Effects of Heart Rate on the Dynamics of Force Development in the Intact Human Ventricle

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Abstract
The effects of heart rate on the dynamics of force development in the ventricle were studied in the intact human heart. In the course of corrective cardiac operations, Walton-Brodie strain-gauge arches were sutured to the right ventricles of 11 patients. The peak contractile force, the time to peak force, and the maximum rate of change of force (df/dt) were determined at various heart rates, produced by electrical stimulation of the right atrium or right ventricle. As the rate was increased, from an average of 72 to 145 per minute, no significant change in peak contractile force was observed. The peak df/dt, however, increased with each increment in heart rate, and the time to peak force decreased. This inverse relationship between peak df/dt and time to peak force accounts for the constancy of the peak force observed within several contractions after heart rate was changed. Also, since df/dt and the time to peak force reflect the intensity and duration of the active state, respectively, an inverse relationship between these factors is implied. Sudden changes in heart rate were associated with transient changes in peak contractile force, attributable to differences in the rates with which peak df/dt and time to peak force reached new constant values. In man, therefore, a "velocity staircase," rather than a "force staircase," occurs with changes in heart rate. The possession of this mechanism apparently permits the human ventricle to maintain its force of contraction and preserve the duration of diastolic filling with increases in rate.