Saturated

Now oximeters are over the counter, less expensive, and ubiquitous, and they have made general aviation pilots safer at all altitudes.

How do pulse oximeters work and how are they useful in the general aviation cockpit? Pulse oximeters emit two types of red light—one that is continuous and one that pulsates very quickly—that is not detectable by the naked eye. The solid light picks up the background color of the circulating blood in the fingertip or the earlobe. The pulsating light analyzes the pulsatile nature of circulation. An extremely complicated computer algorithm calculates the “redness” of the blood, which determines the oxygen content in a relative sense.

The readout is in a percentage of saturation. Most U.S. models are quite accurate, but some of the Chinese imports can have significant variations as well as adverse effects from vibration common in aircraft cockpits. The imports also tend to be less sturdy. The majority of U.S.-made models are manufactured by Nonin or Masimo.

Keep in mind that cold fingers or very low blood pressure can prevent the pulse oximeter from working properly. You should see a green flashing light in order to rely on the pulse oximeter’s reading.

Normal pulse oximetry readings at sea level should be between 95 percent and 100 percent. At 6,000 feet msl where I live, normal saturation should be 90 percent to 95 percent. This is important because people who live for more than 90 days in a row in high-altitude locations compensate for the lower blood saturation by increasing the number of red blood cells circulating in their system.

This is why you will hear people say those who live at altitude have “thick blood.”

Even if flying at lower altitudes like 6,000 to 10,000 feet msl, wearing oxygen for the last few minutes of the flight will make your landings better.The FAA’s 12,500 feet msl/14,000 feet msl rule is based on geography, not physiology. These rules were established in the 1950s when oxygen systems were expensive and heavy. The numbers that eventually came out of the FAA allowed general aviation pilots to fly anywhere in the continental United States without having to carry oxygen, since airplanes can cross most mountain passes within 30 minutes and not have to climb above 14,000 feet where one must use oxygen at any time.

However, many pilots need oxygen at far lower altitudes, particularly at night. The retina is the tissue of the body most sensitive to hypoxia. It is always entertaining to have pilots fly at night around 8,000 feet without oxygen and then have them put on oxygen, thus oxygenating the retina. This causes both the lights in the cockpit and the lights on the ground to brighten like a rheostat has been turned up. If you have not tried this, you should, as it is a dramatic lesson.

My recommendation when we introduced pulse oximetry was for pilots to take their saturation at their home airport. They should use oxygen when their saturation drops five percentage points. They must use oxygen if their saturation drops 10 percentage points.

Oxygen flow rates should be titrated up until the pilot’s saturation is near their home airport’s level. However, there is nothing wrong with using oxygen even when saturations appear normal.

Even if flying at lower altitudes like 6,000 to 10,000 feet msl, wearing oxygen for the last few minutes of the flight will make your landings better. Oxygen will also decrease fatigue on long flights.

It is important to remember that one thing fools pulse oximeters and that is carbon monoxide. Carbon monoxide actually makes blood “redder,” and thus will fool a pulse oximeter. It is always important to have a low-level digital carbon monoxide detector in the cockpit.

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