Evaluation of myocardial properties from image/pressure data: chronic conditions.

Since the wall/cavity ratio of a heart chamber is not a biological constant, fractional cavity-surface motion is not a valid performance index and the stresses most commonly used in the myocardial-mechanics literature are not valid expressions of pulling action or contractility. We have developed a system for analyzing and expressing left-ventricular performance and abilities which avoids these problems. It allows one to estimate the following quantities from left-ventricular image data and arterial pressures: "Fractional midwall excursion", the fractional change in a weighted average of inner- and outer-surface dimensions, which is a valid but preload-dependent expression of performance regardless of wall/cavity ratio. "Fractional midwall excursion rate", fractional midwall excursion divided by EKG-normalized ejection time, which is a relatively preload-independent expression of performance regardless of wall/cavity ratio. "Pressure safety factor", systolic pressure-making ability relative to demanded systolic pressure. "Myocardial fiberstress", the intensity of circumferential pulling force in the wall (pulling force per unit cross-sectional area). "Myocardial growth ability", the anabolic responsiveness to habitual systolic fiberstresses, expressed as the reciprocal of long-term-average systolic fiberstresses. (6) "Contractility", the stress-developing ability of the myocardium, specifically the amplitude of the developed stress-stretch function at peak activation. On the average, these quantities are related as follows: Growth ability determines average systolic fiberstresses; contractility and growth ability (or systolic stress) largely determine safety factor; safety factor largely determines fractional midwall excursion and its rate. We have developed a microcomputer program which evaluates these quantities from image/pressure data and displays them digitally and graphically.