Is it time for personalized cardiac resynchronization therapy

Abstract

Cardiac resynchronization therapy (CRT) in appropriately selected patients can significantly improve left ventricular function and New York Heart Association (NYHA) class. 1-3 CRT trials' inclusion criteria have been based solely on QRS duration with left ventricular (LV) lead implantation in the posterolateral branch of the coronary sinus as the preferred location. Using these guidelines, 60-70% of the CRT recipients respond with improvement in symptoms and/or ejection fraction. Response rate is much higher in individuals with typical left bundle branch block (LBBB) QRS morphology, with QRS duration [150 msec and nonischemic cardiomyopathy. In those who lack these criteria response rate to CRT has been poor. The reason for this poor response rate is multifold including lack of dyssynchrony, presence of scar at the site of lead implantation, and inappropriate LV lead position. To overcome these challenges and improve patient response, implanting physician will benefit from the following information: (1) Is there the presence of significant dyssynchrony? (2) Which is the most dyssynchronous segment of the left ventricle? (3) Can that segment be accessed and used for pacing? This information will be most useful in patients with intraventricular conduction delay (IVCD) and atypical LBBB like QRS pattern rather than typical LBBB. In the presence of typical LBBB, latest activating segment is posterolateral basal left ventricle (LV) and there is usually significant dyssynchrony. 4 Several studies have tried to address one or more of these questions using different imaging modalities. [5][6][7] Ideal imaging modality will have high spatial and temporal resolution to accurately identify the latest contracting segment, will give the implanting physician information about myocardial viability, coronary venous anatomy, and will have high degree of automation and reproducibility. Single-Photon Emission Computed Tomography (SPECT), Echocardiography, and Cardiac Magnetic Resonance Imaging (CMR) all have been evaluated for this purpose and each has its own limitation. There is disagreement on which echocardiographic parameter to use for dyssynchrony assessment and reproducibility has been poor. The largest trial using echocardiographic dyssynchrony assessment-guided CRT placement was negative 6 although the inclusion criteria included only narrow QRS patients. Phase analysis from gated SPECT imaging has been proposed as a relatively straightforward way of assessing dyssynchrony. 8 It uses Fourier harmonic function to approximate change in thickness of myocardium during a cardiac cycle to onset of contraction. This information of phase of onset of contraction is displayed on a polar map. Phase standard deviation and phase histogram bandwidth can then be used to quantify dyssynchrony and also to identify the latest contracting segments. This can then be combined with viability assessment and potentially fused with coronary sinus venograms, 9 thereby providing a road map for precise and individualized LV lead placement.The curren...