Pacemaker implantation is one of the most frequently performed cardiac procedures, with >700,000 new implants Worldwide in a 2009 survey.1 The benefits of implantable pacemakers are undisputed with improvements in symptoms and, for some indications, life expectancy. Traditional pacemakers consist of a battery (or generator) placed under the skin near the collarbone and connected to the heart by pacing leads. The large veins in the chest are used as conduits for the leads to access the heart (See figure 1).
Until very recently, this basic system design hasn’t changed since the inception of implantable pacemakers in the late 1950s, even though there have been many ‘hidden’ technological advancements particularly in device software features.
However, it has been known for some time that placement of the battery near the skin poses a significant risk for developing infection around the pacemaker pocket (see image and table below), particularly after a pacemaker battery or lead replacement procedures.
A dual chamber pacemaker with pacing leads in the right atrium and ventricle and a pulse generator (‘battery’) beneath the left collarbone
Example of a pacemaker wound infection, causing erosion of one of the pacing leads through the skin. Note the multiple scars from previous procedures crossing one another (poor surgical technique like this is a risk factor for pacemaker infection).
In addition, the pacing leads (which connect the battery to the heart) have to be flexible so they do not break when the heart beats or moves with breathing. This same flexibility is also their Achilles heel (pacing leads are often described as the ‘weak link’ of the pacemaker system) as long-term exposure to this constant movement at the shoulder and also within the heart can sometimes result in lead fracture (see figure) or insulation failure. For this reason, traditional pacing leads have a life expectancy of approximately 10 years and then require replacement and potentially extraction (which is associated with a significant risk if they have been in the body a long time).
The goal of doing away with pacing leads altogether and the generator under the skin by miniaturising the entire pacing system so it can be placed entirely within the heart has only now been realised and potentially offers a near-perfect solution to these problems.
Here is a table showing comparison of possible complications relating to traditional pacemaker implants versus the Medtronic Micra
|– pneumothorax (collapsed lung||x|
|– Haemothorax (bleeding in chest)||x|
|– venous obstruction||x|
|-venous thrombosis (clot)||x|
|– femoral vascular injury||x|
|– lead displacement||x|
|– lead fracture||x|
|– lead insulation failure||x|
|-loose header connection||x|
|– cardiac perforation/effusion||x||x|
|– Endocarditis (heart valve infection)||x||?|
|– valve leaflet tethering||x|
|– Twiddler’s Syndrome||x|
|– battery malfunction||x||x|
|– electrical component failure||x||x|
|– early battery depletion||x||x|
|– software malfunction||x||x|
|– mechanical integrity||x||x|
|– device emobilzation (device travels through heart/circulation)||x|
|End of service issues|
|– lead extraction||x|
|– device extraction from pocket||x|
Technological advances in electronics miniaturization and battery chemistry have now made it possible for the development of a device small enough to be implanted entirely within the heart while still providing similar battery life to a traditional pacemaker (see figure below).
The Micra Leadless pacemaker is a miniaturised, single chamber pacemaker system – this means it is designed to pace only within the right ventricle of the heart. It is 93% smaller than traditional pacemakers.2 It is delivered via a catheter (a long plastic tube) inserted through the femoral vein at the top of the right leg and implanted directly inside the right ventricle of the heart (see figure). The Micra device eliminates the need for a separate large battery, a device pocket and insertion of a pacing lead, thereby potentially preventing many of the complications associated with traditional pacing implants but providing the same benefits.
The Micra is a self-contained pacemaker, which has 4 nitinol (memory metal) wires at its tip, which help secure the device to the heart muscle and prevent it from moving subsequently. It is introduced through a large sheath inserted at the top of the right leg. The sheath has a flexible tip helping to steer the catheter to the correct position in the right ventricle (see figures). Despite the differences in size and shape, the Micra device is very similar to standard pacemakers in regards to functionality and features and, by design, is inherently MRI-conditionally safe.
Unlike traditional pacemakers, Micra can be programmed off – this is necessary when the battery expires and a new device is implanted. Animal research has demonstrated that multiple Micra devices can be implanted in the right ventricle simultaneously without compromising heart function.3
Dr Segal implanting the first Micra in a private hospital in the UK
There was a 99.2% implant success rate in the Micra TPS pivotal study published in 2016, which recruited 726 patients worldwide and included 94 different operators.4 There were no infections or Micra dislodgements seen in the trial. In a subsequent post-approval registry of 795 patients treated by 149 different physicians in 97 countries, the implant success was 99.6%.5
There are multiple potential benefits to Micra versus having a traditional pacemaker implant:
· No scar over the chest wall
· No pocket for the pacemaker battery (lower infection risk and no risk of bleeding or clot in the pocket requiring a repeat operation)
· Pacemaker battery not visible and battery cannot cause discomfort under the skin
· No risk of collapsed lung (pneumothorax)
· No risk of bleeding in the lung (haemothorax)
· No risk of blood vessel clot or occlusion due to the long term presence of a pacemaker lead
· No risk of pacemaker lead failure
· No risk of splinting open a tricuspid valve leaflet causing it to leak
· No need for excision of battery from pocket or lead extraction (with its associated risk)
· Longer battery life than traditional pacemakers
· Reduction in overall complication rate versus traditional pacemakers4
· Reduction in re-operation rate versus traditional pacemakers4
· Reduction in hospitalisation rate rate versus traditional pacemakers4
· Pacemaker implantation still possible even if neck and arm veins occluded from previous treatment (e.g. previous traditional pacemaker implants or long term catheter insertion, or radiotherapy)
The overall complication rate in the Micra TPS pivotal study published in 2016 was 4.0%, which compares to a 7.6% complication rate in historical controls from traditional Medtronic pacemaker implant trials and this reflects a 48% reduction in risk.4 In a subsequent post-approval registry of 795 patients implanted with Micra treated by 149 different physicians in 97 countries, there was an overall complication rate of 1.51%, with only 1 dislodgement (0.13% frequency) and only 1 infection (0.13% frequency).5
In addition, there was an 82% reduction repeat operations with Micra versus traditional pacemakers in the Micra TPS pivotal study (0.7% vs. 3.8%, respectively) and a 47% reduction in hospitalisations (2.3% vs. 4.1%).4
Risks of Micra insertion include:
A Micra implant usually takes between 30 minutes and 1 hour and most patients are able to go home later the same day. This is identical to traditional pacemaker implants.
Recovery is almost entirely related to recovery of the puncture site at the top of the leg and this typically needs 2 days of rest and usually a week before vigorous exercise can be recommenced.
The follow up for patients implanted with Micra is identical to patients with traditional pacemakers, with a pacing check 1-month after the implant (performed wirelessly) and then subsequent checks typically every year. For information on life after pacemaker implantation, please see ‘Living with a Pacemaker’ section.
Yes, there is an alternative leadless pacemaker called the Nanostim made by St. Jude Medical (now part of Abbott Healthcare). This is a longer device and uses a screw (or helix) at the tip to secure it to heart muscle. Other manufacturers are designing their own versions. One can also still opt for a traditional single chamber pacemaker (also known as a VVI pacemaker) (please see Pacemaker and ICD section).