Transmural dispersion of myofiber mechanics: implications for electrical heterogeneity in vivo

Abstract

Objectives: We investigated whether transmural mechanics could yield insight into the transmural electrical sequence.

Background: Although the concept of transmural dispersion of repolarization has helped explain a variety of arrhythmias, its presence in vivo is still disputable.

Methods: We studied the time course of transmural myofiber mechanics in the anterior left ventricle of normal canines in vivo (n = 14) using transmural bead markers under biplane cineradiography. In 4 of these animals, plunge electrodes were placed in the myocardial tissue within the bead set to measure transmural electrical sequence.

Results: The onset of myofiber shortening was earliest at endocardial layers and progressively delayed toward epicardial layers (p < 0.001), resulting in transmural dispersion of myofiber shortening of 39 ms. The onset of myofiber relaxation was earliest at epicardial layers and most delayed at subendocardial layers (p = 0.004), resulting in transmural dispersion of myofiber relaxation of 83 ms. There was no significant transmural gradient in electrical repolarization (p = NS).

Conclusions: Despite lack of evidence of significant transmural gradient in electrical repolarization in vivo, there is transmural dispersion of myofiber relaxation as well as shortening.

Figure 1. Schematic Representation of the Heart

The transmural bead set was implanted between the first (D₁) and the second (D₂) diagonal branch of the left anterior descending coronary artery (LAD) to measure finite deformation of the myocardial tissue across the wall. Endo = endocardium; Epi = epicardium; LCx = left circumflex coronary artery; LV = left ventricle.

Figure 2. Bipolar Electrograms From Plunge Electrodes

Red and blue dots mark the time points of steepest portion of initial bipolar QRS waveforms and end of the T wave, respectively. Note the endocardial-to-epicardial direction of depolarization, whereas no clear transmural dispersion of repolarization is observed.

Figure 3. Transmural Fiber Orientation

Values are mean ± SD (n = 14). Fiber angles were measured with reference to the positive circumferential direction. Abbreviations as in Figure 1.

Figure 4. Time Course of Transmural Fiber Strains

(A) Entire cardiac cycle. Different colors represent the percent wall depth from the epicardial surface. Open circles = onset of myofiber shortening; open squares = onset of myofiber relaxation. (B) A closer look at TDShort. (C) A closer look at TDRelax. AoP = central aortic pressure; ECG = electrocardiogram; LAP = left atrial pressure; LVP = left ventricular pressure; TDRelax = transmural dispersion of myofiber relaxation; TDShort = transmural dispersion of myofiber shortening.

Figure 5. Transmural Dispersion of Myofiber Mechanics

Values are mean and error bars indicate SD (n = 14). Open circles = onset of myofiber shortening; open squares = onset of myofiber relaxation. Endo = endocardium; Epi = epicardium; TDRelax = transmural dispersion of myofiber relaxation; TDShort = transmural dispersion of myofiber shortening.

Figure 6. Electrical Activation and the Onset of Myofiber Shortening Versus Mean Wall Depth

Open triangles = electrical activation (n = 4) mean value at each depth (mm); open circles = onset of myofiber shortening (n = 14) mean value at each depth (mm). Abbreviations as in Figure 1.

Figure 7. Electrical Repolarization and the Onset of Myofiber Relaxation Versus Mean Wall Depth

Closed triangles = electrical repolarization (n = 4) mean value at each depth (mm); open squares = onset of myofiber shortening (n = 14) mean value at each depth (mm). Abbreviations as in Figure 1.

Figure 8. Transmural Tissue Coupling

Red and green lines = strains in epicardial (0% wall depth) and endocardial (90% wall depth) layers, respectively. Solid and broken lines = fiber and cross-fiber strains, respectively. AoP = central aortic pressure; ECG = electrocardiogram; IVC = isovolumic contraction; IVR = isovolumic relaxation; LAP = left atrial pressure; LVP = left ventricular pressure.