This was a prospective, randomized, controlled, double-blind trial that was approved by the Ethical Committee of the First Affiliated Hospital of Anhui Medical University, Hefei, China (Approval No. PJ2022-09–43) and was registered in the Chinese Clinical Trial Registry (ChiCTR2200065530, date: 08 November 2022). This study was conducted in the Endoscopic Unit of the First Affiliated Hospital of Anhui Medical University, spanning from January 2023 to June 2023. Written informed consents were obtained from all participating patients.

We recruited elderly patients scheduled for painless gastroscopy. All patients were aged ≥ 60 years, American Society of Anesthesiologists (ASA) physical status II or III, and a body mass index (BMI) between 18 and 24 kg/m2. The exclusion criteria were as follows: severe cardiac arrhythmia; epilepsy; severe dysfunction in the liver and kidney; history of alcohol abuse or drug dependence; history of allergy to soy, milk, propofol, sufentanil, or local anesthetic drugs; and refusal to provide an informed consent form.

Patients were randomly divided into the lidocaine group (group L) and the normal saline group (group N), using a random number table at a 1:1 ratio. The allocation details were concealed using opaque sealed envelopes. A 2% lidocaine at a dosage of 1.5 mg/kg or an equal volume of 0.9% normal saline was prepared in a 20-ml syringe according to the assigned patient groups by a nurse who was not involved in the investigation. The syringes containing the solutions were unlabeled and handed over to an anesthesiologist who administered the medicines, performed general anesthesia, and recorded perioperative data. The anesthesiologist, patients, and endoscopists, were unaware of the group assignment and the contents of the syringes.

All patients were routinely fasted for at least 8 h for solids and 2 h for water. Upon entering the examination room, the patient was positioned on the examination bed in the left lateral position, and peripheral vascular access was established in the right upper limb. Electrocardiography, noninvasive blood pressure, and pulse oximetry were continuously monitored. Patients were administered 100% oxygen using a mask at a 4–6 L/min flow rate.

A single bolus of 2% lidocaine at a dosage of 1.5 mg/kg or an equal volume of normal saline was administered by gradual intravenous injection. After a 2-min interval, anesthesia induction was initiated by administering propofol within 60 s after administering 0.1 μg/kg intravenous sufentanil. The level of sedation was assessed using the Modified Observer’s Assessment of Alertness/Sedation Scale (MOAA/S) (Cohen et al. 2007), which ranges from 5 (responds readily to name spoken in normal tone) to 0 (no response after painful trapezius squeeze). Gastroscopy was performed by the endoscopist when the patient’s MOAA/S score was ≤ 1. The initial induction dose of propofol was set at 1.5 mg/kg for both groups. The Dixon “up-and-down” sequential method (Dixon 1991; Pace et al. 2007) was utilized to determine the dosage of the subsequent patient, with a sequential variable dose of 0.1 mg/kg. That is, if the first enrolled patient coughed or moved during gastroscope implantation after anesthesia induction (defined as ineffective sedation), the propofol dosage for the next patient would be increased by a dose grade of 0.1 mg/kg. Conversely, if the first patient did not cough or move during gastroscope implantation after anesthesia induction (defined as effective sedation), the induction dose for the next patient would be decreased by one dose grade. The Dixon method required at least six pairs of ineffective/effective sedation episodes to determine the ED50 of propofol induction dose. In this study, seven crossover sites were considered sufficient for this purpose. If a patient coughed or moved during the gastroscopy procedure, an additional dose of 0.5–1.0 mg/kg of propofol was administered as a rescue medication.

Intraoperative monitoring was performed to maintain the patient’s heart rate (HR) between 45 and 100 beats/min, mean arterial pressure (MAP) fluctuations within 20% of the baseline value, and pulse oxygen saturation (SpO2) levels between 92 and 100%. If the HR dropped below 45 beats/min, 0.3–0.5 mg of atropine was administered intravenously. Hypotension, defined as a decrease in MAP exceeding 20% of the baseline value, was treated immediately with 3–6 mg dose of ephedrine. If respiratory depression was detected, defined as a minimum SpO2 level below 92%, measures were taken to increase oxygen flow, adjust the mandible position, and provide face mask ventilation if necessary.

The primary endpoint was the ED50 of the propofol induction dose. Secondary endpoints included the added and total doses of propofol, the procedure time (from insertion to withdrawal of the endoscopic), the awakening time (from endoscopic withdrawal to opening eyes), the orientation recovery time (from endoscopic withdrawal to answering questions about name and location), duration of stay in postanesthesia care unit (PACU, from endoscopic withdrawal to reaching a Steward score of 6), common adverse events such as hypotension, respiratory depression, propofol injection pain, nausea-vomiting, and local anesthetic intoxication reactions of cardiotoxic (e.g., increased intervals or widened QRS complex) or neurotoxic (e.g., dizziness, drowsiness, oral metal odor, mouth paresthesia, blurred vision, or convulsion). Additionally, HR, MAP, SpO2, and respiratory rate (RR) were also measured at baseline (T0), after induction (T1), at the end of gastroscopy (T2), and during awakening (T3).

Statistical analyses were performed using IBM SPSS Statistics version 25.0. Normality of the quantitative data was tested, and normally distributed variables were presented as mean ± standard deviation (SD) and compared using independent-sample t-tests. Non-normally distributed continuous variables were reported as medians with interquartile ranges (IQR) and analyzed using nonparametric Mann–Whitney U tests. Categorical variables were expressed as numbers and percentages (%) and tested using chi-square tests or Fisher’s exact tests. Changes in hemodynamic and respiratory variables were analyzed using the repeated measures analysis of variance (ANOVA). The ED50 and 95% confidence interval (CI) of propofol were calculated using the probit method (probability unit regression). GraphPad Prism version 8.0 was used for data visualization. Statistical significance was defined as a P value < 0.05.