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Advances in quantitative echocardiography for resynchronization therapy
John Gorcsan, MD

Many echocardiographic techniques have emerged to quantify regional dyssynchrony, in hopes of improving patient selection and response rate to CRT.  Principal methods include M-mode septal to posterior wall motion delay, pulsed Doppler measures of LV ejection in relation to right ventricular ejection, diastolic filling time as a ratio of cycle length, and several tissue Doppler imaging (TDI) to assess intraventricular opposing wall delay or dispersion of time to peak velocities.  The comparison of LV ejection delay to right ventricular (RV) ejection delay is know as the interventricular mechanical delay (IVMD) which is measured as the time difference in onset of LV ejection to onset of RV ejection using pulsed Doppler in the RV outflow tract. An IVMD difference of  > 40 ms is considered as significant dyssynchrony. IVMD is highly reproducible because of its simplicity, but of only modest predictive value. Recent data has shown that TDI may be highly predictive, but requires training and experience to be reproducable. 

 A more recent advance has been the advent of speckle tracking echocardiography that may be applied to routine two-dimensional grey scale images.  Speckle tracking can measure strain and determine mechanical thickening, independent of passive motion or tethering, which affects both M-mode and TDI velocity data.  Recent data have suggested that a combined approach using TDI longitudinal velocities with an opposing wall delay cut off of > 65 ms and speckle tracking radial strain with a septal to posterior wall delay cut off > 130 ms may yield highly predictive data with respect to CRT response. Currently clinical guidelines continue to utilize the QRS width as a surrogate for dyssynchrony.  This field continues to evolve and further data are needed to precisely define the appropriate role of echocardiographic dyssynchrony information in patient selection for CRT.