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 endsystolic 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
- enddiastolic 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 (τ) of the
exponential ventricular pressure decline that occurs during the isovolumic period
of relaxation. The value for τ varies inversely with ventricular function and reflects
the active, energy consuming process of relaxation. τ 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 pressurevolume relationship is curvilinear
(exponential) when end-diastolic pressures are varied over a wide ränge, but may be
approximated by a linear relationship during low filling pressures. Diastolic stiffness
is influenced by the viscoelastic properties of the heart, pericardical 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.
Key words
Left ventricle - Ventricular function - Contractility - Diastolic stiffness - Active
relaxation
