Oral anticoagulation was stopped prior to implantation: vitamin K antagonists were generally withheld until the INR was ≤ 2 and nonvitamin K oral antagonists were stopped 24 hours before the procedure.

All interventions were performed with patients under conscious sedation. The Micra implantation was performed according to the standard technique described elsewhere.10 Venous femoral access was obtained by ultrasound-guided puncture and a 27-Fr introducer was advanced into the right atrium. The delivery system and the device were then advanced through the introducer and positioned in the septum of the right ventricle. Either an apical-septal or midseptal positions were targeted for implantation. Dye injection was routinely performed to confirm the correct apposition of the delivery catheter to the septum. The device was then deployed and affixed to the myocardium through the 4 nitinol tines. After verification of adequate electrical measurements, the “pull-and-hold maneuver” was routinely performed to confirm the correct anchoring of the device to the myocardium. When electrical measurements or device fixation were not appropriate, the device was repositioned.

An echocardiogram, chest X-ray and electrocardiogram as well as assessment of electrical parameters were performed 24 hours after implantation.

Follow-up was performed at 7 days, and 1, 3, 6 and 12 months after implantation. Subsequent annual follow-ups were then performed. Patients with high pacing threshold at implantation (> 1.0 V at 0.24 ms) or during follow-up (≥ 1.5 V at 0.24 ms) were routinely included in the remote follow-up program, with monthly visits until pacing threshold stabilization, and then every 4 months.

Continuous data are expressed as mean±standard deviation or median (interquartile range) when appropriate. Categorical or ordinal variables are expressed as percentages. All continuous data were tested using the 1-sample Kolmogorov-Smirnov test against a normal distribution. Comparison between groups was made using the Student t-test, the Mann-Whitney U test and the chi-square test, according to the type of variable and its distribution. A P value <.05 was considered statistically significant. The analysis was conducted with the IBM SPSS Statistics version 21.0 (SPSS, Chicago, Illinois).

A Micra pacemaker was implanted in 129 of 353 patients aged ≥ 70 years with an indication for a single-chamber pacing device in our institution during the study period: 41 (31.8%) in patients aged 90 (92.9±2.4) years or older and 88 (68.2%) in patients younger than 90 (83.9±4.1) years. Implantations for previous transvenous pacemaker infection and extraction were performed in 1 (2.4%) patient ≥ 90 years and in 3 (3.4%) patients <90 years. Implantations due to lack of vascular access were performed in 1 (2.4%) patient ≥ 90 years and in 3 (3.4%) patients <90 years. In patients with no specific indication for a leadless device (no previous device infection and/or absence of upper vascular access), Micra implantation was urgent and performed within 24 hours from indication in 85 patients (33 aged ≥ 90 years), and was non-urgent in 37 patients (7 aged ≥ 90 years) (Figure 1).

The baseline characteristics of the study population are shown in Table 1. The older group showed a higher proportion of women (56% vs 36%), a higher prevalence of chronic kidney disease (59% vs 35%), a lower prevalence of structural heart disease (34% vs 56%), and fewer indications for chronic anticoagulant therapy (10% vs 41%).

Advanced atrioventricular block was the main indication for cardiac pacing in both groups (70.7% in patients aged ≥90 years and 69.3% in those aged <90 years), followed by atrial fibrillation with slow ventricular response (19.5% vs 12.5%) and sinus node dysfunction (9.8% vs 18.2%). In 2 (4.9%) patients aged ≥ 90 years and in 9 (10.2%) patients aged <90 years, the indication was an atrioventricular conduction disorder following TAVI implantation.

The main implantation procedure characteristics are shown in Table 2. The Micra pacemaker was successfully implanted in 40 of the 41 (97.6%) patients aged ≥ 90 years and in 87 of the 88 (98.9%) patients aged <90 years (P=.58). The 2 device implantation failures were due to important iliocaval venous tortuosity, which prevented the advance of the 27-Fr introducer in a patient aged ≥ 90 years, and to cardiac perforation in a patient aged <90 years. Procedure and fluoroscopy times were similar in the 2 groups. The implantation was successful after the first deployment of the device in 35 (87.5%) patients aged ≥ 90 years and in 58 (66.7%) aged <90 years (P=.01). No differences were found in the number of repositions required during the implantation: ≤ 2 repositions in 97.5% of the patients aged ≥ 90 years and in 91.9% of those aged <90 years (P=.32).

Electrical measurements at implantation are shown in Table 3. Thirty-nine (97.5%) patients aged ≥ 90 years and 83 (95.4%) of those aged <90 years had a threshold ≤ 1.0 V at 0.24 ms at implantation. Only 1 patient in the group aged ≥ 90 years and 2 in the group aged <90 years had a pacing threshold ≥ 1.5 V at 0.24 ms at implantation (1.5 V; 4.0 V and 1.5 V at 0.24 ms, respectively). In the patient with 4 V, the threshold elevation was observed early after implantation, after removal of the femoral introducer.

Complications at implantation and within 30 days after implantation are shown in Table 4. Cardiac perforation requiring emergent cardiovascular surgery occurred in a patient aged <90 years (0.8%), an 83-year-old woman, early at the beginning of the study (patient #7). There were 2 relevant vascular complications (1.5%): 1 pseudoaneurysm and 1 femoral hematoma, both in the group aged <90 years. The pseudoaneurysm originated in the femoral access used for a temporary pacing catheter, contralateral to the access used for the Micra implantation and therefore not directly related to it. No major complications occurred in the group ≥ 90 years. Length of hospital stay, from pacemaker implantation indication to hospital discharge was similar in both groups (3.0 days [IQR 2.0-5.5] for patients ≥ 90 years, and 3.0 day [IQR 1.0-9.0] for patients <90 years; P=.95). There were no deaths related to the device implantation.

The mean length of follow-up was 342±279 days for the entire population, 230±233 days for patients aged ≥ 90 years and 394±285 days for those aged <90 years. There were 29 deaths during follow-up: 13 in the group aged ≥ 90 years (31.7%) and 16 in the group aged <90 years (18.2%), none of them related to the device. Most deaths (93.1%) were due to noncardiovascular causes. One patient in each group died from heart failure, one due to severe aortic valve stenosis (≥ 90 years), and another due to severe mitral regurgitation (< 90 years); none of these patients were considered suitable for valve replacement.

No differences were observed in electrical parameters between the 2 study groups during follow-up (Figure 2). All patients aged ≥ 90 years and 92.6% of those aged <90 years remained stable with a pacing threshold <1.5 V at 0.24 ms for more than 3 months after implantation.

Figure 2.

Electrical performance characteristics of Micra in the 2 study groups. Data in the graphs are mean values. The vertical solid lines represent standard deviations. No. values are the numbers of patients with available data at each time point.

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The 2 patients aged <90 years with high pacing threshold at implantation showed a reduction in this parameter during follow-up: the patient with a threshold of 1.5 V showed a reduction in this parameter at the first month and it remained below 1.0 V after 2 years; in the other patient, with a pacing threshold of 4 V early after implantation, a reduction to 2.88 V was documented 24 hours after implantation and a progressive increase was then observed to 4.38 V after 6 months of follow-up. The only patient in the group aged ≥ 90 years with a high pacing threshold at implantation (1.5 V) died within the first month due to intestinal ischemia.

In contrast, 4 patients aged <90 years with an adequate pacing threshold at implantation (< 1.0 V), exhibited a progressive increase in this parameter during follow-up: in 1 patient the increase was transient, reaching 1.75 V at 3 months and subsequently decreasing below 1.5 V; in the other 3 patients, the pacing threshold progressively increased during the first year up to 2.88 V, 1.75 V and 1.63 V at 0.24 ms, respectively. Importantly, all these patients underwent remote follow-up surveillance and no reintervention was needed in any of them.

There were no device dislocations needing reintervention, migration nor infection during follow-up.

This is an observational and single-center study that included a reduced number of patients. Patients were selected exclusively on the basis of standard indications for single-chamber implantation and a selection bias cannot be completely excluded. The stimulation mode was carefully chosen for each patient according to clinical criteria and prior to the decision to implant a leadless pacing system.

As previously mentioned, the higher cost of the Micra TPS compared with conventional pacemakers raises the question of the cost-effectiveness of the device, especially in very old patients with high mortality risk due to noncardiovascular causes. The study was not designed to evaluate the cost-effectiveness of the device in this setting, a question that certainly warrants further investigation.