Figure 3.
Analysis of ECT performance: For this representative example, a C-OP ECT is analyzed at an acute electrical pacing rate of 4.5 Hz. Mean σA was 0.29 mN/mm2, σbb was 0.17, RT50 was 63 ms, and normalized systolic energy was 0.53. (a) Representative stress versus time data. Force data were divided by ECT cross-sectional area to calculate stress. A cycle is defined as contraction followed by relaxation. The minimum stress before the onset of contraction is σmin and the peak stress is σmax. Active stress, σA, is the difference between σmax and σmin. Beat-to-beat hysteresis, σbb, is the magnitude of the difference between the σA of an even-numbered cycle and the σA of the preceding odd-numbered cycle, divided by the overall mean σA. (b) Fast Fourier transform (FFT) analysis of the stress versus time signal. An acute electrical pacing frequency was considered captured if it matched the peak frequency of the FFT power spectrum. (c) RT50 calculated for cycle 1 in (a). RT50 is the time for stress to relax by 50% of σA from σmax. (d) Normalized systolic energy calculated for cycle 1 in (a). Stress and cycle time were both normalized to a maximum value of 1. The area of the shaded region under the stress-time curve is related to systolic energy during contraction.