In search of the perfect indicators of left ventricular mechanical dyssynchrony.

Cardiac resynchronization therapy (CRT) is indicated for the treatment of patients with advanced heart failure (HF) symptoms, a left ventricular (LV) ejection fraction (EF) B35 %, and prolonged QRS duration. Unfortunately, the sole use of these current patient selection parameters leads to implantation in an estimated third of patients for whom improvement is not demonstrable. Alternatively, using QRS duration as a surrogate for electrical dyssynchrony may lead to the inappropriate exclusion of large segments of the HF population that may benefit from CRT. In recent years, there has been a push towards towards investigating non-invasive imaging methods to accurately define LV mechanical dyssynchrony and thus improve patient selection for CRT. Quantitative echocardiographic measures derived from techniques such as tissue Doppler imaging (TDI) and strain analysis have been used to identify dyssynchrony, but these methods lack reproducibility and have not consistently been able to discriminate between responders and non-responders to CRT. The use of phase analysis of gated single-photon emission computed tomography (SPECT) myocardial perfusion imaging (MPI) for quantifying mechanical dyssynchrony has been well described in literature. Phase analysis using SPECT MPI can allow the simultaneous assessment of LV function, mechanical dyssynchrony, and location/ extent of scar in a proven very reproducible fashion. Therefore, it has the potential for being an ideal first test in the population of patients being considered for CRT. Different parameters from this phase analysis technique such as peak phase, bandwidth (BW), standard deviation (SD), kurtosis, and skewness describe the characteristics of the timing of onset of myocardial contraction and thus degree and severity of LV mechanical dyssynchrony. Previous investigators evaluating the utility of phase analysis in mechanical dyssynchrony focused on using the cut-off of Phase SD of 43 to define significant dyssynchrony. However, it should be noted that Phase SD is a continuous variable, while it correlates well with increasing measures of dyssynchrony such as QRS duration, its use to dichotomize populations into normal vs dyssynchronous using a single-cut point is still based on single-center data that included only patients with wide QRS duration. To date few investigations have focused on dyssynchrony variables as continuous measures occupying the spectrum between normal and diseased myocardium. As GSPECT MPI gains traction as a viable method for assessment of dyssynchrony, it must be evaluated for its diagnostic capabilities, prognostic power, and ultimately the clinical impact of interventions based on different GSPECT MPI-derived dyssynchrony measure cut-offs. Taking a firm step in that direction, in this issue of the journal, Romero-Farina et al. prospectively evaluated 455 patients who underwent clinically indicated GSPECT MPI. This cohort comprised normal patients defined by normal conduction, LV function, and functional capacity and a group of ‘‘abnormal patients’’ with either myocardial or conduction disease or both. The investigators applied an algorithm to evaluate the diagnostic or discriminant capability of the dyssynchrony indices for normal and abnormal patients, for those with conduction vs myocardial disease, and for those with indications for See related article, pp. 1247–1258