Objectives The goal of the present study was to analyze the effects of different intraaortic balloon counterpulsation (IABC) inflation volumes on effective arterial elastance (Ea) in patients with complicated coronary heart disease and to determine whether Ea can predict the hemodynamic response to IABC. Background Ea (the central aortic end-systolic pressure to stroke volume ratio) incorporates the principal elements of input arterial impedance and has been proved useful to evaluate the effects of afterload reduction in patients with left ventricular (LV) failure. However, although the hemodynamic action of IABC can be considered as a typical example of "pure" afterload reduction, it has never been assessed in terms of changes in Ea. Methods After clinical stabilization, 18 patients treated with IABC for complicated acute myocardial infarction or unstable angina were enrolled in the study. Systemic hemodynamics were measured by use of right cardiac thermodilution catheters with IABC off (control) and IABC on at balloon inflation volumes of 20 and 40 ml, in randomized sequence. Aortic pressure was recorded through the central lumen of the IABC catheter to calculate Ea as the ratio of aortic dicrotic pressure to stroke volume. Results A higher control Ea was associated with a lower control LV stroke work and a larger IABC-related hemodynamic improvement (that was maximal with the 40 ml inflation volume). The increase in LV stroke work was closely related to the decrease in Ea. Accordingly, hemodynamic benefits from IABC were less evident in patients with lower control Ea. In conclusion, effects of IABC were related to both balloon inflation volume and control hemodynamics, reflecting the afterload dependence of a depressed LV function. (Am Heart J 1998;135:855-61.)