Afentanil and Remifentanil for Anesthesia Quality and Recovery

1. Background and Rationale Total intravenous anesthesia (TIVA) with propofol is widely employed for a broad spectrum of surgical procedures due to its favorable pharmacokinetic profile, reduced environmental pollution, and lower incidence of postoperative nausea and vomiting compared to volatile anesthetics. In clinical practice, propofol-based TIVA is routinely complemented by potent, short-acting opioid analgesics to attenuate the stress response to laryngoscopy and surgical stimulation, thereby ensuring hemodynamic stability and providing adequate intraoperative antinociception. Remifentanil, an ultrashort-acting synthetic opioid with a context-sensitive half-time independent of infusion duration, has become a dominant choice for TIVA maintenance. Its rapid metabolism by non-specific tissue and plasma esterases ensures predictable and prompt emergence from anesthesia, making it highly controllable. However, the very properties that confer its controllability also underlie its major clinical drawbacks: the abrupt cessation of its analgesic effect upon discontinuation frequently leads to severe acute postoperative pain, opioid-induced hyperalgesia, and a significant requirement for rescue analgesics during the early recovery phase. Furthermore, its use is associated with notable intraoperative bradycardia and hypotension, and its rapid offset complicates the transition to postoperative analgesia. Alfentanil, another short-acting μ-opioid receptor agonist, presents a distinct pharmacological profile. While also rapidly acting, its analgesic potency is approximately one-quarter that of fentanyl, with an elimination half-life of 8-32 minutes. Critically, alfentanil is predominantly metabolized via hepatic CYP3A4 enzymes, resulting in a context-sensitive half-time that increases with the duration of infusion. This characteristic suggests that, during prolonged surgeries, alfentanil may provide a more gradual offset of analgesia, potentially serving as a built-in "bridge" to postoperative pain management, reducing the incidence of rebound pain and emergence agitation. Additionally, its more sustained receptor occupancy may offer superior hemodynamic stability against surgical stimuli, particularly in patients with cardiovascular comorbidities. Despite these theoretical advantages, high-level comparative evidence remains sparse. Existing studies are predominantly limited by small sample sizes, single-center designs, and an overreliance on "extubation time" as the primary metric, while neglecting comprehensive, patient-centered outcomes such as pain trajectories, cognitive recovery, inflammatory stress responses, and overall perioperative morbidity. Furthermore, there is a notable paucity of data applying rigorous non-inferiority methodology to determine whether alfentanil can match remifentanil's established intraoperative performance while potentially offering superior recovery characteristics.
2. Study Objectives and Hypotheses Primary Objective: To test the hypothesis that the intraoperative anesthesia quality (defined as a composite of hemodynamic stability and analgesic adequacy) achieved with alfentanil is non-inferior to that achieved with remifentanil in adult patients undergoing elective non-cardiac, non-cranial surgery under general anesthesia. Secondary Objectives: To compare recovery profiles, including time to eye-opening, extubation, and attainment of an Aldrete score ≥9 in the post-anesthesia care unit (PACU). To assess postoperative pain severity via Visual Analog Scale (VAS) at multiple time points and the incidence of hyperalgesia. To evaluate the incidence of postoperative adverse events (nausea, vomiting, respiratory depression, shivering) within the first 24 hours. To compare intraoperative propofol consumption between the two opioid regimens. To examine perioperative changes in serum biomarkers of stress and inflammation (cortisol, interleukin-6, C-reactive protein). To assess early postoperative cognitive function using the Mini-Mental State Examination (MMSE). The core hypothesis is that alfentanil is non-inferior to remifentanil regarding the primary composite endpoint (intraoperative rescue events) with a pre-specified non-inferiority margin of 8% for the difference in event rates.
3. Study Design and Setting This is a multicenter, prospective, randomized, double-blind, parallel-group, active-controlled, non-inferiority clinical trial. The study will be conducted across five participating centers in Shandong Province, China: The First Affiliated Hospital of Shandong First Medical University (lead site), The 960th Hospital of the Joint Logistics Support Force, Weifang People's Hospital, Linyi People's Hospital, and Binzhou Yangxin County People's Hospital. The total study duration is 24 months, encompassing enrollment, intervention, follow-up, data analysis, and manuscript preparation.
4. Randomization and Blinding Eligible patients will be randomly assigned in a 1:1 ratio to either the Alfentanil Group (A) or the Remifentanil Group (R). Randomization will be performed using a stratified block design with study center as the stratification factor. Block sizes of 4, 6, or 8 will be randomly generated and concealed to prevent selection bias. This is a double-blind study. The study drugs (alfentanil and its matching placebo, remifentanil and its matching placebo) are prepared and labeled by an independent, unblinded pharmacist according to pre-generated randomization codes. The appearance of the infusions is identical, and infusion pumps are programmed with identical parameters for both groups (volume rates adjusted for drug concentration). All participants, attending anesthesiologists, intraoperative care teams, postoperative follow-up assessors, outcome adjudicators, and the primary statistical analyst remain blinded to group allocation until the final database is locked. Emergency unblinding is permitted through sealed opaque envelopes and an interactive web response system (IWRS) in the event of a medical emergency where knowledge of the assigned opioid is essential for patient safety.
5. Study Population and Recruitment The study will recruit adult patients (aged 18-65 years) with an American Society of Anesthesiologists (ASA) Physical Status of I to III, who are scheduled for elective non-cardiac, non-cranial surgery under general anesthesia with an expected procedural duration of 60 to 240 minutes and anticipated blood loss of less than 500 mL. Participants must have a body mass index (BMI) of 18-30 kg/m². Recruitment will be conducted during preoperative anesthesia consultations. Eligible patients will be provided with a comprehensive explanation of the study objectives, procedures, potential risks, and benefits. Written informed consent, in accordance with the Declaration of Helsinki and Chinese GCP regulations, will be obtained from all participants prior to any study-specific procedures.
6. Study Interventions and Anesthesia Protocol All participants will undergo standardized general anesthesia with endotracheal intubation or supraglottic airway device placement, utilizing a propofol-based TIVA technique. Besides the allocated opioid, all other anesthetic and adjunctive medications are strictly standardized across both groups to ensure comparability.
7. Induction: Group A (Alfentanil): Intravenous alfentanil 30 μg/kg over 30 seconds. Group R (Remifentanil): Intravenous remifentanil 1 μg/kg over 30 seconds. Both groups: Propofol 2 mg/kg (MCT/LCT emulsion) administered over 30 seconds, followed by rocuronium 0.6 mg/kg to facilitate intubation. Intubation is performed once a Bispectral Index (BIS) ≤ 60 and a train-of-four (TOF) count of 0 are confirmed.
8. Maintenance: Group A: Continuous infusion of alfentanil at a starting rate of 0.5 μg/kg/min, titrated within a range of 0.5-2.0 μg/kg/min based on hemodynamic responses (heart rate and systolic blood pressure deviations >20% from baseline). Group R: Continuous infusion of remifentanil at a starting rate of 0.1 μg/kg/min, titrated within a range of 0.1-0.3 μg/kg/min based on the same hemodynamic parameters. Both groups: Propofol is administered via target-controlled infusion (TCI) using the Schnider pharmacokinetic model, targeting an effect-site concentration (Ce) of 2.5-3.5 μg/mL. The depth of anesthesia is continuously monitored using BIS, with the propofol target titrated to maintain a BIS value between 40 and 60. Rocuronium (0.6 mg/kg) is supplemented intraoperatively as clinically indicated.
9. Ventilation and Hemodynamic Management: A lung-protective ventilation strategy is uniformly applied: tidal volume 6-8 mL/kg of predicted body weight, I:E ratio of 1:2, positive end-expiratory pressure (PEEP) of 5-10 cmH₂O, and intermittent alveolar recruitment maneuvers, maintaining end-tidal CO₂ between 35-45 mmHg. The hemodynamic target is mean arterial pressure (MAP) ≥ 65 mmHg. Hypotension (SBP < 80 mmHg) is first managed with ephedrine 5-10 mg IV; if refractory, norepinephrine 4 μg IV is administered. Bradycardia (HR < 40 bpm) is treated with atropine 0.5 mg IV, followed by isoproterenol 0.001 mg IV if ineffective.
10. Cessation and Transition to Postoperative Analgesia: All study opioids and the propofol infusion are discontinued precisely upon completion of skin closure. To ensure a smooth transition, both groups receive a standardized loading dose of sufentanil 0.1 μg/kg IV for transitional analgesia at the end of skin closure. Postoperatively, all patients receive patient-controlled intravenous analgesia (PCIA) with a standardized formulation of sufentanil 1.5 μg/kg, nalbuphine 40 mg, and ondansetron 8 mg diluted to 100 mL in normal saline, with a background infusion of 2 mL/h and a bolus of 0.5 mL with a 15-minute lockout interval. Additional rescue analgesia (sufentanil 5 μg IV) is provided for VAS scores ≥4.
11. Study Assessments and Follow-up Schedule Participants are followed from the time of screening through postoperative Day 3. Assessments are structured into three distinct periods: Screening Period (Day -14 to Day -1): Comprehensive medical history, physical examination, vital signs, routine laboratory tests (hematology, biochemistry, coagulation), and infection markers (HBsAg, HCV, HIV, syphilis) are obtained to confirm eligibility. Baseline MMSE scores and baseline inflammatory markers (cortisol, IL-6, CRP) are collected. Intraoperative Period (Day 1): Continuous monitoring of hemodynamics (heart rate, blood pressure), respiratory parameters, BIS, and propofol consumption is recorded. The primary outcome-intraoperative "rescue events"-is documented in real time. Recovery milestones (time from drug cessation to eye-opening, extubation, and Aldrete score ≥9) are precisely measured. Blood samples for postoperative stress/inflammatory biomarkers are drawn at the end of surgery. Postoperative Follow-up (Day 1, Day 2, Day 3): Participants are assessed for VAS scores (at 30 min, 2 h, 6 h, 12 h, and 24 h), incidence of adverse events (nausea, vomiting, respiratory complications, shivering), and pain hyperalgesia. Cognitive function is re-evaluated using MMSE on Day 2 and Day 3. Serum cortisol, IL-6, and CRP are measured again at 24 hours postoperatively. Physical examinations and vital sign monitoring continue daily until discharge or study completion.
12. Endpoints Primary Endpoint: Incidence of intraoperative "rescue events," defined as a composite of the need for rescue analgesia (administration of sufentanil >5 μg) or the need for vasoactive rescue medication (ephedrine, phenylephrine, norepinephrine, or esmolol) due to hemodynamic instability, at any time during the surgical procedure. Secondary Endpoints: Recovery times: (a) cessation of opioid infusion to eye-opening, (b) cessation to extubation, (c) cessation to achieving PACU Aldrete score ≥9. Incidence and severity of postoperative hyperalgesia. VAS scores at defined postoperative time points (0.5, 2, 6, 12, 24 hours). Total intraoperative propofol consumption. Incidence of adverse events within 24 hours (including PONV, respiratory depression, bradycardia, hypotension). Perioperative changes in serum cortisol, IL-6, and CRP (pre-induction, at end of surgery, and 24 h post-surgery). MMSE scores (baseline, Day 1, Day 2, and Day 3) to assess early postoperative cognitive decline. Safety Endpoints: All adverse events (AEs) and serious adverse events (SAEs) are recorded from the time of informed consent through the end of the active follow-up period. AEs are graded according to the NCI CTCAE version 5.0. Particular attention is paid to respiratory depression (SpO₂ < 90% requiring intervention), severe hypotension, severe bradycardia, and allergic reactions. All SAEs are reported to the Ethics Committee and regulatory authorities within 24 hours.
13. Sample Size Calculation The sample size determination is based on the primary composite outcome. Literature review and internal pilot data indicate an intraoperative rescue event rate of approximately 14% in the remifentanil group. We assume a conservative expected difference of 0% between the alfentanil and remifentanil groups. The non-inferiority margin is set at 8% (i.e., the upper limit of the one-sided 95% confidence interval for the event rate difference [Alfentanil minus Remifentanil] must not exceed 8%). Using a one-sided type I error (α) of 0.025 and a statistical power (1-β) of 90%, the required evaluable sample per group is 396. Accounting for an anticipated 20% dropout rate, the total enrollment target is 990 participants (495 per group).
14. Statistical Analysis Plan The primary analysis will be conducted on the Full Analysis Set (FAS), which includes all randomized participants who receive at least one dose of the study drug and have the primary endpoint recorded. A Per-Protocol Set (PPS) analysis will also be performed for sensitivity; however, the non-inferiority conclusion will be primarily based on the FAS. Missing data for the primary endpoint will be handled using multiple imputation. Baseline characteristics will be summarized using descriptive statistics. Continuous variables will be tested for normality (Shapiro-Wilk) and homogeneity of variance (Levene's test). Normally distributed data will be presented as mean ± standard deviation and compared using Student's t-test; non-normally distributed data will be expressed as median (interquartile range) and compared using the Mann-Whitney U test. Categorical variables will be compared using the Chi-square test or Fisher's exact test. Primary endpoint analysis: The risk difference (alfentanil - remifentanil) in rescue event rates and its corresponding 95% confidence interval (CI) will be calculated. Non-inferiority is declared if the lower limit of the two-sided 95% CI (equivalent to the one-sided 97.5% CI) for the risk difference is greater than -8%. Secondary endpoint analysis: Continuous secondary endpoints (e.g., recovery times, VAS scores, biomarker changes) will be compared using t-tests or non-parametric tests as appropriate. For repeated measures across time points (e.g., VAS, MMSE), a mixed-effects model for repeated measures (MMRM) will be employed to account for within-subject correlations and assess the group-by-time interaction. Categorical safety data will be summarized by frequency and percentage. All statistical tests will be two-sided with a significance level of P < 0.05, except for the non-inferiority test, which is one-sided. All analyses will be performed using SPSS version 22.0 and SAS version 9.4.
15. Data Management, Quality Control, and Monitoring Data are collected using standardized paper case report forms (CRFs) and subsequently entered into a validated electronic database using double-data entry with automated discrepancy checks. To ensure data integrity, 10% of randomly selected CRFs will be manually verified against source medical records. An independent Data and Safety Monitoring Board (DSMB) will be established to conduct interim safety reviews after enrollment of the first 300 participants and subsequently every 6 months. The DSMB is authorized to recommend early trial termination if there is clear evidence of a safety signal that outweighs the potential benefits of the intervention. All study procedures are conducted in accordance with Good Clinical Practice (GCP) guidelines and the institutional Standard Operating Procedures (SOPs). All investigators receive protocol-specific training and GCP certification prior to study initiation. Source data and all essential trial documents will be retained at each site for a minimum of 5 years following trial completion.
16. Ethical and Regulatory Considerations

This study is conducted in compliance with the Declaration of Helsinki (2024 revision), the CIOMS International Ethical Guidelines, the Chinese Measures for Ethical Review of Life Science and Medical Research Involving Human Subjects (2023), and the Chinese GCP regulations. The final protocol, informed consent form, and all participant-related materials have been approved by the central Ethics Committee of the lead institution (The First Affiliated Hospital of Shandong First Medical University) and will be submitted for approval to the ethics committees of each participating center prior to initiation.

All participants provide written informed consent before undergoing any study procedures. Participants are explicitly informed of their right to withdraw from the study at any time without prejudice to their medical care. Confidentiality of participant data is strictly maintained; data are coded using unique participant identifiers, and personal identifying information is stored separately under restricted access.