Abstract Background Analgesia Nociception Index (ANI) is a device based on analysis of the R-R interval and respiratory sinus arrhythmia to assess the balance between sympathetic and parasympathetic activity. The autonomic system is directly affected by load changes. Therefore, monitoring sympathetic tone and its change could theoretically allow tracking of load changes during volume expansion. The aim of the present study was to determine whether changes in ANI are able to track the increase in stroke volume caused by volume expansion (SV). Methods This prospective observational study included mechanically ventilated patients undergoing neurosurgery and benefiting from SV monitoring. Exclusion criteria were cardiac dysfunction, arrhythmias, beta-blockade therapy, and dysautonomia. SV was optimized by fluid administration of 250 ml of crystalloid fluid. A positive fluid increase was defined as a SV increase of 10% or more from baseline. Changes in SV and medium ANI (ANIm) were recorded before and 4 to 5 min after volume expansion. Results Sixty-nine patients had 104 fluid challenges (36 positive and 68 negative). Volume expansion resulted in a greater ANI increase in responders than in nonresponders. The change in ANIm &gt; 5 predicted fluid responsiveness with a sensitivity of 68.4% (95% CI: 67.4% to 69.5%) and a specificity of 51.2% (95% CI: 50.1% to 52.3%). The area under the receiver operating characteristic curve was 0.546 (95% CI: 0.544 to 0.549) and appeared to be affected by remifentanil dose and baseline ANI. Conclusion Changes in ANIm induced by fluid challenge is not able to predict fluid responsiveness in mechanically ventilated patients undergoing neurosurgery. Trial registration Clinical trial registration: NCT04223414.< Réduire