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 observations and laboratory tests to help estimate the appropriate flow.

First, the indication of oxygen therapy

(1) Cardiac and respiratory arrest Persons with cardiac arrest or respiratory arrest caused by 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 No matter what kind of underlying disease it is, it is an indication for 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 ideal for non-ventilated alveoli. Arteriovenous shunts are not helpful.

(3) Tissue hypoxia In cases of decreased cardiac output, acute myocardial infarction, and anemia, there may be no obvious hypoxemia, but tissue hypoxia may occur. 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).

Second, the purpose of oxygen therapy

(1) Correcting 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, and correct the hypoxic blood caused by the imbalance of ventilation/blood flow and diffusion dysfunction. Symptoms cause PaO2 to rise.

(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.

Three, the method of oxygen therapy

At present, oxygen therapy methods can be divided into two categories: low-flow system and high-flow system according to the size 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 supplied 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.

(1) High-flow system oxygen supply The system provides the full inspiratory gas 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 delivery system is the Ventruri mask. The principle is that high-speed oxygen is sprayed 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.

The high-flow system has the following advantages: ①As long as the system is properly adjusted, it can supply a lasting and correct oxygen concentration, and is not subject to changes in patient ventilation; ②It can control the temperature and humidity of the inhaled gas; ③It can monitor the inspired oxygen concentration. However, it should be noted that the high-flow oxygen supply system must meet the peak inspiratory flow rate of the patient, which should generally be at least 4 times the minute ventilation volume to ensure a constant oxygen concentration.

Four，Hyperbaric oxygen therapy

Hyperbaric oxygen uses 100% oxygen higher than 101.325kPa (latm), its purpose is to improve tissue hypoxia and inhibit the growth of anaerobic bacteria during anaerobic infection. Hyperbaric oxygen increases the amount of oxygen dissolved in the blood. When inhaling 100% oxygen at 303.975kPa (3atm), the dissolved oxygen in the plasma can reach 6.6ml, and the physically dissolved oxygen can be utilized by the tissue. The indications for hyperbaric oxygen therapy are ① hemorrhagic anemia; ② carbon monoxide poisoning; ③ acute cyanide poisoning; ④ acute gas embolism; ⑤ gas gangrene, etc.

Five， Monitoring the effect of oxygen therapy

Clinically, it can be judged from three aspects. ① Cardiovascular system response: After oxygen therapy, the consciousness, blood pressure, heart rate, heart rhythm, peripheral tissue perfusion (skin color, etc.) should be observed, and the urine output should be recorded. If the effect of oxygen therapy is ideal, the above indicators should be significantly improved. ②Respiratory system response: After oxygen therapy, dyspnea and shortness of breath should be improved, the breathing movement should be stable, the breathing frequency will be slowed down, and the work of breathing should be reduced. ③ Blood gas analysis showed that PaO2 increased.

Six， Side effects of oxygen therapy and oxygen toxicity

If oxygen therapy is used improperly, the following side effects may occur, and oxygen toxicity may occur in severe cases.

(1) Carbon dioxide retention

In hypoxemia, the decrease in PaO2 can stimulate the carotid sinus chemoreceptors, reflexively excite the respiratory center, and increase pulmonary ventilation. If the patient's breathing is maintained by this reflex excitation (such as pulmonary heart disease), after inhaling a high concentration of oxygen, the increase in PaO2 can eliminate this reflex mechanism, inhibit the patient's spontaneous breathing, and reduce alveolar ventilation, resulting in an increase in PaCO2, and even pulmonary encephalopathy may occur. Therefore, these patients should be given low-concentration oxygen inhalation, and the changes of PaCO2 should be monitored during oxygen therapy.

(2) Absorptive atelectasis

After high-concentration oxygen inhalation, a large amount of nitrogen in the alveoli is flushed out, and the partial pressure of alveolar oxygen gradually increases. When there is bronchial obstruction, the oxygen in the alveoli can be quickly absorbed by the blood flow of the pulmonary circulation, resulting in atelectasis.

(3) Oxygen poisoning

Long-term inhalation of high concentrations of oxygen can cause oxygen poisoning, reducing alveolar surfactant, inhibiting ciliary activity, congesting pulmonary capillaries, increasing permeability, causing alveolar effusion and pulmonary edema. Long-term oxygen toxicity can lead to pulmonary interstitial fibrosis. The risk of oxygen toxicity is determined by two factors. ①Inhaled oxygen concentration; ②Oxygen inhalation time.

1. Symptoms of oxygen toxicity

The early manifestations of oxygen toxicity are symptoms of tracheal irritation, such as uncontrollable dry cough, shortness of breath, and sharp retrosternal pain. These symptoms usually occur within about 6 hours after inhalation of 100% oxygen. The lung function may be normal in the early stage, and the lung capacity will decrease after 18 hours, and then the lung compliance will decrease. ARDS may be associated with pulmonary interstitial and alveolar fluid exudation within 24 to 48 hours. The clinical manifestations of hemoptysis may be due to damage to the pulmonary capillary epithelium. After 3 days, alveolar cells were affected, alveolar surfactant decreased, and bilateral diffuse infiltrates were seen on chest X-ray, and atelectasis was possible. Late stage manifests as pulmonary interstitial fibrosis and multiple organ dysfunction, and even death.

2. Prevention of oxygen

It is currently considered that 60% to 70% of the oxygen inhaled at 101.325kPa (latm) can be used safely for 24 hours; 40% to 50% of oxygen can continue to be used for 24 hours; If the oxygen concentration is greater than 40%, the possibility of oxygen toxicity increases greatly after 2 to 3 days. Therefore, patients who need oxygen therapy should be targeted, and the oxygen concentration cannot be blindly increased due to hypoxemia (if there is a right-to-left shunt in the lung, increasing the oxygen concentration is ineffective). Oxygen therapy should be supplemented with other necessary treatment measures, such as the application of bronchodilators, active expectoration, application of cardiac diuretics, etc. If necessary, PEEP can be used to keep the oxygen concentration below the level that can produce oxygen toxicity. At the same time, the PaO2 can reach a level of 8.0-9.33kPa (60-70mmHg).