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This study evaluated the ability of two Doppler ultrasound-derived parameters, the carotid corrected flow time (FTc) and respirophasic variation in carotid artery blood flow peak velocity (ΔVpeak), to predict fluid responsiveness in spontaneously breathing patients.A total of 53 spontaneously breathing patients were studied before anaesthetic induction for neurosurgery. Carotid FTc, ΔVpeak, and haemodynamic data were measured before and after administration of 6 ml kg−1 colloid. Fluid responsiveness was defined as a 15% or more increase in stroke volume index as assessed by transthoracic echocardiography after the fluid challenge.Twenty-two (42%) patients were fluid responders. The areas under the receiver operating characteristic curves for FTc and ΔVpeak were 0.842 [95% confidence interval (CI) 0.735–0.948, P<0.001] and 0.818 (95% CI: 0.701–0.935, P<0.001), respectively. The optimal cut-off values of FTc and ΔVpeak for fluid responsiveness were 349.4 ms (sensitivity of 72.7%; specificity of 83.9%) and 9.1% (sensitivity of 72.7%; specificity of 87.1%), respectively. The grey zone for FTc was 346.9–361.0 ms and included 28% of the patients, and the grey zone for ΔVpeak was 6.5–10.2% and included 50% of the patients.Using Doppler ultrasound-derived parameters measured at the carotid artery, FTc predicted fluid responsiveness in spontaneously breathing patients better than ΔVpeak. However, further studies are warranted before these parameters are recommended for clinical use.NCT 02843477.