Virtual Disaster Medicine

Training Center (VDMTC)

Module 14

Advanced Airway Techniques

Part 2 - New Generation Supraglottic Ventilatory Devices

Classification of Supraglottic Ventilatory Devices

CUFFLESS PRESHAPED SEALERS - STREAMLINED LINER OF THE PHARYNX AIRWAY^TM

History

The SLIPA™ (Hudson Respiratory Care Inc., Temecula, CA) airway is named as such because it is both an acronym for Streamlined Liner of the Pharynx Airway and because looks like a slipper.⁷⁰  The SLIPA was developed by Dr. Donald Miller, who’s motivation for developing the device was his personal experience with 3 inconsequential aspirations in elective fasted children, who regurgitated and aspirated small quantities of bile-stained fluid during the emergent phase of anesthesia (personal communication).  This triggered the idea of an airway that lined the pharynx, to provide an enlarged cavity for trapping regurgitated liquids before pulmonary aspiration occurs.  This idea not only made the need for a cuff inflation mechanism for sealing to achieve positive pressure ventilation less necessary, but even less desirable, as any cuff inflating mechanism occupies space thus decreasing storage capacity.  In addition, a device without a cuff inflating mechanism decreases manufacture’s costs, thus achieving a less expensive single-use airway device.

Although the SLIPA was launched and commercially available in Europe since June 2004, production stopped when there was a change in the manufacturer of the device (Hudson to Teleflex). At that time 510k or FDA approval was not necessary.  The product should be available for sale again early in 2006 and FDA approval is now pending.

Device Description

The SLIPA is a new type of SGA, fabricated from soft plastic with an anatomically preformed shape that lines the pharynx.  Thus, positive pressure ventilation may be achieved without a cuff inflating mechanism that is required in other SGAs, such as the LMA.  It comprises a hollow, blow-molded chamber shaped like a boot with a toe (T), bridge (B) that seals at the base of the tongue and a heel (H) (Figure 13), which anchors the device in a stable position between the esophagus and nasopharynx.⁶⁴

Towards the tow side of the lateral bulges of the bridge are smaller secondary lateral bulges leaving an indentation (I in Figure 13), the positions of which had been extrapolated from a study on cadavers to coincide with the tips of the hyoid bone.  The design aims at relieving pressure at this vulnerable anatomical site⁶⁵ and should thereby prevent nerve damage to the hypoglossal nerve and perhaps also the recurrent laryngeal nerve from pressure effects that may occur with cuff inflating devices.^66,67,68,69

As the device is hollow, a limited volume of pharyngeal secretions or regurgitated liquids can pass through the anterior hole and by trapped within the airway, thereby providing some protection from aspiration.  The chamber provides a large capacity (50 mL in the equivalent SLIPA [size 53] compared to 3.5 mL in the LMA [size 4]) for providing maximum but limited storage of regurgitated liquids should they arise from the stomach, thus preventing their inadvertent overflow into the trachea.⁶⁴  The safety advantages of simplicity and minimization of aspiration risk of a device without a cuff inflating mechanism necessitates double the number of sizes in order to obtain a good quality seal for positive pressure ventilation.  There are currently six adult sizes: 47, 49, 51, 53, 55, and 57.  Sizes 47, 49, and 51 are suitable for small medium and large females and sizes 53, 55, and 57 are suitable for small, medium and large size males.  The numbers refer to the widest transverse diameter (in mm) at the level of the bridge.  Matching this dimension to the distance between left and right cornu of the thyroid cartilage is a useful and possibly even more precise method of choosing the correct size.

Insertion Technique and Device Removal

Insertion technique differs from the LMA since it passes the oropharyngeal curve with greater ease and it is not helpful to try pushing it up against the hard palate.  Insertion simply requires extension of the head and advancement of the device towards the esophagus until the heel of the device spontaneously locates itself in the nasopharynx.  As with most other SGAs, it is helpful if an assistant lifts the jaw forward during insertion.  Alternatively, the anesthesiologist can lift the jaw forward with their thumb and finger.

The crescent shape of the tow lowers the risk of obstructing the airway whether caused by folding the epiglottis down or by invoking laryngospasm.  The toe of the chamber slips easily into the entrance to the esophagus, where it seals against the cricopharyngeus sphincter.  The bridge in the center of the chamber with its two lateral bulges fits into the pyriform fossar at the base of the tongue, which it displaces away from the posterior pharyngeal wall, thus helping to prevent the epiglottis from closing on the glottis.  The anterior opening in the SLIPA also achieves the same by means of the narrowed lower portion that keeps a particularly long epiglottis from closing completely on the glottis.  The heel of the chamber anchors the SLIPA in position by sliding over the soft palate and nasopharyngeal opening.  Once positioned, it provides a reliable airway with no need for further manipulation.

It is usually not necessary for the SLIPA to be tied or strapped into position.  The hollow chamber is able to flatten to facilitate insertion and once in position, it spontaneously reverts to is preinsertion shape.  Additionally, since there is no cuff to deflate, the SLIPA can be removed once there is return of protective airway reflexes.

Indications and Advantages

The SLIPA is a simple, inexpensive disposable device designed to minimize the risk of aspiration during controlled ventilation.  The SLIPA comprises of a hollow blow-molded soft plastic airway shaped to form a seal in the pharynx, rather than an inflatable cuff.  This design feature of being hollow allows liquid entrapment, thus possibly providing protection against aspiration.  Although the SLIPA may provide this protection, its use is still only recommend in similar types of cases in which other SGAs are recommended.

As with the use of other SGAs, use of the SLIPA does not require paralysis or laryngoscopy and its inherent risks and disadvantages, thus there is less cardiovascular stress than tracheal intubation as well as a decreased incidence of sore throats.  Additionally, it is designed without a cuff.  Since no cuff is necessary for the device to seal in the pharynx, the cost of production of the SLIPA, as compared to other SGAs, is decreased.  Also, because of its unique design, once in place, the need to manipulate and mange the airway is unlikely.

Disadvantages

Appropriate sizing of the SLIPA to the patient is necessary since the dimensions of the airway need to match for this device to form a seal, thus many sizes need to be available and, therefore, there is a greater possibility that the wrong size may be chosen.  Indeed, the literature reflects that when difficulty was encountered, it appeared to be associated with incorrect size selection.  Matching of the width of the thyroid cartilage to the SLIPA’s dimension will aid in the appropriate choice of device size.  Several studies have been conducted on the function of this device in laboratory models only.  Randomized clinical trials may be required to establish the role of the SLIPA in relation to other airway devices.

Comparison to Face Mask Ventilation & Endotracheal Intubation

As previously mentioned, the SLIPA should only be used in similar types of cases in which other SGAs are recommended.  Further study is necessary to determine its usefulness in patients with poor lung compliance or increased airway resistance.  Its use should be avoided in patients with lesions of the oropharynx or epiglottis and patients with an obstruction at or beyond the level of the glottis.  As compared to endotracheal intubation, less cardiovascular stress and fewer sore throats may occur with the use of this device.  Additionally, the use of muscle relaxants is not necessary, unless desired for the operative procedure.  Despite the irregular shape of the SLIPA, it does not pose a greater resistance to airflow than other SGAs or an oral ET.

Guide to Endotracheal Intubation

The use of the SLIPA as an intubation conduit has yet to be evaluated.

Medical Literature

Most of the studies on the SLIPA have been led by the inventor with only two independent reports.  The initial pilot study on 22 patients using only one size of the SLIPA demonstrated a remarkable 91% success rate⁷⁰.  In a study involving 120 patients, the SLIPA was compared with the LMA with 3 sizes to choose from.  In all the basic requirements in performance in a SGA namely, ease of use, success rate, sealing for positive pressure ventilation, stress response to placement and post-operative trauma and sore throat, both devices were comparable.  The success rate for both the SLIPA and cLMA was 59 / 60 patients.  The oropharyngeal leak pressure was found to be greater than with the LMA but this finding was not significant.⁶⁴

In this same report,⁶⁴ a laboratory study using a lung model was done, in which aspiration into the lungs could be quantified in relation to regurgitation volumes during positive pressure ventilation.  The SLIPA, cLMA and pLMA were compared.  Both the SLIPA and pLMA compared favorably with the cLMA.  The effectiveness of the pLMA was limited by volume capacity in the laboratory study.⁶⁴  The cLMA proved to be vulnerable to aspiration occurring if the volume exceeded 3.5 mL, whereas the SLIPA volumes when aspiration began were approximately 50 mL. Table 14 shows the relation of SLIPA size to volume where aspiration may occur.

Table 14 - Capacity of the various sizes of SLIPA to retain fluid in the horizontal and "10º head-down" positions (n=6).  Values are mean ± S.D. (used with permission from Dr. Donald Miller)

While the storage volumes of the SLIPA are probably more than adequate to prevent aspiration for >99% of fasted patients, it may not be adequate for non-fasted patients.  Nevertheless, for resuscitation, it would appear to have some major theoretical advantages over the cLMA.  Not only is the SLIPA storage capacity 10 times greater than that of the LMA, suction into the SLIPA chamber is much less likely to pass into the trachea, possibly stimulating laryngospasm.  Also, large particles of food may easily pass into the chamber (they could obstruct the drainage tube of the pLMA).  In the desperate situation where vomiting is occurring, the chamber can be removed, its contents shaken out and the SLIPA could be reinserted again (theoretically, it may even provide a quicker means initially of removing vomitus than with a suction apparatus).

In a study of its application in gynecological laparoscopies involving 150 patients (50 in each group), the SLIPA was compared with the pLMA and the use of tracheal tubes.  The SLIPA was comparable with the pLMA regarding ease of insertion, oropharyngeal leak or pressure, seal quality, systolic pressure response to insertion and OR time saving when compared to endotracheal intubation.  There were fewer sore throats with the pLMA than with use of an ET (P<0.05) and SLIPA airway.  The oropharyngeal leak pressure of 30 cm H₂0 was not significantly different from that of the pLMA of 31 cm H₂0.

An independent study of new users of the SLIPA resulted in a success rate for the first attempted insertion of 17 / 20 for a single user and a 36 / 40 success rate for multiple users with no discernible learning curve.⁷²  In this study, most users found the SLIPA to be easy or very easy to use as an effective airway for spontaneous or assisted ventilation.  A further study comparing the SLIPA with the SSLM showed the former to have a higher first time insertion success rate and was easier to use.⁷³  Fluid dynamic studies of various SGAs revealed that there was less resistance to gas flow in the SLIPA than through other devices.⁷⁴

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