Key result
Increasing right atrial pacing rates up to 180 bpm in an in vivo canine model significantly decreased both systolic and diastolic mechanical restitution time constants (P<0.05).
Why the study?
Does increasing pacing rate alter systolic and diastolic mechanical restitution in an in vivo canine heart model?
Does increasing pacing rate alter systolic and diastolic mechanical restitution in an in vivo canine heart model?
Increasing heart rate significantly decreases the time constant of mechanical restitution in the in vivo canine heart, providing insight into cellular processes governing contraction and relaxation.
Hypothesis-generating for rate effects on restitution; leaves open translation to human pacing.
INTRODUCTION: It is known that as stimulation frequency is increased in a healthy heart, a corresponding increase in LV contractile function (dP/dt(max)) is observed, i.e., force-frequency relationship. The impact of this relationship on systolic and diastolic mechanical restitution in an ejecting, in vivo preparation has yet to be explored. Understanding this relationship may lead to further insight on the cellular processes that govern the contraction and relaxation of the heart, in addition to providing a safer, more feasible clinical diagnostic tool. METHODS AND RESULTS: Anesthetized canines (n = 8) were paced from the RA at rates of 130, 150, and 180 bpm. At each rate, extrasystoles were delivered at varying intervals. The LV dP/dt(max) and dP/dt(min) associated with the extrasystolic beat were expressed as a percentage of steady-state levels and plotted as a function of the extrasystolic interval to obtain mechanical restitution curves. The systolic restitution time constant length decreased significantly with all increases in heart rate, P < 0.05. In the diastolic case, significant decreases in restitution time constants were seen when heart rate was increased from 130 bpm to 180 bpm, and from 150 bpm to 180 bpm, P < 0.05. CONCLUSION: This study was the first to quantify the finding that the time constant of restitution significantly and consistently decreased with a consistent increase in heart rate. The identification of such behavior may be employed to develop stimulation protocols and chronic diagnostic tools to more safely and sensitively identify and optimize the clinical status of patients receiving pacing therapy.
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Ahlberg et al. (2007) studied Healthy heart (canine model) (n=8). Right atrial pacing at varying rates vs. Lower pacing rates was evaluated on Systolic and diastolic mechanical restitution time constants (p=<0.05). Increasing right atrial pacing rates up to 180 bpm in an in vivo canine model significantly decreased both systolic and diastolic mechanical restitution time constants (P<0.05).
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