Disclosed embodiments include a method for assessing fluid responsiveness implemented in a medical apparatus, a medical system, or a digital computer with one or more processors comprising: (a) measuring an electrocardiogram signal, and (b) computing a dynamic index predictive of fluid responsiveness from said electrocardiogram by calculating the R-wave amplitude variability over one or more respiratory cycles. According to one particular embodiment, the ECG changes in R-wave amplitude are calculated from the DII lead to generate a dynamic index to guide fluid therapy.