Non–Laryngeal Mask Airway Supraglottic Airway Devices

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Chapter 23 Non–Laryngeal Mask Airway Supraglottic Airway Devices

I. Introduction

II. Nomenclature

III. Limitations of the Classic Laryngeal Mask Airway

IV. Efficacy, Safety, and Evaluation of Supraglottic Airway Devices

V. First-Generation Supraglottic Airway Devices

VI. Second-Generation Supraglottic Airway Devices

VII. Pediatric Supraglottic Airway Devices

VIII. Conclusions

IX. Clinical Pearls

I Introduction

Nothing is more fundamental to the practice of general anesthesia than the maintenance of a clear upper airway. The choice of device depends on several factors, including access to the airway, duration of surgery, and risk factors for aspiration. After placement, the cuffed ETT provides a secure airway and protects against aspiration, but placement and removal of an ETT require training and judgment. Although ETTs typically are used without incident, complications ranging from trivial to life-threatening can occur.1

Advanced airway management depends on many airway devices, several of which have been included in the American Society of Anesthesiologists (ASA) difficult airway algorithm.2 The Classic laryngeal mask airway (LMA Classic, LMA North America, San Diego, CA) was introduced into clinical practice in 1988. Since then and particularly in the past 10 years, there has been an explosion of supraglottic airway devices (SADs) designed to compete with the LMA Classic, especially single-use devices. The introduction of single-use devices has been driven by concern about the sterility of cleaned, reusable devices (e.g., elimination of proteinaceous material, risk of transmission of prion disease) and the inability to recycle the device enough to be cost-effective. More than 20 manufacturers produce single-use LMs. Other designs of SADs have been introduced, and they are the main focus of this chapter.

II Nomenclature

The term supraglottic airway device (SAD) is used to describe a group of airway devices designed to establish and maintain a clear airway during anesthesia. SADs have several roles, including maintenance of the airway during spontaneously breathing or controlled-ventilation anesthesia, airway rescue after failed intubation or out of the hospital, use during cardiopulmonary resuscitation, and use as a conduit to assist difficult tracheal intubation. Brimacombe recommended that the term extraglottic airway be used, because many of these devices have components that are infraglottic (i.e., hypopharynx and upper esophagus).3 This textbook describes all airway devices that have a ventilation orifice or orifices above the glottis as supraglottic and those that deliver anesthetic gases or oxygen below the vocal cords (e.g., transtracheal jet ventilation, cricothyrotomy) as infraglottic. Other terms and acronyms include supraglottic airway (SGA), extraglottic airway device (EAD), and periglottic airway device (PAD), but SAD is more widely accepted and is used in this chapter.

Brimacombe and Miller suggested there should be a classification system for this increasingly complex family of devices. Miller4 described three main sealing mechanisms: cuffed perilaryngeal sealers, cuffed pharyngeal sealers, and cuffless, anatomically preshaped sealers. Further subdivision can be made by considering whether the device is single use or reusable and whether protection from aspiration of gastric contents is offered. The practical value of this type of classification is uncertain. Chapters 22 and 27 review the LMA, its variants, and the Combitube.

The acronym LMA is a protected term and should be used to refer to any laryngeal mask airway produced by the manufacturers of the LMA Classic (LMA North America and associated international companies). The acronym LM refers to a laryngeal mask manufactured by anyone other than the original manufacturers.

First-generation SADs are devices that can be considered simple airway tubes. They include the LMA Classic, a flexible LMA (LMA Flexible), and all LMs. They also include the Laryngeal Tube and the Cobra perilaryngeal airway (CobraPLA). They may or may not protect against aspiration in the event of regurgitation, but they are not specifically designed to lessen this risk.

Second-generation SADs have been designed with safety in mind, and they incorporate design features that aim to reduce the risk of aspiration.5 They include the ProSeal LMA (PLMA), i-gel, LMA Supreme, Laryngeal Tube Suction II (LTS-II), disposable version of the LTS (LTS-D), the Streamlined Liner of the Pharynx Airway (SLIPA), and the Baska mask. The efficacy of several of these designs has not been proven.

III Limitations of the Classic Laryngeal Mask Airway

Prior to 1988, choices of airway devices essentially were limited to the face mask and endotracheal tube (ETT). The LMA Classic was designed by Archie Brain in the United Kingdom in the early 1980s, and it was introduced into anesthetic practice in 1988. Its introduction revolutionized airway management (Fig. 23-1). It was soon recognized to be a suitable device to use for many cases that previously were managed with a face mask or an ETT, because the LMA Classic had many advantages over both devices.6 It has been used in approximately 200 million episodes of anesthesia globally. More than 2500 studies on the device have been published. The LMA Classic is considered the benchmark against which other SADs are judged. A 2008-2009 UK census found that 56% of all episodes of general anesthesia were delivered with a SAD as the primary airway,7 and 90% of the devices were LMAs and LMs.

When correctly placed, the mask lies with its tip behind the cricoid cartilage at the esophageal inlet, with the airway orifice facing anteriorly and the cuff encircling the laryngeal inlet. The lateral cuff lies against the piriform fossa, and the upper cuff is located at the base of the tongue. The mask is held in a stable position by the hypopharyngeal constrictor muscles laterally and the cricopharyngeus muscle inferiorly. Inflation of the mask cuff produces a low-pressure seal around the larynx.

Limitations of the LMA include problems with controlled ventilation, airway protection, access through the airway for intubation, reusable design, and absence of a bite block. The first two limitations limit the case mix for which the LMA Classic is suitable.

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

IV Efficacy, Safety, and Evaluation of Supraglottic Airway Devices

The market for SADs is extensive, and in the United Kingdom, which represents only a fraction of the global market, estimated sales are in excess of $10,000,000 per annum. The LMA has five variations (i.e., Classic, Flexible, ILMA, ProSeal, and Supreme) or eight if single-use variants are included. At least 10 other SADs of different design to the LMA are on the market. In the past decade, production has ceased for at least six SADs, and it is inevitable that in the next decade more new devices will arrive and some currently in use will depart.

About 30 distinct single-use and reusable LMs are marketed. They are somewhat different from one another in design and in manufacturing materials and processes. If all variants are included, more than 40 SADs are available for nonintubated adult patients.

A small survey of SAD manufacturers in 2003 examined several devices introduced around that time35 and found that the number of patients in whom the device had been used before marketing was less than 150 all in cases but one. In most cases, no trials were published in peer-reviewed journals before launching the product. One device launched in 2001 and remained without published data 18 months later. Only two of seven devices were compared with the LMA Classic in randomized, controlled trials before marketing, and the largest of them enrolled only 60 patients. This situation has not changed, and many later devices have been introduced with little or no trial evidence of their efficacy.

How does an anesthesiologist start using a new airway device? Typically, a company representative supplies a few anesthesiologists with samples of the new device and provides education in its use. The anesthesiologists try the device on a few patients and form an opinion. Hundreds of anesthesiologists may go through this process, exposing perhaps thousands of patients to relatively untested devices before a consensus is reached. The quality of each evaluation varies with the individual’s practice, experience, and diligence. Companies may use informal comments from one user to encourage other users. Individual uptake may be swayed considerably by limited personal experiences, and new devices can be introduced without adequate evaluation of clinical efficacy or safety, or the devices can be rejected without due cause. Some companies restrict the distribution of new devices to a few hospitals or to experts in the field. Others attempt to collect an informal assessment of the device’s performance each time it is used. Some perform extensive laboratory, model, and clinical evaluations before marketing, but these practices are far from universal.

This raises a question about whether it is acceptable to evaluate new devices in such an ad hoc manner. This process should be contrasted with the introduction of a new drug, which must go through laboratory and preclinical studies even before clinical trials are considered. Results of the three phases of clinical trials are reviewed before release of the drug to the market, and postmarketing surveillance is mandatory and extensive.

What regulations govern the introduction of new medical devices, particularly airway devices? In the European Union, the use of medical devices is controlled by three European Directives as part of European law.36 Some directives are specifically applicable to airway devices, and adherence is overseen by a regulatory body in each member country. The statutory body has responsibility for ensuring that medical devices do not threaten patients’ health and safety. Statutory requirements are largely harmonized throughout Europe, and compliance with one country’s requirements allows distribution and marketing of a device throughout the European Union. Although many countries have mechanisms that are designed to critically examine the efficacy of new technologies (e.g., National Institute of Clinical Excellence [NICE] in the United Kingdom), these bodies often have specific conditions (e.g., for NICE, new technology for new procedures) such that new (airway) equipment designed to do an old job tends to fall outside their areas of inspection and regulation.

The statutory requirements of safety and quality do include a statement to the effect that the device should function as intended by the manufacturer. However, in practice, the statutory assessments focus on production quality control and manufacturing standards. A mixture of self-assessment and external assessment is obtained, depending on the risk that the device may pose to patients. These assessments must be passed to allow continued marketing of a device. Airway devices are considered to be of low or intermediate risk and are primarily subject to manufacturers’ self-assessment. Performance of the desired function, efficacy, and cost-effectiveness are not a focus of these assessments. Passing the statutory requirements and obtaining a Conformité Européenne (i.e., European Conformity [CE]) mark allows marketing of the device throughout Europe, and member countries are not allowed to impose other barriers to trade after a CE mark is applied. The CE mark implies that the device is fit for its intended purpose, but assessment of performance, efficacy, and cost-effectiveness are left to the manufacturers, distributors, and end users in the postmarketing phase.

After a device is marketed, clinical trials are not required to demonstrate efficacy or quality of performance. Manufacturers are legally bound to report serious or potentially serious adverse incidents.37 The statutory body requires reporting of incidences in which “malfunction of or deterioration in the characteristics and performance of a device” leads to “actual or potential patient harm.”37 There is also a mechanism for voluntary reporting of incidents by users. Whether these mechanisms lead to reliable reporting of such incidents and whether these schemes identify devices that are poorly designed or underperform is not clear. Formal assessment of performance may come from postmarketing cohort or comparative studies. However, these studies are uncommon, and they usually are published at some interval after a device has been marketed.

Lack of timely reporting creates another problem. In light of clinical experience and customer feedback, new devices are often redesigned after their initial release onto the market. These “improved” devices go through a similar process to that described earlier, and they are again brought to market. Second-, third-, and fourth-iteration devices may then appear, often under the same name as the original. For example, one SAD was modified three times (the four versions were named identically) in the 18 months after it was initially marketed. Publication delay compounds the confusion, because the unsuspecting reader of journals may not realize that a recently published paper relates to a device that has subsequently been modified. Performance of the old version may be very different from that of the current version. In one review of the SAD with four versions, one half of the cited papers related to previous versions of the device. Although this does not make trials of the previous version of each device completely redundant, it does make interpretation of the limited data more difficult. It would arguably be better to determine the desirable features of a new airway device and use these to assess the design and function of it before and after it is marketed.

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.4648 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 image.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

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