Physiologic Effects of Flight

Practicing medicine at altitude has several challenges related to the effects of altitude on human physiology. This section will explain the details of the atmosphere, physiologic changes based on altitude, and the impact of hypoxia on the body. The information in this unit will form the foundation for future units focused on specific organ systems.

 

The Atmosphere

 

Composition

The atmosphere is primarily composed of oxygen and nitrogen. Regardless of the altitude, oxygen makes up 21% of the atmosphere and nitrogen 78%. The remaining 1% consists of various gases. 

Layers of the atmosphere

• The Troposphere is the layer of the atmosphere from 0-30,000 feet where the majority of rotor-wing (RW) flight occurs.
• The Tropopause is the layer of the atmosphere from 30,000-60,000 feet where the majority of fixed-wing (FW) flight occurs. 
• The Stratosphere is the layer of the atmosphere from >60,000 feet.

Physiologic zones

• The Physiologic Zone is 0-10,000' and is the most common zone for air medical flight, specifically rotor-wing (RW) operations. A healthy human being can breathe within this range without requiring additional oxygen.
• The Physiological Deficient Zone ranges from 10,000' to 50,000.' Without adaptations in this zone, there is insufficient oxygen to support human life indefinitely. When a fighter pilot dons his oxygen mask, he prepares to enter this zone.
• Space Equivalent Zone >50,000' - At this altitude, the pressure is too low to support human life even when breathing 100% oxygen. Flight at this altitude requires a pressurized craft or suit.

Barometric pressure

• Barometric pressure is the weight of air in association with its density. At sea level, the barometric pressure is 760 torr or 760 mmHg. The higher your altitude, the lower the barometric pressure because you have less dense air above you. Think of it like blankets over you in bed when the room is cold. The more blankets piled on, the heavier the feeling and the warmer you are. The fewer blankets covering you, the lighter they will feel and the colder you will become.

 

Hypoxia

Hypoxia is a deficiency in the amount of oxygen reaching the body's tissues. 

Types

• Hypoxic hypoxia is a lack of oxygen in the blood due to an inability to absorb oxygen. Hypoxic hypoxia can be found in patients with COPD, pneumonia, or airway obstruction.
• Histotoxic hypoxia occurs when oxygen is unable to bind to the hemoglobin. Carbon monoxide poisoning is a classic example of histotoxic hypoxia.
• Stagnant hypoxia is the failure to transport oxygenated blood. You can oxygenate all you want, but if the blood is not moving through the rest of the body, such as in cardiac arrest, it will not oxygenate the cells.
• Hypemic hypoxia occurs mainly in hypovolemic trauma patients. Low blood volume means that oxygenated hemoglobin is not getting to tissues.

Stages

There are four main stages of hypoxia:

The Indifferent Stage occurs between sea level and 10,000 feet of altitude. Oxygen saturation is typically between 98%-90%. The first sign of this stage, aside from the actual decrease in saturation, is reduced night vision.

The Compensatory Stage occurs between 10,000-15,000 feet of altitude. Oxygen saturation ranges from 89%-80% without supplemental O2. Symptoms seen are as follows:

• Drowsiness
• Poor Judgement
• Impaired coordination
• Impaired efficiency
• Blurred Vision

The Disturbance Stage occurs between 15,000-20,000 feet. The maximum oxygen saturation can range from 79%-70% without supplemental O2.
Symptoms seen are as follows:

• Drowsiness
• Poor Judgement
• Slurred or Garbled Speech
• Loss of muscle coordination

The Critical Stage happens between 20,000 feet and beyond. Typical maximum oxygen saturation is 69%-60% without supplemental O2 and can occur within 3-5 minutes of initial hypo-oxygenation.
Symptoms seen are as follows:

• Loss of muscle coordination
• Loss of consciousness
• Death

 

Other Terms to Know

ATM = Atmospheres
 Mostly referenced for underwater activities such as deep-sea diving, ATM measures distance in relation to atmospheric pressure. Each ATM = 33 feet of distance. 1 ATM is the atmospheric pressure at the surface of the ocean. Each ATM increment down in depth equals an increase in atmospheric pressure. 2 ATMs is a depth of 33 feet, 3 ATMs is 66 feet, and so on. As the amount of atmosphere increases, the atmospheric pressure increases. 

MSL = Mean Sea Level 
Atmospheric pressure at MSL is 14.7 pounds per square inch (psi) or 760 millimeters of mercury (mmHg). This is the average altitude at the water level of all major seas on earth. If you fly over the ocean, the pilot may relay distances from the ground as "X feet above MSL"

AGL = Above Ground Level 
AGL is the distance between the aircraft and the ground directly below it. Should you be flying over the mountains, the pilot may say that you are 2000' AGL and 6000' MSL, meaning that you are 2000 feet above the physical mountains, but 6000 feet above the level of the ocean.