Fluid administration is one of the most common therapies performed on intensive care patients. However, no clinical evidence is available to establish optimal strategies for fluid management as well as characterizing the effects on the cardiovascular system after therapy initiation. Moreover, fluid overload showed a correlation with worse clinical outcomes. This study aims at characterizing the response to the fluid intervention of intensive care unit patients. We extracted a population of 57 subjects with available electrocardiogram and arterial blood pressure recordings from the MIMIC-III database and evaluated the induced changes in cardiovascular and autonomic indices. We compare autonomic indices obtained from a statistical model of heartbeat dynamics before and after the intervention. Results show significant differences in RR interval, blood pressure, autonomic and Baroreflex activities up to 60 minutes after fluid administration. Specifically, we observed a median increase in RR interval, Baroreflex activity, and overall activity both in pressure and RR time series, as well as a reduction in systolic blood pressure. Specifically, a subgroup of survived patients shows an imbalance toward sympathetic activity, whereas non-survivors have a persistent vagal state after fluid administration. Clinical relevance - The observed differences in autonomic response after fluid administration, together with the assessment of their correlation with patients' mortality, paves the way for the inclusion of heart rate variability indices as markers for assessing fluid responsiveness as associated with ICU patients' state.