ASSESSMENT OF LEFT-VENTRICULAR FUNCTION

There are three distinct phases of the cardiac cycle (contraction, act ive relaxation, and passive relaxation) which should be examined in or der to perform a thorough assessment of global left-ventricular functi on. 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. Indi ces of contractility may be derived from various aspects of the phase of contraction including isovolumic contraction, the endsystolic press ure-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 ej ection phase index; the dP/dt - enddiastolic volume relationship, an i sovolumic contraction phase index; and end-systolic elastance, an inde x derived from the end-systolic pressure-volume relationship. The acti ve phase of relaxation is most commonly assessed by the time constant (tau) of the exponential ventricular pressure decline that occurs duri ng the isovolumic period of relaxation. The value for tau varies inver sely with ventricular function and reflects the active, energy consumi ng process of relaxation. tau is influenced by many of the same factor s which influence contractility. The end-diastolic pressure-volume rel ationship reflects the passive properties of the left ventricle and ma y be used to obtain a measure of diastolic stiffness. The end-diastoli c pressure-volume relationship is curvilinear (exponential) when end-d iastolic pressures are varied over a wide range, but may be approximat ed by a linear relationship during low filling pressures. Diastolic st iffness is influenced by the viscoelastic properties of the heart, per icardical constraint, the atrioventricular pressure gradient, and vent ricular interaction. Knowledge of the indices of the three phases of t he cardiac cycle and their interactions are important to the understan ding and interpretation of ventricular function in health and disease.