Torsion of the left ventricle during pacing with MRI tagging.

The effects of different pacing protocols on left ventricular (LV) torsion were evaluated over the full cardiac cycle. A systolic and diastolic series of magnetic resonance imaging (MRI) scans were combined and used to calculate the torsion of the LV in a canine model. The asynchronous activation resulting from ventricular pacing interferes with the temporal evolution of LV torsion. The torsion of the left ventricle was investigated under three different protocols: 1) right atrial pacing, 2) right ventricular pacing, and 3) simultaneous pacing from the right ventricular apex and LV base. The temporal evolution of torsion was determined from tagged MRI and evaluated over the cardiac cycle. The peak rotation for the atrially paced hearts was 11.1 degrees (+/- 3.5 degrees) compared to 6.1 degrees (+/- 1.7 degrees) and 6.1 degrees (+/- 0.7 degree) for those hearts paced from the right ventricle and from both ventricles, respectively. While biventricular pacing increases the synchrony of contraction, it significantly alters the pattern of LV torsion. From these experiments we have shown that measuring torsion is an extremely sensitive indicator of the existence of ectopic excitation.