When recovery of the cardiac function is detected in assisted hearts, the ventricular assist device can be removed. Due to the invasiveness of the surgical procedure, an accurate assessment of cardiac function is fundamental for the treatment success. The main challenge for the detection of cardiac function during assistance is to know whether the cardiac function index represents the cardiac function after pump removal independently of the pump assist rate. Therefore in this paper we present an evaluation of the influence of the pump over the slope of the preload recruitable stroke work, a cardiac function index. Analyzing data from four acute animal experiments, we found that the pump affects the stroke work, which could be corrected by the end diastolic volume. However, the datas examined was limited and further investigation is necessary.