Hypoxia

Hypoxia, or tissue hypoxia, is defined as a decrease in the supply of oxygen to the body’s cells, which disrupts the normal function of organs and tissues. A total interruption of oxygen supply is called anoxia.

Depending on the cause of insufficient oxygen supply, different types of hypoxia are distinguished:

• Hypoxic or hypoxemic hypoxia: A disorder in the pulmonary ventilation process leads to deficient delivery of atmospheric oxygen to the blood, causing a decrease in arterial oxygen pressure.
• Anemic or hypemic hypoxia: The amount of oxygen transported in the blood is insufficient due to an abnormally low concentration of hemoglobin, the protein responsible for transporting oxygen from the lungs.
• Ischemic, circulatory, or stagnant hypoxia: The respiratory and transport processes function normally, but a circulatory disorder prevents the proper delivery of oxygen to tissues.
• Cytotoxic or histotoxic hypoxia: Cells are unable to properly utilize the oxygen they receive.

Based on its progression, hypoxia can be classified into:

• Acute hypoxia: A sudden decrease in oxygen supply, often due to an airway or arterial obstruction.
• Chronic hypoxia: Develops slowly, usually due to diseases that progressively impair lung function.

Hypoxia, particularly the hypoxic and ischemic types, is one of the most common physiological effects of aviation, caused by altitude and atmospheric pressure changes inherent to flying.

Symptoms

The symptoms of hypoxia vary depending on the level of oxygen deficiency, the individual's physical condition, and their tolerance to oxygen deprivation. The most common symptoms are associated with the central nervous system, which is highly sensitive to oxygen deficiency. These symptoms include:

• Headache.
• Dizziness and nausea.
• Lethargy and drowsiness.
• Mood changes: euphoria, anxiety, or aggression.
• Difficulty concentrating.
• Mental confusion.
• Loss of muscle coordination.
• Pale skin.
• Cyanosis: Bluish discoloration of the skin, especially in the extremities.
• Shortness of breath.
• Vision problems: blurred vision or tunnel vision.
• Tingling sensation in extremities.
• Seizures.
• Loss of consciousness.

During flight, symptoms appear in different phases as altitude increases:

• Indifferent phase: Up to 10,000 feet. Oxygen saturation decreases to 87%. In healthy individuals, no significant alterations occur, though night vision and immediate memory may decline.
• Compensatory phase: Between 10,000 and 15,000 feet. The body activates compensatory mechanisms such as increased lung ventilation and cardiac output. Symptoms include headache, fatigue, drowsiness, respiratory difficulty, and impaired memory and concentration.
• Clinical or symptomatic phase: Between 15,000 and 20,000 feet. Compensatory mechanisms become ineffective, neurological symptoms worsen, and cyanosis appears.
• Critical phase: Above 20,000 feet. Oxygen saturation drops to 65%. The individual experiences seizures and loses consciousness. Without immediate treatment, it can be fatal.

Causes

Each type of hypoxia has specific causes:

• Hypoxic hypoxia:
• Altitude exposure: As altitude increases, atmospheric pressure decreases, reducing the amount of inhaled oxygen. This also occurs in cases of sudden cabin depressurization.
• Lung disorders that impair ventilation, such as pneumonia, emphysema, bronchial asthma, or respiratory failure.
• Inhalation of gases such as nitrous oxide or laughing gas, which reduce oxygen availability while increasing carbon dioxide.
• Apnea or temporary breathing cessation.
• Anemic hypoxia:
• Physiological anemia.
• Nitrate or carbon monoxide poisoning.
• Blood loss.
• Inhalation of carbon dioxide from smoking.
• Ischemic hypoxia:
• Cardiovascular disorders.
• Cold exposure, which can cause arterial vasoconstriction.
• High accelerations and gravitational changes during flight, decreasing blood pressure above the heart while increasing vascular pressure below. This occurs particularly in combat or aerobatic flights.
• Cytotoxic hypoxia:
• Poisoning from substances such as cyanide, carbon monoxide, alcohol, or drugs.

Risk Factors

Factors that increase the likelihood of hypoxia include:

• Intense physical activity: The body requires more oxygen.
• Temperature: Extreme temperatures activate compensatory mechanisms, increasing oxygen consumption.
• Alcohol consumption.
• Smoking.
• Physical condition: Better physical fitness allows for more efficient oxygen use and greater hypoxia tolerance.
• Stress or anxiety: Increased oxygen consumption.

Specific aviation risk factors include:

• Ascent speed: Faster ascent results in quicker symptom onset.
• Altitude: Symptoms worsen as altitude increases.
• Time spent at a given altitude: Proportional to symptom severity.

Complications

Prolonged hypoxia can cause severe damage, particularly in cases of cerebral hypoxia. Brain cells are highly sensitive to oxygen deprivation and can begin dying within minutes of oxygen supply interruption (anoxia). This can lead to coma or death. Even if reversible, coma may result in permanent motor impairments, such as spasms or fasciculations.

Prevention

To minimize the risk of hypoxia during flight, it is essential to ensure proper cabin pressurization and have an emergency oxygen supply. All cabin crew must be trained to recognize and treat hypoxia. Proper use of anti-gravity suits in combat pilots is also crucial. Maintaining good physical condition, avoiding harmful substances such as alcohol, tobacco, and drugs, is key to better tolerating hypoxia.

Which Doctor Treats Hypoxia?

A multidisciplinary team is required, including specialists in intensive care, pulmonology, neurology, and angiology. Aviation-related hypoxia is studied in aerospace medicine units.