The time-varying elastance concept provides a comprehensive description of the intrinsic mechanical properties of the left ventricle (LV) that are assumed to be load independent. Based on pressure-volume measurements obtained with combined pressure conductance-catheterization in 6 open-chest anesthetized sheep, we show that the time to reach end-systole (defined as maximal elastance) is progressively prolonged for increasing ventricle pressures, which challenges the original (load-independent) time-varying elastance concept. Therefore we developed a method that takes into account load dependency by normalization of time-course of the four cardiac phases (isovolumic contraction, ejection, isovolumic relaxation, filling) individually. With this normalization, isophase lines are obtained, that connect points in pressure-volume loops of different beats at the same relative time in each of the four cardiac phases, instead of isochrones which share points at the same time in a cardiac cycle. The results demonstrate that pressure curves can be predicted with higher accuracy if elastance curves are estimated using isophase lines instead of using isochrones (root-mean-square error (RMSE): 3.8±1.0 vs. 14.0±7.4 mmHg (p<0.001), and variance-accounted-for (VAF): 94.8±1.3% versus 78.6±14.8% (p<0.001). Similar results were found when the intercept volume V₀ was assumed to be time-varying (RMSE: 1.7±0.3 vs. 13.4±7.4 mmHg (p<0.001), and VAF: 97.4±0.5% versus 81.8±15.5% (p<0.001). In conclusion, phase-dependent time normalization reduces cardiac load dependency of timing and increases accuracy in estimating time-varying elastance.