A specific introducer sheath for femoral access, the Medtronic MI2355A, is used for implantation. The introducer is 55.7cm long, has an internal diameter of 23 Fr (external diameter, 27 Fr), and possesses a hydrophilic coating to facilitate insertion. Once the introducer is inserted, the patient receives a bolus dose of 3000 IU of heparin, and thereafter, continuous perfusion of heparinized saline solution through the introducer side-port. The delivery catheter is designed to position the device in the right ventricle, with access through the femoral vein. The catheter is 105cm long and has a deflectable, flexible 23-Fr body with a compartment at the distal end to contain the device (Figure 1). The Micra remains joined to the catheter during implantation by a tether that runs along the length of the catheter forming a loop from the delivery system handle to the distal device recapture cone, which enables recovery and repositioning of the device, if needed. The handle also allows adjustment of the tether to advance or retract the distal compartment for engaging and repositioning the device.

Once the steerable catheter is situated in the right ventricle, a site in the septal apical or medial septal region is sought and evaluated to determine its suitability for releasing and attaching the device. This is achieved by injecting contrast medium and filming in 2 complementary radiologic projections for further analysis (Videos 1 and 2 of the supplementary material).

If the position is deemed adequate, the device is deployed, usually in a right oblique projection (Figure 2 and Video 3 of the supplementary material). The electrical parameters of the device are then determined, and if they are appropriate, the degree of fixation is evaluated. The maneuver of pulling and holding for a few seconds is performed, and the movement of the device tines is filmed and then analyzed frame by frame. Fixation is considered adequate when movement of at least 2 of the 4 tines is confirmed (Figure 3 and Video 4 of the supplementary material). Following the pull-hold maneuver, the electrical parameters are again determined and if they show no changes, implantation is considered complete. The tether threads are cut and withdrawn, and finally, the introducer is withdrawn and percutaneous closure is carried out using figure-8 sutures with No. 1 silk thread.

Figure 2.

Initiation of device deployment by retraction of the device cup, whose end in the middle of the Micra is made visible by a radio-opaque marker (arrow).

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Figure 3.

Pull and hold maneuver to evaluate the degree of fixation by putting tension on the system's tether. (A) Baseline image and (B) after gentle pulling and holding, which shows that 2 of the tines are curved outward from the device, the upper one being more markedly engaged (arrows).

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All patients underwent electrocardiography and posterior-anterior and lateral chest radiography before hospital discharge (24 h following the procedure). At 1, 3, 6, and 12 months later, the automated measurements of the device were reviewed, and the adequacy of the programming and pacing parameters was confirmed manually.

An initial descriptive analysis was performed, in which the continuous Gaussian variables are expressed as the mean±standard deviation, and non-Gaussian variables as the median (range). Statistics were carried out with SPSS 19.

The baseline characteristics of the patients are shown in Table.

The outcome of the implantation procedure was satisfactory in all 30 patients selected. Mean age was 79.4±6.4 (66-89) years and 20 patients (66.7%) were men. Mean follow-up was 5.3±3.3 months. At the time of the procedure 28 of 30 patients had permanent atrial fibrillation (AF). The indications for pacemaker implantation were slow AF in 28 patients (93.3%), trifascicular block and syncope in 1 patient in sinus rhythm, who was expected to require a low percentage of pacing, and recurrent episodes of rapid atrial tachycardia in another patient. Of note, 2 patients underwent pacemaker implantation following percutaneous aortic valve implantation. One patient had a mechanical mitral valve and a De Vega tricuspid annuloplasty, and another had severe mitral regurgitation.

In 5 patients with inadequate control of the ventricular response (16.6%), radiofrequency ablation of the atrioventricular (AV) node to improve valve competence was performed in the same procedure, immediately after percutaneous implantation of the Micra pacemaker. Ablation used the same sheath, advancing with 16-Fr and 8-Fr introducers, and the additional time needed (mean, 5±2 minutes) did not significantly prolong the procedure.

Implantation was performed under uninterrupted oral anticoagulant (OAC) therapy with coumerins in 23 patients (76.6%); the maximum international normalized ratio (INR) was 2.4. In 4 other patients who were receiving new oral anticoagulant agents, the dose prior to pacemaker implantation was omitted and therapy was reinitiated on the same day as the procedure. Hence, heparin bridge therapy was not used in any patient.

The device was positioned in the apical region in 20 patients (66.6%), in the medial septal region in 9 (30%), and in the right ventricular outflow tract in 1 (3.3%).

Pacemaker implantation was achieved in all patients, with a low rate of major complications, no displacements, and no systemic infections. Of note, 1 patient experienced a moderate pericardial effusion with no hemodynamic repercussions that was resolved conservatively and did not prolong the hospital stay. The patient was 83 years of age, was not receiving OACs, and the device was placed in the medial septal region. In another patient, the left femoral vein was used to implant the device because the right femoral vein showed excessive tortuosity and a local dissection. The patient was discharged 24hours after the procedure, with no local complications immediately after or during follow-up. No local venous access-related complications occurred in any of the patients.

The pacing variables were excellent at implantation and at short- and mid-term follow-up (Figure 4). Four patients completed 1 year of follow-up with no changes in the sensing or pacing parameters, or in the pacing impedances.

Figure 4.

Changes in the R wave threshold, impedance, and sensing values according to month of follow-up.

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With population aging, current medical practice is facing an increase in the number of pacemaker implantation procedures, which are associated with a far from negligible complication complications, such as infections and hematomas.2–6 These complications can also occur in device replacement procedures.5,10–13

The estimated complication rate in conventional pacemaker implantation ranges from 6% to 10%2,3 and increases to 14.2% when a new lead is implanted, the device must be checked for malfunctioning, or an upgrade of the pacing system is required.2

With regard to cardiac perforations, the rate associated with conventional pacemakers is 1%.2,14,15 The use of active fixation systems has been associated with a low perforation rate. In the study by Cano et al.,15 the rate of clinically relevant cardiac perforations was reported at only 0.8% and the predictors of this event were advanced age and female sex.

The use of the new Micra transcatheter pacemaker system could, at least theoretically, allow minimization of the complications associated with conventional systems, particularly those related to the 2 elements that are eliminated in the Micra: a) the pacing leads and connections, and b) the generator pocket implanted in the prepectoral region. Another factor to consider is the >10-year estimated service life of the device.16 This raises doubts about its use in patients with a long life expectancy, but experimental studies have determined that the right ventricle can accommodate up to 3 such devices.17–19 In addition, acute capture and extraction of the device at a few months of follow-up has been possible, but current knowledge on the amount of fibrosis associated with the implant is insufficient to predict the feasibility of later withdrawal.20–23

The present series has several important particularities. In our opinion, the most important is that the procedure was carried out without interrupting OAC therapy, which did not imply a higher incidence of pericardial effusion or puncture site complications. This is an important, considering that in the pivotal trial8 anticoagulation was not used in 36% of patients vs 64% who received this therapy (heparin bridge therapy in 40% and OAC in 24%). In the study by Pachón et al.,24 7 of the 10 patients included were receiving OACs, but therapy was discontinued for the procedure in 5 of them.24 From our point of view, it seems safe to carry out the implantation procedure while continuing the anticoagulant regimen, a therapy known to be associated with hematomas when conventional pacemaker systems are used. Therefore, safe implantation could be performed in at-risk populations (eg, carriers of valvular prostheses) without the need for heparin bridge therapy.25–27 Among the patients in our series, 2 underwent Micra placement immediately after percutaneous valve implantation, 1 patient had a mitral valve prosthesis, and 2 others had AF and severe mitral regurgitation, all situations in which OAC interruption, even with heparin bridge therapy, would considerably increase the risk of complications.28

Second, for the first time, the versatility of the procedure was seen, as pacemaker implantation was combined with AV node ablation. In our experience (although limited to 5 patients), the combined objective was satisfactorily achieved by taking advantage of the introducer sheath of the Micra system. This is a clear advantage over conventional systems, since a second puncture and the additional cost of a second procedure are avoided. Nonetheless, it may be argued that this is a risky strategy. Based on our previous experience with implantation of this device, we believe that the combined technique could be a safe, convenient strategy when there is no device displacement or malfunctioning and patients are monitored by telemetry for a minimum of 24hours following the procedure. The combination strategy of AV node ablation and pacemaker implantation in the same procedure is now commonly performed in our center and there have been no complications related to dislodgement of the ventricular electrode. Nevertheless, the value of this approach should be confirmed in future studies with larger samples before it becomes widespread practice.

Third, we believe the present study provides clinically relevant information: Although the patients were drawn from clinical practice and we included the learning curve, there was a lower incidence of complications than that reported in the pivotal clinical trial.8 Specifically, that trial had 30 relevant complications (in 26 patients, 18.6%), including ventricular arrhythmia (n=9, 6.4%), femoral complications (n=9, 6.4%), and pericardial effusion without tamponade (n=1, 0.7%). In the study by Reynolds et al.,9 there were 28 major complications (in 26 patients, 3.2%), the most important being embolism/thrombosis (n=2, 0.3%), vascular complications (n=5, 0.7%), myocardial injury (n=11, 1.6%) and problems with the pacing system (n=2, 0.3%).

The total complication rate in the study by Reynolds et al.9 was only 4%, half the value reported in a historical series used for comparison (7.4%). Nonetheless, myocardial injury in patients receiving the Micra device was 1.6%, higher than the 1.1% in the comparison group. The predictors of this adverse event were advanced age, low weight, female sex, chronic obstructive pulmonary disease, steroid use, and previous percutaneous coronary artery revascularization. These are the factors classically associated with the risk of perforation related to the intravenous leads in conventional permanent pacemakers.9,14,15

There was 1 major complication in our series (pericardial effusion) and no major complications at the venous access site. Therefore, our data concur with those of Pachón et al.,24 who also reported an absence of notable complications following the procedure. These results may encourage more widespread implementation of this technique in other centers, providing that the patients selected are compatible with the characteristics of the device.

At the scheduled clinical follow-up visits (24 hours, and 1, 3, and 6 months after the procedure) there were no cases of device displacement or elevation of the pacing thresholds. Moreover, the 4 patients who completed 1 year of follow-up showed an identical trend. The trend in the electrical pacing parameters (Figure 4) was very good, supporting the data described in the study by Reynolds et al.9; that is, a decrease in the acute threshold in the first 24hours, with stable values during follow-up. The same pattern was observed in the impedance values. This information is of particular clinical importance because, together with the automatic capture control function, it enables the device to achieve an estimated service life of 8 to 9 years.

We found that the R wave detection values tended to increase slightly over follow-up. This is likely because this sequence has a relationship with the local microtrauma and injury associated with deployment of the device and the later beneficial effect of steroid elution from the cathode. This has sparked controversy in some forums,29,30 and it is certainly important to adhere to the recommendation to obtain values >5 mV. Nonetheless, we believe that borderline values may be acceptable considering the inherent risk involved in device repositioning. In the study reported by Pachón et al.,24 1 patient had an R wave value at implantation of 4.7 mV that increased to 5.7 mV during follow-up. In our series, 1 patient had excellent threshold and impedance values, and an R wave value of 4.4 mV at implantation, which increased to 5.5 mV in 24 hours. Thus, we believe that, in terms of pacing, it is much more important to have an optimal capture threshold (increased device duration) as well as good impedance of around 700-800Ω, which, in addition to favoring the longevity of the device, is an indirect marker of good fixation to the endocardium (and the safety of the implant).

A limitation of this study is that it was a single-center, nonrandomized study. In addition, the number of patients included and the follow-up are relatively small. Although it provides 1-year follow-up data for the first time, only 4 patients completed this period; hence, there is a need for studies with longer follow-up and including larger numbers of patients. Last, only 2 professionals performed all the procedures; therefore, the results cannot be considered applicable to all centers.