Effect of Positive End-expiratory Pressure on the Gastric Volume

General anesthesia has positive effects in relieving pain and unpleasant memories during surgery by blocking the patient's voluntary movements and reflexes to stimuli. However, it can lead to side effects such as severe impairment or absence of spontaneous breathing, necessitating artificial respiration. Among the conventional airway management methods for artificial respiration, the supraglottic airway device (SGA) has the advantage of reducing hemodynamic changes, intracranial pressure, and intraocular pressure caused by airway stimulation, as it does not pass through the airway, and it reduces airway-related complications. However, a disadvantage is its inability to adequately seal the airway, making it difficult to use in cases where there is a high risk of aspiration or when high airway pressures are required.

Positive End Expiratory Pressure (PEEP) has the advantage of preventing atelectasis, improving oxygen exchange capacity, and preventing pulmonary complications by maintaining lung expansion. However, applying PEEP increases airway pressure, and if the SGA does not adequately seal the airway, there is a possibility of air entering the stomach. If air enters the stomach and increases gastric volume, the risk of postoperative vomiting and aspiration pneumonia increases due to increased gastric pressure. Also, increased intra-abdominal pressure and diaphragmatic elevation can lead to respiratory distress due to decreased lung volume during postoperative recovery.

While there are studies suggesting that PEEP can be safely used with SGA, there is a need for verification as there are no studies objectively measuring changes in gastric volume using methods such as ultrasound when using PEEP with SGA. The main purpose of this study is to quantitatively evaluate the effect of applying PEEP during the use of SGA on gastric volume, aiming to provide objective evidence regarding the potential side effects, such as gastric volume increase. It is expected that this will contribute to improving the quality of patient management through the safe clinical use of SGA and PEEP.

Before entering the operating room, patients will be placed in a sitting and right lateral decubitus (RLD) position to measure the cross sectional area(CSA) of the stomach antrum using ultrasound. One investigator will scan the sagittal plane in the upper abdominal region using a low-frequency ultrasound probe (2-5 MHz) and measure the major diameter (D1) and transverse diameter (D2) of the observed antrum. CSA can be calculated from the measured values as in formula (a) below. The investigator will scan three times for each posture, and calculate the average.

CSA = D1 x D2 x 1/4 ------- (a) Using CSA, gastric volume (GV) is estimated using formula (b) below. GV (ml) = 27.0 + 14.6 × CSA - 1.28×age ------- (b) Midazolam premedication is not administered before surgery. When patients enter the operating room, standard monitoring devices (electrocardiogram, pulse oximetry, non-invasive blood pressure monitor, BIS or SedLine) are attached for vital sign monitoring, and target-controlled infusion of propofol is initiated at a target concentration of 4.0 µg/ml, and remifentanil at a target concentration of 3.0 ng/ml.

After confirming loss of consciousness and loss of spontaneous respiration, SGA is inserted, and the position of the SGA is evaluated and recorded using the fiberoptic scoring system.

During surgery, maintenance of anesthesia is managed with target-controlled infusion of propofol to keep BIS (Bispectral Index) between 40-60 or PSI (Patient State Index) between 25-49. For adequate analgesia during surgery, remifentanil is continuously infused using target-controlled infusion.

At 5 minutes (T0), 30 minutes (T1), and 60 minutes (T2) after SGA insertion, the following values related to mechanical ventilation are measured: FiO2 (Fraction of Inspired Oxygen), PIP (Peak Inspiratory Pressure), RR (Respiratory Rate), lung compliance, SpO2 (Oxygen Saturation), EtCO2 (End-tidal Carbon Dioxide), TV (Tidal Volume), OLP (Oropharyngeal Leak Pressure), leak volume, and leak fraction.

Immediately after surgery, CSAright-lat and GV are measured as described in above. After confirming that respiration and consciousness levels have returned to clinically normal conditions post-surgery, the patient is discharged to the recovery room, and assessments are made for nausea or vomiting, respiratory depression, sore throat, and blood staining.