Fig. 19. The temporal evolution of left ventricular shortening measured by magnetic resonance imaging tagging under different pacing protocols: right atrial pacing (top row), biventricular pacing (middle row), and right ventricular apex pacing (bottom row). Blue, contraction; red, reference state; yellow, stretch. Data are shown for late diastole after tagging (first column), early systole (second column), midsystole (third column), and late systole (last column). These measurements give precise quantification of the regional mechanical consequences of different pacing protocols. BiV, biventricular; RA, right atrial; RVa, right ventricular apex. Image provided by Dr. Eliot McVeigh (National Institutes of Health and Johns Hopkins Medical School). Further details about this study can be found in B.T. Wyman, W.C. Hunter, F.W. Prinzen, O.P. Faris, and E.R. McVeigh (2002), Effects of single- and biventricular pacing on temporal and special dynamics of ventricular contraction. Am J Physiol Heart Circ Physiol. 282, H372-H379.

the same study, the investigators also found that diastolic mean flow was negative in patients after valve replacement, but not in controls.

Interestingly, the usefulness of MRI for assessing the function of cardiac pacing devices has already been proven in experimental animal studies, although currently there are still hurdles for performing cardiac MRI on patients with such devices. More specifically, Prinzen, Wyman, and coworkers (41,42) showed that MR tissue tagging can be used to evaluate mechanical activation in the left ventricle for different pacing sites, as shown in Fig. 19. These investigators were able to show with MRI that the propagation of mechanical activation for right ventricular apex pacing elicited regional variations, whereas left ventricular base pacing did not.

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Essentials of Human Physiology

Essentials of Human Physiology

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