one gets hypoxic at altitudes, which is why it is mandated to provide oxygen to all persons at altitudes above 15,000 ft MSL. Hypoxia has several types, which we will briefly go over in this article.
They are:
- Hypoxic Hypoxia
- Hypemic Hypoxia
- Histotoxic Hypoxia
- Stagnant Hypoxia
Hypoxic Hypoxia is an altitude related hypoxia, caused by a decrease in Barometric Pressure and lapse rate as we climb. It is true, there is roughly 21% oxygen on the ground as we breathe and as we climb in altitude; let’s just say to 35,000 feet…, it is still the same 21% oxygen. The problem is that the oxygen molecules are now farther apart because of the decrease in Barometric pressure.
Hypemic Hypoxia is almost as it sounds. Hypemic almost sounds like Anemia. And we know what Anemia is; it is a lack of circulating red blood cells (hemoglobin) that carry the oxygen around the circulatory system to the vital organs of the heart, lungs, brain, kidneys, etc. Hypemic Hypoxia can be from severe blood loss, or anemia itself.
Histotoxic Hypoxia sounds like Histology, whereas “Histo” means poisons or toxins. This does not necessarily mean someone is doing or taking drugs, which it can though. It can be alcohol, sleeping aids the night before, anti-histamines (Which increases oxygen metabolism, and subsequently can pose dangers with
Stagnant Hypoxia is resultant from poor cardiac output, Cardiogenic Shock (Heart Transplant patients) blood pooling, stagnating blood, pulling excessive G-forces, or can even be the position the patient is loaded in the helicopter. If we fly a head injury patient, and we were to theoretically place them, feet facing forward on a stretcher…as the aircraft departs nose down entering Effective Translational Lift, we are pulling G-forces that may be unnoticed to the crew, but the effects of blood rushing to the head of the patient can be fatal, as the increases in Intracranial Pressure (ICP) causes no blood flow to the patient brain, which results in a permanent damage, usually recognized by stroke like symptoms.
Another commonly confused gas law is Boyle’s Gas Law. Boyle’s gas law is directly related to this above scenario as well. Think about putting a balloon in the back seat of an aircraft. As we climb or ascend, the balloon will expand. As we descend, the balloon will contract or get smaller. This gas law is a direct relationship of Barometric Pressure and expansion of gases. Whether it is a balloon we are talking about, or a patients chest tube drainage system, IV fluids and drip rates, Head Bleed, or collapsed lung (Pneumothorax). The resultant effects are the same; as we climb, gases expand and as we descend, gases contract. An easy way of remembering this is to relate Boyle’s with Balloon.
Henry’s Gas Law deals with expansion of gases from solution, as pressure is reduced. What happens if we have a can of soda that was shook up and we opened it? By shaking the can, we increased pressure inside the can and when the pressure is reduced, we have expansion of gases as well as gases coming OUT OF solution. This is an important detail as there is a very well known condition in dive related injuries termed “The Bends.”
So; in closing, let’s end with one more question from the FAA’s Rotorcraft Helicopter Instructor question test bank, and see if we understand this better now.
What physical change would most likely occur to occupants of an unpressurized aircraft flying above 15,000 feet without supplemental oxygen?
- Gases trapped in the body contract and prevent nitrogen from escaping the bloodstream.
- A blue coloration of the lips and fingernails develop along with tunnel vision.
- The pressure in the middle ear becomes less than the atmospheric pressure in the cabin.
Did we get the correct answer of #2? Let’s try one more.
Hypoxia is the result of
- Excessive nitrogen in the bloodstream.
- Decreasing amount of oxygen as your altitude increases.
- Reduced barometric pressures at altitude.
Answer #3.
In future articles, we will discuss diving after flying and vice versa. Transporting patients that have dive related injuries and the proper means of doing so. We have plans on addressing weather related issues surrounding transport of ill or injured patients, and many others.
In closing; stay safe, stay diligent, always check with an Aviation Medical Examiner or Flight Surgeon about any prescriptions we may be taking, and never forget that if it does not feel right, than it probably isn’t.
