Assessment of left-ventricular function.

Abstract

There are three distinct phases of the cardiac cycle (contraction, active relaxation, and passive relaxation) which should be examined in order to perform a thorough assessment of global left-ventricular function. Accurate measurements of left-ventricular pressure and volume are necessary to assess these phases of the cardiac cycle. The ideal index of contractility is sensitive to the intropic state of the heart, but insensitive to loading conditions, heart rate, and cardiac size. Indices of contractility may be derived from various aspects of the phase of contraction including isovolumic contraction, the end-systolic pressure-volume relationship, the phase of ejection, and the stress-strain relationship. The indices of contractility most commonly employed and arguably closest to 'ideal' are preload recruitable stroke work, an ejection phase index; the dP/dt - end-diastolic volume relationship, an isovolumic contraction phase index; and end-systolic elastance, an index derived from the end-systolic pressure-volume relationship. The active phase of relaxation is most commonly assessed by the time constant (tau) of the exponential ventricular pressure decline that occurs during the isovolumic period of relaxation. The value for tau varies inversely with ventricular function and reflects the active, energy consuming process of relaxation. tau is influenced by many of the same factors which influence contractility. The end-diastolic pressure-volume relationship reflects the passive properties of the left ventricle and may be used to obtain a measure of diastolic stiffness. The end-diastolic pressure-volume relationship is curvilinear (exponential) when end-diastolic pressures are varied over a wide range, but may be approximated by a linear relationship during low filling pressures. Diastolic stiffness is influenced by the viscoelastic properties of the heart, pericardial constraint, the atrioventricular pressure gradient, and ventricular interaction. Knowledge of the indices of the three phases of the cardiac cycle and their interactions are important to the understanding and interpretation of ventricular function in health and disease.