Respiratory variation in carotid artery blood flow peak velocity combined with tidal volume challenge in predicting fluid responsiveness of lung-protective ventilation patients: a feasibility study

Dynamic indices provide high predictable value in fluid management during anesthesia. However, as the emerging concept of lung protective strategy, the application spectrum of dynamic indices is limited in low tidal volume (Vt) due to insufficient intrathoracic pressure and preload variation. Tidal volume challenge (TVC), a functional tool, has proved to help dynamic indices solve the dilemma in low Vt. This study aimed to explore the feasibility and accuracy of respiratory variation in carotid artery blood flow peak velocity (ΔVpeak-CA) combined with TVC to evaluate fluid responsiveness in patients ventilated with lung protective strategies during general anesthesia. We performed lung protective strategies after intubation, including low Vt, PEEP titration and open-lung approach when necessary. We implemented TVC after completing individual lung protective setting. Soon after, fluid challenge (FC) was conducted, and we defined fluid responsiveness as cardiac index (CI)≥15%. Hemodynamic data were collected immediately before TVC(T1), at the end-time point of TVC(T2), immediately before FC(T3) and after FC(T4). Finally，we enrolled 74 patients underwent major open abdominal surgery among whom 45 were fluid responsive. We found TVC improved the predictability of ΔVpeak-CA, with significant AUC difference between ΔVpeak-CA T2and ΔVpeak-CA T1(0.162, p = 0.0052). The ΔVpeak-CA T2exhibited more accurate and easier assessable than ΔVpeak-CA T2- T1, yielding the highest AUC of 0.897 (0.619 - 0.830) (p<0.0001). ΔVpeak-CA T2＞15% predicted fluid responsiveness with a sensitivity of 66.67% and a specificity of 100.00%. In subgroup analysis, we found PRM has limited influence on the predictability, with no significant AUC differences between ΔVpeak-CA T2and ΔVpeak-CA T2 without PRM (0.0369, p=0.4355). Clinical trial registration: Chinese Clinical Trial Registry (ChiCTR2500108553). Registered on September 1, 2025.