Implantation of DDD Pacemaker in a Patient With Dextrocardia and Sick Sinus Syndrome: A Case Report

Background: Dextrocardia is a congenital abnormal position of the heart in which the main part of the heart is in the right chest and the long axis of the heart points to the lower right, cases with combination of dextrocardia and sick sinus syndrome are rare. Case presentation: A 65-year-old female patient was admitted with palpitations and dizziness for 1 week. Mirror-image Dextrocardia and sick sinus syndrome was diagnosed by electrocardiogram (ECG), echocardiography, Holter monitoring, and X-ray. The rarity of the case and the specicity of anatomy brought great challenges to our interventional treatments. Finally, we successfully implanted a DDD pacemaker into the patient. Conclusion: For dextrocardia, using active xation leads in both atrial and ventricular leads is easier to nd the position with optimal sensing and pacing threshold, which also have a lower and more stable pacing threshold and can reduce the incidence of falling off at the same time. During operation, combine the multiple positions under uoroscopy to conrm the leads position, which can improve the success rate of implantation.

echocardiography, Holter monitoring, and X-ray. The rarity of the case and the speci city of anatomy brought great challenges to our interventional treatments. Finally, we successfully implanted a DDD pacemaker into the patient.
Conclusion: For dextrocardia, using active xation leads in both atrial and ventricular leads is easier to nd the position with optimal sensing and pacing threshold, which also have a lower and more stable pacing threshold and can reduce the incidence of falling off at the same time. During operation, combine the multiple positions under uoroscopy to con rm the leads position, which can improve the success rate of implantation.
Background Dextrocardia refers to the congenital abnormal position of the heart in which the main part of the heart is in the right chest and the long axis of the heart points to the lower right [1][2] . It is a rare cardiac malformation that abnormal position and connection can occur in the all parts of the heart and great vessels. Dextrocardia can be divided into three types: the rst is Mirror-image Dextrocardia, in which the relationship between the heart is normal, but they are in the opposite direction, usually accompanied by abdominal situs inversus, and there are few cardiovascular malformations. The incidence of congenital heart disease in these patients is between 2% and 5% [2] . The second is Dextroposition, which is not accompanied by situs inversus, only the heart is located on the right side of the chest, and the anatomical position of the left and right cardiac cavity remains unchanged, which is usually secondary to extracardiac factor (such as lung, pleura, diaphragm, etc.) that squeezes the heart to the right. The third type is Dextroversion, which means that the heart is located on the right side of the chest, but there is no situs inversus, about 90% are associated with other cardiovascular malformations [3] , such as atrial septal defect, ventricular septal defect and so on [4] .
The incidence of dextrocardia is about 1/10000, of which mirror-image dextrocardia accounts for about 40% [5] , and dextrocardia with intracardiac malformation is about 40%~50% [6] . Due to the low incidence of dextrocardia, patients with dextrocardia with arrhythmias are not common in clinic, and cases of pacemaker implantation in these patients are even rarer. But there are also successful cases, among which dextrocardia with sick sinus syndrome and complete atrioventricular block are the main cases reported [7][8][9][10][11] .
The patient reported in this paper is a patient with dextrocardia and sick sinus syndrome. Because of the rarity and anatomical speci city of the case, it has brought great di culties and challenges to our dualchamber pacemaker implantation operation. Finally, we completed the operation successfully, improved the patients' symptoms remarkably, and her quality of life improved as well.

Case Presentation
A 65-year-old female patient was admitted with palpitations and dizziness for 1 week. The patient developed palpitations and dizziness in the past 1 week, which were aggravated after the activity, and had symptoms such as amaurosis and syncope. In addition, the patient had hypertension for many years and is now taking nifedipine for treatment. There were no episode of syncope before and no similar family history. At admission, physical examination showed a blood pressure of 156/73 mmHg, heart rate 56 bpm, the point of maximal impulse was found in the right fth intercostal space approximately 0.6 cm from the midclavicular line. Premature beats can be heard, no cardiac murmurs were auscultated. ECG examination revealed that the morphology of waves in leads avR and avL were opposite to the normal, inverted P and T waves in lead I, R wave in lead V1 to V6 decreases gradually, RV3 RV4 (Fig. 1), The ECG with right-sided precordial leads and reversed limb leads showed that sinus arrest, junctional escape, ventricular premature beats, clockwise transposition (Fig. 2). Echocardiography demonstrated mirror image dextrocardia, the size of the cardiac cavities and its connection with blood vessels are normal. The chest anteroposterior X-ray showed that the heart shadow is on the right (Fig. 3). The Holter monitoring showed an average heart rate of 53 bpm, sinus block or sinus arrest (the longest RR interval of 2.20s), frequent ventricular premature beats (4607 times/24 hours, 2 times in pairs),frequent atrial premature beats (3783 times/ 24 hours), junctional escape beats,ventricular escape beats and dextrocardia.
According to the examination results and clinical symptoms, we diagnosed this patient with mirror-image dextrocardia and sick sinus syndrome, and prepared to implant a dual-chamber pacemaker for her. Before the operation, after we communicated with the patient in detail, the patient agreed to our treatment plan.
The procedure was performed in the anteroposterior view, the patient was in a supine position, the guide wire was placed successfully after the left subclavian vein was punctured under local anesthesia. Active xation leads were used in both atrial and ventricular lead. The ventricular lead was positioned in the right ventricular apex, the pacing threshold was 0.6V, the sensitivity was 11.0mV and the impedance was 560 Ω.The atrial lead was xed to the free wall of the right atrium, The pacing threshold was 0.5V, the sensitivity was 2.5mV, and the impedance was 620 Ω. Connected the ventricular and atrial leads with the pulse generator closely. The nal position of the lead was con rmed using uoroscopic projections (Fig. 4).
The patient had an uneventful recovery and was discharged with the symptoms were signi cantly improved one week later. During the 3 months follow-up, the patient was not admitted to hospital again, and she said that she did not have a repeated palpitations within 3 months.

Discussion And Conclusions
In cardiac pacemaker implantation, the commonly used leads are passive xation lead and active xation lead. The passive xation lead is widely used because of its easy operation and low price. It have "tines" at the end of the lead, so that it can be embedded in the trabecular muscles acutely, after a period of time, the myocardium around it will brose to secure it further [12] . With the continuous development of pacing technology, the utilization rate of active xation lead is getting higher and higher [13] . The tip of active xation lead is a screw, which rotates out of the spiral structure during implantation and rotates into the myocardium, which can be placed anywhere in the cardiac cavity.
In this case, both the atrial and ventricular leads are active xation leads. For the atrial leads, the passive xation leads need to be xed in the right atrial appendage, where the trabecular muscles are dense, however, due to the anatomical speci city of the mirror-image dextrocardia, it is di cult to reach the accurate position by using passive xation leads, which increases the radiation exposure time of the patients, and is easy to fall off.
The active xation leads can be xed anywhere in the atrium, it is easier to nd the position with optimal sensing and pacing threshold, and reduce the operation time, so we x the atrial leads to the free wall of the right atrium. For a long time, many people think that this position is very thin, and it is easy to cause perforation when xing the electrode. However, there is no signi cant difference in postoperative complications (such as perforation, pericardial effusion, dislodgement), pacing sensing, pacing threshold and impedance [14][15] . In addition, the use of active atrial lead xation can reduce the incidence of dislodgement [16][17] , and signi cantly reduce the bed rest time and duration of hospital stay of patients.
Furthermore, atrial active xation lead has a lower and more stable pacing threshold [18] .
Because the anatomical speci city of the mirror-image dextrocardia, the operation during the implantation is opposite to that of a normal person, it is necessary to be fully familiar with and informed its anatomical characteristics and the trend of the blood vessels before the operation to help determine the path of lead implantation. While positioning the leads, we can combine the positive position, lateral position and oblique position under uoroscopy, determine the position of the leads from different angles, do not push blindly and cause surgical complications. During the operation, invert the uoroscopic image,perform the operation under the posterior anterior view [11] which is more in line with the usual operating habits, can increase the success rate of the operation, and can also reduce misoperations, which can cause vascular and cardiac injuries.
Abbreviations ECG electrocardiogram Figure 1 ECG examination revealed that the morphology of waves in leads avR and avL were opposite to the normal, inverted P and T waves in lead I, R wave in lead V1 to V6 decreases gradually, RV3 RV4 ( Figure 1)

Figure 2
The ECG with right-sided precordial leads and reversed limb leads showed that sinus arrest, junctional escape, ventricular premature beats, clockwise transposition ( Figure 2).