Atrial fibrillation is a disorder both of heart rate and heart rhythm. By definition, the heart beats irregularly but also can beat excessively fast or excessively slow. When the heart beats excessively fast, there are 2 main consequences:
- The patient is generally more symptomatic with palpitations, breathlessness, fatigue and exercise intolerance
- There is a likelihood that as the heart is working harder than it needs to, there is a possibility that the heart can weaken in the future. This is known as tachycardia-induced cardiomyopathy
The obvious way to control the rate is to control the rhythm by getting the patient back into normal rhythm (otherwise termed sinus rhythm) – although this may not be possible – especially if the atrial fibrillation is persistent and longstanding. When rhythm control is not possible, doctors aim to control the rate by means of medications such as beta blockers, calcium blockers and Digoxin. Sometimes these medications may not be tolerated by the patient due to side-effects or may for some reason be ineffective at controlling the heart rate adequately. In such patients, there is another very effective non-pharmacological approach which is very effective at achieving rate control. This is called ‘pace and ablate’.
In atrial fibrillation, the atria can beat very fast and the ventricles try and ‘listen’ to the atria and respond by beating fast. The majority of the impulses from the atria do not conduct down to the ventricles because of a structure within the electrical system of the heart called the AV node. The AV node, much like a toll-booth on a highway slows the electrical impulses down and only lets a few through to the ventricles which then respond by contracting. The atria therefore beat at about 400 beats per minute but the ventricles will only go up to 160-180 beats per minute. Nevertheless a ventricular rate of 160-180 can still make the patient feel very symptomatic and increase the risk of tachycardia-induced cardiomyopathy.
In pace and ablate, doctors aim to ablate the AV node thereby completely blocking the impulses from the atria from reaching the ventricles and hence the atrial rate has no effect on the ventricular rate. If no atrial impulses reach the ventricles then the ventricles will start contracting at their own intrinsic rhythm which can be extremely slow (30-40 beats per minute) and therefore a pacemaker has to be put in which then dictates the ventricular rate. Hence in this situation, the atria will continue to fibrillate, the ventricles will no longer ‘listen’ to the atria and instead listen to the pacemaker and contract regularly and at the pre-programmed rate of the pacemaker.
Pace and ablate has been shown to be a very effective strategy at controlling the heart rate in AF. However it does mean that the patient is then dependent on the pacemaker. Despite this, results from research studies have shown that pace and ablate does not impact prognosis adversely and does substantially improve symptoms and quality of life.
In one study of 107 patients, a ‘Pace and ablate’ strategy resulted in a significantly reduced number of physician visits, hospital admissions, episodes of heart failure and reduced need for medications. In addition, other studies have indicated that overall quality of life improves, ventricular function (esp in those patients with tachycardia-induced cardiomyopathy) improves and exercise duration also improves. It is however worth noting that in terms of life-span, ‘pace and ablate’ has not been shown to improve or worsen lifespan.
Another thing worth knowing about is that the ablation of the AV node is usually very effective but upto 3-4% of patients can have a recurrence of AV conduction in the future and may need a second procedure.
One concern patients have is that as they are dependent on the pacemaker, what would happen if the pacemaker suddenly malfunctions. The chances of this happening are exceptionally unlikely but what happens in that scenario is dependent on whether the heart has an underlying ‘escape’ rhythm or not. In a study by Curtis et al (American Heart Journal, 2000), an escape rhythm (11-65/min) was found to be present in 67% of patients who had undergone an ablation but less than half of these had an escape rhythm of >40 beats/minute. Therefore in the majority of patients, the heart will not simply stop if the pacemaker malfunctions but may be so slow that it does not achieve adequate blood flow to the body’s vital organs. It may also result in a blackout or collapse. In the remainder of patients who don’t have an escape rhythm, the heart would stop if the pacemaker malfunctions or the battery reaches end of life but the pacemaker department at the hospital where the pacemaker was implanted will be aware of pacemaker dependency and therefore make sure that they are keeping a very close watch on the pacemaker with enough checks in place to know well in advance that the pacemaker needs changing.
In summary: Pace and ablate will achieve control of poorly controlled heart rates in Afib and reduce the need for medications, improve exercise capacity, improve quality of life and reduce the likelihood of heart failure
However pace and ablate will not impact on prognosis in terms of lifespan, or cure the AF or reduce the need for anticoagulants.
Tags: Pacemakers; pace and ablate; tachycardia-induced cardiomyopathy, heart failure, AF, atrial fibrillation, ablation, AV node ablation