Patients with AF who are above the age of 65, or have co-morbidities such as diabetes, high blood pressure, vascular disease or heart failure are at an increased risk of stroke. It is understood that these patients are at a higher risk of stagnation of blood within the heart; the stagnant blood can form clots within the heart and these clots can get dislodged, travel to the brain, and block the blood supply to areas of the brain resulting in stroke.
The prescription of anticoagulants, which act by reducing the ‘clottability’ of blood, to such patient has been shown to reduce the risk of strokes by about 60%. The problem with anticoagulants however is that they can significantly increase the risk of potentially life threatening bleeding and therefore the management of patients with Afib who are at a higher risk of bleeding poses a great clinical dilemma. Such patients include:
1) Patients with haematological disorders such as thrombocytopenia.
2) Patients with previous life -threatening (especially intracranial) bleeding
3) Patients who are at very high risk of falls
4) Patients who are very poorly compliant with anticoagulants.
If these patients are not prescribed an anticoagulant, they are exposed to a high risk of strokes and if they are prescribed an anticoagulant then they are exposed to a much greater risk of bleeding. It is for such patients, that the WATCHMAN device offers an elegant and very effective alternative solution.
Left atrial appendage occlusion
We have known for a long time that the most likely site for clot formation in AF is a beak shaped structure adjacent to the left atrium called the left atrial appendage. It would be a reasonable assumption that if the left atrial appendage could in some way be mechanically occluded or closed off then there would be no way for clot within the left atrial appendage to get dislodged and travel to the brain. It was this hypothesis that led to the development of a left atrial appendage occlusion device known as The WATCHMAN device.
The WATCHMAN device is a self-expandable nitinol cage which is covered by a layer of PTFE membrane and can be inserted into the left atrial appendage using a keyhole percutaneous technique. It is available in 5 different sizes.
A vein in the groin is punctured with a needle and a wire is passed through the needle and pushed all the way up the heart. Here it enters the right heart. Under x-ray supervision the operator will then puncture the inter-atrial septum and push the wire through this to the left atrium. The device which is coiled up on a catheter is pushed up the wire until it reaches the left atrial appendage and here the device is deployed thereby occluding the appendage. The device has 10 fixation anchors which stabilize the device. One concern with any such device is that it is a foreign body within the heart and there is always a risk of clot formation on the foreign body; however in experimental studies it has been shown that after about 45 days, the patients own tissue covers the foreign body surface by a process called endothelialisation.
How effective is it?
There have been 2 randomised studies which have looked at the safety and effectiveness of the WATCHMAN device in patients with AF.
In the PROTECT-AF study, the WATCHMAN device was found to be no worse (non-inferior) to warfarin in preventing strokes in 707 patients. After implantation of the device, patients had to take warfarin for 45 days (to allow endothelialisation to occur) after which they were switched to Aspirin and clopidogrel for 6 months. After 6 months they remained on Aspirin alone. Interestingly in this study, by 3.8 years, more people in the warfarin group had suffered more brain bleeds compared to those patients in the WATCHMAN group.
A second study, PREVAIL also confirmed that WATCHMAN was non-inferior to Warfarin in preventing strokes in 407 patients with AF. In this study also, patients had to continue on warfarin for 45 days before being switched to Aspirin and clopidogrel.
In both these studies, patients had to take warfarin for at least 6 weeks after the device was implanted but there are undoubtedly a significant proportion of patients in whom anticoagulation is absolutely contraindicated. Would this device be as effective if it weren’t covered with 45 days of anticoagulation? This question was addressed in another study known as the ASAP study. In this non-randomized study, 150 patients with absolute contraindication had a watchman device implanted and after this they were given 6 months of 2 antiplatelet agents (aspirin and clopidogrel/ticlopidine) followed by a single antiplatelet lifelong and the investigators found that this group of patients had lower rates of stroke/systemic embolism compared to reported rates of matched patients who were on a single antiplatelet.
Finally there has been a real world study called EWOLUTION which enrolled 1021 patients with a very high stroke risk (average CHADS2VASC score of 4.5) and in this group 62% were considered to be unsuitable for anticoagulant after implant. In the whole group the stroke rates were significantly lower (84% lower compared to historical estimates)
In summary, the WATCHMAN device offers an innovative and very effective solution to patients who have a high risk of bleeding from anticoagulants. The biggest hurdle to their widespread use is the associated costs involved. The good news is that the NHS have now agreed to fund these devices in the most eligible patients and are anticipating 1000 new implantations within the next year.
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