Assessment of left atrial structure and function in the setting of atrial fibrillation using cardiac magnetic resonance imaging.

Abstract

Atrial fibrillation (AF) is the most common sustained arrhythmia and is associated with significant morbidity and mortality. As a result of AF, patients develop palpitations, syncope, cardiac failure and thromboembolic complications. The management of AF has revolved around the issues of rate versus rhythm control, and stroke prevention. Although rhythm control was thought to be the most desirable approach, several large randomised trials failed to show a significant difference between the two treatment strategies. In fact, rhythm control strategy may be associated with increased adverse outcomes. This has largely been attributed to the poor efficacy and significant toxicity of antiarrhythmic agents. Since the seminal observation that the pulmonary veins play an important role in triggering AF, radiofrequency catheter ablation of AF has evolved rapidly. Although this is a potentially curative treatment for selected patients with AF, there are several issues that need to be addressed, including predictors of success of the procedure, effects of ablation on cardiac structure and function, and implications of AF ablation for long-term stroke risk. Cardiac magnetic resonance imaging (CMR) is now regarded as the “gold standard” non-invasive imaging modality for the assessment of cardiac structure and function. It is thus an ideal tool for initial assessment and follow-up of patients with AF undergoing ablation. The chapters in this thesis are based on CMR studies in patients with AF. Left atrial (LA) volume is an established prognostic marker. Currently, LA volume is commonly measured using the biplane area-length method and applying the method of discs. This involves geometric assumptions, which may introduce inaccuracies. In Chapter 3, the accuracy and reproducibility of biplane area-length method was compared with the “gold standard” volumetric measurement using CMR, in healthy controls and subjects with AF. The biplane area-length method correlated well with the volumetric method in healthy controls, but in subjects with AF, the correlation was less robust and the area-length method was less reproducible. Traditionally, “lone” AF has been defined as the occurrence of AF in the absence of any cardiopulmonary disease. However, the CMR study presented in Chapter 4 demonstrates that despite having no evidence of cardiac abnormalities on echocardiography, subjects with “lone” AF have atrial functional abnormalities and ventricular structural changes compared with controls. The success of AF ablation can be variable depending on the expertise of the centre, clinical and imaging characteristics. Predictors of success following AF ablation have not been well established. The clinical and CMR factors associated with medium-term outcomes following AF ablation are presented in Chapter 5. Following multivariate analysis, increased LA volume and female gender were the only predictors of AF recurrence at one year post-ablation. In the study presented in Chapter 6, CMR was performed at baseline and 12-month follow-up to assess the impact of AF ablation on cardiac structure and function. In subjects with successful ablation, there was evidence of reverse remodelling within the atria and ventricles, with reduction in chamber volumes post-ablation. Importantly, these changes associated with cardiac reverse remodelling after ablation may indeed contribute to the success of ablation.