Comparative evaluation of the specificity and sensitivity of isometric indices of contractility.

It is generally held that a useful mechanical index of contractility is one that is unchanged by changes in initial muscle length, but that changes in appropriate direction and magnitude with a change in contractile state. A number of isometric indices were measured in vitro in isolated cat papillary muscles to quantitate their relative dependence on initial muscle length and contractile state. Results indicate that no one isometric index completely fulfills the definition of an ideal index of contractile state. In particular, indices that change considerably with alterations in contractile state (e.g., maximum dF/dt) are also moderately dependent on preload. Conversely, indices that are moderately insensitive to changes in contractile state (e.g., time-to-peak force) are also insensitive to changes in contractile state. Changes in extrapolated maximum velocity and peak VCE were similar and were less sensitive to changes in contractile state than maximum dF/dt. The most sensitive in vitro index of contractility appears to be maximum dF/dt at a constant preload.