This is the 3755th article of Da Medical Care
The importance of oxygen to the human body is self-evident. For example, if a person lacks water, he can live for a week, but if he leaves oxygen, he will not be able to survive for several minutes. The main role of oxygen in the human body is to participate in the aerobic respiration of cells, through the catalytic action of enzymes, the biochemical energy in nutrients is converted into adenosine triphosphate, supplying cells and organs to maintain normal physiological activities.
Oxygen intake by the human body is a complex physiological process. First, the air is inhaled into the lungs through the chest cage, and then the oxygen in the air diffuses into the capillaries in the alveoli, and is transported to various parts of the body in combination with hemoglobin, at which time the oxygen is separated from the hemoglobin, and then diffuses into the tissues and cells for biochemical reactions. Any obstacle in any part of this process can cause a lack of oxygen in the human body.
In special cases, the body can also produce energy through anaerobic metabolism. For example, when performing strenuous physical activity, because the rate of aerobic metabolism is much slower than that of anaerobic metabolism, muscles will use anaerobic metabolism to quickly supply energy to meet the large need for energy in a short period of time. But consuming the same glycogen, anaerobic metabolism produces only one-seventh of the energy produced by aerobic metabolism, which is obviously a very uneconomical method of productivity and can only be used as an emergency means. Moreover, anaerobic metabolism is only a local energy supply in the tissue, and for the human body as a whole, it is impossible to leave the oxygen supply for a moment.
Since oxygen is so important to people, is it better to inhale more oxygen? From an evolutionary perspective, humans have adapted to surviving in the atmosphere with an oxygen content of 21%, and have adapted to maintaining the partial pressure of blood oxygen in the body at a level of about 100 mmHg. In the 1940s, people inhaled high concentrations of oxygen to preterm infants to improve survival rates, but found that a large number of children were blind, and oxygen inhalation can induce retinopathy in preterm infants. Prolonged high-concentration oxygen inhalation will also damage alveolar epithelial cells, and the oxygen concentration should be controlled below 60% as much as possible. Oxygen inhalation can also easily lead to atelectasis, which aggravates carbon dioxide accumulation in patients with type II expiratory disease. If you are inhaling oxygen at a high concentration under high pressure, such as diving, it is even more life-threatening. For example, inhaling pure oxygen for 2 hours at 2 atmospheres may cause central nervous system damage; if it is inhaled at 3 atmospheres, it may be fatal for several minutes.
Why is it that oxygen that we can't leave for a moment can cause so much harm to the human body? Oxygen is essentially a highly active chemical that produces reactive oxygen species in addition to producing adenosine triphosphate to power cells, which is chemically very reactive and can destroy nucleic acids, proteins and lipid membranes, leading to cell death. Under normal circumstances, 98% of the oxygen entering the body produces energy through aerobic metabolism, and the other 2% can be converted into oxygen free radicals, which can be neutralized by antioxidants in the body without harm. But when we inhale oxygen in high concentrations, or the body is in trauma. In states such as ischemia, the oxygen ingested may be converted into a large number of reactive oxygen radicals with lethality, which flow around in the body, attacking and killing various cells, resulting in the so-called "oxidative stress" imbalance.
Almost all patients in the ICU will receive oxygen, and in the past, doctors used to think that oxygen can improve patient safety and avoid patient hypoxia. However, studies have found that giving oxygen to ICU patients too aggressively may increase mortality. For patients with septic shock and ARDS, which are common in the ICU, the partial pressure of oxygen in the patient should be avoided in patients greater than 100-120 mmHg. For patients with myocardial infarction, oxygen has always been routinely given to avoid myocardial hypoxia. However, high concentrations of oxygen may exacerbate coronary artery contractions. In patients without hypoxemia, oxygen is not only not beneficial, but can also be harmful. In patients after CPR, restricted oxygen is also preferable to free oxygen.
In recent years, ICU doctors have become more and more aware of the harm of hyperoxia to patients, emphasizing that the patient's blood oxygen level should be controlled within the normal range to avoid excessive oxygen inhalation or unnecessary oxygen inhalation. This may in turn increase mortality in critically ill patients.
Author: Shanghai Songjiang District Central Hospital
Emergency Critical Care Department
Wang Xuemin