The mechanical cost of decreasing conduction velocity: a mathematical model of pacing-induced micro-dyssynchrony and lower strain

Background: Pacing can lower conduction velocity (Cvel) 4-10 fold and may cause intra-segmental mechanical delay. Sequential myocardial cell activation and contraction may result in decreased average segmental shortening compared to simultaneous activation via the normal conduction system.

Aim : To simulate the effect of decreasing Cvel on average segmental strain using mathematical modeling.

Methods: The simulation was run using MatLab version 7.4 (The MathWorks, Inc. Natick, Massachusetts). A normal strain-time curve pattern was sampled from a normal human echo study using the 2D strain imaging software (GE Healthcare, Milwaukee, Wisconsin). Contraction was simulated from simultaneous segmental activation (Cvel=∞) through normal activation (Cvel=400cm/sec) to pacing Cvel (100 to 10cm/sec). The simulation generated average strain waveforms for each velocity and peak strain vs. Cvel and time to peak strain vs. conduction velocity curves.

Results

With decreasing Cvel average peak segmental strain was found to be decreased and delayed (figure). The following correlation equation represents the correlation between strain and Cvel: strain= -20.12+27.65 x e ^{(-0.29 x Cvel)}. At the highest pacing Cvel (100cm/sec) average segmental strain dropped by 10%, at 50cm/sec by 30% and at the lowest pacing Cvel (10cm/sec) strain by >90%. Time to peak strain was minimally longer with decreasing Cvel down to 70cm/sec (pacing velocity range). Further decreased velocity dramatically increased time to peak strain of the simulated segment.

Conclusion: The simulation yielded a predictive correlation between slower conduction velocities and decreased and delayed segmental strain. This microscopic dyssynchrony may impair both myocardial systolic (decreased contraction) and diastolic (shorter diastole) function