This study was approved by the Ethical Committee of The First Affiliated Hospital of Anhui Medical University, Hefei, China (Approval No. PJ2022-01–19) and registered in the Chinese Clinical Trial Registry (www.chictr.org.cn; ChiCTR2100053753) on 28/11/2021. All participants were from The First Affiliated Hospital of Anhui Medical University. Informed consent forms were signed by all participants. Patients aged 18 to 65 years with an American Society of Anesthesiologists (ASA) physical status of I–II and body mass index (BMI) of 18 to 30 kg/m2 who were scheduled for day-stay oral–maxillofacial surgery were enrolled in this study between August 2022 and December 2022. Patients with the following conditions were excluded: type 1 or 2 diabetes and other endocrine system diseases, a gastric emptying disorder, psychical and psychological disease, hepatic or renal insufficiency, and inability to communicate. Exit criteria were as follows: a procedure lasting less than 1 h or more than 3 h, the occurrence of severe adverse events during operation, and patients’ refusal to follow-up. The specific aims of the study were to (1) evaluate the QoR-15 questionnaire outcomes and (2) compare the patient well-being and satisfaction with or without Outfast drink 2–3 h before the induction of anesthesia.

The patients were randomly divided into two groups using a random number generator website (https://www.powerandsamplesize.com/) in a 1:1 ratio and either administered the preoperative oral carbohydrate Outfast (O group, Yichang Humanwell FSMP CO., LTD, Yichang, China) or made to fast before surgery (F group). In the afternoon of the day before surgery, a study coordinator independent of the clinical team from the hospitals opened the numbered, opaque, and sealed randomization envelopes and managed participants according to the group allocation. Participants in the F group fasted from midnight the day before surgery. Participants in the O group also fasted but received Outfast (4 ml/kg) 2–3 h before the induction of anesthesia. The same nurse not involved in the study provided patient education which included the day surgery process, surgical precautions, postoperative rehabilitation plan, and psychological care.

Once the patient entered the operating room, an intravenous line was placed. Sodium lactate Ringer’s injection was infused at 6–8 ml/kg before the induction of anesthesia and at a rate of 5–7 ml/kg/h during the surgery. Peripheral capillary oxygen saturation (SpO2), non-invasive blood pressure, electrocardiography (ECG), and bispectral index (BIS) were monitored. Anesthesia was induced with sufentanil (0.3–0.4 μg/kg), propofol (1.5–2.5 mg/kg)/etomidate (0.2–0.3 mg/kg), and cisatracurium (0.2–0.4 mg/kg). After endotracheal intubation, mechanical ventilation with tidal volumes of 8–10 ml/kg and a respiratory rate of 10–12/min was undertaken. Anesthesia was maintained with propofol (4–6 mg/kg/h), remifentanil (0.1–0.2 µg/kg/min), sufentanil (5–10 µg), and cisatracurium (0.2 mg/kg). BIS values were adjusted to 40–60. End-tidal carbon dioxide concentration (EtCO2) was maintained at 35–45 mmHg. Heart rate (HR) was controlled between 50 and 90 beats/min, and circulatory dynamics were maintained at ± 20% of preoperative blood pressure. At the end of the surgery, flurbiprofen axetil (50 mg) was given for pain relief.

In this study, the primary outcome was the QoR-15 score. The QoR-15 ranges from 0 (the poorest quality of recovery) to 150 (the best quality of recovery) and is widely used to assess postoperative recovery. It includes 15 items evaluating five dimensions: physical comfort (five items), emotional state (four items), physical independence (two items), psychological support (two items), and pain (two items) (Stark et al. 2013). The QoR-15 was measured before anesthesia induction and 24 h after surgery.

Secondary outcomes included the assessment of patient well-being and satisfaction, which were measured before anesthesia induction and 24 h after surgery. Seven parameters representing patient well-being (thirst, hunger, mouth dryness, nausea and vomiting, fatigue, anxiety, and sleep quality) were evaluated with a numeric rating scale (NRS; 0—no discomfort, 10—worst imaginable discomfort) (Sada et al. 2014). Patient satisfaction was assessed via a 5-point scale (5—very satisfied, 4—somewhat satisfied, 3—neutral, 2—somewhat dissatisfied, 1—very dissatisfied) (Bopp et al. 2011).

Heterogenous predictor variables included demographic variables and perioperative variables, such as age, gender male (M)/female (F), height, weight, ASA I/II, type of surgery, duration of anesthesia, duration of surgery, dose of propofol, dose of remifentanil, fluid volume, fasting time, hospital length of stay, postoperative exhaust time, and adverse events (defined as aspiration, readmission, return to the operating room, increased length of stay due to medical necessity). We also recorded mean arterial pressure (MAP) and HR at the following time points: before anesthesia (T0), 5 min after endotracheal intubation (T1), surgery finish (T2), and tracheal extubation (T3). Blood glucose concentrations were measured at baseline level, anesthesia induction (Ta), surgery finish (Tb), and postanesthesia care unit (PACU) discharge (Tc), respectively.

A previous study recommended that the minimal clinically important difference (MCID) for the QoR-15 scale was 6.0 with a standard deviation (SD) ± 8.77 (Myles and Myles 2021). In our study, sample size calculation was performed with an online power sample size calculator (https://www.powerandsamplesize.com/). A sample size of 45 patients in each group was estimated necessary to detect such a difference with a power of 90% and an α-error of 5% based on the results of our pilot study (σ = 8.77). Considering potential dropouts and incomplete follow-up, a total of 102 patients were finally enrolled.

All data were analyzed using SPSS version 25.0 (IBM Corporation, USA). The Shapiro–Wilk test was used to determine the normality of the data distribution. Continuous variables are summarized as mean (SD) or median (interquartile range [IQR]), as appropriate. Demographic and baseline data such as age, height, weight, duration of anesthesia, duration of surgery, dose of propofol, dose of remifentanil, fluid volume, fasting time, length of hospital stay, and postoperative exhaust time were compared using the Student t-test. Repeated measures ANOVA was used to compare differences in repeated measures of QoR-15 scores, well-being and satisfaction scores, blood glucose levels, MAP, and HR between the two groups. When a statistically significant group effect was found, the LSD post hoc test was used to compare differences in the distributions of observed indicators between the two groups at different time points. The chi-square test was used to examine the relationship between qualitative variables and independent samples (gender M/F, ASA I/II, and type of surgery). A P value < 0.05 was considered statistically significant.