Interactions between the determinants of LV diastole. In early diastole there is an interaction of LV relaxation, passive elastic properties and myocardial visco-elasticity. During diastasis (mid-diastole) we observe interactions of LV relaxation (if prolonged), LV passive elastic properties myocardial visco-elasticity, and inertial factors. In end-diastole the interaction occurs between LA contraction, LV passive elastic properties, myocardial visco-elasticity, and pericardial function. It is also important to remember that all changes in LV loading conditions (both preload and afterload) influence both LV relaxation and passive elastic properties: for example, an increase in LV afterload prolongs LV relaxation.
LV relaxation. Myocardial relaxation (Table 1) is an active, energy-dependent process, based on the breakdown of the actin-myosin bridges, which begins as early as mid-systole, and depends on the hydrolysis of ATP. The invasive reference measurements of relaxation (obtainable with transducer-tipped high fidelity catheters), which have served as the gold standard for all echocardiographic indices used to estimate this phase, are the peak negative dP/dt ratio (-dP/dt) and the time constant of relaxation tau (τ). The -dP/dt depends on active relaxation, on the preceding LV systolic function and on LV peak systolic pressure. In general, positive inotropic stimulations increase the rate of drop in LV pressure, while the reverse occurs with negative inotropic stimulations. The τ is obtained by measuring the decrease in time of the natural logarithm of LV pressure during the isovolumic relaxation phase and depends on LV preload and afterload, age and heart rate. The τ is less dependent on LV peak systolic pressure and LA pressure than -dP/dt. On the other hand, the measurement of isovolumic relaxation time (IVRT) is determined by multiple variables: aortic pressure, LA pressure, heart rate, age, LV loading conditions and LV mass, and does not provide direct information on LV filling. In general, by keeping the other variables constant, a slowing of the early diastolic drop in LV pressure prolongs IVRT.
LV chamber compliance. To characterize the LV passive elastic properties (Table 1) it is necessary to analyze the LV diastolic pressure-volume curve (Figure 7) (12); the latter is a component of the LV pressure-volume loop, with the important difference that multiple loops obtained by varying LV load (by increasing progressively LV preload through liquid infusion or reducing preload by applying negative pressure to the lower limbs) are necessary to obtain a progressive variation of the end-diastolic pressure-volume point of the loop. This change of the end-diastolic pressure-volume point will describe a mono-exponential curve. The geometric characteristics of this curve (constant k of chamber stiffness) represent the chamber compliance (or its reciprocal, stiffness) of that ventricle.