Clinical application of oxygen therapy

The purpose of oxygen therapy is to improve arterial oxygen partial pressure, oxygen saturation and oxygen content to correct hypoxemia, ensure oxygen supply to tissues, and achieve the purpose of relieving tissue hypoxia. Oxygen, like medicine, should be used correctly. Oxygen therapy is clearly indicated, has its flow, and should be aided by clinical observation and laboratory tests to help estimate the appropriate flow.

1. Indications of oxygen therapy

(1) Cardiac and respiratory arrest

Patients with cardiac arrest or respiratory arrest for any reason should receive oxygen therapy immediately during resuscitation. However, it should be noted that if the patient is not breathing, a simple respirator can be used, or a respirator or an anesthesia machine can be used to pressurize oxygen for tracheal intubation.

(2) Hypoxemia

Regardless of the underlying disease, it is an indication of oxygen therapy. From the oxygen dissociation curve, PaO2 is lower than 8.0kpa (60mmHg), indicating that it is on the edge of decompensation, and a slight decrease in PaO2 will produce a significant decrease in oxygen saturation. According to blood gas analysis, hypoxemia is divided into two types. 

①Hypoxemia with hypercapnia: Hypoxia caused by insufficient ventilation is accompanied by carbon dioxide retention. Oxygen therapy can correct hypoxemia, but it does not help carbon dioxide discharge. If it is not used properly, it can aggravate carbon dioxide retention.

② Simple hypoxemia: generally caused by diffusion dysfunction and imbalance of ventilation/blood flow. Diffusion dysfunction, hypoxemia can be corrected satisfactorily by increasing the concentration of inspired oxygen, but the intrapulmonary shunt caused by the imbalance of ventilation/blood flow, oxygen therapy is not ideal, because oxygen therapy is not helpful for arteriovenous shunts caused by non-ventilated alveoli.

(3) Tissue hypoxia

In cases of decreased cardiac output, acute myocardial infarction, and anemia, there may be no obvious hypoxemia, but tissue hypoxia may be present. At this time, the determination of PO2 in mixed venous blood can be used as an indicator of tissue oxygenation. When oxygen therapy is effective, tissue hypoxia is improved, and the PO2 of mixed venous blood can reach more than 4.67kPa (35mmHg).

2. The purpose of oxygen therapy

(1) Correction of hypoxemia

Oxygen can increase the partial pressure of oxygen in the alveoli, increase the oxygen diffusion capacity, increase the partial pressure of oxygen in the pulmonary capillaries, correct the hypoxemia caused by the imbalance of ventilation/blood flow and diffusion dysfunction, and increase PaO2.

(2) Reduce the work of breathing

The response to hypoxemia is usually an increase in the work of breathing. Oxygen therapy can restore the gas exchange in the lungs to a more normal level to maintain an appropriate alveolar oxygen partial pressure, reduce the total ventilation, reduce the work of breathing, and reduce oxygen consumption.

(3) Reduce the load on the heart

The response of the cardiovascular system to hypoxia and hypoxemia is to increase the heart rate and increase the work of the heart. Oxygen therapy can effectively reduce the work of the heart and reduce the load of the heart.

3. The method of oxygen therapy

At present, oxygen therapy methods can be divided into two categories: low-flow systems and high-flow systems according to the rate of oxygen flow. The airflow supplied by the low-flow system cannot fully meet the needs of the inhaled air volume, so indoor air must be provided to supplement part of the inhaled air; the high-flow system can fully meet the needs of all the inhaled air volumes.

In the past, the flow of oxygen supplied by nasal cannula was used as a low-concentration oxygen supply technology. This so-called "continuous low-flow" oxygen supply was popular for a while. Many people thought that "low-flow oxygen supply" was the same as "low-concentration oxygen supply". Nouns, in fact this view is incorrect. Because the oxygen flow is only related to the flow of all gases, and the concentration of inhaled oxygen is another different concept. The oxygen inhalation concentration provided by various oxygen flow rates is only determined by different equipment and the patient's own factors. Low-flow system oxygen supply can provide low-concentration oxygen or high-concentration oxygen; and high-flow system oxygen supply can also provide oxygen from low concentration to high concentration.

High-flow system oxygen supply

The system provides the full inspiratory volume, in other words, the patient breathes only the gas from the system. The characteristic of high-flow system oxygen supply is that it can provide stable oxygen concentration, including oxygen from low concentration to high concentration, and the inhaled oxygen concentration is from 24% to 70%. Therefore, high-flow oxygen supply is not the inhalation of high-concentration oxygen.

The most commonly used high-flow oxygen system is the Ventruri mask. The principle is that high-speed oxygen is injected through a limited pipe, and a negative pressure is generated around it, that is, the Bernoulli principle of gas flow, and the surrounding air is inhaled from the side hole, so that the air enters the inhalation airflow. By changing the oxygen flow rate and outflow diameter, and adjusting the size of the side holes on the pipe wall, the high volume of inhaled air can be controlled, thereby adjusting the concentration of inhaled oxygen to reach a predetermined level.