Abstract 8814: Onset of Systolic Shortening as a Marker of Electrical Dyssynchrony is Modified by Mechanical Function
Background: In a left bundle branch block (LBBB) dog model we have observed increased delay from regional electrical activation to onset of regional shortening in the late activated lateral wall compared to the early activated septum. We investigated if the apparent increase in electromechanical delay (EMD) is due to a higher load (LVP) or due to a higher rate of rise in LV (dP/dt) at the time of electrical activation.
Methods: In 7 anesthetized dogs with LBBB and LV micromanometers we measured electromechanical delay from onset R in intramyocardial electromyograms (EMG) to onset shortening in the septum and LV lateral wall (Fig.1). LVP and LV dP/dt were measured at time of electrical activation. In an in vitro experiment with 5 rabbit papillary muscles, we measured time from activation to onset shortening: at different isotonic loads (F) and at different load rates (dF/dt), simulating the pressure and rate of LVP rise (dP/dt)at the time of activation, respectively.
Results: In the dog model there was no significant correlation between EMD and LVP at time of electrical activation (p=0.68), while there was a strong correlation between EMD and LV dP/dt (r=0.88, Fig 2). Similarly, in the papillary muscles, EMD was unaffected by isotonic load, but exhibited a close relationship to dF/dt, with r values ranging between 0.85 and 0.99 in the 5 muscles (Fig 3).
Conclusions: The delay from electrical activation to onset shortening depended on the rate of LV pressure rise (dP/dt). These findings suggest that a segment does not shorten until it generates active stress at a rate which is faster than the rate at which the load increases. In late activated segments that start contraction at a higher LV dP/dt, this mechanism causes a further delay in onset of shortening, aggravating mechanical dyssynchrony. This implies that time to onset shortening is modified by LV mechanical function and limits its ability to serve as a measure of electrical delay.
- © 2010 by American Heart Association, Inc.