Lung cancer is one of the malignant tumors with the highest incidence and mortality rate today. Among patients with clearly diagnosed lung cancer, only 20%-30% of them can be treated surgically, and most of them are already in the middle and late stages. According to some studies, 30% of lung cancer patients have tumor tissues invading the central airway (trachea, left and right main bronchus and right middle bronchus), and the symptoms of central airway obstruction such as shortness of breath, hemoptysis or pulmonary atelectasis appear. Therefore, timely and effective removal of central airway obstruction is needed to prolong the survival time and improve the quality of life of patients. In recent years, with the development of respiratory interventional technology, great progress has been made in the interventional treatment and strategy for central airway malignancies. Through interventional treatment, symptoms can be relieved in a timely and effective manner to gain time for further treatment such as chemotherapy and radiotherapy. I. Tools for interventional treatment of central airway malignancies Interventional treatment of central airway malignancies can be performed by rigid bronchoscope and/or bendable bronchoscope. No matter which interventional tool is chosen, it should be based on the specific conditions of the lesion, such as the growth site of the tumor and the type of obstruction, etc. Meanwhile, rigid bronchoscope and bendable bronchoscope have their own advantages, and their combined application can complement each other’s strengths and advantages. (A) Rigid bronchoscope Since 1897, Gustav Killian, a German physician, has been using rigid bronchoscope for the treatment of tumor growth and obstruction. Since 1897, when Gustav Killiam, a German physician, used a rigid bronchoscope to remove a pig bone from the trachea of a male patient, the application of rigid bronchoscopy in respiratory interventions has been developed. The rigid bronchoscope not only keeps the airway open, but also has a side hole at the operating end to connect to the ventilator, which is known as the “ventilating bronchoscope”. Modern rigid bronchoscopes have a lumen that allows bendable bronchoscopes and other instruments to pass through and into the airway to perform various interventions under direct vision. It provides sufficient access for powerful suction through a suction tube and balances the contradiction between operation and airway opening. Rigid bronchoscopy has unparalleled advantages in how to determine the central axis of the airway and maintain balance throughout, avoiding perforation of the airway wall, pneumothorax and vascular damage, and maintaining a clear working surface, but is difficult to achieve for the distal airway, especially for the two upper lung lobar orifices. It is contraindicated when there are cervical spine disorders and maxillofacial injuries. (ii) Bendable bronchoscope Bendable fiberoptic bronchoscope has been widely used since it was invented by Japanese scholar Ikeda in 1965, especially since the emergence of bendable electronic bronchoscope and the invention and updating of the accessory equipment that can perform various treatments through the working orifice, it has shown great advantages in respiratory intervention. The miniaturization and flexibility of operation allow them to reach smaller and narrower airways than rigid bronchoscopes. Interventions with bendable bronchoscopes can be done under local anesthesia in the bronchoscopy chamber, and can be performed under mechanical ventilation with tracheal intubation or tracheotomy, expanding the scope and safety of interventions with bendable bronchoscopes. The high-definition digital image increases the accuracy and safety of bronchoscopic interventions. Rigid bronchoscope and bendable bronchoscope are the main tools for interventional treatment of central airway malignancies, and they complement each other’s advantages. The rigid bronchoscope can effectively avoid the occurrence of serious complications such as hypoxia and asphyxia as a safeguard channel, while the bendable bronchoscope can make a comprehensive examination and lesion judgment of the central airway through the narrow section to formulate an effective treatment plan, especially for the bronchial parts that cannot be reached by the rigid bronchoscope, the bendable bronchoscope can be more useful. Interventional respiratory techniques for central airway malignancies Interventional techniques for central airway malignancies mainly include the following categories: mechanical resection, dilation or stent support; thermal therapy; cold therapy; brachytherapy and photodynamic therapy. The above treatment techniques have their own advantages and disadvantages and can be applied jointly to complement each other. (a) Mechanical resection or dilation 1. Direct resection by rigid bronchoscope: the tip and bevel of rigid bronchoscope are used to directly resect tumor tissues in the central type airway lumen with obvious effect. During resection, the tip of rigid bronchoscope is placed at the base of the tumor tissue as far as possible, and it is gently moved back and forth and forward with certain pressure to bluntly separate and remove the tumor tissue under direct vision (Figure 2), and biopsy forceps are applied to remove the broken tissues, and the suction device is used to aspirate the secretion and oozing blood. The main risk is hemorrhage, although the incidence is low, once it occurs, timely and effective application of direct pressure on the wall of the rigid mirror to stop the bleeding or direct pressure on the cotton ball with hemostatic forceps, and if necessary, application of argon plasma coagulator to stop the bleeding. In general, hemorrhage during mechanical resection of tumor tissues is relatively rare, and the above hemostatic methods can basically be controlled, except when there is a hemangioma, which should be done with caution. Another complication of mechanical resection of central airway malignancy is perforation of the tube wall, which can be effectively avoided as long as the rigid bronchoscope lumen is operated parallel to the airway tube wall. 2.Microdebrider: Microdebrider was first applied to respiratory intervention by American scholar Lunn.W in 2005, and the effect is obvious. Microdebrider consists of three parts: control table, handle and aspiration head. The suction head consists of an external hollow metal tube and an internal rotating blade. While rotating, it can effectively remove debris and secretions by suction. It is fast and accurate in removing tumor tissues in the lumen and has fewer complications. Safety is improved without the concern of intracavitary combustion and high concentration of oxygen supply. However, the application must be done by rigid mirror or laryngeal mask to avoid strong suction to avoid accidental cutting of normal tissues. 3.Airway stent treatment: Airway stent is a prosthesis placed in the airway, which is mainly used in the interventional treatment of central airway malignancy for the treatment of external pressure stenosis caused by the compression of central airway tumor tissue and/or metastatic enlarged lymph nodes and the support of the duct wall after the removal of luminal tumor tissue. Airway stents can enhance the stiffness of the duct wall and resist the pressure of extra-mural tumor tissue on the duct wall to maintain the patency of the duct lumen. At present, airway stents mainly include silicone stents, metal stents and hybrid stents. The most widely used silicone stent is the Dumon stent, which must be placed by rigid bronchoscopy under general anesthesia. Metal stents, also known as self-expanding stents, are divided into coated metal stents and non-coated metal stents, which can be done under rigid or bendable bronchoscopy. In contrast to silicone stents, which are prone to displacement, metal stents are less prone to displacement due to their ability to embed in the airway tissue and surrounding granulation tissue. For laminated stents, they are currently favored because they reduce the incidence of luminal restenosis to a certain extent compared to non-laminated metal stents. For central airway malignant tumors without surgical indication such as infiltration of tumors in the duct wall or extraluminal tumors and metastatic enlarged lymph node compression causing obvious airway obstruction and dyspnea or even asphyxia, airway stenting should be considered to unblock the airway, relieve dyspnea and hypoxia, prolong survival time, improve quality of life and buy time for further treatment. Currently, most scholars consider airway stenosis with a diameter of less than 2/3 of the original lumen and/or associated symptoms as an indication for stent placement. Both silicone stents and self-expanding metal stents are associated with certain complications of treatment. The absolute contraindications are central airway stenosis and loss of lung function distal to the lesion due to extraluminal aneurysm compression. Therefore, in the future, airway stents will not only combine the advantages of silicone stents and self-expanding metal stents, but also some new types of stents such as stents with their own radioactivity and absorbable biomaterial stents will be introduced into the clinic, so that the interventional treatment of central airway malignant tumors can be more effective and the complication rate can be lower. (2) Thermal ablation therapy Thermal ablation therapy is to eliminate lesions by carbonizing, coagulating or vaporizing tissues through heat production. It mainly includes laser, electrocautery and argon plasma coagulation, etc. 1.Laser resection: Laser resection of central airway malignant tumor is to apply optical fiber to transmit laser through the working orifice of bronchoscope to produce thermal effect to eliminate lesions and relieve symptoms. Nd:YAG laser has good coagulation and deep penetration, and it is the most widely used laser for airway intervention, especially for rapid resection of malignant tumors in the central airway lumen, improving ventilation and relieving related symptoms such as hemoptysis, cough, coughing, and coughing. It is especially suitable for rapid resection of malignant tumors in the central airway lumen, improving ventilation, reducing related symptoms such as hemoptysis, cough, dyspnea, clearing secretions and obstructive pneumonia, and the efficiency of lesion removal is over 80%. In the application of laser treatment for central airway malignancies, it is mostly performed under rigid bronchoscopy. Rigid bronchoscope not only has sufficient operating space, adequate ventilation and better view, but also can aspirate smoke and tumor fragments more effectively. After laser coagulation of the tumor tissue, the tumor is mechanically removed with the beveled terminal of the rigid bronchoscope, which takes less time and has fewer complications. However, bendable bronchoscopic laser has good application for the treatment of small lesions, distal lesions, and airway tumor obstruction of less than 50%. Treatment of extraluminal tumor tissue is absolutely contraindicated. 2.High-frequency electrocautery: High-frequency electrocautery is ablative treatment using the thermal effect of electric current. When high-frequency alternating current is conducted to the surface of target tissue through electrodes, due to the high resistance of tissue, the current is converted into heat energy to achieve the purpose of cutting, coagulating and vaporizing tissue and removing lesions. Since high-frequency electrocautery has multiple probes (such as round blunt electrode, electrocautery knife, electrocautery biopsy forceps and trap, etc.), multiple treatment modes (electrocoagulation, electrocutting and mixed mode), and is inexpensive and cost-effective, it has been widely used in the interventional treatment of central airway malignancies. 69%-100% of patients with obstructive airway lesions have achieved rapid relief after electrocautery resection, and symptoms such as dyspnea, cough and hemoptysis have been significantly improved. The symptoms such as dyspnea, cough and hemoptysis were significantly controlled. For the interventional treatment of central airway malignancies, bronchoscopic electrocautery is a safe and effective treatment measure. Since high-frequency electrocautery is a contact treatment, the front end of the electrode tends to adhere to the carbonized tissue, which needs to be removed in time, and the smoke generated stimulates coughing and is not conducive to treatment. At the same time, the oxygen supply concentration should be controlled below 40% when performing high-frequency electrocautery to put a stop to the fire in the airway. When performing electrocautery on patients with severe obstruction of malignant tumor tissues in the tracheal lumen, in order to prevent bleeding causing asphyxia, electrocoagulation or mixed mode should be applied as much as possible at the beginning to reduce the bleeding situation and improve the safety of treatment. Argon plasma coagulation (APC): APC is a kind of non-contact electrocautery, which is performed by ionized argon plasma as a conductor between the APC electrode and the target tissue to form a high-frequency current that is converted into heat energy on the surface of the tissue to produce a cautery effect. After dehydration and vascular occlusion, the conductivity of the tissue surface will be reduced and the current will be automatically transferred to the surrounding areas with low electrical resistance to make the cautery more uniform, and the depth is usually 2-3mm, which is not easy to cause airway wall perforation. In the treatment of central airway malignancies, argon plasma is first sprayed on the tumor surface to produce tissue dehydration and superficial vascular coagulation until the tissue is inactivated, and then biopsy forceps are used to remove the inactivated tissue by cautery. This is repeated several times until most or all of the tumor is removed. In addition, APC is often used to treat airway restenosis due to its high safety profile, such as resection of tumors and granulation tissue growing in the stent lumen and at the margins, and for rapid hemostasis of hemorrhagic lesions due to superficial airway lesions because of its good blood coagulation properties. However, APC is, after all, a kind of electrocautery, and in order to prevent burning in the airway, the oxygen concentration administered during treatment is controlled at less than 40%, and the operation time is relatively prolonged due to the continuous removal of coagulated necrotic tissue during the treatment. In conclusion, APC has the advantages of fast onset, simple technique, high safety and low price in the treatment of central airway malignancies under bronchoscopy, and has good application prospects. (iii) Cryotherapy Cryotherapy is the repeated freezing-thawing cycle of tumor tissues at a very low temperature of -40℃ to achieve tumor necrosis. At present, liquid nitrogen, N2O and CO2 are commonly used, and CO2 is the most used in respiratory interventions in China. The principle is that high-pressure CO2 gas is released through small holes, throttled and expanded to produce low temperature, and the lowest temperature can reach -80℃, forming a certain size of ice ball at the front of the cryoprobe. The effectiveness of cryotherapy depends on the following aspects: the lowest temperature that can be reached, the speed of freezing and thawing and the number of cycles, and the water content in the tissue. In the interventional treatment of central airway malignancies, the frozen metal probe is placed on the surface of the tumor tissue or pushed into the tissue to form the largest volume of ice sphere around it, which is continuously frozen for 1 min-3 min, and then re-tempered for several freeze-thaw cycles, moving the probe until all the visible tumor tissue is frozen and the tissue is inactivated in situ, without the need to remove the frozen tissue. The other way is “freeze-thaw”, in which the metal head of the freezing probe is placed on the surface of the tumor tissue or pushed into the tissue to form an ice ball, and the probe and its adherent tissue are removed under the frozen condition. As there is a clear demarcation between frozen and non-frozen tissues, the blood flow of frozen tissues is reduced and platelet emboli are formed, so it is not easy to bleed, but for non-frozen tissues, especially when non-frozen tissues are supplied by bronchial arteries, forcible “freeze-taking” will lead to tearing of tissues and bleeding, sometimes serious hemorrhage. This is a point that should be noted when “freezing”. Since the effect of freezing is related to the water content of the tissue, and cartilage and fibrous tissues contain less water, the chance of wall perforation is greatly reduced in the intervention of malignant tissue in the central airway. In addition, cryopreservation does not cause combustion and smoke production in the airway, and can be performed with high concentrations of oxygen inhalation, without the risk of electric shock and radiation exposure, which greatly improves safety. The disadvantage is that the maximum effect of treatment is delayed and is not suitable for emergency treatment of acute, severe central airway malignancy obstruction. Although many scholars advocate the application of cryotherapy under rigid bronchoscopy [20], the effectiveness and advantages of cryotherapy under bendable bronchoscopy are still recognized by many scholars.