International Journal of Heart Rhythm

: 2016  |  Volume : 1  |  Issue : 1  |  Page : 50--54

Implantation and Clinical Performance of an Entirely Leadless Cardiac Pacemaker

Chu-Pak Lau1, Keping Chen2, Kathy Lai-Fun Lee1, Yan Dai2, Shu Zhang2,  
1 Department of Medicine, Division of Cardiology, The University of Hong Kong, Hong Kong Special Administrative Region, China
2 State Key Laboratory of Cardiovascular Disease, National Center for Cardiovascular Diseases & Fuwai Hospital, Chinese Academy of Medical Sciences & Peking Union Medical College, Beijing 100037, China

Correspondence Address:
Chu-Pak Lau
Department of Medicine, Division of Cardiology, The University of Hong Kong, Hong Kong Special Administrative Region, Hong Kong


Background: Entirely leadless pacemakers (LPMs) address limitations of conventional pacemakers that include complications related to the pacing leads, their connections, and pacemaker pockets. The aim of this study was to describe early implantation experience and clinical efficacy of LPM. Methods: A total of eight patients received an LPM (Micra™ Transcatheter Pacing System, Medtronic plc, Minneapolis, MN, USA). LPM was transvenously deployed using a 23 F sheath, and actively fixed by 4 nitinol tines. Results: On average, the patients were 74.3 ± 8.1 years, and 50% were female. All had indications for a ventricular demand (VVI) pacemaker, and ejection fraction was 66.4% ±7.4%. Except for one patient, all were implanted from the right femoral vein. The LPM was deployed either at the right ventricular apex (63%) or at the septum (37%). At implantation, pacing threshold at 0.24 ms was 0.69 ± 0.35 V, and R wave was 8.1 ± 2.9 mV. Successful pacing sites were reached at a median of 1 attempt (range 1–3), and the mean procedure and fluoroscopic times were 74 ± 19 min and 11.0 ± 5.8 min, respectively. 50% were on uninterrupted anticoagulation, and there were no acute complications including groin hematoma. Both pacing threshold and R wave improved at 1 month compared to acute implant value (0.46 ± 0.11 V and 14.5 ± 5.6 mV, respectively, P< 0.05 compared with implant). Between 1 and 3 months follow-up, there was no change in pacing or sensing threshold. The average percentage of ventricular pacing was 65% ± 26%. The intracardiac accelerometer was activated in 3/8 patients, and the satisfactory rate response profile during activity of daily living was achieved. Battery longevity was estimated to be more than 8 years in all patients. Conclusion This study documents excellent implantation success of the Micra™ LPM with stable pacing and sensing and satisfactory rate response profile.

How to cite this article:
Lau CP, Chen K, Lee KL, Dai Y, Zhang S. Implantation and Clinical Performance of an Entirely Leadless Cardiac Pacemaker.Int J Heart Rhythm 2016;1:50-54

How to cite this URL:
Lau CP, Chen K, Lee KL, Dai Y, Zhang S. Implantation and Clinical Performance of an Entirely Leadless Cardiac Pacemaker. Int J Heart Rhythm [serial online] 2016 [cited 2021 Oct 27 ];1:50-54
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Full Text


Since the implantation of the first permanent pacemaker in 1958,[1] the pacing system for bradycardia has not substantially changed. This still requires introduction of a lead(s) through a venous system to the heart, with the lead fixed either actively or passively in the endocardium. After achieving the desired lead position, the lead(s) has to be connected to the header of a pulse generator. The pulse generator needs to be housed in either a subcutaneous or a submuscular pocket in the chest wall. Acute implantation complications, therefore, include pneumothorax, lead dislodgement, cardiac perforation, and pocket hematoma. The pacing lead is the weakest component of the system, which can be crushed, fractured, or abraised on the long term. Pacemaker in its pocket can cause pain or become eroded. System infection is the most dreaded complication, which has been reported to occur in up to 3.5%.[2],[3] Above all, the presence of a scar and the pacemaker which can be felt are constant reminders and a psychological burden to some patients that their lives are under machine control.

Entirely leadless pacemakers (LPMs) overcome many of these complications of conventional pacing.[4],[5] This concept was introduced as early as 1970,[4] but is only recently available clinically in two systems. The Nanostim™ leadless pacemaker (Nanostim Inc., Sunnyvale, CA, USA; now acquired by the St Jude Medical Inc., St. Paul, MN, USA) uses an active screw acting as a cathode.[6] The Micra™ Transcatheter Pacing System (Medtronic plc, Minneapolis, MN, USA) uses 4 nitinol tines for endocardial fixation.[7] Both systems have been reported to show good safety and efficacy outcome.[8],[9] The aim of the study is to examine the early experience of Micra™ in an Asian patient cohort.



Consecutive patients presenting to the Hong Kong Sanatorium and Hospital in Hong Kong Special Administrative Region and Fuwai Hospital in Beijing, China, who were suitable and agreed for LPM were recruited. In Hong Kong, the Micra™ was clinically available. In Beijing, the patients had consented to the Medtronic Micra™ Transcatheter Pacing Study.[9] The clinical demographics are summarized in [Table 1]. All have AHA/ACC/HRS Class I indication of pacing. 88% had atrial fibrillation. Their mean age was 74.3 ± 8.1 years, and the mean left ventricular ejection fraction was 66.4% ±7.4%. Most patients had significant comorbidities including hypertension (n = 7), valvular disease (n = 3), diabetes (n = 2), and other conditions.{Table 1}

Device description and implant procedure

The Micra™ has been described in detail.[5],[7],[9],[10] In brief, it is 25.9 mm long, 6.7 mm in diameter, and has a volume of 0.8 cc. It has an estimated longevity of 12.5 years based upon use condition [8] and is programmable by Medtronic Model 2090 programmer. The maximum pacing output is 5 V at 1 ms. It is both 1.5T and 3.0T MR compatible.[11] The Micra™ has a triaxial accelerometer. By selecting the best responding axis to body activity, rate responsive, ventricular demand (VVIR) pacing can be effected. The device is fixed by 4 self-retracting nitinol tines, and fixation is achieved when at least 2 tines are attached to the endocardium. The Micra™ is deployed using a 23 F catheter sheath, introduced from one of the femoral veins. After positioning the sheath in the mid right atrium, the sheath is carefully passed across the tricuspid valve and positioned to face the right ventricular (RV) septal surface at or above the apex [Figure 1]. An apical position was initially recommended; however, with experience to avoid perforation, a septal position above the apex is now preferred. This position is confirmed with fluoroscopy during contrast injection through the sheath. The Micra™ is then deployed by retracting the sheath, with the nitinol tines initially in the extended position. Once it reaches the endocardial surface, further withdrawal of the sheath allows full self-retraction of the tines to the flexed position due to memory properties of the nitinol and catches the endocardium in the process. The cathode is then pressed against the endocardium. Thus, the Micra™ is an actively fixed pacemaker, but the cathode does not penetrate the endocardium. Stability of attachment is tested by pulling on the tether to assess extension of the tines under fluoroscopy. Thereafter, electrical parameters are confirmed using external telemetry. A satisfactory R wave (≥5mV), pacing threshold (≤1.0V at 0.24 ms), and impedance of 400–1500 Ω were required before the completion of deployment. After ≥3–5 deployment attempts, a lower R wave and a higher pacing threshold (up to ≥3 V) depending on patient pacing/longevity need were acceptable. If these parameters are not met, the device can be recaptured by tracking the sheath over the tether and repositioned to a different site. Only when both satisfactory fixation and electrical parameters were achieved was the tether cut, and device fully deployed. The sheath was then removed; hemostasis achieved with purse string subcutaneous sutures was put in place right after venous access. Anticoagulation was not reversed in those patients on anticoagulation.{Figure 1}


The patients were discharged after at least an overnight stay and were seen in the outpatient at 1 month (between 4 and 6 weeks) and 3 months. The clinical performance and electrical parameters of the Micra™ were examined. At 3 months after implant, VVIR function was activated in all patients from Hong Kong. Details of vector selection and sensor programming have been described elsewhere.[13] As the orientation of the pacemaker inside the heart is variable depending on the implant, a choice of one of the three vectors available is needed to avoid interference of cardiac motion and posture and to optimize rate response. In brief, the vector that gave a rate during casual walking of about 90–100 bpm was chosen, with the sensor vector giving no excessive high rate with postural changes. Thereafter, sensor adjustment was done automatically using rate profile optimization.


Descriptive statistics were presented as mean ± standard deviation, or as median (and range) if nonparametric. Student's paired t-test was used to compare implant R wave and threshold to those at 1 and 3 months of follow-up. A statistical significance was considered if P < 0.05.


Successful implantation was achieved in all patients. With the exception of one patient, the right femoral vein was used [Table 2]. The remaining patient had a hemodialysis catheter on the right femoral vein, and the LPM was implanted from the left femoral vein. Detail of this case is separately reported.[12] The mean implant time was 74 ± 19 min, and the median number of attempts was 1 (range: 1–3). The fluoroscopic time was 11.0 ± 5.8 min (with a median of 9.7 min). Most (63%) were implanted in the septal aspect of the RV apical septum, and 37% were in the low to mid RV septum. 7/8 patients had an implant threshold of <1 V at 0.24 ms. At implantation, the mean pacing threshold was 0.69 ± 0.35 V at 0.24 ms and the R wave was 8.1 ± 2.9 mV. In one patient (Patient 2), the pacing threshold was 1.38V at 0.24 ms at the third site attempted. This was considered acceptable in view of her age and the infrequent pacing requirement. Anticoagulation was used in 50% of patients, and this was continued during implantation uninterrupted. No heparin was administrated during the procedure. The chest radiography of a typical patient after implant is shown in [Figure 2].{Table 2}{Figure 2}

There was improvement in both pacing threshold and R wave at 1 month compared to baseline ([0.69 ± 0.35] V to [0.46 ± 0.11] V and [8.1 ± 2.9] mV to [14.9 ± 5.6] mV, both P < 0.05) [Table 3]. Between 1 and 3 months, sensing and pacing parameters were stable, without significant differences between them. Different rate responsive vectors were used in three patients in whom VVIR pacing was activated (vector 1 in two patients and vector 3 in one patient). The rate response profile of a typical patient with atrial fibrillation is shown in [Figure 3].{Table 3}{Figure 3}


This study documents an implant success rate of 100%. A successful pacing site was achieved with a median number of 1 attempt. There was no device dislodgement. The implant threshold was 0.69 ± 0.35 V at 0.24 ms, well below the twice safety margin that the device can provide. There was no acute implant complication, including no groin complications in the 50% of patients on uninterrupted anticoagulation during implantation. Up to 3 months of follow-up, these electrical parameters were stable. Based on the mean of 64.7% ±26% pacing, device longevity was estimated to be >8 years. Rate response was satisfactory. Thus, in patients with indication of VVI/VVIR pacing, LPM is an alternative.

The implantation success rate reported for the Nanostim™ leadless pacemaker [7] and Micra™ TPS [9] was 95.8% and 99.2%, respectively. A high success rate was also found in this study. In our series, the pacing threshold was ≤1 V in all except 1 patient, who had an implant threshold of 1.38 V at 0.24 ms at the third attempted RV site. We considered that it was acceptable in view of the age and frequent pacing requirement. Interestingly, her pacing threshold at 3 months decreased to 0.75 V. While complications related to implanting a conventional pacemaker such as pneumothorax and pocket hematoma would not occur with LPM implantation, femoral venous access problems, cardiac perforation, and device dislodgement may potentially occur. Using purse-string sutures before catheter introduction, avoidance of deployment at the RV free wall, and “pull-test” to ensure fixation, these complications were not encountered with the Micra™ TPS in the present study. In patients on anticoagulation, interruption of anticoagulation is a concern and pacemaker pocket hematoma is a significant problem. This appears to be an advantage of intracardiac transcatheter pacemaker system. Chronic problems related to leads, their connections, and pacemaker pockets would not occur with LPM. The infection rate is likely to be significantly lower although data in this regard are limited.

There was no delayed dislodgement in any of our patients, and the electrical parameters were stable. Durability is encouraging with this new technology. Estimated longevity is acceptable in this cohort of patients.

The performance of intracardiac accelerometers is different from those in the pacemaker pocket. There is no control of LPM orientation at implant in the heart, and selection of accelerometer vector on an individual basis is needed. Cardiac signals may also affect accelerometer readings. In the limited experience in this series, rate response profile to daily activities was satisfactory. Further studies on the long-term durability and sensor performance of LPM will be needed.


Our data show that the Micra™ LPM could be implanted with a high success rate and without significant complications. Early clinical performance, both in electrical parameters and sensor rate response, was satisfactory.

Financial support and sponsorship

Micra TPS Global Clinical Trial was sponsored by Medtronic plc for the patients in China.

Conflicts of interest

There are no conflicts of interest.


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