A Problems with Controlled Ventilation

The LMA Classic performs well during spontaneous ventilation, but it is so widely used during controlled ventilation that this is no longer considered an advanced use. The LMA Classic is an alternative to anesthesia with a face mask or an ETT. Its introduction has transformed the routine practice of anesthesia, and face mask anesthesia has become uncommon. However, the LMA Classic is not suitable for all cases, and good case selection is the key to successful use.

The LMA Classic usually seals the pharynx with a pressure of 16 to 24 cm H₂O, and this airway leak pressure is rarely above 30 cm H₂O. This relatively low- pressure seal means that when positive pressure is applied to the LMA Classic, gas leakage is common. Studies have shown a 5% failure rate for achieving an expired tidal volume of 10 mL/kg,⁸ an audible leak in 48% of patients when ventilating to peak pressures of 17 to 19 cm H₂O, and a detectable leak rate as high as 90%, with an 8% failure rate for adequate ventilation.^9,10 Devitt and colleagues applied increasing peak airway pressures while ventilating through an LMA Classic. They found that as the airway pressure rose from 15 to 30 cm H₂O, the incidence of audible leak rose from 25% to 95%, and the leak fraction ([inspired minute volume−expired minute volume]/inspired minute volume) rose from 13% to 27%.¹¹ As the airway pressure increased, the incidence of airway leak into patients’ stomachs rose from 2% to 35%. These findings indicate that the LMA Classic has a relatively low-pressure airway (pharyngeal) seal and that as higher airway pressures are applied, there is a risk of loss of ventilating gases and gastric inflation. Loss of ventilating gases is associated with hypoventilation, loss of anesthetic agent, and environmental pollution, and gastric inflation that may increase the risk of regurgitation.

B Problems with Airway Protection

Several factors affect the risk of and protection against aspiration during use of a SAD. A device with a good pharyngeal seal minimizes the risk of air leak into the esophagus during positive-pressure ventilation, especially if airway pressures rise. A correctly positioned drain tube that is not obstructed by mucosa can vent gas entering the upper esophagus and minimize the risk of gastric inflation. If regurgitation does occur, a correctly positioned drain tube can vent liquid (and solids if large enough) so that it bypasses the oral cavity and alerts the anesthesiologist to the occurrence of regurgitation.

Several factors determine whether regurgitant fluid can enter the glottis: the seal between the tip of the SAD and the upper esophagus (i.e., esophageal seal), the bulk of the SAD in the hypopharynx and oral cavity, the pharyngeal seal, and perhaps the presence of a sump. Reliable placement of a gastric tube through a SAD enables stomach emptying and reduces the risk of regurgitation and aspiration. Many protective factors depend on correct positioning of the SAD. A device that is reliably placed in the correct position is likely to provide better protection. A malpositioned device likely disrupts the pharyngeal and esophageal seals, may displace or obstruct the drain tube, and may lead to high airway pressures during positive-pressure ventilation. The ability to check correct device position (e.g., PLMA) is another benefit. Among available SADs, the ProSeal LMA has the most evidence for protection against regurgitation and aspiration.

The LMA Classic is not regarded as providing protection against aspiration of regurgitated gastric contents and is contraindicated for patients who are not fasted or who may have a full stomach. The LMA Classic has a pharyngeal seal that is usually in the range of 16 to 24 cm H₂O. Its tip obturates the upper esophagus, and it has an esophageal seal of 40 to 50 cm H₂O, but it has no drain tube.¹² Despite this design, cadaver work shows the LMA Classic protects the glottis from regurgitant esophageal fluid considerably more efficiently than the unprotected airway.¹³

Soon after the introduction of the LMA Classic, several small studies raised concerns about the ability of the LMA Classic to protect the airway from regurgitant matter and therefore from pulmonary aspiration. Early concerns were raised that the LMA Classic, sitting at the back of the throat, might stimulate a swallowing reflex, especially during light planes of anesthesia, leading to relaxation of the upper and lower esophageal sphincters and increasing the risk of regurgitation and aspiration. A physiologic study recorded a fall in the lower esophageal barrier pressure of 4 cm H₂O during LMA Classic anesthesia, compared with a 2 cm H₂O rise during face mask anesthesia.¹⁴ A study using swallowed methylene blue capsules demonstrated a 25% incidence of soiling of the inner portion of the LMA Classic on removal, and a small study reported a 2% incidence of aspiration, which occurred during spontaneous and controlled ventilation.^15,16

As experience has accumulated, the evidence of a fundamental problem with aspiration has been reevaluated. Until 2004, 16 years after its introduction, there were no published reports of fatal aspiration during use of an LMA Classic. In 2004, Keller and colleagues published a series of three cases of serious morbidity, including one death from aspiration during LMA Classic anesthesia.¹⁷ Each case had risk factors for aspiration, and on reviewing all 20 published reports of aspiration during use of an LMA Classic, the investigators found identifiable risk factors in 19 of 20 cases. In the accompanying editorial, Asai listed more than 40 factors that increased the risk of aspiration.¹⁸ Several large studies have shown a low rate of aspiration; Verghese and Brimacombe reported a series of 11,910 uses (40% with controlled ventilation, 19% during intra-abdominal surgery, and 5% with a duration longer than 2 hours).¹⁹ Insertion success rate was 99.8%, the incidence of airway-related critical incidents was 0.16% during spontaneous ventilation and 0.14% during controlled ventilation, and there was one case of aspiration. Bernardini and Natalini reported three aspirations in a series of 35,630 LMA Classic uses for controlled ventilation (1 of 11,877).²⁰ In an editorial, Sidaras and Hunter²¹ estimated an incidence of confirmed pulmonary aspiration during LMA Classic use of 1 in 11,000, and Brimacombe and Berry’s meta-analysis calculated a risk during elective surgery of 1 case in 4300 operations.⁶ This is similar to the rate of aspiration reported by Warner and colleagues in a study of 214,000 patients predating use of the LMA, in which aspiration occurred in 1 in 4000 elective operations.²²

Although the risk of aspiration is relatively low in expert hands, this rate is achieved primarily by careful and appropriate case selection, expert insertion, and meticulous management of the airway after insertion. The Fourth National Audit Project of the Royal College of Anaesthetists and Difficult Airway Society (NAP4) in the United Kingdom studied major airway complications of 2.9 million episodes of general anesthesia and found that aspiration was the most common cause of airway-related deaths.¹ One third of these complications occurred during maintenance with a LM or LMA in place, and for many of the patients, the risk of aspiration made this unwise.

Aspiration remains a significant cause of morbidity and mortality during anesthesia, and all anesthesiologists should consider the patient’s risk for aspiration before selecting the appropriate airway device. In making that selection, they should consider the degree of protection provided by the airway device, particularly when using a SAD.

C Problems with Accessing the Airway for Intubation

The LMA Classic sits over the vocal cords in more than 90% of cases and may be used as a conduit for intubation, but several factors limit the ease of this application.²³ The internal lumen of the device is relatively narrow, limiting the size of ETT that can be passed. Size 4 and 5 LMA Classic devices accommodate most manufacturers’ cuffed ETTs with internal diameters (IDs) of 6.0 and 6.5 mm, respectively. A tube of adequate length must be used to exit the LMA Classic and reach the midtrachea; an ETT of approximately 29 cm can be placed through a size 5 LMA Classic. Across the distal end of the airway tube are two flexible bars forming a grill that prevents the tongue from impeding insertion and the epiglottis from causing obstruction after placement; these bars may act as an impediment to intubation through an LMA Classic. The angle at which an ETT exits the mask of the LMA Classic means that blind insertion frequently leads to esophageal intubation. Brimacombe reported blind intubation with an ETT through the LMA Classic to have a first-time success rate of 52% and overall success rate of 59%.²⁴ Use of a bougie is less successful (32% of first attempts and 45% overall), and even fiberoptically guided techniques have a failure rate of 18%. After intubation has been achieved, removal of the LMA Classic without displacement of the ETT is cumbersome. Overall, direct intubation through the LMA Classic is a far from ideal technique.

The technique is dramatically improved if an Aintree intubation catheter (AIC, Cook Critical Care, Bloomington, IN) is used.²⁵ The hollow AIC (ID of 4.6 mm, external diameter [ED] of 7.0 mm, length of 46 cm) is placed over a fiberscope, the scope and AIC are negotiated through the LMA Classic into the midtrachea, and the fiberscope then is removed, followed by removal of the LMA Classic. Care must be taken to ensure the AIC is not advanced too far, especially if gases are passed through it, as this risks barotraumas. This technique can be performed with or without the use of a Bodai adapter (Sontek Medical, Lexington, MA). The Bodai adapter allows oxygen and gas administration through the attached breathing circuit during exchange of the LMA to an ETT. The AIC remains in place, and a suitably sized lubricated ETT is then advanced over the catheter. Although the AIC technique does not appear in current airway guidelines, its use is simple, has a high success rate, and is widely reported.^26,27

D Reusable Design

The LMA Classic is reusable and designed to be used up to 40 times. An in vitro study suggested that the LMA Classic and ProSeal LMA may be reused up to an average of 130 and 80 times, respectively, before showing signs of failing the preuse tests recommended by the manufacturer, and in vivo work supports use up to 60 times.^28,29

After use, the LMA Classic is cleaned (decontaminated) before sterilization by autoclave (up to 137° C for 3 minutes with the cuff fully deflated), and it is stored in sterile packaging thereafter. A 2001 bench-top study demonstrated that routine decontamination and sterilization failed to remove all proteinaceous material from airway devices and from the LMA Classic in particular.³⁰ At the same time, there was increasing public awareness about variant Creutzfeldt-Jakob disease (vCJD), especially in the United Kingdom. Concerns grew that residual prions, the infective, misfolded proteins responsible for vCJD, might remain and be passed from patient to patient. Several national bodies recommended using single-use devices “wherever possible,”³¹ even though the estimated risk of such cross-contamination was 1 to 10 cases in 100,000 patients.³² Brimacombe and coworkers described the rush toward single-use LMs as “driven by fears of the unknown and scientific misinformation.”³³ Since then, the risk of vCJD has fallen dramatically, and the risk of transmission is likely to be vanishingly small.³⁴ This risk must be balanced against other risks introduced by alternative equipment.³⁵ Blunt and Burchett found that even a small deterioration in safety as a result of using a single-use device of poorer quality in place of a reusable device increased the overall risk to patients and went against the recommendation of the Spongiform Encephalopathy Advisory Committee (SEAC).³²

E Absence of a Bite Block

The LMA Classic lacks a bite block and is prone to obstruction by biting in the agitated patient during emergence. Use of a bite block (e.g., rolled gauze placed between the molar teeth) is recommended until the LMA Classic is removed, and failure to adhere to this recommendation can lead to airway obstruction, hypoxia, and postobstructive pulmonary edema.³⁶ The LMA Flexible also has no bite block, but the intubating LMA (ILMA), ProSeal, and Supreme LMA do.

A Desirable Features of Supraglottic Airway Devices

Certain issues influence the anesthesiologist’s choice of an airway for an individual patient. Should the anesthesiologist choose a single-use device, a device that can be reused most often, the cheapest device, or the device that causes the least trauma? Do all devices maintain the airway reliably? To what extent do any of them protect the airway from regurgitation and pulmonary aspiration? Which devices enable safe and effective positive-pressure ventilation? Will the device enable access to the airway for intubation if required? Are there differences in ease of insertion and ability to ventilate patients’ lungs between the various devices? How often are manipulations needed to maintain a clear airway during anesthesia? Which devices are tolerated best during emergence? What are the relative incidences of airway trauma and postoperative pharyngolaryngeal morbidity? Unfortunately, in most cases, remarkably few of these data are available. The manner in which new medical devices are regulated contributes to this situation.

The characteristics of an ideal SAD include efficacy, versatility, safety, reusability, and cost. The device should be made of good-quality, nontoxic materials and have a long shelf-life. Reusable devices should be robust enough to allow many uses and cleaning cycles without damage or deterioration of performance, and other desirable features may reflect the preferences of anesthesiologists and their patients.

The anesthesiologist wants a device that is inserted reliably on the first attempt, producing a clear airway for spontaneous and controlled ventilation. It should enable maintenance of hands-free anesthesia in a variety of patients in various head and neck positions. Device performance should be consistent and predictable, and it should allow emergence without complications. The incidence of airway trauma and postoperative sore throat should be acceptably low. Design features or clinical evidence for protection against aspiration is highly desirable, and the ability to reach the trachea through the device may also be a factor in choosing a SAD. It should function reliably as a rescue device and for difficult airway management. The ideal device should not cause intraoperative complications, trauma, or pharyngolaryngeal morbidity for the patient.

The anesthesiologist should be able to insert the SAD despite a limited mouth opening (most require 2 to 3 cm) and using a light depth of anesthesia (dose range for different airways varies approximately twofold). SADs requiring a muscle relaxant for insertion are of limited use.

All SADs may cause airway obstruction from epiglottic downfolding. This problem can be reduced by ensuring correct insertion technique and by using devices designed with a slim leading edge and a large airway orifice. The slim profile of the deflated Laryngeal Tube (LT, King Systems, Noblesville, IN) and LMA Classic and the deflation device and tip flattener that are provided with the PLMA are examples of designs that minimize the tip size. The epiglottis may cause obstruction by entering the orifice of the airway device. Several design features are aimed at avoiding this problem, including epiglottic bars (e.g., LMA Classic, LMA Flexible, some newer LMs), a large orifice that is too big to become obstructed (e.g., PLMA, i-gel), multiple airway holes (e.g., Combitube, LT), and an orifice with protective fins (e.g., Supreme LMA).

The first-time insertion success rate should be high, and initial insertion should require a minimum of manipulations. With current devices, the first-time insertion success rate ranges from less than 70% to more than 95%. The average number of manipulations required for insertion ranges from less than one manipulation in 25 cases to more than one per case.

The anesthesiologist requires an airway that does not require manipulations after insertion or repositioning during anesthesia, enabling hands-free anesthesia. The most functional devices require an intervention in less than 1 in 25 cases, but others require intervention in two thirds of cases.

The airway should be stable when the head and neck position varies, such as during rotation to improve surgical access or when the head and neck are repositioned for additional procedures. Limited evidence suggests that the stability of different SADs under these circumstances varies.

Intraoperative complications (e.g., airway obstruction, loss of airway, regurgitation, laryngospasm) should be uncommon. The published incidence of minor complications with existing devices ranges from less than 10% to 60%. Serious and minor repetitive complications or the need to perform repeated or continuous manipulations to maintain the airway may force early removal of the airway. This is the ultimate failure of the airway device, and it occurs with an incidence of less than 1 in 50 cases to more than 1 in 5 cases.

The ideal SAD should be reliable for spontaneous and controlled ventilation. Among the existing devices, several versions of one device function poorly during spontaneous ventilation, and another is designed specifically to facilitate controlled rather than spontaneous ventilation. Several devices that produce a low-pressure seal with the airway may be unsuitable during controlled ventilation because of the risk of gastric inflation and regurgitation. The role of second-generation SADs in protection of the airway is discussed later.

The ideal airway device causes no trauma to the airway. The incidence of trauma to the airway with the latest SADs, as evidenced by blood visible on the device, ranges from close to 0% to more than 50%. SADs commonly cause sore throat, dysphonia, and dysphagia, but these symptoms are usually minor, transient, and less common than after tracheal intubation. The possibility of nerve injuries is a greater concern. The ideal airway minimizes or eliminates both of these problems. However, the intracuff and mucosal pressures vary in intensity and location with different SADs. The overall incidence of sore throat varies from less than 10% to more than 40%. Clinically significant nerve injury is rare with all SADs, and the relative risks of individual devices are unknown.

In addition to maintaining the airway during anesthesia, a SAD may enable access to the airway. It is easy for this role to be overemphasized, because during routine anesthesia, it is rare that the SAD needs to be changed for an ETT, and when this exchange is required, the SAD usually can be removed before intubation is attempted conventionally. If the SAD is likely to be needed as a conduit for planned tracheal intubation or for management of a difficult airway, the device may be selected accordingly. The intubating LMA (ILMA) and Cookgas intubating laryngeal airway (ILA) are designed specifically for these roles, and several others (e.g., i-gel, PLMA) are likely to perform similarly well. Several techniques may be used, including blind use of an ETT or bougie and light-guided and fiberoptically guided techniques with an ETT or exchange catheter. Such techniques require the larynx to be visible from the airway orifice and the internal diameter of the SAD lumen and its orifice to be of adequate caliber. For various devices, the ability to view the laryngeal inlet from the airway orifice ranges from more than 90% to less than 40%. Variations in length and diameter limit the techniques that may be used with certain SADs. The proximal orifice of some SADs is too small to admit an ETT with an ID larger than 5.0 mm, whereas others accommodate ETTs with an 8.0-mm ID. At their distal end, grills, bars, small orifices, and difficult angles may impede or prevent access to the trachea.

Although most devices are used by anesthesiologists, SADs may be used by individuals with less experience for anesthesia, out-of-hospital rescue, or resuscitation. The ideal airway should therefore be intuitive to use, have a high success rate for the naive user, and be easy to learn. The few available data suggest that insertion and airway maintenance by nonanesthesiologists and by naive users varies considerably among devices.

Many assume that reusable devices may be replaced by cheaper, single-use devices, and some think that single-use devices are intrinsically preferable. However, many single-use devices differ from the reusable devices they seek to replace in design and in the materials used. Some modifications appear to be minor, but the implications for performance have generally not been evaluated. The work on single-use laryngoscopes and intubation bougies provides evidence that changes in product material may alter performance considerably.^38,39 Data on the current versions of the single-use LM and comparisons between these and the LMA Classic remain largely unavailable.

No single device meets all the criteria for the ideal SAD. Some criteria are incompatible with others. For instance, a device that is large enough to accommodate a standard-sized ETT and that incorporates an adequate-caliber drain tube is unlikely to be as easily inserted as a smaller device. Epiglottic bars reduce airway obstruction but hinder instrumental access to the trachea. A single-use device is less likely than a more expensive reusable device to be made of the best materials to optimize handling characteristics and minimize pharyngolaryngeal trauma. It is likely that several different airways will always be needed for use in different clinical situations.

B Efficacy Versus Safety

SADs may have several roles, and different designs and performance characteristics may be required for each role. The LMA Classic was originally used almost exclusively to provide anesthesia for brief, peripheral operations that were performed in slim patients, usually during spontaneous ventilation. The popularity of the LMA Classic has led to an evolution in practice such that SADs have become increasingly used for longer, more complex operations and for obese patients. SADs are increasingly used for laparoscopic and open intra-abdominal surgery, and their use during controlled ventilation has become commonplace.

Many of these expanded indications can benefit patients, but they also raise questions about efficacy and safety. During spontaneous ventilation, the LMA Classic provides a clear airway and enables hands-free anesthesia in more than 95% of cases. Efficacy of the LMA Classic for controlled ventilation diminishes rapidly as lung resistance increases (e.g., obesity, laparoscopy), and rates of hypoventilation and gastric distention increase, raising concerns about the safety of its use in these situations.

In the past decade, more than 40 SADs have been introduced. Most are attempts to mimic and compete with the LMA Classic to appeal to purchasers and managers. Anesthesiologists are more interested in SAD designs that improve performance (i.e., efficacy and safety) and thereby increase their clinical utility.

Many studies comparing SADs have been inadequately powered to determine efficacy, and none has addressed the issue of safety directly. Efficacy depends on several factors, including ease of insertion, manipulations required to maintain a clear airway throughout anesthesia, and tolerance during emergence. Efficacy during controlled ventilation requires the ventilation orifice of the SAD to be positioned over the larynx, and the SAD must seal well within the laryngopharynx (i.e., pharyngeal seal).

Safety encompasses avoidance of complications occurring at all stages of anesthesia and afterward. Prevention of aspiration requires a good-quality seal within the laryngopharynx and esophagus (i.e., esophageal seal) to prevent gas leaking into the esophagus and stomach and to prevent regurgitant matter passing from the esophagus into the airway. A functioning drain tube enables regurgitant matter to bypass the larynx and be vented outside, protecting the airway and giving an early indication of regurgitation to the anesthesiologist. Studies have shown that the extent of esophageal seal varies considerably among SADs. Those with a drain tube can effectively vent regurgitant fluid if the drain is not occluded.^12,40,41

C Structured Approaches to Evaluation of New Devices

New airway devices should undergo mandatory assessment of manufacturing quality and clinical performance before marketing. The characteristics of the ideal SAD outlined earlier provide a checklist against which function can be assessed. Several methods have been recommended.^35,42,43 Cook described a three-stage evaluation process³⁵:

In stage 1, the bench models include airway manikins and others, such as those specifically designed to test aspiration risk.⁴⁴ This stage is limited by lack of fidelity of available manikins.⁴⁵ With the increasing use of SADs during resuscitation, during out-of-hospital rescue, and by non-anesthesiologists, there is an urgent need to develop realistic manikins for testing and training. Data acquired from such studies require intelligent interpretation and knowledge of the relative performance of different manikins.^46–48 Results of manikins studies are considerably limited, and at best, they may be used to evaluate basic information on device performance and durability and to identify major conceptual or design problems. Appropriate bench testing may lead to further development of a device before starting clinical studies.

In stage 2, a cohort study may be used for the first assessment of clinical performance in patients. This approach enables full clinical evaluation of the new device under routine clinical conditions. Functions that can be tested include ease of insertion, pharyngeal seal, airway resistance, stability of the device in different head and neck positions, ease of passage of a gastric tube, positioning of the airway over the larynx, and suitability for fiberscopic or catheter exchange techniques. Learning curves can be examined. A cohort study also enables assessment of function during spontaneous and controlled ventilation and determination of airway trauma or pharyngolaryngeal morbidity. The cohort must be large enough to enable identification of common problems, but unless it is very large, it cannot detect uncommon or rare problems. For instance, for an event that does not occur in a cohort study of n cases, the 95% confidence interval (CI) for frequency of that event is approximately 1 in .⁴⁹ For example, if no nerve injuries occur in a cohort study of 100 cases, the upper limit of the 95% CI for risk of nerve injury is 1 in 33. A cohort of at least 100 patients is a reasonable compromise between being large enough to identify important uncommon events and remaining a practical size.

Stage 3 employs a randomized, controlled trial. After successful completion of bench and cohort evaluations, the need for further modifications of the device should be considered. Significant modifications necessitate repetition of the early evaluations. On successful completion of the early evaluations, the new device should be compared with its best existing competitor. In many cases, this is the LMA Classic. The randomized, controlled trial must be of adequate size to identify clinically important differences in function. Studies may be designed to test the hypothesis that the devices perform differently (i.e., superiority-inferiority trials) or that the test device does not perform significantly less well than the benchmark device (i.e., noninferiority trials).⁵⁰ Power calculations can be based on data acquired from phase 2, but trials of at least 100 patients provide more comprehensive and clinically useful comparisons. Economic evaluation of cost-effectiveness of the new device may take place at this stage. Data from the three phases of evaluation can be used to determine what role the new airway device has in the market.

In an ideal world, a license for only one aspect of airway care (e.g., spontaneous breathing only, controlled ventilation in patients with good pulmonary compliance, airway maintenance when tracheal access is likely to be necessary) would be offered based on the results of research. License extensions could be granted in light of further research. Current legislation designed to encourage market competition means that no such limits are imposed, as they are in drug development.

Implementation of the suggested methodology would still result in only 200 to 300 uses of the device in patients before release to market. Because this number is not enough to identify uncommon or unexpected problems, complications, and advantages, the proposed method of evaluation does not obviate the need for postmarketing surveillance or reporting of adverse incidents. A formal method of postmarketing evaluation could be developed. For instance, the first 5000 devices marketed could have evaluation cards attached, which would be returned after use. Some SAD manufacturers have used such a system in the past without making the resultant information available to the profession. Alternatively, the manufacturer could be required to seek reports of all adverse incidents for the first 2 years after release, similar to the Yellow Card system for new drugs that applies in the United Kingdom.

A second structured approach to evaluating and choosing new devices was made by Wilkes and colleagues.⁴³ In this proposal, a central body of experts would coordinate research to evaluate new devices, review available evidence and provide national recommendations on devices reaching standards of acceptability. Although potentially of value for a large population (e.g., a country), the barrier it may create to free trade and the likelihood of legal challenges are problems.

The UK Difficult Airway Society (DAS) has proposed a guideline whereby purchasers could adopt a minimum level of evidence before making a pragmatic decision about the purchase or use of an airway device.⁴² This minimum level of evidence (i.e., level 3b: a case- or historical-controlled cohort study) would form the basis of a professional standard to guide those with responsibility for selecting airway devices.⁵¹ Devices without this minimum level of evidence would not be purchased. The investigators argue that widespread adoption of this professional standard would lead to situations in which it was in the interests of manufacturers and purchasers to acquire such evidence and the DAS would support both parties in setting up research with this aim. The strength of this approach lies in purchasers driving the need to raise the evidence bar and manufacturers being encouraged to perform clinical trials at an early stage in device development. This approach is not anticompetitive because it creates no barriers to manufacturers bringing a device to market, but it does raise the level of expectation of the community of purchasers about what they wish to purchase.

Whether any of these methods of structured evaluation are officially adopted, each has potential advantages for the manufacturer, clinician, and patient. For successful devices, the manufacturer would have robust data to support performance claims and a clearer vision of the likely advantages and applications of the new device. This would enhance marketing and raise credibility. For devices that performed poorly, the manufacturer could avoid the expense of large-scale production and marketing of devices that would ultimately fail to achieve market share. The clinician would have better evidence on which to base medical decisions. Researchers would have clearer ideas about how a new device might be evaluated to define function further and investigate wider indications for use. The patient would be less likely to be exposed to unnecessary risk due to the use of an unevaluated device.

Anesthesiologists and patients expect equipment to be effective and safe during anesthesia. The relationship between manufacturer and clinician (acting for the patient) is symbiotic. Care of patients can improve only through a sustained effort by clinicians and manufacturers to improve the medical devices used during anesthesia. In this respect, much has been achieved in airway care in the past 20 years, and the practice of anesthesia has been transformed. Innovation is expensive, and much of the cost of advances or improvements accrues during research and development. This cost is borne entirely by the manufacturer. A more open relationship between interested parties and the early involvement of objective, structured evaluation of new airway equipment are recommended. This approach could prevent undertested or underdeveloped products from coming to market and thereby protect patients medically and manufacturers legally. An evaluation program should encourage and support equipment manufacturers to achieve these goals.

A Generic Laryngeal Masks

Since 2005, many manufacturers have produced LMA-like devices designed to compete with LMAs. They included single-use (mostly polyvinyl chloride [PVC]) and reusable (PVC and silicone) devices. Because the term laryngeal mask airway (LMA) is registered, the newer devices are referred to as laryngeal masks (LMs). The manufacturers assert that the main driving force for the introduction of single-use devices has been the concerns about the sterility of cleaned, reusable devices. The financial opportunities of the SAD market are another reason for the increase in these devices.

For patent reasons, all LMs are different from the LMA Classic, and much variation exists between LMs. The patent on the basic design of the LMA Classic lapsed in 2005, but the patent for the epiglottic grills remained in place until 2008. All LMs designed before 2008, except those made by the original manufacturers of the LMA Classic, do not have the epiglottic bars at the distal end of the airway tube; some recent entrants into the market do. Some LMs have angulated stems, and others have enlarged masks. Some early LM designs featured more bulky masks and cuffs than the LMA Classic, and this design and the stiffer PVC material of which they were made impeded insertion. Particularly with the larger masks, there is a possibility of entrapping the tongue and drawing it backward during insertion (causing trauma) and downfolding the epiglottis after it is inserted (causing obstruction and trauma).

For cost reasons, most single-use LMs are made with a PVC cuff, which increases the rigidity of the device. PVC is cheaper than silicone and reduces the cost of these devices for the manufacturer and purchaser. PVC is less permeable to nitrous oxide than silicone, and during nitrous oxide administration, the increase in LM cuff pressure in the early phase of anesthesia is considerably less than when using a silicone device.^52,53

PVC has two disadvantages. First, it is more rigid than silicone. Brain examined PVC as a material for LMA construction in the mid-1980s but rejected it because of its rigidity.⁵⁴ The increased rigidity of PVC LMs may cause problems, such as increased trauma to the airway during use and an increase in pharyngolaryngeal morbidity postoperatively. The rigidity, especially in masks with thicker cuffs, may lead to folds and wrinkles in the partially inflated cuff, which may affect the airway seal or create channels that enable regurgitant fluid to reach the larynx, altering efficacy and safety. The impact of these effects is not known for many devices because of the lack of published evidence. Some manufacturers use a softer form of siliconized PVC, which may obviate the problems described.

The second concern is the toxicity of phthalates, such as di-2-ethylhexyl phthalate (DEHP) or dioctyl phthalate (DOP), which are used in the manufacturing process to render the PVC softer. These chemicals are not linked to the plastic matrix and leach out slowly during use. They are considered potentially carcinogenic, mutagenic, and reprotoxic,⁵⁵ and there is particular concern about phthalates in products that may be placed in the mouth (e.g., children’s toys, airway devices). The issue has been considered important enough by some for bills banning PVC in some products to be brought before the legislatures of some U.S. states and the European Parliament. Phthalates are banned from use in cosmetics in Europe, and in May 2005, the European Parliament voted to make permanent a temporary ban on six phthalates (including DEHP) in PVC used in children’s toys and called for investigation into health care equipment.⁵⁵ It is easy to overstate the importance of phthalates in the safety of PVC LMs, but the unquantified low risk they pose bears comparison with the unquantified low risk of transmission of prion disease when using a reusable device. Alternative plasticizers, such as no-DOP formulas and di-isononyl-cyclohexane dicarboxylate (Hexamoll DINCH) are available for use in PVC applications, although at considerably increased cost. In the past few years, an increasing number of low-cost silicone LMs designed for single use and reuse have been introduced.

The greatest concern about the profusion of LMs on the market is the lack of rigorous evaluation. Typically, there is no robust evidence to inform whether the single-use or reusable LMs perform similarly to equivalent LMAs or to inform which LM performs best. The limited evidence does show that all LMs and LMAs are not equivalent. Of the approximately 30 devices available, only 2 have substantial publications that compare their efficacy with that of LMAs: the AuraOnce LM (Ambu Inc., Glen Burnie, MD) and the Portex Soft Seal LM (SSLM, Smiths Medical ASD, Keene, NH). For the other devices, there appears to be a void of published evidence. A publication from the National Health Service (NHS) Centre for Evidence-Based Purchasing illustrates the difficulty in determining the relative merits and demerits of these competitors to the LMA Classic. The document listed more than 25 alternative standard LMs and reported a total of 18 comparative trials for the devices. Some of these studies were of poor quality. This publication record contrasts with the more than 2500 publications on the LMA Classic.⁵⁶

B Features of Laryngeal Masks

C Single-Use Laryngeal Masks

Several single-use LMs and LMAs including flexible devices have been introduced in the past few years. Performance must be assumed because performance evaluations and comparative trials have not been performed.

In the United Kingdom, the Department of Health advises that all airway equipment used for tonsillectomy should be single use, and this recommendation is endorsed by the Royal College of Anaesthetists and other professional bodies. NICE did not consider LMAs when examining the risk of vCJD transmission. In this climate, the single-use LMA Flexible is likely to have a significant role. Two single-use LMs—the AuraOnce and the SSLM—have a significant body of published literature that allows a balanced analysis of their performance.

1 Ambu AuraOnce Laryngeal Mask

The Ambu AuraOnce LM was designed and produced between the fall of 2002 and February 2004, when it was launched by the Danish medical device manufacturer Ambu. Ambu offers a range of LMs. The Ambu AuraOnce is a single-use LM with a preformed curve. The Ambu Aura40 is a reusable version of the Ambu AuraOnce. The Ambu AuraSraight is a single-use device with a more conventional curve to the stem. The Ambu AuraFlex is a single-use, reinforced, flexible LM, and the Ambu Aura-i is a modification of the AuraOnce that is designed to facilitate intubation in a fashion similar to that of the ILMA.

The Ambu AuraOnce is the most investigated model. It is a sterile, single-use product made of PVC with a preformed curve designed to replicate human anatomy (Fig. 23-2). This angled curve is designed to ensure that the patient’s head remains in a natural position when the mask is in use and to facilitate insertion without exerting force on the upper jaw. A reinforced tip reduces the risk of the device folding back during insertion. Reinforcing internal ribs built into the curve of the airway tube provide a degree of flexibility and enable the airway tube to conform to individual anatomic variations and adapt to a wide range of head positions without loss of function.

Figure 23-2 Ambu AuraOnce laryngeal mask as delivered. The red “tab” keeps the pilot balloon valve open so the cuff pressure is maintained at atmospheric pressure.

The Ambu AuraOnce is molded in one piece with an integrated inflation line and no epiglottic bars at the airway orifice. The cuff is elliptical and is shaped to lie in the hypopharynx at the base of the tongue. The absence of epiglottic bars means there is no barrier to flexible fiberoptic devices. The product is sterile and latex free and is available in adult and pediatric sizes 1 through 5. The recommended size is based on the weight and height of the patient.

The Ambu Aura40 is the reusable, silicone version of the Ambu AuraOnce.⁴⁰ Its built-in curve is designed o replicate the natural human anatomy and offers the same features as the Ambu AuraOnce. It can be steam autoclaved up to 40 times.

The Ambu Aura-i (Fig. 23-3) is a modification of the Ambu AuraOnce that is designed to facilitate intubation in a fashion similar to that of the ILMA. It has a shorter, wider, and more rigid airway tube than the Ambu AuraOnce, and proximally, a rigid plastic sleeve acts as a handle for insertion and as a bite block. It is designed for use during routine airway maintenance and as a device for airway rescue and fiberoptic intubation. Sizes 3, 4, and 5 accommodate ETTs up to sizes 6.5-, 7.5-, and 8-mm ID, respectively. Ambu has released a single-use videoscope that may be used with the Ambu Aura-i.

Figure 23-3 The Ambu Aura-i laryngeal mask with a tracheal tube passed through it illustrating its role as a conduit. Inset shows markings on posterior of stem.

a Application

Familiarity with the manufacturer’s warnings, precautions, indications, and contraindications is necessary before use, and the patient should be at an adequate depth of anesthesia (or unconsciousness) before attempting insertion. Before insertion, the following guidelines should be observed:

• The size of the Ambu AuraOnce must be appropriate for the patient. Use the manufacturer’s guidelines combined with clinical judgment to select the correct size. Always have a spare Ambu AuraOnce ready for use.

• Resistance or swallowing may indicate inadequate anesthesia or inappropriate technique, or both. Inexperienced users should choose a deeper level of anesthesia.

• The Ambu AuraOnce is inserted in a fashion similar to that described for the traditional LMAs in Chapter 22. Excess force must be avoided at all times. When the mask is fully inserted, resistance is felt. If the cuff fails to flatten or curls over as it is advanced, it is necessary to withdraw the mask and reinsert it. In case of tonsillar obstruction, a diagonal shift of the mask is often successful.

Intubation techniques through the Ambu AuraOnce are the same as those described earlier for the LMA Classic. The preformed curve may make this slightly less easy than in the more smoothly curved LMA Classic, but this difficulty is easily overcome with lubrication. The absence of bars at the end of the airway tube is an advantage over the LMA Classic.

The Ambu Aura-i is a single-use device that may be used in preference to the Ambu AuraOnce if intubation through the device is anticipated or planned.

b Indications, Advantages, and Disadvantages

The Ambu AuraOnce has several features of potential advantage over some other LM designs. Its cuff is soft and flexible but has a reinforced tip; together with the preformed curve of the stem, these features facilitate insertion. Because it is molded in a single unit, it is free from ridges that may injure the airway during insertion, and component parts cannot separate during use.

Its disadvantages are those of all LMs. If used for controlled ventilation, the manufacturers recommend limiting peak airway pressure to 20 cm H₂O and the tidal volume to 8 mL/kg².

c Medical Literature

Approximately 25 publications about the Ambu AuraOnce are listed on PubMed.

Cohort Studies

A multicenter study of the clinical performance of the Ambu AuraOnce was conducted by Hagberg and colleagues to evaluate ease of insertion, insertion success, airway seal, and ventilation.⁵⁷ Device placement was successful in all 118 nonparalyzed, anesthetized patients on the first or second attempt (92.4% and 7.6%, respectively). Adequate ventilation was achieved in all patients, and the vocal cords could be visualized by fiberoptic endoscopy in 91.5% of patients. Complications and patients’ complaints were minor and were quickly resolved. The investigators reported that the curvature of the tube facilitated insertion and that the large, soft cuff allowed a higher oropharyngeal leak pressure than is commonly found in other LMs. The reinforced tip of the Ambu AuraOnce may avoid folding, which can lead to air leakage, a common problem with other LMs.

Genzwuerker and coworkers studied ventilation using the Ambu AuraOnce in different head positions in 30 patients.⁵⁸ Five different head positions were used: head on a standard pillow, head rotated 90 degrees to the left side, head rotated 90 degrees to the right side, head with chin lift on a standard pillow, and head flat on a table without a pillow. No changes in the performance of the device were observed in any position. The Ambu AuraOnce may be a useful SAD for cases in which head movement may be necessary during surgery.

Randomized, Controlled Studies of Efficacy

Shariffuddin and Wang compared the Ambu AuraOnce with the LMA Classic during controlled ventilation in 40 patients in a randomized, crossover design.⁵⁹ The mean ± standard deviation oropharyngeal leak pressure was higher for the Ambu AuraOnce (19 ± 7.5 cm H₂O) than for the LMA Classic (15 ± 5.2 cm H₂O, P = 0.004). The Ambu AuraOnce also required fewer insertion attempts (P = 0.02) but more manipulations to achieve a patent airway (P = 0.045). Values for time to insert the device and intraoperative performance were similar.

Sudhir and associates performed a randomized, crossover study comparing the Ambu AuraOnce and LMA Classic in 50 patients.⁶⁰ Success rates for the first attempt success were similar (92% for the Ambu AuraOnce and 84% for the LMA Classic, P = 0.22). The volumes of air required to inflate the cuff to produce an effective airway seal were similar, but the cuff pressure was lower for the Ambu AuraOnce (median, 18 cm H₂O) compared with LMA Classic (27 cm H₂O, P = 0.007). Complications were similar in both groups.

Ng and coworkers compared the Ambu AuraOnce with the LMA Classic and reported the Ambu AuraOnce to be easier to insert, but they found no difference in time to insertion,⁶¹ successful insertion on the first attempt, oropharyngeal leak pressure, hemodynamic response to insertion, or complications of placement.

Francksen and colleagues compared the performance of the Ambu AuraOnce and the LMA Unique in 80 patients undergoing minor routine gynecologic surgery.⁶² They demonstrated that the time to insertion and failure rate were comparable and that oxygenation and ventilation variables were adequate with either device. Median airway leak pressures were slightly higher with the Ambu AuraOnce than with the LMA Unique (18 versus 16 cm H₂O, P < 0 .013). No gastric inflation was observed with either device in this small study.

Francksen and coworkers also studied 120 patients scheduled for routine minor obstetric surgery who were randomly allocated to size 4 Ambu AuraOnce, LMA Unique, or SSLM.⁶³ The Ambu AuraOnce was fastest to insert by a few seconds. All three had high first-attempt insertion success rates (all >95%). Rates of subjectively rated excellent insertion were 75% for the LMA Unique, 70% for the Ambu AuraOnce, and 65% for the SSLM. All devices were judged acceptable.

Lopez and colleagues compared four single-use LMs (i.e., Ambu AuraOnce, Solus LM, LMA Unique, and SSLM) in 200 patients with ASA physical status class I, II, or III who were undergoing elective ambulatory surgery and for whom airway management was performed by inexperienced residents.⁶⁴ The Ambu AuraOnce (78%) and LMA Unique (80%) performed best in terms of ease of insertion, with the Solus requiring most reinsertions. Optimal ventilation was best with the LMA Unique (94%). Airway leak pressure was 27.3 mm Hg for the SSLM, 23.7 mm Hg for the Ambu AuraOnce, 22.1 mm Hg for the LMA Unique, and 20.9 mm Hg for the Solus. Blood staining occurred most frequently with the SSLM (38%).

Other Studies

Gernoth and colleagues compared the performance of the Ambu AuraOnce with the LMA Classic in 60 patients whose cervical spines were immobilized with an extrication collar before elective ambulatory interventions.⁶⁵ Insertion time, number of insertion attempts, and airway leak pressures were comparable (25.6 ± 5.3 cm H₂O for the Ambu AuraOnce and 26.5 ± 6.5 cm H₂O for the LMA Classic). The investigators concluded that both devices were suitable for rapid and reliable airway management in patients with cervical immobilization.

Using an in vitro test, Zaballos and coworkers examined the Ambu AuraOnce, LMA Classic, PLMA, LMA Unique, and i-gel using magnetic resonance imaging (MRI).⁶⁶ The Ambu AuraOnce and i-gel did not lead to artifacts, whereas each of the LMAs did.

Maino and associates examined the effect of nitrous oxide exposure on intracuff pressure of PVC and silicone LMs and the LMA Classic in vitro.⁶⁷ All cuffs were initially inflated with air to a pressure of 60 cm H₂O. The findings were not surprising given the physicochemical properties of PVC and silicone. When exposed to 66% nitrous oxide, the cuff pressure rose considerably more in the silicone cuffs than in the PVC devices. Among the PVC devices, the lowest increases in cuff pressure after 60 minutes of exposure were found in the Solus and LMA Unique (13 and 15 cm H₂O, respectively), and the increases in the cuffs of the Ambu AuraOnce and SSLM were notably higher (28 and 31 cm H₂O, respectively).

Literature Summary

Overall, the published literature shows that the Ambu AuraOnce performs similar to the LMA Unique and similar to or better than some other single-use LMs. Studies comparing it with the LMA Classic usually report almost equivalent performance. Only one study compared the Ambu AuraOnce with second-generation SADs. There are few reports of adverse events with the Ambu AuraOnce.

2 Soft Seal Laryngeal Mask

The SSLM is a disposable device made of PVC. Like the Ambu masks, it is one of few LMs with a body of published evidence on which its performance can be judged. Product development for this mask began in November 1998. Several prototypes were tried with features designed to improve insertion and performance. Evaluation included construction of a model oropharynx and testing in this replica and cadaver models. The final product was commercially launched in Australia in May 2002 and in the United States in January 2003, making it one of the earliest LMs to come to market.

The SSLM is a tubular oropharyngeal airway with a mask and an inflatable peripheral cuff attached to the distal end (Fig. 23-4). It is designed to produce an airtight seal around the laryngeal inlet to provide a secure airway suitable for spontaneous or controlled ventilation during general anesthesia.

Figure 23-4 The Soft Seal laryngeal mask.

(Courtesy of Smiths Medical ASD, Keene, NH.)

The SSLM is available in sizes 1 through 5. The appropriate size is based on the weight and size of the patient. For adults, mask sizes 3, 4, and 5 are suitable for patients weighing 30 to 50 kg, 50 to 70 kg, and more than 70 kg, respectively. In broad terms, size 3 is suitable for small female patients, size 4 for many male and female patients, and size 5 for larger male patients.

Smiths Portex introduced a single-use silicone LM that includes epiglottic bars. It is available only outside North America.

a Application

Insertion technique for the SSLM is similar to that recommended for LMAs in terms of patient position, device checking, preparation, and lubrication. Although many insertion techniques are used, insertion with the cuff partially inflated is recommended for the SSLM. If a muscle relaxant is administered before device insertion, use of a triple airway maneuver (i.e., mouth opening, head extension, and jaw thrust) should decrease the incidence of epiglottic downfolding.⁶⁸ The cuff should be inflated with air until a “just-seal” pressure is obtained. A maximum intracuff pressure of 60 cm H₂O is recommended.

The SSLM may be used as an intubation conduit. As with other LMs, this can be performed blindly (using an ETT or a bougie) or under fiberoptic guidance. Due to high failure rates for the blind technique, a fiberoptically guided technique is recommended, and use of an Aintree Intubation Catheter, as described previously, is a recognized and successful technique. The SSLM has a wider ventilation orifice than the LMA Classic; larger SSLM sizes can accommodate up to a 7.5-mm ETT. This and the absence of aperture bars facilitate intubation.

b Indications, Advantages, and Disadvantages

Indications for the SSLM are the same as for the LMA Classic, which include routine anesthesia in fasted patients. The device can be used as a rescue airway in the “cannot intubate, cannot ventilate” (CICV) or the “cannot intubate, difficult to ventilate” (CIDV) situation, or it can be used as a primary airway for rescue of an unconscious patient whose airway is compromised or at risk. In all circumstances, the clinician must balance the risk of pulmonary aspiration against the benefits of obtaining an adequate airway or oxygenation of the patient.

The limitations of use of the SSLM are the same as for other LMs. The large bowl of the device and its PVC construction have been found by some to inhibit easy insertion. These limitations affect many PVC LMs.

c Medical Literature

There are approximately 27 publications about the SSLM in the medical literature.

Randomized, Controlled Studies of Efficacy

Van Zundert and colleagues performed a randomized, controlled trial of the use of the size 4 SSLM and LMA Classic during spontaneously breathing anesthesia in 200 adult patients.⁵³ Insertion time, success, and fiberoptic position were equivalent. Further analysis of the cuff pressure change during nitrous oxide anesthesia, with the cuff pressure initially established at 45 mm Hg, showed that it increased in the LMA Classic to a mean of 100 mm Hg and that the mean final pressure in the SSLM was 47 mm Hg (P < 0.001).⁶⁹ There was a greater incidence of sore throat in the LMA Classic group at 2 hours postoperatively but not at 24 hours.

Similar to the low rates of trauma reported in the study by van Zundert’s team, Hagberg and colleagues demonstrated that partial cuff inflation (30 mL of air) enhanced ease of insertion and minimized mucosal trauma.^70,71

Shafik and associates performed a crossover comparison of the SSLM and LMA Classic in 60 patients.⁷² The primary outcome measure was first-attempt insertion, for which success rates were equivalent (92% for the SSLM and 96% for the LMA Classic), as was ease of insertion.

Lopez and coworkers compared the SSLM with the LMA Classic in a randomized trial enrolling 60 patients.⁷³ Most performance characteristics were equivalent between devices, with the exception of a difference in airway seal (23 ±4 cm H₂O for the SSLM and 20 ±4 cm H₂O for the LMA Classic) and the fact that three patients in the SSLM group required the airway changed to an ETT.

Hanning and colleagues studied the SSLM and LMA Classic using a crossover design in a study of 35 healthy patients during paralyzed ventilation anesthesia.⁷⁴ The oropharyngeal leak pressure was higher with the SSLM than the LMA Classic (21 versus 16 cm H₂O).

In a crossover study, Brimacombe and colleagues compared the SSLM and LMA Unique in 90 healthy, paralyzed, anesthetized patients undergoing routine peripheral surgery.⁷⁵ The LMA Unique was superior to the SSLM in terms of ease of insertion, fiberoptic position, and mucosal trauma but similar in terms of oropharyngeal leak pressure and ease of ventilation.

In another randomized, crossover study, Paech and associates compared the SSLM and the LMA Unique in 168 anesthetized, spontaneously breathing patients.⁵² The investigators reported that although both devices performed equivalently for first-time placement, the SSLM was subjectively rated more difficult to insert and more likely to cause mucosal trauma. However, the fiberoptic view of the larynx was better through the SSLM, and it more frequently provided a ventilation seal at 20 cm H₂O. In contrast to the LMA Unique, its cuff pressure did not increase during nitrous oxide anesthesia. In this study, there was a larger proportion of females, a smaller mask size was used for male and female patients, and the SSLM was inserted with a partially inflated cuff.

Cook and colleagues compared the SSLM with the LMA Unique in a randomized trial enrolling 100 patients.⁷⁶ The study was stopped early because of a high rate of airway trauma in the SSLM group. The investigators reported that the SSLM required more attempts for successful insertion (P = 0.041), more manipulations (P < 0.0001), failed more often (P = 0.013), and caused more complications (P = 0.048) than the LMA Unique. In 14% of SSLM uses, insertion or ventilation failed, and its use had to be abandoned. Leak pressure was higher with the SSLM (26.5 versus 20.5 cm H₂O, P = 0.005). Ventilation and fiberoptic view in those successfully inserted were not different for the two devices. The investigators also reported more complications during maintenance and sore throat postoperatively in recovery and at 24 hours for the SSLM. The SSLM was fully deflated before insertion in this study.

Francksen and coworkers studied 120 patients scheduled for routine minor obstetric surgery who were randomly allocated to the size 4 SSLM, Ambu AuraOnce, or LMA Unique.⁶³ The SSLM was slightly slower to insert than the Ambu AuraOnce and as fast as the LMA Unique. All three had high success rates (all >95%) for first-attempt insertions. Rates of subjectively designated excellent insertions were 75% for the LMA Unique, 70% for the Ambu AuraOnce, and 65% for the SSLM. All devices were judged acceptable.

Van Zundert and colleagues compared the SSLM with the LMA Unique and the CobraPLA in a study of 320 patients breathing spontaneously.⁷⁷ Insertion with the SSLM or LMA Unique was easier than with the CobraPLA (P < 0.02), but success rates and overall time to ventilation were similar. The SSLM and CobraPLA had higher airway seal than the LMA Unique (P < 0.001). Anatomic position was best with the CobraPLA, followed by the SSLM and LMA Unique. Blood staining occurred most frequently with the CobraPLA.

Hein and coworkers performed a crossover, randomized comparison of the SSLM and the SLIPA when inserted by trained medical students in 36 anesthetized patients.⁷⁸ Success rates for first-attempt insertions were similar (67% for the SSLM and 83% for the SLIPA), as were overall success rates (89% for the SSLM and 94% for the SLIPA). Time to ventilation with the SLIPA was faster, and there was an overall preference (67%) among participants for the SLIPA.

Tan and associates performed a randomized comparison of the SSLM, LMA Unique, and LMA Classic inserted by novice medical officers for anesthesia.⁷⁹ The novices had a total of five attempts with each mask. The SSLM was significantly slower to insert than the LMA Classic. The SSLM also had the lowest insertion success rate (80% for the LMA Classic, 77% for the LMA Unique, and 62% for the SSLM), although differences were not statistically significant. Although the SSLM achieved an airway seal of 4 to 5 cm H₂O higher than the other devices, blood on the airway was most common with the SSLM (32%) compared with the LMA Unique (9%) and the LMA Classic (6%), and sore throats were reported most frequently after use of the SSLM (42%) or LMA Classic (41%) compared with the LMA Unique (14%).

Other Studies

Boonmak and colleagues studied the use of the SSLM (sizes 3 and 4) for guiding fiberoptic intubation with a 6.0- or 6.5-mm ETT in 60 patients with normal airways.⁸⁰ The glottis was fully visible in 45% of patients after SSLM placement. Blind tracheal intubation succeeded in 5% of attempts, but with fiberoptic guidance, the success rate rose to 85%.

Danha and associates evaluated fiberoptically guided intubation through the SSLM and LMA Classic in 42 healthy patients with normal airways; a 6.0-mm, nasal, right-angle ETT was used for all evaluations.⁸¹ Total intubation times and overall success rates were not different for the two devices. The success rate for first-attempt intubation through the SSLM was 76%.

Kuvaki and coworkers compared two insertion techniques for the SSLM in 100 patients randomized to different insertion techniques.⁸² The SSLM was inserted in the two groups by a direct or a rotational technique, both without intraoral digital manipulation. The primary outcome measure was successful insertion at first attempt. The success rate for first-attempt insertion was higher with the direct technique (98%) than with the rotational technique (75%, P = 0.002), but insertion time was a few seconds faster with the latter method (P = 0.035). Final mask position over the larynx and airway morbidity were similar in both groups.

Keller and colleagues examined the pressure exerted in cadavers by the SSLM and LMA Unique on the larynx and pharyngeal wall during sequential inflation.⁸³ The SSLM had a lower elastance and lower intracuff pressure for a given inflation volume. Mucosal pressure increased with cuff inflation at most locations, and values were not different for the two devices at any site or cuff volume.

Literature Summary

A significant body of research has evaluated various aspects of the SSLM performance and compared it with other first-generation devices. Several studies comparing the SSLM with the LMA Classic and LMA Unique showed broadly equivalent performance, whereas others indicated the SSLM performed substantially less well. The airway seal of the SSLM is higher than that of the LMA Classic or LMA Unique. Insertion technique may be important. Some studies report the SSLM to be less easily inserted than the LMAs, and some report high rates of blood staining. Inflation with air before insertion appears to improve SSLM insertion and reduce trauma.

D Intubating Laryngeal Airway

The Cookgas (St. Louis, MO) Intubating Laryngeal Airway (ILA) is distributed by Mercury Medical (Clearwater, FL). The ILA is a hypercurved intubating laryngeal airway invented by Daniel Cook. The ILA took more than 8 years to develop. In December 2004, it was introduced into clinical practice for airway management as a SAD or as a conduit for tracheal intubation. A single-use device has been launched.

The ILA is manufactured of medical-grade silicon and is latex free (Fig. 23-5). Ridges are located in the airway tube and the mask. The ridges below the airway connector were designed to improve the tube seal. They also allow easy removal of the connector during intubation through the device. The airway tube of the ILA is curved in a manner designed to mimic the anatomic curve of the upper airway; the design aims to eliminate the need to bend the tube further during use, which can lead to kinking.

Figure 23-5 A to C, The Cookgas Intubating Laryngeal Airway. A, Reusable device with tracheal tube inserted. B and C, single-use and reusable devices showing “keyhole-like” airway orifice.

The ILA tube accommodates large standard ETTs (≤8.5 mm for the larger ILA). The mask portion is large and cuffed, and it has a keyhole-shaped airway outlet designed to direct the ETT toward the laryngeal inlet. Three internal ridges located in the distal portion of the mask are designed to approximate the anatomic shape of the posterior pharynx and to create increased airway stability, smooth insertion, and improved airway alignment. When the ILA cuff is inflated, these ridges move against the posterior larynx and improve the anterior mask seal. The ILA was initially launched with three adult sizes available (2.5, 3.5, and 4.5), and two pediatric sizes were added (1.5 and 2). The increased range offers the possibility of use in small children and infants. The 2.5 is suitable for some children and adolescents 20 to 50 kg (smallest suitable ETT is 5.0 mm), the 3.5 is used in small adults (50 to 70 kg), and the 4.5 is used in large adults (70 to 100 kg). They are available in half-sizes to fit a broader range of patients.

After intubation, the ILA is removed using the Cookgas ILA Removal Stylet (Fig. 23-6), which is specifically designed for this use. The stylet stabilizes the previously inserted ETT and allows controlled removal of the ILA without dislodging the ETT from the trachea. The removal stylet consists of an adapter connected to a rod. The adapter is tapered from bottom to top and has horizontal ridges and vertical grooves. The ridges engage and grip the ETT, and the grooves create an airway passage for spontaneously breathing patients during removal of the ILA. The taper enables the stylet to accommodate standard ETTs in many sizes (5.0 to 8.5 mm). The stylet is manufactured from polypropylene and is reusable up to 10 times after washing in detergent, but cannot be autoclaved.

Figure 23-6 The Cookgas Removal Stylet is used with the Cookgas Intubating Laryngeal Airway.

E Laryngeal Tube

The VBM Laryngeal Tube (VBM Medizintechnik, Sulz, Germany) and King Laryngeal Tube (King Systems, Noblesville, IN) are SAD devices that were introduced to the European market in 1999 and to the United States in February 2003.⁹¹ Between 1999 and 2002, several modifications were made to the original version of the laryngeal tube (LT), including a softer tip, a change from cuff inflation by separate pilot tubes to a single pilot tube, and alterations to the ventilation orifices and proximal cuff. Among the several distinct LTs, some are first-generation and others second-generation SADs.

There are five versions of the LT: the reusable LT, the disposable LT (LT-D), the reusable Laryngeal Tube Suction II (LTS-II), a disposable version of the LTS-II (LTS-D), and the Gastro-Laryngeal Tube (Gastro-LT). The LTS-II, LTS-D, and LT-G are considered later in “Second-Generation Supraglottic Airway Devices.” The LT is designed for use during spontaneous breathing, controlled ventilation, and airway rescue.

The LT consists of an airway tube with two inflatable balloons or cuffs designed to lie above and below the laryngeal inlet (i.e., pharyngeal and esophageal, respectively), and both cuffs are inflated through a single pilot tube and balloon (Fig. 23-7).^91,92 When the device is inserted, it lies along the length of the tongue and extends distally into the proximal esophagus. Proximal and distal cuffs sit in the oropharynx and esophageal inlet, respectively. Inflation creates a seal, and ventilation occurs through orifices between the cuffs. Proximally, a 15-mm standard male adapter enables connection to the anesthetic circuit, and distally, several small airway orifices lie between the balloons (Fig. 23-8). Three black lines proximally indicate approximately correct depth of insertion.

Figure 23-7 The VBM laryngeal tube (reusable versions, adult sizes shown). Also shown is a dedicated color-coded syringe (top left) for use with the LT and an inflation and cuff pressure monitor device (bottom left).

Figure 23-8

Laryngeal tube orifices.

The LT is made of silicon and is designed to be reused up to 50 times. The airway tube is slim, curved, and relatively short, with an average diameter of 11.5 mm and a blind tip. Seven sizes are available for use in neonates up to large adults (Table 23-2).^91,92 LT size selection should be based on the patient’s weight for sizes 0 to 2 and on height for sizes 3 to 5. The disposable version (LT-D) is sold in sizes 2 to 5. Adult and child sizes (but not infant and newborn sizes) are supplied with a reusable, silicone bite block.

After correct device insertion, the proximal cuff lies in the hypopharynx and the distal cuff in the upper esophagus. Both cuffs have a high-volume, low-pressure design to avoid ischemic damage to mucosa and to achieve a good seal. The original version of the LT was designed with a single ventilation opening between the two cuffs in the ventral part of the tube. The current version has two main ventilation orifices.

The proximal cuff lies close to the proximal airway orifice, and there is a V-shaped dent in the pharyngeal cuff. As the cuff is inflated, soft tissue is deflected from the airway opening, helping to maintain a patent airway. The distal end of the airway tube slopes toward the distal ventilation orifice. The tip of the LT is a closed wedge shape, and it is surrounded by the distal cuff. Because there is a single inflation line, both cuffs inflate almost simultaneously.

In addition to the main ventilation orifices, there are two side eyelets, with one on either side of the main ventilation orifices. This design allows improved collateral ventilation if the epiglottis obstructs the main ventilation orifice. The latest version of the LT-D has three eyelets on each side. The efficacy of this design feature has not been formally tested. The LT-D is 1.0 cm longer than the LT to facilitate deeper placement of the device, aid correct positioning of the two main ventilation orifices, and improve the location of the proximal cuff beneath the tongue, so that the epiglottis is more likely to be raised with cuff inflation, in much the same manner as with a Macintosh laryngoscope blade.

F Cobra Perilaryngeal Airway

The Cobra perilaryngeal airway (CobraPLA, Engineered Medical Systems, Indianapolis, IN) (Fig. 23-9) was designed by David Alfery. It was based on a modification of the Guedel oral airway and was marketed in 1997. The initial idea was to modify the Guedel airway to accomplish mask ventilation in the most difficult airways encountered. It has since been adapted to create a SAD. The proximal portion of the airway was modified to enable attachment to an airway circuit, a circumferential cuff was added proximal to the distal breathing hole, and the distal end of the device was modified to form a cobra head shape. Later refinements included a distal flexible tip (tongue) and an internal ramp inside the cobra head to help guide an ETT toward the glottis. A modification of the CobraPLA, the CobraPLUS, incorporates an integrated temperature probe and a distal gas-sampling port.

Figure 23-9 The Cobra perilaryngeal airway (CobraPLA) with cuff inflated (top) and deflated (bottom).

The CobraPLA is made of PVC and polycarbonate. It consists of a breathing tube with a proximal, standard 15-mm adapter; a circumferential, inflatable cuff proximal to the ventilation orifice; and a distal, widened cobra-like head that surrounds the ventilation orifice. The anterior surface of the head consists of a grill of soft bars through which gas exchange takes place. The bars are soft enough to allow instrumentation of the larynx and upper airway if required. When in the proper position, the head lies in front of the laryngeal inlet and is designed to seal the hypopharynx. In this respect, it is different from many other SADs, because the distal tip lies proximal to the esophageal inlet. Internal to the head, there is a ramp to direct the breathing gas (or ETT) into the trachea. The head and anterior grill are designed to deflect the epiglottis off the head, preventing the epiglottis from obstructing the ventilation orifice. The cuff is circumferential and is designed to lie in the hypopharynx at the base of the tongue. When inflated, it raises the base of the tongue, exposing the laryngeal inlet, and it effects an airway seal, allowing positive-pressure ventilation to be carried out.

The device is named Cobra because of the shape of the distal part of the airway; when turned over and looked at on end, it appears similar to the head of a cobra snake. This shape is designed to enable the device to pass more easily along the hard palate during insertion and to hold soft tissues widely away from the laryngeal inlet after the device is in place. Perilaryngeal describes its anatomic location and refers to the fact that the widened, distal Cobra end pushes soft tissues away from the laryngeal inlet.

The CobraPLA is available in eight sizes (1.5 through 6) designed to be used according to the weight and size of the patient. The recommended size is that which comfortably fits through the patient’s mouth. Appropriate sizes are no. 3 for most female patients, no. 4 for most men, and no. 5 for larger men. When unsure about the appropriate size, especially when learning placement, use of the smaller of any two sizes under consideration is advisable. When a practitioner is comfortable with the insertion technique, especially when a muscle relaxant has been administered, use of a larger size may also be successful. The most important consideration is to choose a size that fits through the patient’s mouth without undue difficulty, and the same device size may be appropriate for patients of considerably different weight, depending on their body habitus. The manufacturer suggests that two or more sizes may be acceptable for a particular height. Despite a relatively high weight, some patients may have a small mouth that necessitates a smaller CobraPLA.

Agro and colleagues suggested a modified method of size selection: no. 3 for less than 60 kg, no. 4 for 60 to 80 kg, and no. 5 for more than 80 kg.¹⁰⁰ In this study, relatively large CobraPLAs were used by skilled operators with techniques that included a combination of scissoring the mouth open and performing a jaw lift (i.e., Agro maneuver) in patients who had been given muscle relaxants. If using Agro’s range or a relatively large CobraPLA, the cuff inflation volume can be reduced from the maximum recommended by the manufacturer. Use of the larger sizes achieves a higher degree of airway seal.

The CobraPLUS is a modified version of the CobraPLA which in its adult version has an integrated core temperature–measuring device and in its pediatric version also has a distal gas-sampling post within the device head. This is of particular interest in newborns and infants, in whom very rapid respiratory rates and low tidal volumes result in inaccurate gas-sampling values if using more proximal sampling sites.¹²⁴

1 Application

The CobraPLA insertion technique is simple. After appropriate checks of integrity and patency, the cuff is fully deflated and folded back against the breathing tube. A lubricant is liberally applied to the front and the back of the Cobra head and to the cuff, with care taken to avoid obstructing the anterior grill. With the patient’s head and neck in the sniffing position, the mouth is opened with a scissor maneuver using the nondominant hand and gently pulling the mandible upward. The CobraPLA tip should not be directed against the hard palate, as is often done when inserting an LMA Classic because this may increase the curve that the device tip must take at the back of the mouth and make insertion more difficult. The distal end of the CobraPLA is instead directed straight back between the tongue and hard palate. As the device is advanced into the mouth, an anterior jaw lift assists insertion. Pushing the jaw downward makes insertion more difficult. Modest neck extension (without a jaw lift) may aid passage of the device as it turns toward the glottis at the back of the mouth.

As the CobraPLA is advanced to the back of the mouth, it often turns caudally toward the larynx with minimal resistance as the flexible distal tip (tongue) guides the device downward. Sometimes, a gentle push past posterior resistance is needed. The CobraPLA is correctly seated when modest resistance to further distal passage is encountered as the device tip reaches the glottis. When the position is correct, the flexible tip lies under the arytenoids, the ramp-grill lifts the epiglottis, and the cuff lies in the hypopharynx at the base of the tongue.

After insertion, the cuff should be inflated initially with less than the maximum volume recommended until there is no leak with positive-pressure ventilation (i.e., minimal leakage technique). Providing an adequate depth of anesthesia is achieved, the cardiovascular response to CobraPLA insertion is similar to insertion of other SADs and therefore less than during laryngoscopy and tracheal intubation.

Manual ventilation assists the anesthesiologist in confirming correct placement and may determine the pressure at which an audible leak occurs. Indicators of correct placement are good lung ventilation, a normal capnographic trace, and no gastric insufflation.

Cuff inflation should be enough just to achieve a seal, and overinflation should be avoided. A cuff pressure gauge should be used to monitor intracuff pressure (approximately 60 cm H₂O). If adequate ventilation is not achieved, the CobraPLA may be inserted too far and should be withdrawn 1 to 2 cm. Spontaneous or mechanical ventilation can be used as indicated. Ventilation with an airway pressure above 25 cm H₂O should be avoided, even when testing for ventilation and cuff seal, because gastric insufflation may occur at pressures over this level. Low-pressure ventilation may be achieved by setting a low inspiratory flow rate and then adjusting the tidal volume. Pressure-controlled or pressure-limited modes of ventilation may also be useful to minimize peak airway pressure.

Some practical tips are worth considering. First, the patient should be at an adequate depth of anesthesia before insertion is attempted. Laryngospasm may occur during insertion if the patient is at too light a level of anesthesia. Second, if one CobraPLA is found to be unsuitable, an alternative size should be inserted before abandoning this technique. Third, if the CobraPLA is not inserted far enough, cuff inflation may cause tongue protrusion and a poor airway seal; the device should be advanced further or a smaller CobraPLA chosen. The cuff should not be visible at the base of the tongue when the mouth is opened. Fourth, if the CobraPLA is inserted too far, past the laryngeal inlet, ventilation is impossible. This may occur while learning the technique or if too small a CobraPLA is used. Pulling back the airway 1 to 2 cm usually resolves the situation. The CobraPLA is considered to have a steep learning curve, and the technique can be mastered in 5 to 10 insertions.¹⁰⁰

Removal should be performed as the patient regains consciousness. When the patient is able to respond to simple commands (e.g., “open your mouth”), the CobraPLA cuff may be partially deflated and the device gently withdrawn from the patient. Partial deflation of the cuff enables it to squeeze secretions up and out of the mouth as the CobraPLA is removed.

The CobraPLA may be used as an intubation conduit. Its internal diameter is larger than the corresponding LMA Classic, and the largest CobraPLA has a 12.5-mm ID. This means that intubation through a CobraPLA with a relatively large ETT without an intubation guide or exchange catheter is feasible. Provided the proximal connector is removed, because the CobraPLA is shorter than many other SADS, a standard ETT may be passed through it far enough that the cuff lies below the vocal cords. This potentially makes the procedure technically easier than through an LMA Classic, and it does not require an extra-long ETT.

A standard ETT may be used for intubation through a CobraPLA, ideally guided by a fiberscope. As the fiberscope and ETT exit the ventilation orifice, the soft bars of the grill separate easily without impediment to advancement. The internal ramp directs the fiberoptic bronchoscope anteriorly, and if the larynx is anterior to the ventilation orifice, this approach should enable prompt intubation. The large diameter of the CobraPLA permits the passage of an adequately sized ETT (Table 23-3). The CobraPLA can be left in place with its cuff deflated if desired.

TABLE 23-3 Sizing for Passage of an Endotracheal Tube in a CobraPLA

Cobra Size	ETT Size (mm)
	3.0
1	4.5
	4.5
2	6.5
3	6.5
4	8.0
5	8.0
6	8.0

ETT, Endotracheal tube; PLA, perilaryngeal airway.

An alternative technique involves use of a lightwand to guide intubation, as has been described for other SADs.^99,125,126 The technique is described in the “Laryngeal Tube” section. Blind intubation through the CobraPLA by advancing an ETT through it or by using a bougie guide may be successful, but it has a significant failure rate, and the preceding techniques are far preferable when the equipment is available.

G Tulip Airway Device

The Tulip airway device (Marshall Medical, Bath, United Kingdom) is a first-generation single-use SAD. It was designed by a British anesthetist Amer Shaikh and brought to market in 2010. It was designed for easy placement by anesthesiologists and others who are trained in its use. Its novel feature is that one size is intended to fit all adults.

Like the CobraPLA, the Tulip was named to reflect its overall shape and appearance. It is a simple airway tube with a distal airway orifice and surrounding distal cuff (Fig. 23-10). The device has a proximal, 15-mm standard connector. It is made of PVC and is designed for single use. The distal cuff is described as an inflatable polyhedral beveled cuff, which is designed to inflate below the soft palate, behind the tongue, above the epiglottis, and within the oropharynx.

Figure 23-10 The Tulip airway device.

The Tulip’s design is similar to that of a cuffed oropharyngeal airway (COPA), but it has a larger, more distal, asymmetrical cuff and a shorter, softer, more smoothly curved stem. The stem of the Tulip is curved, and proximally, it has depth markings in three colors to indicate the correct depth of insertion for small (green), medium (orange), and large (red) adults.