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5.9 Electromechanics of the Heart 333

Knowledge of the spatiotemporal relationship between electrical excitation and

mechanical contraction is important for understanding cardiac physiology and pathophysiology.
ECGI maps cardiac excitation; tagged MRI images cardiac contraction. By combining application
of these two noninvasive methods, we studied the electromechanics of the normal human heart
in situ, in 20 healthy adults. In all subjects, earliest epicardial activation was between lateral RV
and anterior RV in mid or basal regions. Latest activation was in basal LV (Figure 5.34, A and B).

Repolarization is uniform, without steep repolarization gradients (Figure 5.34C; Left male, Right
female). The mean epicardial ARI had a median value of 238 msec. Females had longer mean
epicardial ARI than males (female median: 261 msec; male median: 229 msec, p= 0.001). Female

ARIs were longer in both ventricles, with greater gender difference in the RV. LV contraction was
synchronous and reached peak strain about 430 msec from QRS onset. A representative 3D
electromechanical sequence is shown in Figure 5.34D; it is animated in Movie 10 (https://youtu.
be/yxOdOmCcdIM).