Methods of Neonatal Oxygen Therapy

Oxygen therapy is an important part of neonatal respiratory therapy. The function of oxygen therapy is to provide an appropriate concentration of oxygen to increase blood oxygen partial pressure and blood oxygen saturation, so as to ensure the oxygen supply of tissues and eliminate or reduce the effect of hypoxia on the body. Negative Effects.

1. Indications for oxygen supply

There is no objection to the need for more oxygen for children with severe dyspnea, but whether to give oxygen to children with moderate dyspnea should be determined according to blood oxygen monitoring. Usually when inhaling air, oxygen should be considered when PaO2 is lower than 50mmHg. Because the oxygen dissociation curve is steep when PaO2 is lower than 50 mmHg, a slight decrease in PaO2 can cause a significant decrease in blood oxygen content.

2. Oxygen supply method

(1) Nasal cannula method: It is a low-flow oxygen method, but the actual FiO2 cannot be accurately estimated. Commonly used rubber tube or silicone tube is placed in the nasal vestibule, and the oxygen flow is 0.3-0.6L/min. This method is simple and suitable for neonates with mild disease. The disadvantage is that it can cause pain in the nasal alar, nasal secretions can block the mouth of the catheter, the catheter will be twisted, the child will open his mouth and cry, which can reduce the oxygen supply; too high flow can cause irritation of the nasopharynx and make the child uncomfortable.

(2) Paranasal tube method: open a narrow hole about 1cm long beside the nasal cannula, fix it in front of the nostril, close the broken end on one side, and connect the other side to the air source for oxygen supply, the flow rate is 0.5~1L/min. It is suitable for children in the convalescent period or those with less severe hypoxia. This method is similar to the nasal cannula method, and FiO2 cannot be accurately estimated.

(3) Oxygen supply by mask: The usual oxygen flow rate is 1-1.5L/min, which can be applied simultaneously with atomization inhalation. This method does not have the shortcomings of nasal oxygen supply, but pay attention to fixing the mask so that it is aimed at the mouth and nose of the child, so as not to affect the effect. At the same time, the mask should be removed intermittently to check the compressed parts of the skin, especially the ridge of the nose, to prevent skin damage.

(4) Oxygen supply to the head box: oxygen supply to the head box can often provide relatively stable FiO2. O2 and compressed air are often mixed, and the actual final FiO2 can be calculated by an air - oxygen mixer, or by the flow of oxygen and compressed air, respectively. Generally, the total flow required is 5~8L/min, and the oxygen concentration can be adjusted as required. When using the head box, attention should be paid to:

① Warm and humidify the input gas so that the temperature in the head box is in the neutral temperature range of the child, otherwise the cold air blowing on the baby's head and face will cause a cold reaction;

② The flow rate should be sufficient. If the flow rate is less than 5L/min, CO2 may accumulate in the box;

③ If the flow rate is too large, such as more than 12L/min, the head temperature of the child will decrease due to the rapid airflow, which will eventually lead to neonatal hypothermia.

3. Oxygen concentration 

The oxygen concentration depends on the patient's needs. The general oxygen supply concentration is to keep the patient's PaO2 at 50-80mmHg ( premature infants 50-70mmHg). To achieve the child's oxygen requirements without adverse consequences such as brain, eye, lung, FiO2 and PaO2 or arterial oxygen saturation (Sa02) must be monitored. If PaO2 is higher than 90~100mmHg, the blood oxygen is too high, and there is a risk of causing ROP in premature infants ( see Chapter 23, Section 2 for details ). In the early stage of severe RDS, FiO2100% may be required to maintain PaO2 at 50mmHg, and when it recovers, if FiO2 is not adjusted to decrease accordingly, the generated PaO2 may be > 200mmHg and cause oxygen toxicity. Therefore, it is important to adjust FiO2 at any time according to the measured PaO2.

4. Monitoring of inspired oxygen concentration and arterial blood oxygen level

(1) Inhaled oxygen concentration: FiO2 is generally monitored with an oxygen concentration analyzer, preferably continuous monitoring and recording. For example, an oxygen-free concentration analyzer can refer to the indicated value of the air - oxygen mixer, or use the air - oxygen mixing ratio of different flow rates to adjust ( see the table below ) to achieve an approximate appropriate oxygen concentration.

氧流量：The oxygen flow rate；压缩空气流量：compressed air flow

(2) Monitoring of arterial blood oxygen level: Monitoring of arterial blood oxygen level is performed at least every 4 hours in neonates with severe disease, and the measurement time for very severe children depends on the disease. For newborns who use assisted breathing, it is generally measured once within 15 to 20 minutes after adjusting the ventilator parameters to judge whether the adjustment is appropriate. In stable patients, measurements can be made every 6 hours or more. PaO2 is an index of the physical dissolved oxygen level in arterial plasma, and its measurement is generally carried out through arterial blood sampling. PaO2 only reflects the blood oxygen level at the time of blood collection, and cannot be observed continuously. Therefore, in the process of oxygen therapy, if no arterial ( umbilical artery or radial artery ) cannulation is placed, it must be punctured multiple times, which will cause the patient to suffer from excessive pain stimulation and also cause excessive blood loss. Although arterial capillary blood gas analysis is convenient for clinical work, it is difficult to standardize the method, and the measured blood oxygen value varies greatly, so it cannot be used as the basis for adjusting oxygen therapy.

Transcutaneous oxygen partial pressure measurement (TePO2) is a relatively non-invasive blood oxygen monitoring method. Under normal circumstances, the oxygen required for skin metabolism is automatically regulated and supplied by the skin blood flow, and the PO2 on the skin surface is zero; the principle of TePO2 is that the electrode placed on the skin warms the skin to 42~44 °C, making it hyperemia and local perfusion. Increase, so that oxygen can diffuse through the skin; TePO2 meter electrode contains the same device as the blood oxygen measurement. When the skin temperature is 42~44 ℃, the measured TePO2 value is similar to the PaO2 of arterial blood. When PaO250~100mmHg, TePO2 has a good correlation with PaO2, so it can be used for clinical dynamic observation. The disadvantages of TePO2 are:

① When the skin perfusion is poor, such as shock and low temperature, the TePO2 decreases, and the correlation with PaO2 is poor;

② The technical operation is complicated, time-consuming, and requires high requirements; The measurement site should be replaced in time every 3-4 hours to prevent local burns ;

③ It is necessary to measure PaO2 regularly to understand the accuracy of TePO2. Based on the above shortcomings and limitations, this method has been gradually replaced by transcutane ousoxygen saturation ( TcSO2).

Arterial oxygen saturation: It can reflect the oxygenation state and oxygen content level of the blood, and can be measured by percutaneous pulse oximeter. According to the different absorption characteristics of hemoglobin and oxyhemoglobin on light, the red light (660nm) and infrared light (940nm) that can penetrate the blood are respectively irradiated, and the photodiode is used to pair, and the light signal after irradiation ( take only the pulsating capillary Vascular bed signal ) processing to obtain the value of SaO2. The sensor is placed on the end of the limb ( finger, toe ), the tip of the nose or the skin of the earlobe for measurement. When SaO2 is measured in the range of 70% to 100%, the measured blood oxygen saturation (SaO2) of each pulse is closely related to the PaO2 measured by the blood gas analyzer. However, since the oxygen dissociation curve is S -shaped, in the flat part of the curve, when PaO2 increases greatly, SaO2 changes very little; pulse oximeter is not sensitive to the determination of hyperoxemia, when SaO2 > 95%, PaO2 often changes. More difficult to predict, can exceed 100mmHg ( see the figure below ).

(3) Target oxygen saturation for preterm infants: There is no consensus on the maintenance of optimal blood oxygen saturation. Some studies have shown that maintaining SaO2 at 85%-89% can increase the mortality of premature infants and the chance of NEC, but reduce the incidence of ROP ; there are also studies suggesting that setting the target SaO2 > 95% will prolong the final oxygen therapy time. The tolerance of premature infants to hypoxia and the sensitivity to hyperoxia injury varies greatly among individuals. It has also been reported that the following schemes have been proposed: In order to prevent the damage to the retina and lung caused by hyperoxia, for premature infants < 29 weeks or < 1250g, SaO2 can be kept at 88%~-92%, and the alarm value is set at 85%~93% until 36 weeks of age; for those > 29 weeks, SaO2 can be set at 88%~95%, and the alarm value is set at 85%-97 % ; Under the above setting, PaO2 will rarely be > 100mmHg[10]. The advantages of pulse oximeter are non-invasive and accurate. When the oxygenation in the body changes, the instrument can display within a few seconds, and it is easy to operate, does not require calibration, is easy to master, and can continuously monitor arterial blood oxygen levels. The relationship between SaO2 and PaO2 can be found by the oxygen dissociation curve. When the vascular bed pulsation of the child is significantly reduced, such as hypothermia, hypotension, and the application of high-dose vasoconstrictor drugs, the accuracy of the pulse oximeter will be affected; strong light environments ( such as strong light therapy ), carbon oxygen or high-speed iron Increased hemoglobin, etc. can interfere with the measured value. When fetal hemoglobin is more than 50%, due to its high affinity for oxygen, the SaO2 value displayed by the instrument is often greater than 95%. The above problems should be paid attention to in the monitoring of neonates, especially premature infants.

5. Monitoring of ventilation function 

In the process of oxygen therapy, the monitoring index of ventilation function is mainly the level of blood carbon dioxide (PaCO2). Since CO2 levels are greatly affected by respiration and change rapidly, real-time monitoring is important. Arterial blood gas is the gold standard for reflecting CO2, but it is an invasive procedure. In hypocapnia, the lung volume may be too large, which may lead to potential lung damage, and it can also cause decreased cerebral blood flow and potential brain damage, so it should be avoided as much as possible in clinical practice. There is no uniform definition of neonatal hypercapnia. To reduce lung damage, PaCO2 can be allowed to be between 50 and 65 mmHg.

Transcutaneous partial pressure of carbon dioxide (TePCO2): is a relatively non-invasive blood carbon dioxide monitoring method. The principle of TePCO2 is that the electrode placed on the skin warms the skin to 42~44 ℃, which makes it hyperemia, increases local perfusion, and enables carbon dioxide to diffuse through the skin; the measured TcPCO2 value is slightly higher than the PaCO2 of arterial blood. The disadvantages of TePCO2 are:

① When the skin perfusion is poor, such as shock and low temperature, the correlation with PaCO2 is poor;

② The technical operation is complicated, time-consuming, and requires high requirements; The measurement site should be replaced every 3-4 hours to prevent local burns;

③ At normal PaCO2, TePCO2 is about 4 mmHg higher than the blood gas value, but the difference can be very large in hypercapnia.

End-tidal carbon dioxide (carbon dioxide inend expiraory gas, EtCO2) : EtCO2 is used for non-invasive monitoring of the ventilation effect during mechanical ventilation, or a monitoring method to confirm whether the tracheal intubation is in the respiratory tract, and can also be used for monitoring during anesthesia. The commonly used end-tidal carbon dioxide meter uses the emission and absorption of infrared light ( wavelength 4.26um) to monitor the CO2 concentration. It is generally placed in series monitoring at the tracheal intubation interface during mechanical ventilation, but it may increase the dead space. There is also monitoring by flow measurement, that is, by sampling at the nasal or oral catheter, for non-intubated patients. The main influencing factors of EtCO2 are that local temperature and humidity affect the newborn 's faster breathing rate, and the ventilator has continuous airflow interference with the accuracy of the measurement. to the actual PaCO2 value.

Other ventilation function monitoring: tidal volume monitoring is in the dedicated neonatal spirometer or most artificial ventilator monitoring modules, with monitoring parameters such as tidal volume, flow - volume loop, etc. These parameters have certain reference significance for guiding respiratory therapy. If bronchomalacia is common in children with BPD, real-time monitoring of the flow - volume loop can determine the optimal value of positive end-expiratory pressure (PEEP) to combat airway collapse. Neonatal breathing rate is relatively fast, tracheal intubation often does not have a closed air bag, and the airway leakage rate is high during monitoring.