Abstract
Since its introduction in the 1980s, the advantages of the laryngeal mask airway (LMA) were quickly recognized and LMAs progressively replaced tracheal intubation in many cases. Because positive pressure ventilation results in higher airway pressures in obese patients and because these patients were previously believed to be at increased risk of regurgitation, obesity has been considered as a contra-indication to the use of an LMA. The risk of pulmonary aspiration during general anesthesia in obese patients has been reconsidered. Furthermore, during the past decade, several new supraglottic airway (SGA) devices with high leak pressures have been introduced into clinical practice. These newer SGAs are potentially more suitable for use in obese patients. In this chapter, we review the potential role of SGA devices in the management of obese patients and propose a practical guide to help deciding whether or not the trachea of an obese patient requiring mechanical ventilation should be intubated.
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Keywords
- Obese Patient
- Tracheal Intubation
- Laryngeal Mask Airway
- Positive Pressure Ventilation
- Pulmonary Aspiration
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.
1 Introduction
Obesity has become common and a serious societal problem throughout the world. Consequently, anesthesiologists frequently must care for morbidly obese (MO) patients. In order to secure their airway tracheal intubation has been advised for all MO patients requiring muscle relaxants and/or controlled ventilation. Intubation was considered necessary since obese patients were considered at risk of regurgitation and aspiration of gastric contents and also to assure adequacy of ventilation because of their decreased respiratory system compliance and increased airway resistances. However, our knowledge of the pathophysiologic changes and the anesthetic management of MO patients have evolved during recent decades. The risk of regurgitation and aspiration in MO patients has been reconsidered and new (SGA) devices are now used as alternatives to the tracheal tube in lean patients. This raises the question whether tracheal intubation is still required in all MO patients requiring muscle relaxants and/or controlled ventilation. This issue remains controversial. In this chapter we will provide practical recommendations for airway management, the current assessment of the risk of regurgitation and aspiration, the characteristics of the newer SGA devices, and our own experience in anesthetized MO patients.
2 Specificities of Mechanical Ventilation in Anesthetized Obese Patients
2.1 Ventilation Results in Higher Airway Pressures
Morbid obesity produces several important pathophysiologic changes of respiratory mechanics. Functional residual capacity (FRC) is decreased reflecting an alteration in the balance between the outward expansion force of the chest wall and the inward elastic recoil of the lung. This change is caused by the fatty infiltration of the rib cage and the increased weight of the abdominal contents. Muscle relaxation during anesthesia decreases the antero-posterior diameter of the chest, allows the abdominal content to displace the diaphragm cranially, and consequently further reduces FRC. This decrease in FRC and to a lesser extent an increase in pulmonary blood volume both lower lung compliance of MO patients. Whether their chest wall compliance is also reduced remains a matter of debate. In addition, the airway resistance of obese patients is increased. Because of the decreased respiratory system compliance and the increased airway resistance, positive pressure ventilation of obese patients results in higher airway pressures compared with lean patients.
2.2 Greater Benefits of Peep
Positive end-expiratory pressure (PEEP) increases the end-expiratory lung volume and partially counteracts deleterious effects of both obesity and anesthesia on respiratory mechanics and gas exchanges. Addition of 10 cmH2O PEEP has been shown to improve arterial oxygenation in anesthetized MO patients, but not in normal-weight patients [1]. Besides oxygenation, PEEP also improves the compliance of the respiratory system in anesthetized obese and non-obese patients. In MO patients the benefits of PEEP appear at higher levels than in non-obese patients [2]. Moreover, ventilation of MO patients without PEEP after a recruitment maneuver (RM) quickly leads to recurrence of atelectasis [3]. Therefore 10 cmH2O PEEP should be applied in MO patients particularly during both open and laparoscopic surgery.
The reduction in airway pressure associated with the improvement in compliance secondary to PEEP only partly outweighs the increased airway pressure produced by PEEP. Ventilation of obese patients with PEEP results in higher airway pressure compared to ventilation without PEEP. In 20 anesthetized MO patients (BMI = 45 ± 9 kg m−2) placed in the supine position and ventilated with 10 cmH2O PEEP, the average peak and plateau airway pressures were 32 and 21 cmH2O, respectively [2].
2.3 Potential Need for Lung Recruitment
In spite of the application of preventive measures such as placing the patient head-up and using CPAP throughout the pre-oxygenation phase, the induction of anesthesia in MO patients nevertheless results in atelectasis [4]. This atelectasis can be reversed only by lung RMs, which raise the airway pressure at least up to 40 cmH2O [5]. When surgical conditions favor recurrence of atelectasis, which occurs during laparoscopic surgery or major open abdominal surgery, RMs need to be repeated on a regular basis.
2.4 Some Surgeries Increase Airway Pressures
The laparoscopic approach is favored in MO patients since it allows significant reduction of the postoperative restrictive pulmonary syndrome [6]. However, during carbon dioxide pneumoperitoneum, the plateau airway pressure comes close to or above 30 cmH2O in obese patients ventilated with 10 cmH2O PEEP [7]. Surgery in a head-down position reduces thoraco-pulmonary compliance and consequently further increases airway pressures.
2.5 Increased Risk of Regurgitation and Aspiration of Gastric Content?
Fasted MO patients have been traditionally considered to have excessive acidic gastric juice volume and to be at increased risk of pulmonary aspiration during induction of general anesthesia [8]. For this reason, rapid sequence induction was recommended and the use of a SGA device contra-indicated. Whether obesity per se actually increases the risk of regurgitation and aspiration is now controversial. Morbid obesity is indeed frequently associated with risk factors for pulmonary aspiration like esophageal reflux [9] and diabetes [10]. Since the incidence of lung aspiration during anesthesia is extremely low, the combination of large gastric juice volume and low gastric pH has been used as a surrogate of risk. Whereas this combination was previously considered common in MO patients, recent data indicate this is no more frequent than in lean patients and in otherwise healthy MO patients who do not receive opioid premedication [11]. In a large survey of pulmonary aspiration during the perioperative period, a BMI > 35 kg m−2 was not identified as being a risk factor [12]. Together these data suggest that in MO patients without comorbidities that predispose to regurgitation, there is no increased risk of pulmonary aspiration during the perioperative period [13].
3 Supraglottic Airway Devices
As we have discussed, controlled mechanical ventilation in MO patients produces higher airway pressures than in lean patients. Obese patients require high levels of PEEP and RMs which further increase airway pressures, and as a consequence, airway pressures frequently rise above 30–35 mmHg. In addition, in the past MO patients were considered to be at increased risk of gastric content aspiration. For these reasons tracheal intubation has been recommended for control of the MO patient’s airway. Now, the risk of pulmonary aspiration no longer is believed to be increased in MO patients compared to non-obese patients so tracheal intubation might no longer be mandatory in these patients.
The laryngeal mask airway (LMA) was first developed in the 1980s to maintain upper airway patency. Since then the number of different SGA devices has exploded since the introduction of the LMA Classic™. Currently these SGA devices are used as an alternative to an endotracheal tube to ventilate lean patients. As such, they provide a number of advantages compared to an endotracheal tube. Insertion of a SGA causes less stimulation of the sympathetic nervous system and tolerance of these devices requires lighter levels of anesthesia; neuromuscular blockade is not needed. During emergence from general anesthesia hypertension, coughing, and bucking associated with the endotracheal tube can be avoided with a LMA. Induction and emergence from anesthesia may be quicker using a SGA device. However, SGA devices have two main limitations. First, they offer poor protection against aspiration of gastric contents. Despite this, the reported incidence of aspiration associated with the LMA (~2 per 10,000) is not statistically different from an endotracheal tube [14]. Second, their suitability to deliver positive pressure ventilation is limited by their leak pressure, above which alveolar hypoventilation and potentially gastric insufflation occur. Leak pressures of most LMA lie between 20 and 25 cmH2O [15].
3.1 LMA Classic™
The SGA era started with the introduction of the LMA Classic™ which still remains the most widely used device. A disposable version, the LMA Unique™, was subsequently introduced to eliminate concerns about transmission of infectious agents. These LMAs are suitable for positive pressure ventilation with peak airway pressures up to 20–25 cmH2O. Use of the LMA Unique™ was studied and compared with the i-gel™, a recently introduced SGA device with a non-inflatable cuff, in a population of moderately obese patients (BMI ≤ 35 kg m−2) undergoing non-laparoscopic surgery in the supine position. The mean leak pressures were 17.5 cmH2O and 24 cmH2O for the LMA Unique™ and i-gel™, respectively. Audible air leak occurred during the delivery of a tidal volume ≤ 5 ml kg−1 in 36 % of patients with the LMA and 24 % of patients with the i-gel™. Patients at increased risk for pulmonary aspiration were excluded and no instance of aspiration was noted [16].
3.2 LMA ProSeal™ and LMA Supreme™
The LMA ProSeal™ and LMA Supreme™ offer two advantages over the LMA Classic™. First, they have an additional lumen allowing a drainage tube to be placed in the esophagus. Second, their larger cuffs provide a better airway seal and consequently allow for a higher leak pressure. The use of LMA ProSeal™ was studied in modestly obese patients (mean BMI = 34 kg m−2) [17]. As compared to the LMA Classic™, the LMA ProSeal™ results in a reduced leak fraction of the tidal volume at lower cuff pressures. Unfortunately, the leak pressure of both LMAs was not reported [17]. The efficiency of the LMA ProSeal™ was also tested during laparoscopic surgery. In that study, the mean leak pressure was 34 cmH2O. The LMA ProSeal™ was reported to be as effective as an endotracheal tube for ventilation during laparoscopy. However, four of 16 (25 %) of the obese patients assigned to the LMA ProSeal™ group had to be crossed over to the endotracheal tube group [18]. This suggests that the LMA Proseal™ did not deliver adequate positive pressure ventilation in MO patients undergoing laparoscopic surgery despite its leak pressures being almost twice as high as those of the LMA Classic™.
Keller et al. used the LMA ProSeal™ as a temporary ventilatory device in 60 MO patients (BMI = 43 [35–60] kg·m−2) before laryngoscope-guided tracheal intubation [19]. Oropharyngeal leak pressure was 32 ± 8 cmH2O. Positive pressure ventilation at 8 ml kg−2 tidal volume was possible without oropharyngeal air leaks in 95 % of patients and without gastric air leaks in all patients. In this study the LMA ProSeal™ was used only for a short period before tracheal intubation. Additional studies are needed before recommending use of the LMA ProSeal™ in MO patients for longer periods, although it is unlikely that airway quality deteriorates with time. Finally, MO patients can be ventilated with a LMA Supreme™, but this was only grossly tested at the induction of anesthesia [20]. Interestingly, a positive correlation between BMI and oropharyngeal leak pressure for the LMA ProSeal™ and LMA has been reported [21]. Perhaps fat deposits in the neck allow the pharynx to better adapt to the shape of the cuff or reduce the pharyngeal volume. It is important to note that PEEP was not used in any of these studies.
4 Practical Considerations for Airway Management in Obese Patients
4.1 When Can a SGA Device be Considered?
As discussed, several SGA devices have been successfully used to ventilate MO patients during surgery. However, data remain sparse and the cited studies enrolled only selected populations of MO patients. Moreover, the anesthesiologists involved in these studies were experienced using SGA devices. Experience with these devices in obese patients remains limited because endotracheal intubation has been the “gold standard” for airway management of MO patients.
We believe that a SGA device is an acceptable alternative to an endotracheal tube in modestly obese patients with no risk factors for aspiration especially when scheduled for peripheral surgery. Ophthalmic and head and neck surgeries may also benefit from the use of these devices since they prevent the coughing associated with an endotracheal tube. The patient’s cardiac status can also be considered when considering a SGA device since insertion results in less cardiovascular response compared to laryngoscopy and endotracheal intubation and to better hemodynamic stability at lighter levels of anesthesia. Finally, a SGA should be used in cases of unexpected difficult intubation as a rescue device. For these indications the LMA Proseal™ and the i-gel™ are theoretically more suitable for obese patients since they have higher leak pressures than the LMA Classic™. The i-gel™ appears easier to position than the LMA Unique™ in obese patients [16]. The LMA Proseal™ seems more difficult to insert than the LMA Classic™. Several attempts at insertions are sometimes needed [22]. Successful insertion on first attempt is higher with the LMA Supreme™ [23]. However, this device has a lower oropharyngeal leak pressure than the LMA ProSeal™. Since increasing BMI is associated with higher airway pressures during positive pressure ventilation, SGA devices should be selected depending on their leak pressure in the following order: LMA Classic™ or Unique™, i-gel™, LMA Supreme™, and then LMA ProSeal™.
When using a SGA we recommend the following to lower the airway pressures: use a tidal volume 6 ml kg−2 of ideal body weight, with an I/E ratio of 1:1.5, and adjust the respiratory rate for an end-tidal PCO2 less than 40 mmHg.
4.2 When Should The Trachea Be Intubated?
If no benefits are expected from a SGA then an endotracheal tube should be used. We consider tracheal intubation mandatory in the following situations:
4.2.1 Severity of Obesity
When the BMI exceeds 40–45 kg m−2, especially if fat is centrally distributed, the respiratory system is likely to be affected to such an extent that adequate ventilation would result in peak airway pressures > 35 mmHg and could probably not be achieved with a SGA device. The adverse respiratory consequences of obesity are indeed proportional to its severity and affected by the topography of fat distribution [24]. Furthermore, these patients are very likely to require PEEP and repeated RMs, which cannot be assured with a SGA since leak pressure will be exceeded.
4.2.2 The Type of Surgery
Ventilation of obese patients undergoing intra-thoracic, intra-abdominal, or laparoscopic surgery results in high airway pressures since these procedures greatly reduce the compliance of the respiratory system. Furthermore, in these surgeries the benefits of PEEP and RMs are well-established. The probability of exceeding the leak pressure of SGA is therefore very high. Adequate ventilation of MO patients during these procedures requires endotracheal intubation.
4.2.3 Patient Position
Intraoperative positions that decrease the respiratory system compliance, such as the head-down position, can also increase airway pressures above the leak pressure of a SGA.
4.2.4 Patient Comorbidities
One of the major limitations of SGA devices is poor protection against pulmonary aspiration. No significant episodes of regurgitation were reported in obese patients ventilated with a SGA in earlier studies of obese patients with no additional risk factors for regurgitation. As with for non-obese patients if risk factors for regurgitation (e.g. full-stomach, active esophageal reflux, insulin-dependent diabetes, bowel obstruction) are present, the trachea of MO patients must be intubated. However, the recent national audit by the British Medical Association did report a significant association between obesity and aspiration when first generation SGA devices were used. The safety of the use of second generation LMAs (LMA Supreme™, LMA ProSeal™) in obese patients is currently unknown.
5 Which Device to Use in Case of an Emergency?
Tracheal intubation of MO patients has traditionally been regarded as potentially difficult. Optimizing patient’s position and in particular using the ramped position can at least partly overcome this problem [25]. SGA devices must be considered early in the algorithm to solve a “cannot intubate, cannot ventilate” scenario. Any available SGA is theoretically suitable to achieve sufficient ventilation in this situation.
Our preference would be to select a SGA that allows tracheal intubation. Two choices are the intubating laryngeal mask LMA Fastrach™ and its modified version allowing continuous video-endoscopy, the LMA Ctrach™. In a study which included 52 MO patients, adequate ventilation was achieved with the LMA Ctrach™ in all patients although further manipulation of the LMA after the first insertion attempt was required in one-third of the patients. Intubation was performed in all patients through the LMA Ctrach™ but a second attempt was required in 8 % of the patients [26]. In case of emergency, we therefore recommend using the SGA most familiar to the anaesthesiologist. A SGA with incorporated video-endoscopy should be helpful to optimize positioning of the mask and to facilitate intubation. An alternative is to perform a fiberoptic intubation through a LMA Fastrach™ once adequate ventilation has been achieved. If tracheal intubation is difficult but ventilation remains possible, video-laryngoscopy should be used before intubating LMA.
6 Conclusions
New SGA devices with high leak pressure provide an alternative to endotracheal intubation for positive pressure ventilation in MO patients scheduled for peripheral surgery. Furthermore, during certain other operations and for obese patients with cardiac comorbidities, a SGA may be indicated. The LMA ProSeal™ and the LMA Supreme™, and to a lesser extent the i-gel™, seem the most appropriate devices currently available. In MO several factors and conditions (extreme obesity, intra-thoracic and intra-abdominal [open and laparoscopic] surgeries, need for PEEP and RMs) may preclude the use of a SGA. In the absence of an actual benefit from using a SGA, safety considerations favor ventilation through an endotracheal tube in obese surgical patients.
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Hans, G.A., Joris, J.L. (2013). Do Supraglottic Airways Have a Role in the Anesthetic Management of Obese Patients?. In: Leykin, Y., Brodsky, J. (eds) Controversies in the Anesthetic Management of the Obese Surgical Patient. Springer, Milano. https://doi.org/10.1007/978-88-470-2634-6_16
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