Muscle activation is depicted by myocardial elastance (stiffening). In the example with a temporal delay (left panel), both regions of the heart contract with similar force (contractility) but one has a phase delay relative to the other. The vertical difference between curves determines when one area of muscle is stiffer than the other and thus when reciprocal wall contraction and stretch (dyssynchrony) will be observed. This rises early during isovolumic contraction and peaks at end-systole/early diastole. This can be treated by cardiac resynchronization therapy (CRT). The right panel shows an example where the 2 regions of the heart are stimulated at the same time but there is a disparity in contractility, so one territory is stronger the other another. As muscle activation progresses, there will also be dyssynchrony, because the stronger wall pushes out the weaker one, and in early relaxation the weaker wall now appears to contract. This is typical of ischemic heart disease. The vertical difference between curves again identifies the discoordinate wall motion (i.e., dyssynchrony) that would be observed (based on times when maximal regional shortening is observed around the heart, or the variance in its magnitude). However, this dyssynchrony is probably not amenable to a pacing (i.e., CRT) strategy.

Dyssynchrony is assessed using 3-dimensional circumferential strains, calculating a vector sum index that not only provides information about the dispersion of strain values during the heart beat but further amplifies this if they are geographically clustered (i.e., the whole lateral wall is late) (45). Dyssynchrony gradually rises to end-systole and then declines, so dyssynchrony during systole and diastole would be observed. Biventricular pacing (cardiac resynchronization therapy) reduces dyssynchrony during both periods. HF = heart failure; LBBB = left bundle branch block; LV = left ventricular.