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 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)

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