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Airway management in the obese

Hendrik Drinhaus MD
10 May, 2016  

Obesity poses a growing challenge to healthcare worldwide, which apart from increasing the risk for diverse medical conditions can have serious implications for airway management and ventilation in anaesthesia, intensive care and emergency medicine

Hendrik Drinhaus MD 
Jochen Hinkelbein MD
Department of Anaesthesiology and Intensive Care Medicine
University of Cologne, Cologne, Germany
Obesity, defined as a body mass index (BMI) of >30kg/m2, is an ever-increasing challenge to public and individual health. According to a common definition, individuals with a BMI of >30kg/m2 are classified as ‘obese’, >40kg/m2 as ‘morbidly obese’ and >50kg/m2 as ‘super-obese’.1,2
The prevalence of obesity has doubled between 1980 and 2008 and today about one in five Europeans are obese.3 Not only does obesity predispose to chronic conditions such as diabetes, cardiovascular or musculoskeletal disease, which entail a substantial cost and disability burden to European societies, but it can also pose serious challenges to health care practitioners in anaesthesia, intensive care or emergency medicine with regards to airway management and ventilation in obese patients.
This article outlines the relevant physiological and anatomical changes and presents current strategies for airway management and ventilation in the obese. Main issues addressed include optimal preoxygenation, management of difficult ventilation and intubation, and lung-protective ventilation strategies.
Alterations of respiratory physiology
Obese individuals have a higher metabolic rate, necessitating higher minute ventilation, achieved by increased respiratory rates rather than enlarged tidal volumes, which actually tend to be decreased. Lung and chest-wall compliance is reduced, while airway resistance tends to be increased, resulting in a higher work of breath compared with non-obese individuals. Expiratory reserve volume and thus – given a virtually unaltered pulmonary reserve volume – functional residual capacity are reduced in an exponential fashion inversely related to BMI. Especially in paralysed patients in supine position, FRC may drop below the closing capacity of the lung, resulting in small airway collapse and atelectasis. Alveolar-arterial gradient of oxygen (AaO2) may be increased, chiefly as a result of a dysbalance between ventilation and perfusion of the lungs.1,4,5 The described alterations imply serious consequences for apnoea tolerance during anaesthesia, as discussed below.
Positioning and preoxygenation
As apnoea tolerance is reduced in obese subjects due to the mechanisms described above, optimal preoxygenation is needed all the more in obese patients in order to prevent hypoxaemia during induction of anaesthesia and airway management. Whereas preoxygenated normal weight patients theoretically have an apnoea tolerance of up to approximately eight minutes, it may decrease significantly in obese patients.
The concept of preoxygenation is to wash out nitrogen from the functional residual capacity (FRC) and to replace it with oxygen, thereby extending the duration during which oxygen can diffuse into the blood even without respiratory movements. In obese patients, time from onset of apnoea to oxygen desaturation is significantly shorter than in non-obese individuals. Concurrently, longer times may be needed to establish an airway and to ventilate the patient, as difficult mask ventilation and intubation is more prevalent in obese patients. Therefore, healthcare practitioners involved in airway management of the obese must pay particular attention to obtain the best preoxygenation possible. 
Head up or sitting position allows for the diaphragm and the abdominal organs to be shifted in a caudate direction, permitting the lung to expand more freely and to increase FRC. It has been demonstrated that longer apnoea times until oxygen desaturation can be achieved when positioning the obese patient head up or sitting during preoxygenation. 
To facilitate tracheal intubation by aligning the axes of mouth, pharynx and larynx, the ramped position can be applied, which means to position the auditory meatus and the sternal notch at the same level by supporting neck and shoulders with, for example, pillows or blankets. Continuous positive airway pressure (CPAP) and pressure support ventilation (PSV), performed as non-invasive ventilation, applying a tight fitting mask to the patient’s face, can also increase apnoea tolerance. 
Another item of ideal preoxygenation is apnoeic oxygenation, which is a passive flow of oxygen into the alveoli during apnoea: Between the injection and onset of narcotics and neuromuscular blockers this can be achieved by leaving the tight-fitting oxygen mask on the face. During laryngoscopy nasal or pharyngeal insufflation of oxygen at a high flow can enable a mass flow of oxygen into the lungs.6,7
Aspiration risk and protection
Obese patients are at risk of pulmonary aspiration due to increased gastric volumes, lowered gastric pH and higher prevalence of gastroesophageal reflux disease.8,9 Therefore, rapid sequence induction (RSI) and intubation is usually employed. However, current guidelines do not include an explicit statement on RSI and some authors doubt its necessity in fasted obese patients for elective procedures.10
Difficult airway management (DAM)
Difficult airway management (DAM) is more likely to be encountered in obese patients. This does not only include difficult intubation, but also difficult mask ventilation, difficult placement of supraglottic/extraglottic airway devices (SGAs, EGAs) and difficult transcutaneous access to the trachea.
There is, however, a controversial debate as to whether obesity per se (and if yes, which grade of obesity) is a risk factor for difficult intubation and as to which commonly used predictors of difficult intubation can be relied upon in the obese. In an analysis of almost half a million cases of airway management, a BMI of >30kg/m2 was associated with both difficult mask ventilation and difficult intubation, implying an increased risk of a potentially catastrophic “cannot-ventilate-cannot-intubate-situation” 11
Most, albeit not all, studies identify Mallampati grade 3 or 4, enlarged neck circumference, and male gender as predictive of difficult airway management in obese patients.1,12 In day-case surgery patients with a high likelihood of undiagnosed of obstructive sleep apnoea (OSA), difficult laryngoscopy, multiple attempts at intubation and awake fibre-optic intubation were more frequent. 13
It should be kept in mind that airway management in the obese is even more challenging in emergency situations (for example on the intensive care unit or in preclinical emergency medicine) than for scheduled anaesthesia in the theatre.14 Trained personnel, appropriate equipment, and concise algorithms familiar to all team members are crucial for successful management of the difficult airway.
Using a video laryngoscope for endotracheal intubation (ETI) may facilitate ETI and reduce the time for tube positioning. Optimising visual conditions by using a video laryngoscope, however, does not necessarily lead to easier tube placement during ETI and cannot guarantee intubation success.15
Alternative airway devices
As aspiration risk is increased in the obese, endotracheal intubation is traditionally believed to be mandatory in these patients. Recently, this belief has been challenged by some and the scheduled use of supraglottic airway devices in obese patients does not seem to be inconceivable anymore. Potential benefits of SGAs that are discussed include ease and speed of use, no necessity of neuromuscular blocking, or reduced incidence of coughing. 16 Main drawback of SGAs is their inferiority in protecting from aspiration of stomach contents. 
Data on the incidence of aspiration during the use of SGAs in obese patients is scarce. In a Cochrane analysis of two studies using the ProSeal laryngeal mask airway (PMLA), no conclusions could be drawn about pulmonary aspiration or other serious complications. The same analysis found improved peripheral oxygen saturation and a trend towards less coughing and laryngospasm for the PMLA, compared with a tracheal tube.17
Given the paucity of the data available, we do not deem it prudent to give advice as to whether and when to use or not to use supraglottic airway devices at the current time. Independent of their indication as a routine airway in the obese, SGAs are valuable tools when dealing with difficult airway management. Preference should be given to new devices which permit higher airway pressures and insertion of a gastric tube.
Lung-protective ventilation strategies
During mechanical ventilation of obese subjects, be it intraoperatively or on the intensive care unit, it is vital to provide lung protective ventilation in order to minimize the risk of ventilator induced lung injury. Mainstays of protective ventilation are positive end-expiratory pressure (PEEP), low tidal volumes (6–8ml/kg) based on predicted rather than actual body weight and alveolar recruitment manoeuvres. Inspiratory fraction of oxygen should be kept as low as reasonably possible in order to prevent atelectasis. Caution must be applied in hemodynamically instable patients, as elevated PEEP or recruitment manoeuvres can lead to cardiovascular compromise.7,12 
Postoperative strategies
In a risk model derived from an analysis of the impact of pre-existing conditions on perioperative adverse events, perioperative respiratory complications were calculated to be four times more likely in obese than in non-obese patients.18 Obese patients in ambulatory surgery experienced more respiratory complications and might also need a longer postoperative recovery and observation, making scheduling of ambulatory surgery more complicated.1 Despite the higher frequency of respiratory adverse events, the number of unplanned hospital admissions over night was not increased in obese patients.19
The grade of OSA, invasiveness of surgery, the type of anaesthesia and the need of postoperative opioids could be identified as main risk factors for perioperative complications in outpatient surgery for patients with OSA.1 Patients with a BMI of 40kg/m2 or less can probably undergo ambulatory surgery safely, whereas the ‘super obese’ (BMI >50kg/m2) were found to be at an increased risk of perioperative adverse events.19
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