Flow-pulse response: A new method for the characterization of ventricular mechanics

To determine the mechanical properties of the left ventricle as a pump, we developed a new technique based on impulse-response concepts. In 11 isolated canine hearts contracting isovolumetrically, steplike changes in ventricular volume (ΔV=1-3) ml) were produced by flow pulses lasting 35-50 ms; such steps could be applied at any phase of systole. The resultant change in ventricular pressure (AP) between the perturbed beat and the preceding undisturbed beat was examined during the time of the pulse and the remainder of contraction. The ratio AP-AV, a measure of ventricular viscoelasticity, varied throughout systole in characteristic patterns. Following a volume step early in systole, AP/AV increased steadily during contraction (max values, 3.6-11 mmHg/ml). A volume step imposed late in systole, however, produced a bimodal AP/AV with the first transient peak in phase with the flow pulse. Analyzing these patterns revealed that the mechanical components contributed differently to AP/AV depending on the time of the flow pulse: for early pulses, elasticity dominated; for late pulses, a resistive component was more evident. Using pulse-response measurements. we could predict the course of ventricular pressure during ejections having arbitrary flow contours. Measured and predicted pressures agreed within 15%. Thus, the flow-pulse technique provides a comprehensive method to study ventricular mechanical properties.