Relationship between systolic time intervals and heart rate during atrial or ventricular pacing in normal subjects

Systolic time intervals (STI) are regarded as good indices of cardiac performance in many heart diseases. It must be considered, however, that they are temporally related to the cardiac contraction cycle and, therefore, may be modified by heart rate changes. Thus, it is necessary to define the possible relationship between STI and heart rate changes. In this study, changes in heart rate were induced by atrial and ventricular pacing. Tachycardia caused a proportional decrease of left ventricular ejection time (LVET) (y=275.142-1.0025 x, r=0.76, p less than 0.001 for atrial pacing and y=298.28-0.691 x, r=-0.75, p less than 0.001 for ventricular pacing, respectively), but did not modify the pre-ejection period (PEP) and the isometric contraction time (ICT) and the electromechanical interval (QS1). These results demonstrate that while LVET must be corrected for the changes in heart rate, no correction of PEP and ICT is necessary.