Scientific letter Implantation of the Micra Transcatheter Pacing System: Initial Experience in a Single Spanish Center

To the Editor,

The number of pacemakers implanted continues to increase as a result of increased life expectancy. According to the 2013 Spanish Pacemaker Registry,¹ the number of first implants per million inhabitants was 567. Despite improved technology and more extensive experience, complications related to pacemaker implantation are still significant, particularly in relation to transvenous leads and the pacemaker pocket.²

Single-component leadless pacemaker systems have been developed to reduce complications. In these systems, the pacemaker and lead are integrated in a single unit, thereby eliminating the risks associated with the leads, pacemaker pocket, and connections.³

In the present letter, we report our initial experience in a single center with implantation of the Micra transcatheter pacemaker system (Medtronic Ibérica, S.A.), approved for clinical use in Europe in June 2015. The device consists of an 8-cm³ capsule, measuring 25.9 × 6.7 mm, designed to provide bipolar VVI-mode cardiac pacing of the right ventricle, with an estimated longevity of 9.6 years. The device also has automated pacing capture threshold management and can be used in magnetic resonance imaging studies without any body restrictions in 1.5 and 3 Tesla systems. The pacemaker is affixed to the ventricular endocardium by 4 self-expanding electrically inert nitinol tines. The device is positioned with the pacemaker and unexpanded tines at the distal end of a steerable catheter, which lies inside an introducer (23-F internal diameter and 27-F external diameter), inserted via the femoral vein. Before the pacemaker is definitively released from the catheter, and once it is positioned in the ventricle, the operator must confirm that the electrical parameters are appropriate and that the system is well affixed to the endocardium by a fluoroscopically guided pull-and-hold test. If the device is not properly affixed or the electrical parameters are deficient, the pacemaker can be removed and the implantation procedure repeated.

The Micra pacemaker was implanted in 10 patients with a standard indication for a permanent pacemaker and with a clinical profile and indication appropriate for VVI pacing. In this initial phase, patients without their own cardiac rhythm were excluded. Prior to implantation, the patients and family members were informed of the characteristics of the new system and the more limited clinical experience with respect to conventional systems. Informed consent was obtained. The general characteristics of the patients and basic data on the procedure are summarized in the Table. The mean age of the patients was 77.1 ± 5.1 years, 6 were men and 4 were women, and 8 had permanent atrial fibrillation as the basal rhythm and 2 had sinus rhythm. The Micra pacemaker was successfully implanted in 10 patients (Figure) by the same operator (M. P.), who had extensive experience in placement of conventional implantable cardiac devices and in electrophysiological procedures with femoral venous and arterial approaches. The first fixation attempt was definitive in 7 patients. In the remaining 3, repositioning was required, although in all patients only 1 further attempt was needed due to deficient electrical parameters after the initial implantation. The mean values of the R wave and impedance were 12.7 ± 4.8 mV and 739 ± 161 ohms, respectively. The mean duration of the implantation procedure was 44.6 ± 7.5 minutes, while fluoroscopy lasted 10.03 ± 2.5 minutes. No patients experienced sustained arrhythmias or blocks during implantation. The femoral puncture site was closed with a Figure 8 suture, with no incidents of note (2 of the patients continued anticoagulant treatment with acenocoumarol during implantation). The clinical and electrical data are summarized in the Table. Of note is the absence of complications requiring intervention and the low rate of repositioning even though these were the first implantations of the new device. At follow-up (mean duration, 55 ± 33 days, range 27-112 days), thresholds (all = 1 V at a pulse width of 0.24 ms), R wave (13.4 ± 5.1 mV), and pacing impedance (633 ± 138 ohms) were very stable.

Table.

Patient Characteristics and Implantation and Follow-up Data

	Patient 1	Patient 2	Patient 3	Patient 4	Patient 5	Patient 6	Patient 7	Patient 8	Patient 9	Patient 10
Sex	Male	Female	Male	Female	Male	Male	Male	Male	Female	Female
Age, y	70	78	69	80	75	80	81	86	77	75
Weight, kg	88	70	106	46	98	90	85	82	76	59
Height, cm	160	155	174	155	178	173	181	172	162	164
Concomitant conditions	Hypertension Dyslipidemia Exsmoker Diabetes mellitus	Hypertension Dyslipidemia Diabetes mellitus	Hypertension Dyslipidemia Exsmoker	No	Hypertension Dyslipidemia	Hypertension	No	Hypertension Obstructive sleep apnea	Hypertension Chronic renal failure COPD	Hypertension Chronic renal failure
Indication	Intermittent syncope due to AF	Slow symptomatic AF	Intermittent AF	Erratic control of AF	Bradycardia-tachycardia syndrome	Bradycardia-tachycardia syndrome	Slow AF	Permanent 1st degree AV block and paroxysmal 3rd degree block	Erratic control of AF	Slow AF
Basal rhythm and frequency, bpm	AF	AF, 48	AF, 54	AF, 85	SR, 55	AF, 80	AF, 40	SR, 80	AF, 95	AF, 60
QRS, ms	100	90	95	140, left bundle branch block	100	110	95	110, left anterior hemiblock	100	110
LVEF, %	50	60	55	65	60	70	71	60	61	65
Heart disease	Ischemic	Severe mitral and aortic stenosis; refused surgery	Severe mitral regurgitation due to mitral prolapse, with surgical repair	Valvular: mechanical mitral prosthesis and tricuspid annuloplasty	Moderate degenerative AoVD	Hypertensive, mild mitral regurgitation	Mild degenerative AoVD	Mild degenerative AoVD	Moderate AoVD, moderate mitral regurgitation	Mechanical mitral prosthesis due to rheumatic disease
Tricuspid value regurgitation and systolic pulmonary pressure	Grade II, 40 mmHg	Grade II, 50 mmHg	Grade III, 49 mmHg	Grade II, 43 mmHg	Grade III, 25 mmHg	Grade II, 35 mmHg	Grade I, 32 mmHg	Grade I, 30 mmHg	Grade II, 37 mmHg	Grade II, 50 mmHg
Medications	Acenocoumarol ARB ASA Carvedilol Statins Amlodipine	Acenocoumarol ARB Insulin Statin Furosemide	Acenocoumarol ACEI Bronchodilators Statin	Acenocoumarol Furosemide Diltiazem Bisoprolol Spirolactone	Dabigatran Nebivolol ARB Furosemide	ARB Hydrochlorothiazide ASA	Acenocoumarol	ACEI	Acenocoumarol ARB Nitrates Furosemide Atenolol	Acenocoumarol Digoxin Furosemide ARB
Particular considerations	No	No	No	Previous implantation of transvenous pacemaker, explanted after 1 month due to complicated hematoma	No	No	No	No	No	Temporary epicardial pacemaker after cardiac surgery
Total implantation time	51	55	57	43	36	38	37	45	45	39
Fluoroscopy time	15	10.7	13.4	9.5	7.5	8	8.1	10.5	10.3	7.3
Anticoagulation during implantation	No	No	No	Yes, INR 2.8	No	No	No	No	No	Yes, INR 2.2
Anesthesia	Conscious sedation and analgesia and local anesthetic	Conscious sedation and analgesia and local anesthetic	Conscious sedation and analgesia and local anesthetic	Conscious sedation and analgesia and local anesthetic	Conscious sedation and analgesia and local anesthetic	Conscious sedation and analgesia and local anesthetic	Conscious sedation and analgesia and local anesthetic	Conscious sedation and analgesia and local anesthetic	Conscious sedation and analgesia and local anesthetic	Conscious sedation and analgesia and local anesthetic
No. of repositioning procedures and reasons	1, threshold > 1.5 V	0	1, threshold > 1.5 V	0	0	0	0	1, threshold > 1.5 V	0	0
Final position	Apical	Midseptal	Midseptal	Apical	Apical	Midseptal	Apical	Midseptal	Apical	Midseptal
Implantation
R wave, mV	18.2	14.2	10.4	11.5	4.7	12.5	6.2	16.4	20	12.8
Impendence, ohms	920	520	910	610	660	560	700	990	720	800
Threshold in V at 0.24 ms	1.25	1.25	0.5	0.25	0.5	0.25	0.25	0.5	0.63	0.25
Complications	No	No	No	No	No	No	No	No	No	No
Programming	VVIR 50-110 bpm	VVIR 60-120 bpm	VVIR 60-140 bpm	VVIR 55-120 bpm	VVI 50 bpm	VVI 50 bpm	VVIR 60-130 bpm	VVI 50 bpm	VVI 50 bpm	VVI 60 bpm
Follow-up, d	112	112	62	62	40	40	33	33	27	27
Follow-up
R wave, mV	20	13	11	12.5	5.7	20	6.4	13.7	19.8	15.1
Impendence, ohms	470	500	880	750	500	580	670	530	600	700
Threshold at 0.24 ms	1	0.88	0.88	0.38	0.38	0.38	0.38	0.5	0.38	0.88
Battery voltage and estimated longevity	3.14, > 8 y	3.14, > 8 y	3.15 V, > 8 y	3.16 V, > 8 y	3.17 V, > 8 y	3.16 V, > 8 y	3.15 V, > 8 y	3.16 V, > 8 y	3.15 V, > 8 y	3.16 V, > 8 y
Complications	No	No	No	No	No	No	No	No	No	No
% pacing	8	60	25.5	27.5	3.3	7.5	90.6	21.3	13	25

ACEI, angiotensin converter enzyme inhibitor; AF, atrial fibrillation; AoVD, aortic valve disease; ARB, angiotensin receptor antagonists; ASA, acetylsalicylic acid; bpm, beats per minute; COPD, chronic obstructive pulmonary disease; LVEF, left ventricular ejection fraction; SR, sinus rhythm; V, volts.

Figure.

A: Image of completely hidden (left) and partially released (right) device in the steerable implantation catheter. B: Radiological detail of the implanted pacemaker in patient 4. C: Posteroanterior chest X-ray in 2 patients with midseptal position and 2 patients with apical position.

The initial results in terms of electrical performance and safety of the implantation procedure for the Micra transcatheter pacemaker are promising in the series presented here. However, to date there are no studies in humans comparing this new system with conventional pacemaker systems. The largest study reported is the Micra Transcatheter Pacing Study. This was a prospective, multicenter, noncomparative study in which the Micra system was successfully implanted in 719 of 725 attempts (99.2%).⁴ Major complications were reported in 25 patients, with no cases of systemic infection or dislodgement of the pacemaker. The electrical performance at 6 months of follow-up, assessed in 297 patients, was very satisfactory, with a mean pacing capture threshold of 0.54 V at 0.24 ms, an R-wave of 15.3 mV, and an estimated longevity of 12.5 years (94% of devices had a projected longevity of more than 10 years). This performance was similar to that of modern conventional pacemakers.

Future studies will provide information on the long-term safety and efficacy of this new permanent pacing system. These studies should also aim to confirm the potential benefits of this device compared with conventional lead-based devices.

Conflicts of interest

M. Pachón is a proctor for the Micra device. M.A. Arias is member of the editorial board of Revista Española de Cardiología.