The leadless pacemaker was developed to avoid
many of the potential complications encountered
with traditional transvenous pacemakers.
In December 2012, the world’s first leadless
pacemaker was implanted in a series of 11
patients at Homolka Hospital in Prague, Czech
Republic, heralding the start of a new era in
cardiac pacing. 7 These small, capsule-shaped,
self-contained devices are implanted directly into
the right ventricle, and are entirely intracardiac,
eliminating the need for a lead or subcutaneous
pocket, and thereby avoiding many of the
attendant complications.
Their major limitation is the restriction to
single-chamber ventricular pacing, and thus to
a comparatively small subgroup of patients who
require pacing – predominantly those with
concomitant atrial arrhythmia, those in whom
there is an expectation of only infrequent pacing,
or the elderly.
There are two kinds of leadless pacing system:
the Nanostim™ Leadless Cardiac Pacemaker (LCP)
by St Jude Medical (now Abbott), and the Micra™
Transcatheter Pacing System (TPS) by Medtronic.
Currently only the Micra™ has Food and Drug
Administration (FDA) approval for use in the
U nited States. Some of their characteristics are
summarised in Table 1, along with those of
a standard transvenous single chamber
pacemaker. 8,9
Micra™ Transcatheter Pacing System (TPS)
The Micra™ TPS is a 25.9mm x 6.7mm device
weighing 2g, with a volume of 0.8 cm 3 . Figure 2
illustrates the difference in size between it and
Table 2
Electrical parameters of the Micra™ TPS and the
Nanostim™ LCP at implantation and six months
Parameter
Micra™ TPS
Mean capture threshold
Mean R wave amplitude
Mean impedance
Nanostim™ LCP
Mean capture threshold
Mean R wave amplitude
Mean impedance
Figure 3
Four flexible nitinol
tines affix the Micra™
to the right ventricular
myocardium
Reproduced with permission
of Medtronic, Inc.
13
HHE 2018 | hospitalhealthcare.com
Implantation Six months
0.63V
11.2mV
724 Ω 0.54V
15.3mV
627 Ω
0.82V
7.8mV
700 Ω 0.53V
9.5mV
486 Ω
a standard single chamber pacemaker. At the
distal end of the device, four flexible nitinol
(nickel titanium) tines affix the Micra™ to the
myocardium. At the proximal end is a docking
button by which it attaches to the delivery
catheter during the implantation procedure,
and from which it can later be snared for retrieval
if required.
Implantation of the Micra™ takes place via the
femoral vein through a large introducer sheath.
The steerable delivery catheter enables the device
to be positioned in the right ventricle (RV). As the
device is deployed, the fixation tines emerge from
the delivery system and engage with the
myocardium (see Figure 3). Device stability and
electrical parameters are then tested, and, if
needed, the Micra™ can be retrieved and
positioned in a different location, multiple times
if necessary. When a satisfactory position is found
with good electrical parameters, a tether is cut,
the delivery catheter and sheath are withdrawn,
and haemostasis achieved at the access site. 10
The largest study of the Micra™ TPS, an
international, multi-centre, single arm study,
included 725 patients who underwent attempted
implantation. Of these, 719 (99.2%) were
successfully carried out. At six months, 96.0% of
patients remained free from major
complications. 11 The remaining 4.0% included 11
cardiac injuries, five puncture site complications,
two cases of thromboembolism, two pacing issues
and eight other events. One death from metabolic
acidosis in a dialysis-dependent patient was felt
to have been contributed to by a prolonged
procedure. There was no incidence of
radiologically apparent device dislodgement, no
systemic infections and no failures in telemetry.
Comparison was made in this study with
a historical control cohort of 2667 patients who
had undergone transvenous pacemaker
implantation. The 4.0% major complication rate
in the Micra™ group compares with 7.4% in the
transvenous pacemaker group (hazard ratio 0.49;
95% CI 0.33–0.75, p=0.001). The rates of
hospitalisation (2.3% vs 3.9%) and system revision
due to complications were also lower in the
Micra™ group (0.4% vs 3.5%). There was no
significant difference between the two cohorts
in rates of cardiac injury, access site issues
or problems with pacing. 11
In 98.3% of patients capture thresholds were
low and stable, and paired electrical data from
implantation and six-month follow up indicate
a tendency toward improvement over time.
Electrical parameters are shown in Table 2. 11