Mapping of regional myocardial strain and work during ventricular pacing: experimental study using magnetic resonance imaging tagging
Frits W. Prinzen, PhDa,
William C. Hunter, PhD*,
Bradley T. Wyman, MSc* and
Elliot R. McVeigh, PhD*
a Department of Physiology, Cardiovascular Research Institute Maastricht, University of Maastricht, Maastricht, The Netherlands
* Department of Biomedical Engineering, Johns Hopkins University Medical School, Baltimore, Maryland, USA
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Figure 1 Examples of MRI tagging images, acquired during LVbase pacing. Presented are short-axis images at the papillary level, with taglines oriented at an angle of 0 (SA 0) and 90° (SA 90) with the circumference and long-axis images with taglines at an angle of 90° with the long axis (LA 90). In plane, resolution was 1.25 mm (readout direction) x 3 mm (phase encoding direction) and slice thickness was 7 mm. Images were taken every 20 ms, frame 0 being the time when taglines were parallel. Note the inward bending of taglines at the LV anterior wall and the outward bending in the septum in the SA0 image of frame 4, indicating early systolic shortening and stretching in these regions, respectively.
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Figure 3 Midwall circumferential strain as a function of time in a high papillary cross-section of one experiment. The "tracings" are displayed according to the anatomy of the LV, as if the LV was cut open along the mid septum. Only every other material point is shown (12 out of the 24 in circumferential direction; see Methods). The approximate pacing site is indicated by an asterisk, and was actually 2 cm below this cross-section during RVapex pacing and slightly above it during LVbase pacing. Zero strain is the length at the time of placing the taglines (time = 0 ms). The ejection phase is indicated by the broken lines.
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Figure 4 Midwall fiber stress–fiber length diagrams in the same regions and the same experiment as presented in Figure 3. Fiber length = 1, is defined as the length at estimated zero cavity volume (see Methods). Actual values were calculated according to eq. 2. Format is the same as in Figure 3.
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Figure 5 Systolic circumferential strain (top), external work (middle) and total work (bottom) during RA, LVbase and RVapex pacing, as measured at two sites: RVapex (striped bars) and LVbase (closed bars). Mean values ± SD from seven experiments are presented. *p < 0.05 compared with the same site during RA pacing,  p < 0.05 compared with the same site during pacing at the RVapex and #p < 0.05 compared with the opposite region during pacing at the same site.
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Figure 6 Maps of local external work during RA, RVapex and LVbase pacing (from top to bottom) from the same experiment as presented in Figures 3 and 4. External work values are presented as gray levels, zero being black (see scale bar). The LV wall is represented as a circle with the base located at the outer contour and the apex in the middle. Location of septum, anterior and lateral wall are indicated.
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Figure 7 Cumulative histograms of midwall external work in the LV wall. For each experiment values from all 192 sites of the LV were normalized to the mean value during RA pacing (=1). Presented are the mean curved from the seven experiments. Thin horizontal lines indicate SD of the 0.1 through 0.9 values during each mode of pacing. Solid line = RA pacing, dotted line = RVapex pacing and broken line = LVbase pacing. *p < 0.05 as compared with RA pacing, #p < 0.05 between RVapex and LVbase pacing.
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-L) relations in regions with early, normal and late activation are presented. PE = potential energy, EW = external work. External work is determined as the area of the