Cardiac magnetic resonance imaging is establishing itself as a powerful tool in combination with echocardiography in the assessment of cardiac function post-ablation procedures
John Whitaker MB ChB MRCP
Matthew Wright MB BS PhD
Department of Cardiology,
St Thomas’ Hospital and King’s
College London,
Westminster Bridge Road,
London, UK
The use of percutaneous radiofrequency ablation strategies to treat cardiac arrhythmias is well established. Originally the technique was used in the treatment of accessory pathways responsible for supraventricular tachycardias.(1) Such accessory pathways were identified by their characteristic intracardiac electrograms and energy delivered based upon electrophysiological characteristics. Subsequently, percutaneous radiofrequency ablation has gained wider use as a treatment for atrial fibrillation (AF).
AF is a common cardiac arrhythmia that is associated with a great burden of mortality and morbidity. It is characterised by uncoordinated electrical activity in the atria leading to loss of mechanical function of the atria. In 1998, Haissaguerre et al(2) made the seminal observation that the majority of paroxysmal AF was initiated by ectopic electrical activity originating from the pulmonary veins.
They demonstrated the ability to treat paroxysmal AF by using percutaneous catheter-guided radio frequency ablation of the atrial tissue surrounding the pulmonary veins in order to isolate these ectopic foci from the rest of the atria. The anatomy of the pulmonary veins opens them to two potential catheter-based approaches designed to achieve electrical isolation. Single, successive points may be ablated in sequence, with the aim of creating an unbroken line of electrically inert tissue around the ostium of the vein.
Alternatively, a catheter may be designed to form a single, large lesion that circumnavigates each pulmonary vein. Such tools depend on a balloon that enters the pulmonary vein such that unbroken contact is made between the balloon and the entire ostium of the vein before energy is applied across all areas of contact. Treatment of persistent AF involves not only isolation of the pulmonary veins, to stop the triggers for AF, but also ablation of the substrate of the left atrium.(3) The majority of work performed in the electrophysiology lab is now substrate based, be it for AF or ventricular tachycardia (VT). Substrate-based strategies rely less on specific electrophysiological characteristics and more on anatomically guided procedures. This change of approach requires a new set of tools to visualise accurately the catheter and the heart in 3-D and 4-D space.
Focused delivery of energy to the myocardium, with the aim of creating electrically inert scar tissue, is the fundamental tool of the contemporary electrophysiologist. Catheter ablation was traditionally carried out under fluoroscopic guidance alone. There are significant limitations to fluoroscopy as a sole means of guiding the delivery of radiofrequency ablation therapy. Most significantly, the visual information it provides about the anatomy of the structures involved is in two dimensions.
Fluoroscopy provides no information about the impact of the delivered therapy, which is monitored indirectly by assessment of intracardiac electrograms, changes in temperature of the catheter tip and a change in impedance.(4) Due to the destructive nature of ablation therapy, it is of paramount importance that the delivery of this energy be precise and kept to the minimum required in order to achieve the desired endpoint. To facilitate this goal of accuracy and efficiency, electrophysiologists have developed a number of imaging modalities. Advanced imaging now commonly forms part of both the pre-assessment of patients and, increasingly, the real-time guidance of delivery of therapy during the procedures. These provide exquisite anatomical information about the cardiac structures involved, often in three dimensions, as well as tools to assess the impact of the delivered therapy in real time. Furthermore, current imaging techniques are demonstrating themselves to be invaluable in the follow up of patients after such procedures.
Pre-assessment imaging
Prior to proceeding to interventional electrophysiological procedures for the treatment of cardiac dysrhythmias, knowledge of the cardiac anatomy of the patient is crucial. Transthoracic echocardiography (TTE) is a mandatory requirement in the workup prior to any ablation procedure. TTE will reliably identify structural cardiac disease that may be a cause of the dysrhythmia (for example mitral valve disease in AF) and demonstrate gross cardiac anatomy and function crucial to planning any interventional procedure4 and gives an indication of the the likelihood of success. Most significantly, the left atrial size has been shown to be a predictor of the likelihood of maintenance of sinus rhythm following ablation in long-lasting persistent AF.(5) However, there is further crucial information that cannot be assessed by TTE alone.
The restoration of sinus rhythm is associated with a risk of thromboembolism, which is further increased by the introduction of catheters into the atria. There is the potential for intracardiac catheters to dislodge thrombus that is present in the left atrial appendage (LAA). Transoesophageal echocardiography (TOE) is the gold standard investigation for excluding LAA thrombus. For this reason, routine TOE in the pre-procedural period is mandatory as a safety requirement.(3) For those patients in whom TOE is contraindicated, other imaging modalities have been employed to exclude LAA thrombus, including cardiac computed tomography (CT)(6) and, in some cases, cardiac (c) MRI.
Further anatomical information is of great value in planning interventional procedures. Of particular interest with regard to atrial fibrillation is the position and existence of each of the pulmonary veins, of which there is substantial normal variation.(4) The most accurate methods for establishing the size and shape of the pulmonary veins is CT or cMRI.(7) If balloon-based ablation catheters are to be used for an ablation procedure, then a reliable knowledge of the diameter and position of these structures is vital.(4)
Cardiac MRI has been demonstrated to be able to quantify the degree of atrial fibrosis in pre-procedural subjects undergoing ablation procedures. An increasing degree of atrial fibrosis is associated with increasing risk of thromboembolism. This has been shown to allow reliable thromboembolic risk stratification as well as providing more accurate estimates of the likely success of procedures, and an indication of the degree of ablation that might be necessary to achieve success. Such information will likely prove to be of great importance as the population of patients deemed suitable for catheter ablation grows and access to these procedures comes under greater pressure, making accurate identification of those likely to benefit from intervention as well as those in more urgent need of intervention valuable.(8,9)
Periprocedural imaging
Catheter ablation procedures for the treatment of paroxysmal AF result in freedom from AF for between 60 and 85% of patients.(3) A significant proportion of these patients relapse due to incomplete electrical isolation of the targeted area at the index procedure. Due to the low, but significant, risk of serious complications as well as the exposure to radiation and the cost involved, there exists a great incentive to optimise the delivery of therapy at the index procedure to minimise the need for subsequent ablation procedures. A variety of tools have been developed in pursuit of more effective assessment of the delivered therapy in real time.
Electroanatomical modelling is a process whereby a reconstruction of the internal boundaries of the cardiac chamber of interest is generated.
Magnetic fields and impedance measurements are used to localise the position of the intracardiac catheter in space. By registering the position of the catheter at various points of the chamber boundaries, a 3-D geometric reconstruction of the internal boundaries of that chamber can be generated. This information can be automatically integrated with information from pre-procedural CT imaging using powerful software that combines these two forms of information (Figure 1). The result is to give great accuracy and detail to the real-time three-dimensional imaging available during procedures.(10)
Recently, the ability to guide radiofrequency ablation catheters under direct, real-time MRI guidance and characterise the lesions delivered has been demonstrated in animal models.(11) If this concept were extended to clinical practice, it raises the possibility of greater accuracy in delivery of energy and more comprehensive ablation procedures.
The traditional tool for monitoring the effect of therapy is the monitoring of intracardiac electogram amplitude. The diminution of local intracardiac electrograms may be the result of true tissue necrosis (the goal of delivered therapy). However, difficulty persists in interpreting this diminution in the size of electrograms may also be the result of transient phenomena resulting from delivered therapy such as tissue oedema or haemorrhage,(12) as well as the bipolar electrogram being recorded from proximal to the actual ablation site. In these cases, conduction across these regions may be recovered once these acute responses resolve, resulting in electrical reconnection of an area that had appeared electrically isolated at the index procedure, and therefore potential relapse of AF.
The contact between an ablation catheter and the myocardium can be quantified as contact force (CF), the force that exists between the two objects when they are in direct contact. A proposed explanation for some instances of inadequate lesion formation during ablation procedures is inadequate contact between ablation catheter and myocardium, while excessive CF between catheter and tissue can result in tissue damage and complications. Difficulty in judging the safest and most effective force between the catheter and the myocardium has occurred because procedures are carried out with limited visual information regarding the position of the catheter with regard to the specific contours of the atrial wall with which it is in contact. Studies have demonstrated that there is considerable intra- and inter-operator variability of contact force, and this may explain the wide variation in clinical outcomes.
Recently catheters capable of directly measuring the force, or the degree of electrical contact, have been developed.(13) Some systems measure the mechanical force directly, giving greater control over the lesion depth and protecting from complications associated with excess energy delivery. The so-called electrical contact index (ECI) is an alternative method that has been demonstrated to be a reliable way of indirectly assessing the contact between the ablation catheter and the left atrium by measuring local electrical impedance.(14) It may be that these technologies will allow safer manipulation of ablation catheters throughout the procedures, where it has demonstrated high contact forces (CF) even outside of delivery of radiofrequency energy.
A new and exciting tool has recently been shown to have the potential to more accurately identify true tissue necrosis in real time. In animal experiments, a novel catheter that incorporates radiofrequency ablation technology alongside a high frequency ultrasound probe was used to deliver lesions that were monitored simultaneously with ultrasound imaging.(12) The potential for identifying true transmural lesions and distinguishing these from haemorrhage, which may masquerade as effective lesions on electrograms, was convincingly demonstrated. The potential also exists, for the first time, to tailor the energy delivery to the end point of transmurality of the lesion, which potentially allows for greater safety in clinical practice, as excessive energy delivery could be avoided and complications resulting from this potentially reduced (Figure 2).
If further studies demonstrate its effectiveness and safety in clinical practice, this technology has the potential to deliver permanent electrical isolation at index procedure leading to a reduction of recurrent arrhythmias and the need for subsequent procedures.
Post-procedural
The anatomical and electrophysiological characteristics of the atria evolve throughout normal life. This occurs regardless of the presence or absence of previous interventional procedures. It is established that effective ablation procedures result in an improvement in the mechanical function of the atria (as expected with the restoration of sinus rhythm). It has now also been demonstrated that successful ablation procedures result in morphological remodelling of the atria and improvements in left ventricular function.(15,16) Imaging is crucial in monitoring the evolution of cardiac chamber morphology and function following ablation procedures. While echocardiography remains the primary tool in assessing cardiac function post ablation procedures, cMRI is establishing itself as a powerful tool in combination with echocardiography and is able to provide additional detailed information relating to the evolution of the lesions themselves.
Cardiac MRI is able to identify and characterise lesions resulting from radiofrequency ablation in ventricular tissue in long-term follow-up.(17) Cardiac MRI is likely to have an important role in following the progression of lesions resulting from atrial ablation procedures. Cardiac MRI has been shown to distinguish accurately between scar types created during ablation procedures that are associated with different clinical outcomes.(18) Specifically, the use of late gadolinium-enhancing cMRI can visualise acute atrial injury effectively(19) and there is evidence that cMRI may be useful in predicting gaps in the electrical isolation of atrial regions20 which could, in the future, better predict those patients whose index procedure was more likely to require revision.
Conclusions
It is clear there are many exciting developments as electrophysiologists strive for ever more powerful and accurate imaging tools. As more data are gathered, the role of each of these promising new tools will need to be clarified. In a competitive environment, each will have to prove its benefit in terms of clinical outcomes and cost effectiveness. The search for the optimal strategy in the expanding and heterogenous groups undergoing ablation procedures goes on.
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