Figure 2. Representative pressure-volume trace (PV loop) with the phases of cardiac cycle and correlated parameters measured in skinned myocardial preparations.

Early isovolumic pressure development, dP/dt_max, and t_d during phase 1 is affected by the magnitude (P_df) as well as the rate (k_df) of XB recruitment. k_df is Ca²⁺ sensitive due to the influx of Ca²⁺ during IVC. k_rel is a measure of XB turnover, so it may also contribute to the isovolumic pressure development during phase 1. ET during phase 2 is influenced by k_rel and shortening velocity. Since Ca²⁺ is absent at the end of the ejection phase, k_rel determines ET by altering the timing of cooperative deactivation of thin filaments. Early isovolumic relaxation during phase 3 can be correlated with k_rel as well because XB detachment rates influence early diastolic relaxation. dP/dt_max, maximum rate of pressure development; ET, ejection time; IVC, isovolumic contraction; IVR, isovolumic relaxation; k_df, rate of XB recruitment; k_rel, rate of XB detachment or XB turnover; LV, left ventricular; t_d, time to dp/dt_max; and XB, cross bridge.