Ventilation and Gastric Insufflation During Three Levels of Inspiratory Pressure During Mask Ventilation

Pulmonary aspiration of gastric contents is one of the leading causes of general anesthesia-related mortality. Gastric insufflation during positive pressure mask ventilation increases the gastric volume, and consequently the risk of regurgitation. Thus, positive pressure mask ventilation should be avoided during rapid sequence induction of anesthesia when full-stomach is suspected (e.g. inadequate fasting hours, gastrointestinal obstruction, delayed gastric emptying, and lower esophageal disease) Although avoidance of positive pressure mask ventilation during induction would potentially minimize the risk of aspiration, this would lead to rapid hypoxia . Hence, finding a proper regimen for mask ventilation would avoid hypoxia which might be serious in high-risk patients.

Pressure-controlled face mask ventilation previously proved to be the least regimen to cause gastric insufflation in comparison to manual, and volume-controlled mask ventilation during induction of anesthesia. Later, a pressure of 15 cmH2O during face mask ventilation had been reported optimum to achieve the balance between adequate ventilation and reduced gastric insufflation in non-paralyzed patients. This finding was not yet replicated in paralyzed patients who represent the majority of population who receive mask ventilation during induction of anesthesia. We hypothesize that in paralyzed patient, the optimum pressure during face mask ventilation might be lower than the pressure which was previously reported in non-paralyzed patients. However, no studies to the best of our knowledge had confirmed this hypothesis.

Gastric insufflation was previously evaluated using auscultation with stethoscope, microphone placed in the epigastric area, or esophageal manometry. Recently, gastric antrum ultrasound was used successfully to gastric insufflation in real time by measuring the cross sectional area of gastric antrum before and after face mask ventilation. This newly developed method is more sensitive than the auscultatory method and less invasive than esophageal manometry method.

Upon arrival to the operating room, routine monitors (ECG, pulse oximetry, and non-invasive blood pressure monitor) will be applied, intravenous line will be secured, and routine pre-medications (ranitidine 50 mg and midazolam 3-5 mg) will be administrated. End-tidal CO2 monitoring will be initiated after induction of general anesthesia and starting face-mask ventilation. Induction of anesthesia will be achieved using fentanyl (2 mcg/Kg), propofol (2 mg/Kg), and rocuronium (0.6 mg/Kg). 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 randomly allocated to receive the assigned pressure-controlled mask ventilation. The 3 study groups will receive the planned inspiratory pressure, without positive end-expiratory pressure, with a respiratory rate of 15, and inspiratory to expiratory ratio of 1:2 for 180 seconds.

Assessment gastric insufflation during mask ventilation will be achieved by ultrasound assessment of gastric antrum (in the sagittal plane between left lobe of the liver and pancreas at level of the aorta) and auscultation by two blinded investigators to the assigned pressure. The ultrasound screen will be positioned to be not visible to the investigator responsible for auscultation. Gastric insufflation will be identified by comet-tail sign by gastric ultrasound and gurgling sound by auscultation.

Gastric antrum cross sectional area \[ D1(longitudinal diameter) X D2 (anteroposterior diameter) X π /4\] in between contractions in supine position before face mask ventilation and after insertion of endotracheal tube will be recorded. gastric insufflation will be identified if gastric antrum cross sectional area increased by \> 30% after endotracheal intubation in relation to the baseline.

During mask ventilation, the following parameter will be recorded at 30, 60, 90,120, 150 and 180 seconds; peripheral oxygen saturation, the end-tidal carbon dioxide concentration , peak airway measured pressure, and tidal volume Demographic data (age, sex, weight, height, BMI, comorbidities) will be recorded.

Eligibility Criteria:
Inclusion Criteria:

* adult patients (above 18 years)
* American society of anesthesiologist I-II
* scheduled for elective non-cardiac surgery under general anesthesia

Exclusion Criteria:

* increased risk of difficult mask ventilation
* patients at risk of aspiration
* Patients with craniofacial anomalies
* BMI \>35 kg/m2
* pregnant patients