Cardiac pacing in infants and children has increased significantly in recent years, although less than 1% of all infants and children receive pacing therapy. Infants and children have special physiological and pacing characteristics that are different from those of adults. Before deciding on pacing therapy for infants and children, it is important to fully understand their physiology and pacing characteristics and to select the appropriate pacemaker, electrode lead, pacing mode, and pacing site. Physiological characteristics of infants and young children that affect pacing include small size, thin venous diameters (e.g., subclavian veins, cephalic veins, etc.), thin subcutaneous tissue in the chest and abdominal wall, and often combined with structural abnormalities such as heart defects. At the same time, infants and children are growing and changing in size. Pacemaking indications for infants and children According to the 2008 American College of Cardiology (ACC)/American Heart Association (AHA)/Heart Rhythm Society (HRS) pacing indications for infants and children include: 1. Severe second- or third-degree atrioventricular block with symptoms, cardiac insufficiency, or decreased cardiac output; 2. Sinus bradycardia with symptoms. Sinus bradycardia should be defined in relation to the expected heart rate at the actual age of the child; 3. Severe second- or third-degree atrioventricular block that is irreversible after surgery (persisting for 7 days after surgery without improvement); 4. Congenital third-degree atrioventricular block with wide QRS wave escape rhythm, complex ventricular ectopic rhythm, or left ventricular insufficiency; 5. Infants with congenital third-degree atrioventricular block with a ventricular rate <55 beats/min, or with congenital heart disease with a ventricular rate <70 beats/min; 6. conduction block associated with cardiomyopathy; 7. conduction block associated with long QT syndrome. Among these, congenital third-degree AV block that is difficult to recover after surgery or congenital third-degree AV block with complex preconditioning (e.g., corrected transposition of the great arteries) is the most common and important indication for pacemaker placement. Third-degree AV block that does not recover by 7-10 days after congenital surgery is often associated with a risk of sudden death and therefore pacemaker placement is recommended. It should be noted, however, that some surgically related third-degree AV blocks, such as those above the Hirschsprung bundle, can be recovered. Endocardial pacing and epicardial pacing (1) Pacing electrode leads There are two types of pacing electrode leads: epicardial electrode leads and endocardial electrode leads. The former is placed on the surface of the heart through surgical open-heart surgery, whereas the latter is placed on the endocardial surface through a venous puncture route (see the accompanying table for a comparison of the two). Factors that influence the choice of electrode lead type include the weight of the infant, the diameter of the subclavian vein, the presence of a cardiac defect, and the magnitude of the risk of embolism. In summary, the current trend in infant and child pacing is toward transvenous placement of endocardial electrode leads. Endocardial leads are recommended for larger infants and children, whereas epicardial leads are primarily used for infants with small body weights or difficulties with the venous route. (2) Infant weight and pacemaker size A global trend is to try to place endocardial leads in infants and children. Generally, endocardial leads are used in infants >20 kg because the subclavian vein is too small for endocardial leads to cause venous occlusion. Some institutions also choose endocardial leads for children weighing <15 kg. Of course, the use of endocardial leads in children weighing <10 kg has also been reported. Complications include embolism of the circulation, venous embolism at the pacemaker lead, pulmonary artery embolism, and superior vena cava syndrome. The incidence of symptomatic pulmonary embolism in infants and children with structurally intact hearts is approximately 0.6-3.5% with the use of endocardial electrode leads. The risk of embolism with endocardial electrode leads is significantly higher in infants and children with combined precordial or intracardiac structural defects. Thus, in infants and children with combined precordial or intracardiac structural defects, the application of endocardial electrode leads usually has a lower pacing threshold but a 2-fold increased risk of developing embolism in the body circulation, which is not reduced by the application of warfarin and aspirin. Embolism of the body circulation occurs not only in children with right-to-left shunts but also in children with left-to-right shunts (e.g., small ventricular septal defects or atrial septal defects). Therefore, epicardial electrode leads are recommended for children with combined intracardiac structural defects. In children with severe heart failure, multiple previous open-heart surgeries, multiple organ failure, and other children at high risk for open-heart surgery, endocardial electrode leads are still recommended, even when combined with endocardial structural defects. (3) Venous occlusion and tricuspid regurgitation In infants and children with small veins, venous occlusion (partial or complete occlusion) is a major complication of endocardial electrode leads, with an incidence of about 30%. Adults also have some rate of venous occlusion, mainly related to the presence of multiple electrode leads in the vein. Animal studies have found a procoagulant, thrombogenic effect of the interaction between the polyurethane and silicone electrode lead insulation, which has not been demonstrated in humans. Selection of endocardial leads for infants and children needs to take into account the possibility of future venous occlusion and the need to reserve leads for future growth. The diameter of the infant's innominate and superior vena cava, the length from the innominate vein to the superior vena cava, and the distance from the superior vena cava to the right atrium correlate positively with the infant's height; the diameter of the vein becomes thicker with age until age 10. As infants and children grow and develop, the veins into which the electrode leads are placed become longer, so a coil is made to reserve the electrode leads. Usually the coil is placed in the atrium or the inferior vena cava. However, this is not a one-size-fits-all solution. In one child, the ventricular lead failed 5 years after pacemaker placement because the ventricular lead was so tightly attached to the endocardium at the inferior vena cava that, even though an electrode reservoir was left, the reservoir lead did not release as expected because of the adhesion of the lead to the vein. Transvenous placement of ventricular electrode leads can cause regurgitation of the tricuspid valve or increase the existing tricuspid regurgitation, but the regurgitant flow is generally small and has little hemodynamic impact. However, there have been reports of ventricular electrode leads causing severe tricuspid regurgitation or even requiring tricuspid valve replacement after placement. For growing infants and children with endocardial ventricular leads or adults with right heart system disease, careful follow-up is needed to monitor tricuspid regurgitation. (4) Lead failure and reinsertion The incidence of lead failure in infants and children is as high as 15 - 27%. In infants < 12 years of age, independent risk factors for lead failure include the presence of a preexisting heart and the use of epicardial leads. Infants and children are more likely than adults to have a break in the insulation of the lead. Many factors, such as the pulling of the lead by the developing infant, the narrow space between the ribs (for epicardial leads) and between the subclavian vein and the ribs (for endocardial leads), also contribute to lead insulation breakdown. Epicardial leads often fail due to increased pacing thresholds caused by fibrosis and pericardial adhesions after previous cardiac surgery. There are two types of epicardial leads: unipolar and bipolar; hormone-coated epicardial leads prevent a gradual increase in pacing threshold. Winged passive endocardial electrodes (e.g., Medtronic's Capture Epi) are currently the mainstay; however, spiral active fixed electrode leads (e.g., St. Jude's Myodex electrode lead) are now being used more and more each year. Infants and adult patients with congenital heart disease can actively select the ideal pacing site with Medtronic's SelectSecure leads. The system involves feeding a 4.1F hormone-coated lead (Medtronic SelectSecure 3830) to the desired pacing site through a reorientable 8F sheath. The smaller diameter of this electrode lead reduces the risk of electrode lead insulation breakdown and reduces push resistance. Since the 8F sheath is too large for infants and children, the 5F sheath is now being used. For small infants and children (especially for newborns or premature infants weighing less than 4 - 5 kg), smaller pacemakers should be chosen. St. Jude's Microny II 2526T is the smallest single-chamber pacemaker available on the market today. The single-chamber pacemaker, 33 X 33 mm, is only 6 mm thick, has a volume of 5.9 cm3 and weighs only 12.8 grams. A typical single-chamber pacemaker has a volume of 8 - 11 cm3 and a total weight of about 17 - 23 grams. The basic pacing frequency is adjustable from 40 to 160 beats/min and the output voltage is up to 4.5 V. An automatic threshold capture function automatically tests the pacing threshold of the ventricular electrode leads. With the automatic threshold capture function, the life of the pacemaker can be extended. Generally, dual-chamber DDD mode is chosen for adult atrioventricular block, which requires two electrode leads for atrial and ventricular pacing. Due to the small diameter of infant veins, it is difficult to feed two electrode leads and easy to cause venous occlusion. Therefore, a VDD pacemaker with only one electrode lead and atrial sensing without atrial pacing is a good choice for infants and children with third-degree AV block and normal sinus node function, which can effectively reduce the occurrence of venous occlusion. If epicardial electrode leads are chosen, the epicardial electrode leads in the atria cannot be placed via the subxiphoid route and must undergo an open chest or sternotomy. The maximum heart rate of infants and children is also a consideration. The average heart rate in infants is greater than 100 beats/minute and can increase to 180 - 200 beats/minute during crying. In infants with atrioventricular block, the atrial rate may be rapid and exceed the maximum programmable upper tracking frequency. This upper tracking frequency is controlled by the post-ventricular atrial nonstimulus and interatrial interval. If the infant's heart rate exceeds the maximum programmable upper tracking frequency, symptomatic 2:1 AV block may occur. Therefore, a single-chamber VVI or VVIR (with frequency response) pacemaker may be an option for small infants with rapid ventricular rates. In children with precordial heart, DDD pacemakers with epicardial electrode leads may lose DDD pacing if the amplitude of atrial waves is too low, and cardiac function may deteriorate due to asynchronous contractions between the right and left ventricles caused by frequent right ventricular pacing. In children with sick sinus node syndrome, atrial pacers have a lower incidence of atrial fibrillation, fewer embolic complications, and a lower incidence of atrioventricular block compared with right ventricular apical pacing. In patients with sick sinus node syndrome, the less ventricular pacing, the lower the chance of atrial fibrillation and cardiac insufficiency. In children with poor sinus node function, unnecessary right ventricular pacing can increase the occurrence of atrial fibrillation, worsen cardiac function, reduce myocardial electrophysiological remodeling, repolarization instability, and further have proarrhythmic effects, so atrial pacing is recommended. In children with a high degree of atrioventricular block, ventricular pacing is unavoidable. In this case, the choice of ventricular pacing site is also very important, regardless of whether epicardial or endocardial pacing is chosen. Endocardial pacing is either Hirschsprung bundle pacing or Hirschsprung bundle parasternal pacing. However, there are problems with this type of pacing: difficult electrode lead insertion, high pacing threshold, and high power consumption. Moreover, the stability of long-term Hirschsprung or Hirschsprung parasternal pacing is difficult to guarantee. In contrast, right septal pacing is a better option because it is easier for the physician to perform and has a lower pacing threshold compared to the pacing voltage at the Hirschsprung bundle. For infants and children requiring epicardial pacing, the left ventricular apical region is an ideal pacing location. The left ventricular lateral wall is also an alternative pacing site. In infants and children with left ventricular insufficiency due to excessive right ventricular pacing, the pacing pattern can be changed to biventricular pacing or Hirschsprung bundle pacing, which may improve left ventricular remodeling. Children with pathologic sinus node syndrome are prone to atrial flutter and intra-atrial fold tachycardia. Pacemakers with an atrial anti-tachycardia pacing pattern are selected to treat both atrial tachycardia and sinus bradycardia. Generally, the pacemaker capsule is placed in the subclavian region when the endocardial electrode lead is placed via the venous system. In infants, the subcutaneous tissue in this area is weak, so the electrodes are often introduced into the abdomen from under the clavicle through a sheath behind the rectus abdominis muscle. In children who cannot place the pacemaker in the subclavian region for aesthetic reasons, the pacemaker capsule can also be placed in the axillary region. For newborn infants with comorbid gastrointestinal disease (e.g., necrotizing enterocolitis) or who are expected to undergo abdominal surgery or are expected to require retroperitoneal dialysis, the pacemaker should be placed either on the chest wall or under the axilla. VI. Infection of the pacing system The high rate of infection of the pacing system in infants and children is a major complication of infant pacing, due to the fact that infants and children need to undergo multiple pacemaker or electrode lead changes for pacing. In adults, the incidence of pacemaker bag infections is about 1 - 2%, often requiring removal of the entire system. In contrast, the infection rate in infants and children is around 7.8% (4.9% for epidermal infections, 2.3% for capsular bag infections, and 0.5% for positive blood cultures). The main causes of infection are trisomy 21 and pacemaker replacement. The management of pacing system infections can be found in the relevant 2010 American Heart Association guidelines. Infections can lead to conditions such as sepsis, bacteremia, pacemaker exposure, redundancy, or infective endocarditis. Once infection has occurred, the entire pacing system, including the electrode leads, needs to be removed. If the epicardial leads need to be removed, a complete or partial thoracotomy or sternotomy is required. Therefore, it is important to weigh the risks and benefits when deciding to remove the leads and/or the pacemaker. In infants and children, the electrode leads are more tightly adhered than in adults for the same amount of time after pacemaker placement. Removal techniques that can be used include laser sheaths, mechanical sheaths, and electrosurgical sheaths (with radiofrequency energy). The success rate can be over 90% with the use of laser sheaths for hormones in experienced centers. The main complications are pericardial tamponade and subclavian vein occlusion. For centers that have not been able to perform laser sheath removal using a combination of methods such as mechanical sheaths, the success rate can also be about 80%. Sometimes, in complex cases where the electrode leads are severely adherent at multiple sites, open-chest, extracorporeal circulation is required to remove the abandoned or infected electrode leads. In conclusion, infant cardiac pacing has its own specificities. Before deciding on pacing therapy, several special conditions accompanying infant pacing need to be considered: (1) small body, fine veins, developing body, possible combined structural heart defects and cardiac insufficiency. (2) Infants and children who are physically weak are suitable for epicardial pacing only. Epicardial pacing has problems such as high surgical trauma, complicated operation, long operation time, high pacing threshold, and easy pacing dysfunction; endocardial pacing is currently the trend in infant and child pacing, while endocardial pacing needs to be performed after correction of the structural heart defect, and complications such as venous occlusion and thrombosis, and pacing system infection need to be noted. (3) The need for multiple pacemaker changes during the lifetime of infants and children undergoing pacing therapy, the increased risk of pacing system infection, and the risk of electrode lead removal should all be taken into consideration. The most advanced technology and a reasonable pacing mode should be applied, taking into account the specificity of each child; at the same time, careful and long-term follow-up is needed for each infant and child pacing to ensure the proper functioning of the pacemaker.