Pulmonary aspiration of gastric contents during surgery is a rare but potentially catastrophic event. Current guidelines recommend that patients should fast for six hours for solid food and two hours for clear liquids. Despite this, many patients fast considerably longer than two hours for clear liquids, often as a result of logistical challenges.

In response to this, the Sip-to-Send concept was developed where patients are permitted to drink small amounts of clear liquid (170-200 ml per hour) until they are called to the operating department. The introduction of Sip-to-Send has been well received by patients and has been associated with increased patient satisfaction.

For planned general anaesthesia in the operating department, the incidence of pulmonary aspiration is estimated at approximately 1 per 4,500 anaesthetics. Because of this low incidence, it is in practice unrealistic to design studies using aspiration as the primary endpoint to demonstrate non-inferiority of new fasting regimens. For example, a study with a non-inferiority margin of 20%, power of 90% and a one-sided α of 0.025, based on an incidence of 1 per 4,500, would require approximately 2.6 million patients in each study arm.

Gastric ultrasound represents a rapid, non-invasive technique for estimating gastric volume by measuring the cross-sectional area of the antrum and subsequently calculating volume. Several studies have shown that cross-sectional area correlates reliably with gastric volume, with good inter- and intra-observer reliability.

Sip-to-Send has been established practice at the Day Surgery Department at Haukeland since October 2023. The purpose of this study is to investigate whether there is a clinically relevant change in gastric volume as a result of this liberal fasting regimen.

Adult patients scheduled for general anaesthesia or deep sedation at the Day Surgery Department (DKS) will be recruited prospectively on the day of surgery by a member of the research team. Potential participants will be identified through review of the operating theatre scheduling system and, where necessary, review of the medical record for planned elective patients against the study inclusion and exclusion criteria. An exemption from the duty of confidentiality under Section 29 of the Norwegian Health Personnel Act will be sought for the necessary medical record reviews undertaken for recruitment purposes. Only necessary information will be reviewed, and informed consent will be obtained before any inclusion in the study.

The primary hypothesis of the study is that Sip-to-Send is not inferior to standard fasting, assessed as the within-individual change in gastric volume (primary analysis).

Following inclusion, a non-invasive gastric ultrasound examination will be performed using a curvilinear ultrasound probe on the abdomen. The examination will be performed both in the supine position and in the right lateral decubitus position. A video recording of each ultrasound examination will be made (120 seconds per examination), stored in coded and anonymised form, and later assessed by another member of the research group who is blinded to the patient's fasting status.

The development and validation of automated methods for measurement of antral cross-sectional area forms part of the secondary aims of this research project. Any future use of the video recordings beyond the purposes of this project will require separate regional ethics committee approval.

After the first ultrasound examination, the patient may drink up to 200 ml of clear liquid during the subsequent hour, in accordance with the Sip-to-Send protocol. The antral cross-sectional area will be calculated using the ultrasound machine's freehand tracing function when the stomach is at rest. The measurement will be performed by tracing the gastric serosa, defined as the outermost hyperechoic line. The total volume of liquid consumed will be documented.

After one hour, a new ultrasound examination will be performed, again in the supine position and in the right lateral decubitus position. The anaesthetist responsible for the patient will be informed of the ultrasound findings and whether these correspond to normal fasting status (less than 1.5 ml/kg) or indicate a "higher-risk" stomach in accordance with established threshold values. The anaesthetist will be free to adjust the anaesthesia plan on the basis of these findings. Both the originally planned anaesthesia and the anaesthesia actually performed will be documented.

The antral cross-sectional area will be measured and documented at both examinations. Gastric volume will be calculated and recorded using the following formula:

Gastric volume (ml) = 27.0 + (14.6 × cross-sectional area in cm²) - (1.28 × age)

The study will begin with an initial feasibility phase including up to 15 participants. The purpose of this phase is to evaluate recruitment procedures, the consent process, protocol timing and the feasibility of perioperative gastric ultrasound within ordinary clinical patient pathways. Participants in this phase will undergo the same observational procedures as planned for the main cohort, without any additional interventions or risk. Data from the feasibility phase will be recorded in a separate dataset and will not be included in the main analyses of the study. These data will be used only to assess protocol adherence, examination success, data completeness and integration into clinical workflow. Separating the feasibility phase from the main cohort ensures methodological transparency and reduces the risk of bias related to procedural adaptation or operator learning effects during the start-up phase.

This is a prospective, non-inferiority cohort study with a paired within-individual design. Each patient serves as their own control. The study will be conducted as a single-centre study. There is no separate intervention arm, as the liberal fasting regimen is already established as standard practice for patients at the Day Surgery Department.

Intervention

There is no separate intervention arm in the study. The liberal fasting regimen represents established standard practice in the department and is used for all patients treated at the Day Surgery Department.

Patients attend hospital having completed preoperative fasting in accordance with current guidelines. The first ultrasound examination is conducted in standard preoperative fasting status.

Patients are then permitted to drink up to 200 ml of clear liquid, such as water, cordial or coffee, per hour until they are called to surgery, in accordance with established Sip-to-Send practice at the Day Surgery Department. The second ultrasound examination is performed after one hour of the liberal drinking regimen. Sip-to-Send practice is standard treatment at the Day Surgery Department.

The study does not involve randomisation or allocation of treatment; the only research-related procedures are two ultrasound examinations and data recording. Information from the ultrasound may nevertheless form part of the anaesthetist's clinical assessment in the same way as other perioperative information.

Sample Size and Statistical Analysis Plan

Analysis of the primary outcome will be performed as a non-inferiority analysis. Each participant serves as their own control.

Statistical planning

• One-sided α = 0.05.
• Power = 90%.
• Non-inferiority margin (Δ = 0.3 ml/kg).
• Standard deviation for the paired difference: 0.6-0.8 ml/kg.
• Gastric volume calculation: Volume (ml) = 27.0 + (14.6 × cross-sectional area (cm²)) - (1.28 × age) (11).
• Patients will be recruited until 86 patients have usable ultrasound images that can be analysed.
• Expected loss from the study (dropouts or unusable ultrasound examinations) is approximately 20%, based on previous experience.

The primary analysis will be conducted as a per-protocol analysis. Sip-to-Send will be considered non-inferior if the upper one-sided 95% confidence limit for the mean paired difference in gastric volume is ≤ 0.3 ml/kg.

Detailed statistical calculations for the number of patients to be recruited are included in the appendix.

The study is designed to test non-inferiority; no confirmatory superiority analysis is planned. Descriptive statistics, including point estimates and uncertainty, will be presented for both ultrasound examinations.

Data from the feasibility phase will be analysed descriptively to report recruitment rates, protocol adherence, examination success and data completeness. No analysis of clinical outcomes will be performed on this dataset.

Randomisation / Allocation / Blinding

Participants serve as their own controls, and there is therefore no randomisation or allocation to treatment groups. All patients are permitted to drink up to 200 ml of clear liquid per hour while waiting for surgery, as this is standard practice at the Day Surgery Department.

The operator performing the ultrasound examinations will be blinded to how much liquid the patient has drunk until both examinations have been completed. The ultrasound findings, together with the documented volume of liquid consumed, will then be made available to the anaesthesia team responsible for the patient's anaesthesia.

If the anaesthesia team is not familiar with the interpretation of gastric volume and its significance for aspiration risk, they will be informed of the findings and whether the patient is considered to have a low or increased risk of aspiration. The anaesthesia team will be free to adjust the anaesthesia plan on the basis of this information. The final choice of anaesthetic technique will be documented.

Data Collection and Data Management

All data collected in connection with the study will be coded and stored on secure research servers with access restrictions. A separate linkage key (key log) containing personally identifiable information will be stored separately from the study data on the research server.

Ultrasound examinations stored for secondary analyses will be retained in anonymised form on the research server, in accordance with applicable procedures and regulations.

Ultrasound video sequences will initially be stored temporarily on the ultrasound machine in coded form for internal quality control and blinded measurement analysis on the same device. The recordings will subsequently be transferred to the hospital's secure research servers, where they will be stored in de-identified form for further analyses and the development of automated measurement methods.

Data processing will take place in accordance with applicable data protection legislation and internal guidelines at the health trust. Only authorised members of the research group will have access to the study data.

Data from the feasibility phase will be stored separately from the main cohort on the research server. A separate linkage key will be used, and all data will be kept separate from the main cohort. Video recordings made as part of the feasibility phase may be used for the development of automated interpretation of ultrasound images.

The data controller is Helse Bergen.

Safety Assessments

This is a low-risk, physiological and pragmatic study. The study involves no new intervention or exposure beyond what is already established clinical practice. The liberal fasting regimen has been in use at the Day Surgery Department since February 2023 and appears safe, well tolerated and preferred by both patients and healthcare personnel.

All gastric ultrasound findings will be communicated to the responsible anaesthesia team, who may adjust the anaesthesia plan where this is considered clinically appropriate. Any adverse events will be recorded as part of the study. Adverse events include, among other things, aspiration, postponement of surgery as a result of ultrasound findings, and discomfort related to the examination. Any incidental clinically relevant findings will be managed in accordance with current clinical procedures.

The study will be monitored by the principal investigator to ensure adherence to the protocol.

Ethics and Consent

Participation in the study requires written informed consent from adult patients. The study involves no change to standard treatment beyond the performance of two non-invasive abdominal ultrasound examinations. The ultrasound examinations may provide additional information regarding the patient's fasting status and gastric volume, and this information may be used by the anaesthesia team when planning anaesthesia. Participation in the study does not affect the patient's rights or further treatment.

Feasibility and Workflow Potentially eligible patients will be identified using the operating theatre scheduling system. Written informed consent and verification of inclusion and exclusion criteria will take place upon the patient's arrival at the Day Surgery Department.

The first ultrasound examination will be performed after admission to the Day Surgery Department. The second ultrasound examination will be performed after the patient has followed the liberal fasting regimen for one hour. The study will not result in changes to or delays in the patient's planned treatment.

The estimated additional time required for participation in the study is approximately 15-20 minutes. The ultrasound examination is painless and non-invasive.

Finances

There are no direct costs associated with conducting this study, and there is no need for external funding. The ultrasound examinations will be performed using equipment already in clinical use at the Day Surgery Department.

Dissemination Plan

The study results are planned to be presented at both national and international scientific meetings.

A manuscript is planned for publication in a peer-reviewed, PubMed-indexed journal within perioperative medicine or anaesthesiology. Authorship will be in accordance with international guidelines and actual contribution to the study.

The study will be registered in ClinicalTrials.gov after approval from the regional ethics committee and before inclusion of the first participant.

Inclusion Criteria:

• Able to give written consent
• Adult patient (> 18 years of age)
• Planned general anaesthesia or deep sedation
• Fasted for at least six hours for food and two hours for clear fluids upon arrival
• Expected to wait at least one hour before surgery

Exclusion Criteria:

• Unable to give written consent
• Active gastrointestinal obstruction
• Pregnancy
• Ongoing opioid use
• Previous stomach surgery - gastric banding / Roux-en-Y / sleeve gastrectomy
• Use of GLP-1 agonist (Wegovy)
• Type 1 or 2 diabetes

Study population includes all adult patients for planned general anaesthesia or deep sedation at the day surgery unit at Haukeland University Hospital

There are no plans to share patient data outside of our institution. We have not asked for permission to do this via our regional ethics committee.