Airway Management in Bariatric Surgery: A Challenge for Anesthesiologists

| February 16, 2012

Column Editor: Stephanie B. Jones, MD
Dr. Jones is Vice Chair for Education, Department of Anesthesia, Critical Care and Pain Medicine, Beth Israel Deaconess Medical Center, Boston, Massachusetts

This ongoing column is authored by members of the International Society for the Perioperative Care of the Obese Patient (ISPCOP), an organization dedicated to the bariatric patient.

This month’s column by Riccardo Taddei, MD; Nora Terrasini, MD; Cedrick Zaouter, MD; Erik Arbeid, MD; and Thomas M. Hemmerling, MSc, MD, DEAA

Drs. Taddei, Terrasini, Zaouter, and Arbeid are from the Department of Anesthesia, University of Pisa, Italy. Dr. Hemmerling is from the Department of Anesthesia, McGill University, Montreal, Canada, and is a board member of the ISPCOP.

Funding: No funding was provided.

Disclosures: The authors report no conflicts of interest relevant to the content of this article.

Bariatric Times. 2012;9(2):28–29

ABSTRACT
Airway management in patients with obesity can be challenging; diligent preoperative assessment of some airway parameters can help the anesthesiologist to properly assess the risk for intubation and avoid life-threatening complications. The most important parameters during that assessment are the Mallampati score and the neck circumference. Extensive pre-oxygenation before the actual intubation attempt is also of critical importance; oxygen should best be delivered in a head-up position. The so-called ramped position is ideal for intubation; it guarantees the best direct laryngeal view. When in doubt that direct laryngoscopy can safely be performed, awake fiberoptic intubation is the safest choice. Most drugs during anesthesia induction are based on lean body weight, which is calculated using special formulas and does not take fat into account. After surgery, patients should be extubated in the sitting position when fully awake and muscle function has fully recovered. Following these simple considerations will increase the level of safety when managing the airway of the morbidly obese patient.
Airway management is one of the challenging periods of anesthesia; the risk of complications in anesthesia is highest during the induction period where general anesthesia is induced and the airway needs to be secured using either an endotracheal tube or laryngeal mask airway to allow artificial ventilation during surgery. Since the 1970s, the number of individuals with clinically significant obesity (body mass index [BMI]≥30kg/m2) is on the rise in the United States, with a prevalence of 32 percent in 2008.[1] This leads to an increasing number of patients with obesity undergoing anesthesia and surgery, and with special considerations regarding airway management.[2]

Preoperative assessment
Difficult or failed intubation in patients with obesity is more common than in patients without obesity.[3] The predictors of difficult intubation in patients with obesity are modified. Anesthesiologists usually evaluate the neck circumference at the level of the thyroid cartilage and the so-called “Mallampati score.” The Mallamati score uses the visibility of the base of uvula, faucial pillars, and soft palate to separate patients in four classes, with class 1 defining patients where these structures are best visible and class 4 patients with least visible structures. The higher the class, the higher the probability of a difficult intubation. In patients with morbid obesity (BMI>40 kg/m2), Mallampati class 3 and 4 and a neck circumference of 43cm or more is related to increased risk of difficult intubation.[4–6] The thyromental distance and the range of movements of neck and larynx must also be considered.

When these parameters are normal, BMI is a weak predictor of difficult intubation.[7,8] However, BMI of 26kg/m2 or more is related to difficult bag-mask ventilation,[2,9] defined as the necessity to perform a two-handed mask ventilation technique or the necessity of operator change. Therefore, preoperatively, identifying situations and patients at risk for airway management problems is the key for optimal care.

Pre-oxygenation
Positioning the patient in a 25- to 90-degree head-up position10,11offers improved pre-oxygenation conditions because of the reduction of the cephalad diaphragmatic displacement that decrease functional residual capacity (FRC) in the supine position. The pre-oxygenation is typically performed delivering 100 percent oxygen via a facemask. Using this position, peripheral oxygen saturation falls less rapidly during induction compared to the supine position, with a delay of approximately 60 seconds.[10,11]

Nasopharyngeal oxygen insufflation during intubation attempts via a 10 Fr catheter in the nasopharynx has also been recommended to delay apneic desaturation.[5]

Intubation
During intubation, the ramped position has been proven to improve laryngeal view and intubation conditions compared to the standard sniffing position.[5,12,13] In the ramped position, the external auditory meatus and the sternal notch are horizontally aligned, reproducing the same alignment of the intubation axis that the sniffing position produces in patients without obesity.[12] It is necessary to prepare a proper ramp with a head and shoulder support to obtain this position (Figure 1).

Patients with obesity have an increased risk of pulmonary aspiration only if predisposing conditions are present,[14,15] such as symptomatic gastroesophageal reflux, diabetes mellitus, and pregnancy or during emergency surgery. A rapid sequence induction (RSI) is recommended in these cases. It consists of pre-oxygenation, administration of rapidly acting hypnotic agents and muscle relaxants, and the absence of bag-mask ventilation before laryngoscopy and intubation.[14] Awake fiberoptic intubation is indicated only in patients with obesity with anticipated high risk of problematic tracheal intubation, as for patients without obesity.[2,16]

The use of video-assisted devices, such as the fiberoptic laryngeal mask airway (CTrach,™ LMA North America, San Diego, California) and other video-laryngoscopes have been shown to improve the visualization of the larynx and require a lower dose of propofol when compared to the traditional Macintosh laryngoscope.[17–19] In one study,[17] the use of video-assisted devices reduced the intubation time and improved the arterial blood oxygenation during airway management compared to the Macintosh laryngoscope.

The use of the intubating laryngeal mask airway in patients with obesity has been proven to be simpler, with less airway adjusting maneuvers and lower number of failed blind tracheal tube insertion attempts, in comparison to patients without obesity.[20] Therefore, this device can be a valid rescue device if ventilation or intubation becomes difficult.

Inducing Anesthesia
Anesthesiologists use three different body weight calculations for drug dosing. The lean body weight (LBW) is the sum of all body structures without fat; the ideal body weight (IBW) describes what a person should ideally weigh and is influenced by age, muscle-fat ratio, height, sex, and bone density. The total body weight (TBW) is defined as the actual body weight a person has when weighed on a balance. Even though our present knowledge of the influence of obesity on drug pharmacokinetics is limited, dosing drugs according to lean body weight (LBW) appears to be the most appropriate dosing principle for the majority of hypnotic agents,[21] including thiopental (3–4mg/kg) and propofol (1–2.5mg/kg).

The most commonly used opioids during induction are fentanyl (2–6µg/kg) and remifentanil (0.1–1.0µg/kg/min). These drugs are appropriately dosed using LBW, resulting in similar plasma concentrations as lean patients who were given TBW-based infusion.[21]

Succinylcholine (1mg/kg) dosed on LBW or TBW is the muscle relaxant of choice in patients with morbid obesity,[21] because of its rapid onset and short duration of action, which allows earlier resumption of spontaneous breathing. Rocuronium (0.6mg/kg) and cisatracurium (0.15mg/kg) have a prolonged action in patients with obesity when they are dosed based on the TBW. A dose based on IBW is recommended for these drugs.[21]

Extubation
Airway obstruction at extubation is more frequent in patients with obesity than in lean patients. Difficulties in mask ventilation and tracheal intubation should be taken into account in deciding whether to leave the patient intubated after surgery.[22] Also comorbidities, such as cardiopulmonary diseases, length and type of surgery, BMI, and presence of obstructive sleep apnea (OSA) should be considered. Prior to extubation, the patient should be fully awake (i.e., alert, oriented, and cooperative), and fully recovered from neuromuscular blockade. Extubation in the reverse Trendelenburg or semi-upright position is recommended because these positions reduce diaphragmatic compression. Extubation over an airway exchange catheter should be considered if re-intubation is expected to be difficult.[22]

Patients with obesity have an increased risk of postoperative hypoxemia. Oxygen therapy and/or continuous positive airway pressure (CPAP) can be utilized in the post-anesthesia care unit (PACU) in order to prevent periods of oxygen desaturation during the 24 hours following surgery.[23]

Recommendations

Airway management in patients with obesity should not—a priori—be considered more difficult than in lean patients, when necessary precautions are taken. Predicting difficult intubation with a careful airway examination based on Mallampati score and neck circumference is more important than only considering BMI. Head-up position of 25- to 90-degree during pre-oxygenation is recommended.

Intubation should be performed in the ramped position, and RSI should be considered if predisposing conditions for pulmonary aspiration are present.
Hypnotic induction agents, such as propofol (1–2.5mg/kg) are dosed on LBW, as are opioids (e.g., fentanyl 2–6mcg/kg). Non-depolarizing muscle relaxants (e.g., rocuronium 0.6mg/kg) should be dosed on IBW, and succinylcholine 1mg/kg should be dosed on LBW or TBW.

Extubation should be performed in the reverse Trendelenburg or semi-upright position only when the patient is fully awake and has fully recovered from muscle relaxation.

References
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18.     Andersen LH, Rovsing L, Olsen KS. GlideScope videolaryngoscope vs. Macintosh direct laryngoscope for intubation of morbidly obese patients: a randomized trial. Acta Anaesthesiol Scand. 2011;55(9):1090–1097.
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Category: Anesthetic Aspects of Bariatric Surgery

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