Optimal Anesthesia for Morbidly Obese Patients

Before surgery, ultrasound will be used to assess pretracheal soft tissue depth and height and width of tongue base. The distance from the skin to the anterior aspect of the trachea will be measured at three levels: vocal cords, thyroid isthmus, and suprasternal notch. With the patient in a seated position, the convex transducer of a portable ultrasound device (LOGIQTM, GE Healthcare, Chicago, IL, USA) will be introduced to the skin of the neck in the submental region coronally, immediately cephalad to the body of the hyoid bone, and then in the area between the hyoid bone and the symphysis of the mandible. Maximal width of tongue base, tongue base height, and maximal height of mid-tongue will be measured. The laryngoscopy intubation will be performed using a size-3 or -4 Macintosh (Rüsch Inc., Duluth, GA, USA) blade and a styletted endotracheal tube. The laryngoscopic view will be graded according to Cormack and Lehane's classification with external laryngeal pressure applied.

For all patients, the wrist will be extended and taped to a board to maintain wrist extension. All patients will receive local skin anesthesia at the anticipated puncture site. All radial artery catheterizations will be performed by anesthesiology residents with similar levels of experience in both blind-palpation and ultrasound-guided radial arterial catheterization. In the ultrasound group, a linear vascular probe of portable ultrasound device (LOGIQTM, GE Healthcare, Chicago, IL, USA) will be applied to the skin to localize the radial artery and a 20-gauge catheter will be inserted distal to the transducer and directed according to the ultrasound image. In the palpation group, the radial artery will be identified by palpation, and the cannula will be directed by continuous or intermittent palpation of arterial pulsation. An attempt is defined as a new penetration of the skin with the needle, followed by an unlimited number of subcutaneous needle redirections.

In the HFNC group, preoxygenation will be performed using HFNC (Optiflow™, Fisher \& Paykel Healthcare, Auckland, NZ), nasal prongs set at 30 L/min flow of heated and humidified 100% oxygen. In the facemask group, patients will breath spontaneously with an anesthetic facemask and 100% oxygen 15 L/min. Gas flow for HFNC or facemask can be adjusted depending on patients' tolerance. During laryngoscopy intubation, HFNC will be left in place with the nasal flow escalated to 50 L/min of 100% oxygen in order to achieve apneic oxygenation. In the facemask group, the facemask will be removed when apnea occurs. After tracheal intubation, correct placement of the endotracheal tube will be confirmed by capnography and the nasal prongs of the HFNC group will be removed.

"At the operating room, a M-Entropy™ sensor and S/5™ module (GE Healthcare, Helsinki, Finland) will be applied to all patients' forehead before induction of anesthesia according to the manufacturer's recommendations. Patients will be randomized to Sevoflurane or Desflurane as the single volatile anesthetics for general anesthesia.~Besides, patients will be randomized to M-Entropy group or controls. In the M-Entropy group, dosage of volatile anesthetics will be adjusted to achieve the Response and State Entropy value between 40 and 60 from the start of anesthesia to the end of surgery. In the usual care group, dosage of volatile anesthetics will be titrated according to clinical judgment. M-Entropy monitoring will be continued in the usual care group, but the Entropy number and EEG waveform will be concealed from the anesthetist in charge."