Figure 1. A TI based on cardiac twitch T-time integrals correlates with the type and severity of myocardial growth.

(A) Depiction of the relationship between the TI and the degree and severity of cardiac growth (modified from ref. 7). The TI is determined by subtracting the area under a twitch T curve of a cardiomyocyte with perturbed contractility from that of a “normal” or “WT” cardiomyocyte. A positive TI correlates with concentric hypertrophy, whereas a negative TI correlates with eccentric hypertrophy (7). (B) Top panel: Theoretical 300 ms twitch T traces of a WT cardiomyocyte (black) and 4 potential variants with altered twitch T magnitude and kinetics. All twitches are represented as a percentage of the T_peak of the WT twitch, which is shown as a dashed black trace with each variant twitch for comparison. Twitches were generated using a simple exponential model, as in ref. 8. Bottom panel: The area under the T-time trace (left ordinate) for each corresponding twitch above and the resulting TI (right ordinate) calculated as the difference in the area under the twitch T-time curve between WT and each variant twitch. Blue and gray correspond to twitches with decreased and increased (respectively) twitch-time integrals compared with WT (black). We note that because the TI encompasses contraction and relaxation kinetics, the absolute value of the TI does not depend on the peak twitch tension. T, tension; TI, tension index.