Background. Obesity is chronic condition characterized by excess adipose tissue and is associated with several comorbid conditions including diabetes, obstructive sleep apnea, and hypertension. Obesity is commonly defined by a body mass index (BMI)≥30 kg/m2. Severe obesity is defined by a BMI ≥40 kg/m2. Obesity impairs respiratory function and increases risk of postoperative complications.

Respiratory pathophysiology

1. Excess adipose tissue results in compression of the lung parenchyma and upper airway obstruction resulting in a restrictive pattern of pulmonary pathology

2. Respiratory pathology in a patient with obesity is further characterized by:

   1. elevated pleural/intrathoracic pressures

   2. reduced FRC

   3. atelectasis and shunting

   4. small airway closure, air trapping, and iPEEP

   5. increased work of breathing

   6. obstructive apnea

Preoperative evaluation

1. Complications are increased in patients with truncal (central) rather than peripheral adiposity. Truncal adiposity has a more direct effect on the lung parenchyma and is associated with metabolic dysfunction.

2. Patients should be screened for the presence of obstructive sleep apnea (OSA) as it is highly co-prevalent and often underdiagnosed in patients with obesity. Patients with OSA who use continuous positive airway pressure (CPAP) at home should continue this therapy during the perioperative period. Always consider that the obese patient may have sleep-disordered breathing (Chapter 1: Evaluating the Patient Before Anesthesia).

3. Airway examination and history of previous airway management are important to evaluate risk of difficult intubation and/or mask ventilation.

4. Thorough cardiopulmonary assessment is important to assess perioperative risk in patients with obesity. Signs that may indicate an increased risk include:

   1. Baseline Spo2 with arterial blood gas analysis if Spo2 <95% on room air.

   2. Evidence of daytime hypercarbia (eg, elevated serum bicarbonate on basic metabolic panel) suggesting obesity hypoventilation syndrome

   3. Decreased functional capacity

Intraoperative considerations

1. Airway. The risk of difficult mask ventilation is elevated in obese patients. Due to a decreased FRC, oxygen desaturation can be rapid with the induction of anesthesia. Considerations for prevention of hypoxemia include:

   1. Ensuring complete denitrogenation with 100% Fio2 administration.

   2. Applying CPAP during preoxygenation to increase end-expiratory lung volume. For patients with known OSA, CPAP should be applied at their home pressures level if known. If the pressure level is not known or there is no diagnosis of OSA, at least 10 cmH₂O should be applied to prevent lung collapse. For patients undergoing intubation in the intensive care unit who are unable to tolerate CPAP, high flow nasal cannula (HFNC) is better tolerated and will provide low amounts of PEEP and extend the time until desaturation.

   3. Positioning the patient with their head elevated (>30°) and off their chest will optimize positioning for laryngoscopy while increasing FRC. Reverse Trendelenburg combined with a back-up position on the bed can be used to facilitate a ramping position where the auditory meatus is in line with the sternal notch.

Maintenance of anesthesia

1. Pharmacology. Obesity may alter the pharmacokinetics of anesthetic drugs. Optimal methods for weight-based dosing in obesity are unclear and vary by drug class. Patients with obesity should be monitored closely throughout the perioperative period and drugs that decrease respiratory drive (eg, opiates) should be titrated carefully. Administration of large doses based on total body weight in patients with obesity can be dangerous. To mitigate risk, the following can be considered:

   1. The use of intraoperative depth of anesthesia monitoring is recommended, particularly when total intravenous anesthesia is administered.

   2. Assessment of neuromuscular blockade by train-of-four monitoring, preferably acceleromyography, is essential for the maintenance of adequate depth of paralysis and its complete reversal prior to emergence.

   3. The use of maintenance anesthetics that have low fat-blood solubility coefficients (sevoflurane, desflurane) and are less lipophilic will facilitate quicker emergence.

   4. The administration of long-acting opioids should be limited, and ideally spared. If used, patients should be monitored postoperatively with pulse oximetry and capnography.

2. Mechanical ventilation

   1. Patients with obesity are more susceptible to lung and small airway collapse during mechanical ventilation due to increased transpulmonary pressures.

   2. The mass load on the lungs from truncal adipose tissue requires high levels of counteracting airway pressure throughout the respiratory cycle to prevent lung collapse. Patients with obesity are less prone to alveolar overdistention, even when high airway pressure and recruitment maneuvers are applied, due to elevations in pleural pressure.

   3. Ventilator settings

      1. Either pressure (PCV)- or volume (VCV)-controlled modes can be used during controlled ventilation. With PCV, the inspiratory pressure needs to be set sufficiently high to overcome airway collapse, otherwise, inadequate alveolar ventilation can occur and plateau/driving pressures will be overestimated.

      2. Patients with obesity benefit from ventilatory approaches that maintain end-expiratory lung-volume and prevent lung collapse. For most patients with mild to moderate obesity, a PEEP of 6 to 8 cmH2O is appropriate.

      3. In patients with severe obesity, higher levels PEEP and recruitment maneuvers may be required. A recruitment maneuver, or transient increase in mean airway pressure, should be applied prior to PEEP titration, targeting a plateau pressure up to 50 cmH₂O. Recruitment maneuvers can help to restore FRC.

      4. Patients with obesity may benefit from advanced respiratory monitoring. Pleural pressure can be estimated using an esophageal pressure catheter. PEEP can be titrated to maintain a transpulmonary pressure (PEEP − end-expiratory esophageal pressure) of 0 to +2 cmH₂O.

      5. Electrical impedance tomography (EIT) is a noninvasive, radiation-free imaging technique that measures regional ventilation based on variation in the electrical conductivity of the thorax. EIT can measure regional lung collapse and overdistention and may be a useful tool for PEEP titration.

Emergence. Multiple factors can be manipulated to optimize emergence. Extubation should occur with the patient in upright position to maximize FRC. For patients with severe obesity, CPAP should be administered after extubation. CPAP can be applied via a portable ventilator or a Boussignac mask. CPAP should be continued in the postanesthesia care unit until the patient is fully awake. Patients should be closely monitored after extubation and narcotics should be administered sparingly given the increased risk for postoperative respiratory depression.