Ventricular ejection pressure and stroke-volume approximation (${\hat{p}}_a=(C_a{p}_a^{+}+V^{+}-V_0)/(C_a+C_s)$)

After a rapid switch to systolic compliance $C_s$, negligible valve resistance equalizes arterial and ventricular pressures. Neglecting runoff during this short stage conserves combined stored volume and gives the displayed pressure. Thereafter, with closed mitral valve, negligible venous pressure and constant $C_s$, the pressure decays as ${\hat{p}}_a{e}^{-t/[R_c(C_a+C_s)]}$. At firing duration $\Delta t_F$, the stroke volume is $V^{+}-V_0-C_s{\hat{p}}_a{e}^{-\Delta t_F/[R_c(C_a+C_s)]}$. Each approximation follows from an explicit valve or time-scale separation condition.