Cardiac Magnetic Resonance Imaging Guided Left Ventricular Lead Placement

Cardiac Resynchronisation Therapy (CRT) is currently recommended for patients with heart failure who have symptomatic left ventricular (LV) systolic impairment and a prolonged QRS duration.1, 2 Up to a third of patients post CRT implantation do not derive the anticipated clinical benefit. The reasons for this are multifactorial, with patient selection and successful LV lead implantation likely to be key factors.

The mechanism by which CRT exerts its clinical benefits is fundamentally through the correction of mechanical dyssynchrony. However, despite much research in this area the optimal measures of dyssynchrony for the selection of suitable candidates for CRT have not been established. The current guidelines were revised in light of the PROSPECT trial which failed to prove validity and reproducibility in complex echocardiographic variables of dyssynchrony.3 The 12 lead electrocardiogram (ECG) remains the most widely used criterion for the assessment of dyssynchrony in patients being considered for CRT, with patients with a broad QRS complex (>150ms) appearing to benefit the most.4, 5

Although the definition of left bundle branch block (LBBB) is well established, the precise electrophysiological characteristics remain poorly understood. An arbitrary 'cut off' of 120 milliseconds was recommended by the New York Heart Association (NYHA) in 1948 for its definition.6 This has subsequently become enshrined in the literature. The presence of LBBB, a heterogeneous entity, is associated with both electrical and mechanical abnormalities within the left ventricle.7 Septal and lateral wall delay frequently occur in this setting, with delayed activation of the lateral LV wall forming the basis for bi-ventricular pacing. It is well documented within populations with left ventricular impairment that there is prolongation of the QRS complex which is associated with an adverse prognosis.8

The success of CRT is reliant upon achieving an acceptable position of the left ventricular lead during implantation. The LV lead position needs to be anatomically stable to minimise the risk of lead displacement and also to avoid diaphragmatic capture. Furthermore, patients with myocardial scar tissue in the lateral LV segments as detected on CMR are known to have a worse outcome following CRT 9 and pacing such sites may potentially be pro-arrhythmic.10 It is not known whether CMR guided placement of the LV lead in order to avoid sites of myocardial scar and fibrosis can result in an improved clinical outcome in these patients.

A recently published study corroborates that myocardial scar in the region of activation of the LV lead may have a detrimental effect on the delivery of CRT. A consecutive series of 397 patients with ischaemic cardiomyopathy were imaged prior to the implantation of CRT. Using the complex echocardiographic technique of 'speckle tracking', myocardial scar was demonstrated to have an adverse effect on patients' outcomes. It remained an independent predictor of adverse clinical outcome. Notably due to the complexity of the technique, the presence of myocardial scar was validated using CMR.11

CRT response is a contentious subject. It is well recognised within the literature that approximately 30-40% of patients do not appear to improve clinically following CRT implantation.12 However, the inter-study variability of what has been considered as a marker of response has been wide and several different variables have been utilised.13 Several of the studies have also been small, single centre, and non-randomised. There is currently a lack of consensus in what constitutes 'response' vs 'non-response' following CRT, which may be either defined in terms of markers of LV reverse remodelling or changes in the clinical indices of heart failure or a combination of them both. In an effort to rationalise the endpoints of CRT trials, clinical composite scores have been devised inclusive of both imaging based and clinical endpoints. However, the correlation between both LV remodelling and clinical endpoints when compared using correlation coefficients is marginally better than chance. The realisation that clinical improvement post CRT implantation does not necessarily accompany mechanical remodelling has also confused the issue.

Rationale for Study The aim of the present study is to provide pilot data, the results of which should increase our understanding of the mechanisms by which CRT improves clinical outcomes in patients with heart failure.