[Effect of chronic cardiac pacing on cross-bridge activation rate in the human left ventricle]

Our active cross-bridge model has theoretically established that the cross-bridge activation rate constant of the left ventricular myocardium (Ka), which may correspond to the rate constant for the binding of Ca2+ with troponin C, can be approximately expressed as a simple formula: Ka = 3/electromechanical systole (sec-1). To determine whether this constant has the potential to be used as a clinical index of cardiac adrenergic tone, we investigated the chronic effect of cardiac pacing on the value of Ka in 15 subjects with a pacemaker in full pacing rhythm. The value of Ka (7.10 +/- 0.50 vs 6.62 +/- 0.39 sec-1, p < 0.05) and that of Ka corrected for heart rate (Kac) (7.63 +/- 0.50 vs 6.60 +/- 0.41 sec-1, p < 0.01) were both decreased significantly at one month or more after permanent pacemaker implantation. Another eight subjects were investigated to evaluate the effect on Kac of acute left bundle branch block due to intermittent right ventricular pacing. Intermittent pacing did not cause a significant change of the Kac value (7.46 +/- 0.43 vs 7.29 +/- 0.39 sec-1). The decrease of Kac during chronic cardiac pacing might reflect reduced cardiac adrenergic activity, since acute left bundle branch block due to intermittent right ventricular pacing did not affect this parameter.