Can hyperbaric oxygen therapy cause oxygen toxicity?

Author: Liu Yaling Beijing Tiantan Hospital, Capital Medical University

Reviewer: Du Juan, deputy chief physician, China Rehabilitation Research Center

Will hyperbaric oxygen therapy cause oxygen poisoning? This is a topic of great concern to many clinical medical staff, patients undergoing hyperbaric oxygen therapy, and their families.

1. What is the high pressure in hyperbaric oxygen?

The high pressure in hyperbaric oxygen is relative to normal pressure. Normal pressure is 1 standard atmospheric pressure. Generally, the air pressure at sea level under standard atmospheric conditions at 0°C is called 1 standard atmospheric pressure, which is 760 mmHg. Atmospheric pressure changes with altitude. The air pressure is low at high altitudes, such as Tibet, which has a high altitude and low air pressure, while the air pressure is high below sea level. The average altitude of Beijing is more than 40 meters. We roughly think that the living environment is 1 atmosphere, which is the normal pressure just mentioned, and anything higher than this pressure becomes high pressure. Of course, air pressure is also related to temperature changes. The pressure cookers commonly used in our lives use the high-pressure environment formed by rising temperature.

2. Are hyperbaric oxygen and hyperbaric oxygen therapy the same?

Although the difference between hyperbaric oxygen and hyperbaric oxygen therapy is only the word "treatment", their meanings are very different. Hyperbaric oxygen is a relatively broad concept. As long as oxygen is inhaled in an environment higher than 1 absolute atmosphere (ATA), it can be called hyperbaric oxygen. Theoretically, its lower limit of air pressure is 1ATA; the upper limit of air pressure can be infinite. However, hyperbaric oxygen therapy has very strict requirements on ambient air pressure and oxygen inhalation time. As early as 2003, the American Undersea and Hyperbaric Medical Association recommended that the maximum dose of hyperbaric oxygen used in clinical treatment be limited to: 3ATA for 2 hours (except for special diseases). Hyperbaric oxygen exceeding this dose may cause oxygen poisoning.

3. The process of hyperbaric oxygen therapy

The hyperbaric oxygen chamber is a huge, airtight metal chamber that can withstand high pressure. Normal pressure air from the outside can be continuously transported into the chamber. Since the metal chamber is pressure-resistant, the volume of gas it contains is relatively constant. As the amount of gas in the chamber continues to increase, the gas pressure in the chamber continues to rise, and high pressure is formed in the chamber. During hyperbaric oxygen therapy, the patient needs to go through three stages in the hyperbaric oxygen chamber: pressure increase, pressure-stabilized oxygen inhalation, and decompression.

(Hyperbaric oxygen chamber for patients to breathe oxygen while sitting)

Figure 1 Copyright image, no permission to reprint

The pressurization stage refers to the process in which the air pressure in the cabin gradually rises from normal pressure to the target high pressure after the patient enters the cabin and the hyperbaric oxygen cabin door is closed. The pressurization time is generally 15 to 30 minutes. During this stage, the gas breathed by the patient is compressed air with a uniformly rising air pressure in the cabin.

The pressure stabilization stage refers to the process in which the patient intermittently or continuously inhales 100% pure oxygen at the target high pressure, which lasts about 50 minutes to 1 hour.

The decompression phase refers to the process of reducing the air pressure in the cabin from the target high pressure to normal pressure, which usually takes 15 to 30 minutes. During this process, the patient breathes compressed air with the cabin pressure continuously decreasing. After the decompression is completed, the cabin door is opened and the patient leaves the cabin. The entire hyperbaric oxygen therapy process is over.

IV. Historical Oxygen Poisoning Incidents

The study of oxygen poisoning has a history of more than 140 years. In 1878, French physiologist Paul Bert first described the epileptic symptoms of skylarks with body convulsions at an air pressure of 15 to 20 ATA, which was called the Paul Bert effect. He believed that this was a manifestation of brain tissue damage caused by oxygen poisoning.

Soon after, in 1899, J Lorain Smith found that rats that inhaled 73% oxygen for four days under normal pressure developed fatal pneumonia. This was called the Smith effect, which is a manifestation of lung damage caused by oxygen poisoning.

Later, scientists divided oxygen poisoning into central, pulmonary, ocular and hemolytic types according to the site of organ damage caused by oxygen poisoning. The corresponding damage sites are brain tissue, lungs, eyes and blood system.

From the above history, it is not difficult to see that oxygen poisoning can occur in high-pressure environments as well as in normal-pressure environments. Brain tissue damage caused by oxygen poisoning is more common in short-term high-pressure environments (air pressure is more than 3 times normal pressure). Lung tissue and eye damage caused by oxygen poisoning is more common in normal-pressure environments, when high-concentration oxygen is inhaled for a long time.

5. Under what circumstances will oxygen poisoning occur?

This is related to the imbalance between the human body's oxidation system and antioxidant system. Under normal pressure, the oxidation system and antioxidant system of a normal body are in dynamic balance to maintain the body's health. Simply put, after the human body inhales oxygen, a series of substances with oxidative properties such as reactive oxygen are produced after oxygen metabolism, which we call the oxidation system. Normal levels of reactive oxygen play an important role in cell signal transmission and maintaining human functions. However, if the content of reactive oxygen increases abnormally, it will cause serious damage to the cell structure. Of course, the human body also has an antioxidant system, including some active enzymes. When inhaling oxygen under high-pressure oxygen, the initial physiological reaction of the human body is an increase in oxidative substances such as reactive oxygen, followed by the active enzymes with antioxidant capacity in the human body. Under conventional high-pressure oxygen therapy, the oxidation system and the antioxidant system can achieve dynamic balance; if the air pressure is too high or the oxygen inhalation time is too long, the amount of reactive oxygen produced will exceed the body's antioxidant system's clearance capacity, and the oxidation system and the antioxidant system will be unbalanced, causing damage to tissues and organs. This is why oxygen poisoning occurs.

6. Will hyperbaric oxygen therapy cause oxygen poisoning?

Scientific research shows that the occurrence of oxygen poisoning is closely related to the ambient air pressure, oxygen concentration and oxygen inhalation time of the human body. Whether it is the Paul Bert effect caused by oxygen poisoning brain tissue damage or the Smith effect caused by lung damage, it is unlikely that the above-mentioned air pressure, oxygen concentration and oxygen inhalation time will be reproduced in the clinical treatment of patients in various hospitals in China, and oxygen poisoning will hardly occur. Despite this, scientists and hyperbaric oxygen professionals still hold a rigorous and cautious attitude towards oxygen poisoning; this is an important reason why the dose of hyperbaric oxygen therapy is strictly limited to 3ATA 2 hours. This limited range neither reaches the air pressure several times or even dozens of times higher than normal pressure required for oxygen poisoning to cause brain tissue damage, nor does it reach the high concentration of oxygen inhalation time of several days required for pulmonary and ocular oxygen poisoning. Therefore, oxygen poisoning rarely occurs clinically.

(Hyperbaric oxygen chamber for patients to breathe oxygen in supine position)

Figure 2 Copyright image, no permission to reprint

The domestic hyperbaric oxygen therapy program is generally 1.6-2.8ATA for 1 hour of continuous or intermittent oxygen inhalation. The target pressure and duration of oxygen inhalation are lower than the 3ATA for 2 hours recommended by the American Undersea and Hyperbaric Medical Association, so the probability of oxygen poisoning is lower. Due to the inconsistency of hyperbaric oxygen therapy programs, patients, and oxygen inhalation equipment, the incidence of hyperbaric oxygen poisoning fluctuates between 1/10,000 and 9/10,000.

Although the incidence of oxygen poisoning is extremely low in a hyperbaric oxygen therapy environment, we still need to remain vigilant. Brain tissue damage caused by oxygen poisoning can manifest as tremors in the face or limb muscles, weakness in the hands and feet, and convulsions in the limbs. Lung damage caused by oxygen poisoning is mainly manifested by chest pain, irritating dry cough and other symptoms. Of course, the above symptoms are not specific and are likely to be caused by other reasons. This requires a hyperbaric oxygen professional to carefully identify them according to the patient's condition and treatment plan.

Therefore, as to whether hyperbaric oxygen therapy can lead to oxygen poisoning, our conclusion is: First, hyperbaric oxygen can cause oxygen poisoning under conditions of excessively high air pressure and excessively long oxygen inhalation time; second, oxygen poisoning generally does not occur under the current conventional domestic hyperbaric oxygen therapy regimen.