Phase plane determination of the mechanical properties of the myocardium.

A phase-plane diagram, obtained by plotting force versus the rate of force during isometric contractions, was used for determination of the mechanical properties of rabbit left atria. Considering the muscle as a two component mechanical analog we found: (a) the initial slope of the phase-plane diagram (m1) is proportional to the product KVmax, where K is the stiffness of the series elastic component, and (b) the final slope of the phase-plane diagram (m3) is equal to the product -Kb, where b is a constant with dimensions of velocity, and may be related to the absolute rate of energy liberation. It is shown that m1 increases with positive inotropic interventions (frequency increase, epinephrine, post-extrasystolic potentiation, elevation of calcium and increase in temperature) but does not change as a function of muscle length. The final slope M3 increases with frequency, temperature and epinephrine; decreases with calcium and stretch, and does not change with post-extrasystolic potentiation. The results also suggest that the change in m3 depends on the particular inotropic intervention. This supports the idea that separate mechanisms are responsible for the process of contraction and relaxation.