An early phase of slow myocardial activation may be necessary in order to benefit from cardiac resynchronization therapy

Abstract

Background Not all patients with a QRS duration longer than 140 milliseconds respond to cardiac resynchronization therapy (CRT). The same QRS duration may correspond to different spatiotemporal patterns of myocardial activation that influence response to CRT. Methods Electrocardiographic ima'ging based on 80 chest wall electrodes was used to construct the spatiotemporal myocardial activation map in 46 consecutive patients before CRT. The cumulative percentage of myocardium activated was plotted against time expressed in terms of quintiles of the overall QRS duration. Changes in the left ventricular ejection fraction and end-diastolic diameter, maximum oxygen consumption per minute, brain natriuretic peptide level, and 6-minute walk distance after 6 months of CRT were compared across different patterns with 1-way analysis of variance. Results Data from 34 patients were available for analysis. Four spatiotemporal patterns of myocardial activation could be identified: triphasic (fast-slow-fast) (13), uniform (8), fast-slow (7), and slow-fast (6). The overall QRS duration was similar in the 4 groups (166 ± 19 vs 138 ± 21 vs 157 ± 26 vs 152 ± 37 milliseconds, P = not significant [NS]). The ejection fraction showed a trend of greater increases for the triphasic (6.5% ± 7.0%) and slow-fast (15.5% ± 6.4%) patterns than for the uniform (4.0% ± 13.3%) and fast-slow (8.0% ± 6.1%) patterns ( P = NS). The end-diastolic diameter showed a trend of greater decreases for the triphasic (−3.7% ± 5.3%) and slow-fast (−7.0% ± 6.7%) patterns than for the uniform (0.8% ± 6.7%) and fast-slow (0.0% ± 4.6%) patterns ( P = NS). The maximum oxygen consumption per minute showed a trend of greater increases for the triphasic (1.2 ± 4.2 mL/kg/min) and slow-fast (4.1 ± 2.7 mL/kg/min) patterns than for the uniform (0.1 ± 4.1 mL/kg/min) and fast-slow (1.0 ± 2.1 mL/kg/min) patterns ( P = NS). The brain natriuretic peptide level decreased significantly more for the triphasic (−450 ± 1269) and slow-fast (−3121 ± 1512) patterns than for the uniform (762 ± 1036) and fast-slow (718 ± 2530) patterns ( P = .0003). The 6-minute walk distance increased significantly more for the triphasic (29 ± 89) and slow-fast (40 ± 23) patterns than for the uniform (6 ± 87) and fast-slow (37 ± 45) patterns ( P = .0003). Conclusions Different spatiotemporal patterns of myocardial activation exist among patients with broad QRS complex and may affect response to CRT. An early phase of slow myocardial activation (the triphasic fast-slow-fast and the slow-fast patterns) may be necessary for a patient to benefit from CRT.