BackgroundRemifentanil can inhibit the hemodynamic responses caused by endotracheal intubation, but the effect-site concentration of it required to control intubation responses when combined with etomidate has not been demonstrated. The purpose of this study was to determine the effect-site concentration of remifentanil blunting tracheal intubation responses in 50% and 95% of patients (EC50and EC95) during etomidate anesthesia.MethodsAmerican Society of Anesthesiologists physical status (ASA) I-II elective surgical patients receiving target-controlled infusion (TCI) of remifentanil, followed by etomidate and rocuronium for anesthesia were enrolled. The Belive Drive A2 monitor was used to calculate the MGRSSI (Maygreen Sedative state index) of hypnotic effect and the MGRNOX (Maygreen Nociception index) of nociception. The MGRSSI and the MGRNOX value were generated every 1 s. Mean arterial pressure (MAP) and heart rate (HR) were measured every minute, noninvasively. Using the modified Dixon’s up-and-down method, the concentration of remifentanil was determined based on the intubation response of the previous patient. The cardiovascular response during endotracheal intubation was defined as positive when MAP or HR is 20% higher than the pre-intubation value. A probit analysis was used for calculating EC50, EC95and 95% confidence interval (CI).ResultsThe EC50and EC95of remifentanil blunting tracheal intubation responses were found to be 7.731 ng/ml (95%CI: 7.212–8.278 ng/ml) and 8.701 ng/ml (95%CI: 8.199–11.834 ng/ml). There were statistically significant increases in HR, MGRSSI and MGRNOX value to tracheal intubation in the positive responses group compared to the negative group. The most common adverse event was postoperative nausea and vomiting, which occurred in 3 patients.ConclusionRemifentanil effect-site concentration of 7.731 ng/ml is effective in blunting sympathetic responses to tracheal intubation in 50% of patients when combined with etomidate anesthesia.Trial registrationThe trial was registered at the Chinese Clinical Trials Registry (www.chictr.org.cn, registration number: ChiCTR2100054565, date of registration: 20/12/2021).