Gastric Insufflation During Facemask Ventilation at Different Levels of End-expiratory Pressure in Obese Patients

Gastric insufflation occurs when the inspiratory pressure exceeds the lower esophageal sphincter pressure. Thus, it is desirable to avoid excessive positive pressure during mask ventilation after induction of anesthesia and keeping the inspiratory pressure \<15-20 cmH2O.In patients with obesity the lower compliance of the respiratory system usually requires higher inspiratory pressures to maintain adequate ventilation making these patients more prone to gastric insufflation. This high risk of gastric insufflation can be aggravated by the use of positive end-expiratory pressure (PEEP) which is recommended to avoid lung atelectasis. The application of PEEP during mask ventilation increases the risk of gastric insufflation as it reduces the pressure threshold at which gastric insufflation occur The optimum ventilatory strategy during mask ventilation should achieve the balance between adequate lung ventilation and avoiding gastric insufflation. In obese patients, it is not clear whether the use of PEEP during mask ventilation would increase the risk of gastric insufflation or not.

We hypothesize that using zero end-expiratory pressure (ZEEP) or low PEEP during mask ventilation would reduce the risk of gastric insufflation in comparison to high PEEP.

Upon arrival to the operating room, routine monitors (electrocardiogram, pulse oximetry, and non-invasive blood pressure monitor) will be applied, intravenous line will be secured. End-tidal CO2 monitoring will be initiated after induction of general anesthesia and starting face-mask ventilation. All patients will be positioned in the ramped position (achieved by elevation of the head and shoulders till achieving alignment of sternal notch and external auditory meatus). Preoxygenation will be achieved by pressure support ventilation with 5 cmH2O and FiO2 of 0.8 without PEEP for three minutes. Induction of anesthesia will be achieved using fentanyl (2 mcg/Kg lean body weight), propofol (2 mg/Kg lean body weight), and rocuronium (0.6 mg/Kg ideal body weight). After loss of verbal response, mask ventilation will be achieved by appropriate size face mask and oropharyngeal airway with 100% oxygen and double hand jaw thrust head tilt maneuver. The included patients will be receiving volume-controlled ventilation adjusted to deliver tidal volume of 8-10 mL/kg (ideal body weight), at I:E ratio of 1:2, inspiratory pause of 0.5 s, respiratory rate of 12 breath per minute, FiO2 of 0.8.

The 3 study groups will receive the planned ventilatory strategy for 120 seconds.

Assessment gastric insufflation during mask ventilation will be achieved by ultrasound assessment of gastric antrum (at the sagittal plane between left lobe of the liver and pancreas at level of the aorta). Gastric antral cross-sectional area (CSA) \[ (longitudinal diameter) X (anteroposterior diameter) X π /4\] will be assessed in between contractions before face mask ventilation and after insertion of endotracheal tube. \[9\] The proportion of change in the CSA will be calculated as (delta CSA %= \[CSA after intubation - baseline CSA\] / baseline CSA X 100). Significant gastric insufflation will be identified if the CSA increased by \> 30% after endotracheal intubation in relation to the baseline.

Intermittent gastric auscultation will be performed during mask ventilation at 30, 60, 90, 120 seconds by a blinded investigator (the presence of gastric insufflation will be defined as a gurgling sound).

Eligibility Criteria:
Inclusion Criteria:

* adult patients,
* with American Society of Anesthesiologists physical status II,
* body mass index \>35 kg/m2,
* scheduled for elective surgery under general anesthesia.

Exclusion Criteria:

* Patients at increased risk of difficult mask ventilation: Mallampati classification \>2, presence of beard, limited neck extension, limited jaw protrusion, patients with history of obstructive sleep apnea or STOP-Bang score\>2
* Patients at risk of aspiration or history of esophageal reflux.
* Patients with craniofacial anomalies, and pregnant patients