Ventricular-arterial coupling. In the cartoon, arterial elastance is the "mouth" of the whale. Big mouth means low arterial elastance (no resistive and well compliant), able to easily accommodate large stroke volume energetically ejected; small fish mouth means higher arterial elastance (highly resistive and no compliant), able to accommodate only small stroke volume (low contractile-energy launched). Understanding the performance of the left ventricle (LV) requires not only examining the properties of the LV itself, but also investigating the modulating effects of the arterial system on left ventricular performance. Interaction of the LV with the arterial system, termed ventricular-arterial coupling, is a central determinant of cardiovascular performance and cardiac energetics. Ventricular arterial coupling is indexed by the ratio of left ventricular systolic elastance index (systolic pressure/end-systolic volume) to arterial elastance (E a, ratio of end-systolic pressure by stroke volume). Although in the resting state ventricular-arterial coupling is maintained in a range that maximizes the efficiency of the heart, when the system is stressed, energy efficiency is sacrificed in favor of cardiac efficiency, manifested by an increase in the coupling index (i.e., a greater relative increase in ventricular contractility than arterial load). Ventricular arterial coupling is normally set toward higher left ventricular work efficiency, whereas in patients with moderate cardiac dysfunction, ventricular and arterial properties are matched to maximize stroke work at the expense of work efficiency.