Figure 2. A, Evaluation of load sensitivity of cardiac relaxation in a typical papillary muscle (group 2, patient number n [center dot] 4) during control conditions previous to the addition of the plasma. Upper panel, muscle shortening length (mm) plotted versus time. Lower panel, muscle force normalized per cross-sectional area (mN/mm²) plotted versus time. t1 was the time to the end of isotonic relaxation of the twitch loaded at 50% of peak active isometric force at Lmax. t2 was the time at which the fully isometric twitch relaxes to the same level of load. Load sensitivity of relaxation was quantified by measuring tRi, i.e., the t1:t2 ratio. The relaxation of rat myocardium was typically load-sensitive with t1 < t2, thus leading to a tRi value = 0.83. The onset of isometric relaxation occurred sooner when the muscle had shortened (t1) than in the fully isometric twitch (t2), and the corresponding force traces during relaxation were clearly dissociated. This illustrated the influence of load/shortening on relaxation time course. B, Evaluation of load sensitivity of cardiac relaxation 30 min after the addition of reperfusion plasma (T30) in same muscle as A. Upper panel, muscle shortening length (mm) plotted versus time. Lower panel, muscle force normalized per cross-sectional area (mN/mm²) plotted versus time. At T30, t1 = t2, thus leading to a tRi value of 1. The corresponding force traces were superimposed, indicating that load/length no longer modulated relaxation time course. A marked negative inotropic effect was also observed.