1.The principle and characteristics of microwave ablation
(1) Microwave ablation principle: Microwave is an electromagnetic wave between radio frequency and ultrasound, with a wave frequency between 1 GHz (1 billion oscillations per second) and 300 GHz. The microwave ablation instrument emits high frequency electromagnetic waves at 2.45 GHz, which are transmitted to myocardial tissue by microwave probes (FLEX2, FLEX4, FLEX10), causing rapid rotation and vibration of bipolar water molecules and frictional heat generation. The electromagnetic field can propagate through blood, normal myocardial tissue and scar tissue. These characteristics of microwave make it very suitable for ablation of atrial myocardium. The penetrating power of microwave on the tissue mainly depends on the ablation time and power.
(2) Microwave ablation characteristics: the point ablation with radiofrequency as energy, its penetrating power is weak, there is a certain amount of radiation damage to the surrounding tissue, the surface can form charring or crusting, microwave ablation is different from it.
It has the following characteristics.
①Tissue penetration ability is strong, can quickly form a continuous transmural blocking line. Unlike radiofrequency ablation, where heat is conducted by a heat conductor to produce heat passively, microwave ablation is directly heated by microwave energy. The degree of heating of the tissue does not depend on the size of the current passed through the electrode, and the energy attenuation of the microwave through the fat and blood is very small, therefore, microwave ablation does not require a high current to have strong tissue penetration, through the local necrotic myocardium and scar tissue, ablation is not easy to recur;
②By adjusting the power and time of ablation, the depth of tissue damage can be controlled more precisely, thus reducing complications;
③The microwave energy is concentrated and does not cause damage to the surrounding tissues.
④Because of the good penetration of microwave energy, the energy is mainly deposited in the tissue, microwave ablation can obtain deep tissue ablation without overheating the tissue surface and forming charring or crusting, which reduces the risk of thromboembolism.
⑤ Microwave ablation under direct vision can directly check the coherence of the ablation path and the connection.
2.Microwave ablation equipment
Microwave ablation equipment mainly includes microwave emitter and treatment probe. The microwave ablation device emits 2.45 GHz electromagnetic waves with an energy output range of 20-75 W at 5 W. The probe is equipped with a thermocouple to record the internal temperature of the device during the ablation process (automatically displayed on the microwave emitter), and the microwave emitter will automatically stop working when the temperature exceeds 55°C. After the temperature drops to 40°C, the ablation can continue to prevent uncontrollable damage to the myocardial tissue caused by high temperature. The temperature can be lowered to 40℃ and then continue to ablate. Currently, there are three commonly used microwave therapy probes: FLEX 2, FLEX 4 and FLEX 10, of which FLEX 2 was first used in clinical practice and is only suitable for endocardial ablation. The ablation unit of FLEX 4 is 4 cm in length, and its ablation probe is unidirectional and retractable, The FLEX 10 is a newly developed and FDA-approved ablation probe with a length of 20 cm and is mainly used for epicardial microwave ablation in the minimally invasive route. The sheath of the probe is made of polytetrafluoroethylene, which is easy to handle. The operator can control the ablation segment by the handle, and the contact angle extends from the treatment probe 2 cm at a time, and ablates with 65 W for 90-120 s until the ablation is completed. The output power of the generator is 65 W, but due to the inherent loss of the coaxial cable, there is a 40% power loss from the generator to the antenna, therefore, only 39 W is actually emitted from the treatment probe.
3, microwave ablation indications and contraindications
(1) Indications: In theory, microwave ablation of atrial fibrillation is suitable for all types of drug-refractory chronic, persistent or paroxysmal atrial fibrillation, especially for patients with coronary heart disease who need valve replacement surgery or coronary artery bypass grafting (CABG). It is also indicated for patients with symptomatic drug-refractory isolated atrial fibrillation. In general, the left ventricular ejection fraction should be greater than 0.30 in patients with mitral valve replacement or coronary revascularization, and greater than 0.40 in patients with CABG.
(2) Contraindications: It is contraindicated in patients with left atrial or pulmonary artery thrombosis, especially when ablation is performed by the transthoracoscopic epicardial route.
In addition, caution should be exercised in the following cases: emergency or foreseeable increased surgical risk; history of stroke or myocardial infarction within the last 3 months; congestive heart failure (NYHA class VI cardiac function); severe obstructive or restrictive respiratory disturbances; renal and/or hepatic insufficiency.
4.Microwave ablation method
Anticoagulants should be stopped for 4 days before the procedure, if needed, heparin can be used instead. Anti-arrhythmic drugs can be used until 1 day before surgery. Preoperative esophageal ultrasound is performed to evaluate the size of the left atrium and the thrombus condition. Commonly used ablation procedures can be divided into transendocardial route, small incision transepicardial route, and endoscopic epicardial route. Microwave ablation routes have been inconsistently reported in the literature, but are based on a modified maze III procedure in which blocking lines are created around the four pulmonary veins to prevent conduction of potentials from the pulmonary veins to the adjacent atria; blocking lines are created in other parts of the atria (such as the right atrium and left atrial isthmus) to interrupt the large foldback loop. To reduce the occurrence of atrial flutter, many authors have also ablated between the posterior wall of the inferior vena cava inlet and the posterior tricuspid valve annulus. Because of the special structure of the left auricular commissure, it is easy to cause blood stagnation and is the main site of thrombus formation. In some cases, the left auricle has been dissected or sutured at the same time. The latest minimally invasive technique is to deliver the Flex10 probe through three small 1-cm-diameter incisions in the right anterior axillary line to ablate the pulmonary veins and atrial tissue, while exposing and removing the left auricle with a robot-assisted device. The key to successful surgery is clear exposure of the left atrium and accurate positioning of the probe. Minimally invasive microwave ablation is often performed using two blocking lines: the first line crosses the atrial sulcus, passes behind the inferior vena cava, follows the transverse and oblique sinuses, circles the four pulmonary veins, and returns to the atrial sulcus from behind the inferior vena cava; the second line is drawn between the left and right sides of the upper and lower pulmonary veins.
Most patients have atrial fibrillation at the time of surgery, and sinus rhythm transformation after surgery. However, most patients may develop intermittent atrial fibrillation 3 to 4 days after surgery. These patients should be cardioverted electrically. If there are no contraindications, amiodarone or sotalol is routinely administered postoperatively. In addition, anticoagulation with warfarin is required to control the INR between 2 and 3. The atrial fibrillation load can be evaluated by ambulatory electrocardiography on the same day, 1 month, 3 months, 6 months and 1 year after surgery, and the cardiac structure, such as left atrial size and thrombus, can be evaluated by cardiac color ultrasound.
5.Pathological changes
The pathological changes in the acute phase of microwave ablation tissues are mainly coagulative necrosis and peripheral inflammatory cell infiltration. At 6 months after the procedure, the dense scar tissue will form a well-defined line of block with the surrounding myocardial tissue.
6.Complications
Because of the concentrated energy of microwave ablation, no damage to surrounding tissues, ablation depth can be controlled, generally no additional cardiac damage, no proven injury due to microwave technology. However, some patients cannot undergo endoscopic ablation due to adhesions between the lung and the pleura, and can be replaced by microwave ablation through a small thoracic incision via the extrapleural route. In addition, some patients developed severe sinus bradycardia or nodal rhythm after ablation, which was considered to be related to the suppression of the sinus node or atrioventricular node by long-term atrial fibrillation rhythm, and a permanent pacemaker was required for those who could not recover.