Virtual Disaster Medicine

Training Center (VDMTC)

Module 14

Advanced Airway Techniques

Part 2 - New Generation Supraglottic Ventilatory Devices

Classification of Supraglottic Ventilatory Devices

CUFFED PHARYNGEAL SEALERS - COBRA PLA^TM

History

The Cobra PeriLaryngeal Airway^TM (CobraPLA, Engineered Medical System, Indianapolis, IN) (Figure 11) was introduced into the anesthetic community in 1997 when Dr. David Alfery modified a well known instrument helpful in airway management, the Guedel oral airway.  The initial idea was to modify the Guedel airway in order to accomplish mask ventilation in the most difficult airways encountered.  The first changes consisted of lengthening and widening the distal end of the Guedel airway and placing a slot in the widened end to accommodate the epiglottis.  This modification of the airway functioned quite well to hold soft tissues away from the laryngeal inlet, but a decision was soon made to convert it into a supraglottic airway device.

Thus, the proximal portion of the airway was attached to a breathing tube, adding a circumferential cuff proximal to the distal breathing hole, modifying the shape of the distal “cobra head” portion, putting a 15 mm adapter on the proximal end, and adding a “grill” to the distal anterior surface.  Later refinements to the product included a distal flexible “tongue,” an internal ramp inside the Cobra head to help guide an ET into the glottis, and a distal gas-sampling port on the pediatric models.

Device Description

The CobraPLA consists of a breathing tube with a circumferential inflatable cuff proximal to the ventilation outlet portion, a 15 mm standard adapter, and a distal widened Cobra head which holds soft tissues apart and allows ventilation of the trachea.  When in proper position, the Cobra head lies in front of the laryngeal inlet and seals the hypopharynx.  In this respect, it differs from many other SGAs as the distal tip lies proximal to the esophageal inlet.  Internal to the Cobra head, there is a ramp to direct the breathing gas (or an ET) into the trachea.  Over the distal anteriorly placed breathing hole of the Cobra head, there is a soft grill.  This feature helps defect the epiglottis off the Cobra head, preventing the epiglottis from obstructing the breathing hole.  The bars of the grill are flexible enough to allow for passage of an ET when that maneuver is performed.  The cuff is circumferential and is shaped to reside in the hypopharynx at the base of the tongue.  When inflated, it raises the base of the tongue exposing the laryngeal inlet, as well as affecting an airway seal, thus allowing positive pressure ventilation to be carried out.

The device is named “Cobra” due to 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 special shape allows the device to pass more easily along the hard palate during the insertion procedure and holds soft tissues widely away from the laryngeal inlet, once in place.  “Perilaryngeal” refers to the fact that the widened distal Cobra end pushes soft tissues away from the laryngeal inlet and describes its anatomic location.

The CobraPLA is available in 8 sizes according to the weight and size of the patient (Table 11).  The proper size is that which comfortably fits through the patient’s mouth.  Generally, the size indication is a #3 for most female patients, a #4 for most men, and a #5 for larger men.  When unsure of the size appropriate size, especially when learning placement technique, it is advisable to pick the smaller of any two sizes under consideration.  Once a practitioner is comfortable with insertion technique, and especially when a muscle relaxant has been administered (providing maximal jaw relaxation which aids passage of the CobraPLA), it is possible to choose the larger of two sizes under consideration.  Again, the most important consideration is to choose a size which fits through the patient’s mouth without undue difficulty.  There is a range of potential weights of patients for any given CobraPLA size.  This is because of body habitus differences in patients with equivalent sized mouths.  In fact, the size 3 CobraPLA has been successfully used in patients ranging < 40 - 130 kg (personal communication, Dr. David Affrey).  The manufacturer’s suggested range for each size is indicated in Table 10.  It can be noted that for most weights you will consider 2 or more sizes, and that there is no suggested “upper limit” for any given size.  The reason for this is that a patient with a relatively high weight may have a very small mouth in relation to his or her weight.

Table 11 - Range for choosing the Cobra size suggested by Agrò et al.

Table 10 - CobraPLA sizes

Agrò et al.² suggest the following range for choosing the CobraPLA size: size 3 < 60kg, size 4 between 60-80 kg, size 5 >80 kg (Table 11).  In this study, relatively large-sized CobraPLAs were used because the author was skilled in the insertion technique, scissoring the mouth open and performing jaw lift (Agrò maneuver), and patients were given muscle relaxants.  When using Agrò’s range the cuff inflation volume can be reduced from the maximum recommended by the manufacture (Table 12A, 12B).  In this fashion, the cuffs can function similar to the cuffs on ETs, as high volume/low pressure cuffs.  In addition, use of these larger sizes allows considerably higher cuff sealing pressures to be obtained (if desired) over those which result from choosing lower sizes.

Table 12A - CobraPLA cuff inflation volumes suggested by manufacturer.

Table 12B - Cuff inflation volumes suggested by Agrò et al.

A newly patented CobraPLUS will be released in the near future.  It combines the advancement of the CobraPLA “plus” monitoring of core temperature on all adult sizes.  For pediatric sizes, the CobraPLUS combines both core temperature and distal CO₂ monitoring⁷⁷.

Insertion Technique and Device Removal

The CobraPLA is easily inserted in most cases, with the insertion technique very simple to carry out.  First, 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 being made to avoid obstructing the anterior situated grill.  The patient’s head and neck are positioned in the sniffing position and the mouth is opened with a scissor maneuver by one’s non-dominant hand, gently pulling the mandible upwards.  The CobraPLA tip should not be directed against the hard palate, as is often done when inserting a LMA⁴.  This maneuver could make insertion more difficult because of the increasing of the curve that the device tip must take at the back of the mouth.  Rather, the distal end of the CobraPLA is directed straight back between the tongue and hard palate.  When the Cobra head is inserted in the mouth, an anteriorly directed jaw lift maneuver should be effected with the non-dominant hand while inserting the device with the dominant hand.  Conversely, pushing the jaw downwards will make insertion more difficult.  In addition, modest neck extension (without jaw lift maneuver) may aid passage of the device, as it turns towards the glottis at the back of the mouth.

When the CobraPLA is advanced to the back of the mouth, it often turns caudally towards the larynx with minimal resistance, as the flexible distal tip (or “tongue”) guides the device downwards.  Alternatively, in some situations a gentle push past posterior resistance is required to orient the CobraPLA towards moving to its final position.  The CobraPLA is correctly seated when modest resistance to further distal passage is encountered as the device tip reaches the glottis.  When positioning is correct, the flexible tip lies under the arytenoids, the ramp/grill lifts the epiglottis and the cuff lies in the hypopharyx at the base of the tongue.

The smallest-sized CobraPLAs have a unique feature in that the distal gas-sampling port is located in the head of the Cobra, directly adjacent to where exhaled gas leaves the trachea.  This is especially useful in newborns and infants where very rapid respiratory rates and low tidal volumes result in inaccurate gas sampling values when obtained from the Y-circuit connector to a supraglottic device.  Distal gas sampling from the Cobra head removes much of the dead space from the gas sampling in the smallest patients and, resulting in more accurate end-tidal gas sampling levels.

Following proper positioning of the CobraPLA, the cuff can be inflated.  Initial cuff inflation should be done with less than the maximum volume recommended until there is no leak obtained with positive pressure ventilation (minimal leakage technique).  When an adequate depth of anesthesia is achieved, the cardiovascular response to the CobraPLA insertion appears be similar to that following placement of other supraglottic airway devices and less than with laryngoscopy and endotracheal intubation.

Manual ventilation is performed to confirm correct placement and to measure the pressure at which an audible leak occurs.  Indicators of correct placement are auscultation of neck, bilateral lung sounds and chest excursion, absence of gastric insufflation, and positive capnometry.  It is not advisable to ventilate with over 25 cm H₂O of airway pressure, even when testing for ventilation and cuff seal, because gastric insufflation may occur at pressures over this level.

As previously mentioned, it is recommended to inflate the cuff with only enough air to achieve a good seal.  Never overinflate the cuff.  If possible, a cuff pressure gauge should be utilized to monitor intracuff pressure (60 cm H₂0 approximately).  If positive pressure ventilation is carried out, ventilation pressures should be limited to approximately 20-25 cm H₂O.  This is accomplished by setting a low inspiratory flow rate and then adjusting the tidal volume.  If an adequate ventilation is not achieved, it is possible that the CobraPLA is inserted in too far; in that case it must be pull back 1-2 cm.  Halothane, enflurane, isoflurane, sevoflurane, desflurane and total intravenous anesthesia are all acceptable for maintenance of anesthesia with the CobraPLA.  Patients may be allowed to breath spontaneously or be mechanically ventilated accorded to the desires of the anesthesiologist.

With respect to insertion of the CobraPLA, some practical points should be born in mind.  First, an adequate depth of anesthesia should be achieved before insertion is attempted.  Laryngospasm may occur and has been attributed to attempting to insert the device at too light a level of anesthesia.  Second, if the size of the CobraPLA is unfit for the patient, it can be removed and a new size reinserted with minimal trauma to the oropharynx.  Third, if the CobraPLA is not inserted far enough, inflation of the cuff may cause the tongue to protrude from the mouth of the patient and an adequate seal may not be achieved.  In this case it should be advanced further or a smaller sized CobraPLA chosen for use when properly positioned.  The cuff should not be visible at the base of the tongue when the mouth is opened.  Fourth, it is possible to advance the CobraPLA past the laryngeal inlet, in which case ventilation will not be possible.  This is most often encountered on initial insertions (as one is learning insertion technique) and when a smaller-sized CobraPLA is used.  The solution to this problem is merely to pull back the airway 1 – 2 cm and reattempt ventilation.  This step can be repeated as necessary.  Overall, proper insertion technique of the CobraPLA can be easily mastered within 5 – 10 insertions, and often on the first attempt².

When the patient responds to simple commands such as “open your mouth,” the cuff may be partially deflated and the CobraPLA gently withdrawn from the patient.  The partial deflation of the cuff allows it to squeeze secretions up out of the mouth as the CobraPLA is removed.

Indications and Advantages

The precise indications for use of the CobraPLA have not been fully established, but they should be similar to other supraglottic airways.  However, it is fundamental to emphasize that this device can not be relied on to protect the upper airway from aspiration in anesthetized patients.  Thus, elective use should be confined to patients not at risk of regurgitation or vomiting of gastric contents.

The CobraPLA is designed for spontaneous and controlled ventilation.  It can be used as a “rescue airway” in either the CICV or CIDV scenarios.  Because the CobraPLA does not provide airway protection, it is advisable to facilitate endotracheal intubation in order to secure the airway, if the patient is considered a risk for aspiration.

Another important advantage of the CobraPLA that is particularly useful in the management of emergency airway problems is that the insertion technique is very simple, and even when used by personnel with little or no experience in supraglottic devices (as evidenced by physicians’ experience when beginning their residency training), success is often obtained².  Thus it could be useful for those who undertake airway management infrequently.  Currently, a large clinical study is underway evaluating the use of the CobraPLA in the out-of-hospital emergency airway situation when difficulty in establishing an adequate airway is encountered.

There are additional advantages afforded by the Cobra PLUS (Figure 12), which involves monitoring of core body temperature, as well as distal CO₂ monitoring in the pediatric sized devices.

Disadvantages

Given that airway pressures should be limited to no greater than 20 – 25 cm H₂O, the CobraPLA may not be appropriate for patients with reduced lung compliance or increased airway resistance.  The major disadvantage of the device is that it does not protect against aspiration and does not secure the airway as effectively as the tracheal tube.  In addition, the CobraPLA can not prevent nor treat airway obstruction at or beyond the larynx.

Comparison to Face Mask Ventilation & Endotracheal Intubation

In contrast to the face mask, the use of the CobraPLA is associated with less dead space ventilation and no gastric inflation, if reasonable airway pressures are used.  Furthermore, it allows the anesthesiologist to have hands-free for other important tasks (such as administration of drugs).  Compression of eyes and facial and infraorbital nerves is avoided, and operating room pollution from vapors and anesthetic gas is less likely.

Compared to an ET, the CobraPLA is easier to place, avoids laryngoscopy and its associated problems, and is less invasive.  Furthermore, the insertion of this device does not require the use of muscle relaxant drugs.

Guide to Endotracheal Intubation

The Cobra PLA may be used as a intubation conduit.  This may be done by loading a standard ET onto a fiberoptic broncoscope (FOB) and then passing it into the lumen of CobraPLA.  When the device is correctly positioned and the distal breathing hole is opposite the vocal cords, the internal ramp directs the FOB directly towards the larynx.  Once the FOB has passed through the vocal cords, the ET can be advanced into the trachea and the FOB removed.  The large diameter of the CobraPLA permits the passage of an adequately sized ET, as indicated in Table 13.  The CobraPLA can be left in place with its cuff deflated while ventilation is carried out by the ET.

Table 13 -  Passage of ET into the CobraPLA

When a FOB is not available and  the Trachlight^TM (TL) without rigid stylet is available, it is also possible to execute a semi-blind technique for airway rescue ^5,6,7.  Once the CobraPLA has been inserted and an emergency airway successfully achieved, the TL, with a standard ET tube is loaded on to it is inserted into the lumen of the CobraPLA.  When a bright halo of the TL is trans-illuminated in the anterior part of the neck underneath the proximal trachea, it indicates that the breathing hole of the CobraPLA is directly in front of the vocal cords and the TL is correctly positioned in the trachea⁸.  The ET can then be passed over the TL and the TL removed.  Blind intubation through the CobraPLA by simply advancing an ET through it or by using a bougie guide may be successful, but passage into the trachea is not consistently obtained, so the above described techniques are far preferable in the emergency situation.

Unlike other supraglottic devices, it has been made short enough to allow use of a standard ET to be passed through it far enough that the cuff resides below the vocal cords.  Thus when used in this manner it is not necessary to use either an especially long ET (such as a nasal RAE or microlaryngoscopy tube), to remove the CobraPLA or to do additional maneuvers to accomplish this goal.

Medical Literature

There is limited medical literature relating to the use of the CobraPLA because it is such a new medical product.  The first report of use of the CobraPLA in a peer-reviewed journal appeared in 2003 by Agro. et al¹which involed 28 anesthetized and mechanically ventilated patients.  Although the investigating team had only experienced insertion of the CobraPLA in mannequins when the study commenced, they still reported successful insertion in 100% of patients within 10 ± 3 seconds.  Immediate ventilation was achieved in 57% of patients while 43% required a positioning maneuver (e.g. pulling back) to achieve success.  The need for adjusting the position of the CobraPLA was related to an increased body mass index.  Oxygen saturation always remained >98% and there were no complications.  In a later study, Agro et al. studied a series of 110 patients².  Successful insertion was observed in every patient (no failures) in 6.8 ± 2 seconds.  Again, there were no adverse airway events or significant complications.  Agro was able to achieve mean cuff leak pressures of 34 cm H₂O using very low cuff inflation volumes by choosing relatively large sized CobraPLAs.

Akca⁹ compared the LMA-Unique to the CobraPLA in a randomized series of 81 patients.  He reported insertion times, airway adequacy, number of repositioning episodes, and minor complications to be similar in both groups of patients.  However, he found that the cuff sealing pressure of the CobraPLA was significantly greater than the LMA-Unique (23 ± 6 vs. 18 ± 5 cm H₂O).  Akca also found cuff sealing pressures to be lower than those found by Agro which was most likely due to the fact that Acka used smaller sized CobraPLAs.  Finally, Gaitini et al¹⁰ presented a study which compared the LMA-Unique, the CobraPLA, and the Pharyngeal Airway Xpress (PAXpress).  In this study, which comprised 25 patients in each group, Gaitini found that the 3 devices were equivalent in providing safe and effective airways, with the CobraPLA having significantly higher cuff leak pressures (33 ± 6 cm H₂O for the CobraPLA, 24 ± 5 cm H₂0 for the PAXpress and 20 ±5 cm H₂O for the LMA-Unique) and a better fiberoptic score (i.e. better positioning in front of the vocal cords) than with the other two devices.

Szmuk et al¹¹ have recently described the use of the CobraPLA in an elective case with a known difficult airway.  Szmuk reported use in a 2.3 kg 3 week old infant undergoing G-button placement for feeding access.  This infant suffered from Desbuqois Syndrome, a rare condition characterized by multiple congentital anomalies including hypoplastic midfacies, subluxation of C5-6, and thoracic abnormalities.  As this constellation of findings results in an airway that is known to be difficult to intubate, the authors successfully managed the airway using a size ½ CobraPLA.

To date there are 3 reports of an “airway rescue” using the CobraPLA.  In the first, Agro et.al.¹² describe a patient who experienced sudden airway obstruction following extubation after undergoing a total thyroidectomy.  As the initial intubation in this patient had been difficult, the airway was emergently secured using a CobraPLA.  Following this maneuver, an ET was advanced into the trachea using bronchoscopic guidance.  In the second report, Szmuk et.al.¹¹ discuss a patient undergoing a cadaver kidney transplant in whom oro-tracheal intubation proved impossible to achieve and where bag and mask ventilation was carried out only with difficulty.  A CobraPLA was inserted, ventilation easily achieved, and an ET passed into the trachea under fiberoptic guidance.  The operation was then carried out with the CobraPLA being left in place, with the CobraPLA and ET being removed as a unit at the end of the case.

In the third report, Agrò et al¹³ used the CobraPLA  in a 71-yr-old male patient with respiratory failure during the performance of a cricothyrotomy following the Griggs technique.  The FOB, through the CobraPLA permitted the internal view of the tracheostomy site so that the anesthesiologist could observe the needle and guidewire entering the trachea by ensuring the proper placement of the introducer and dilator while having continuous airway control.

From these initial reports, it appears as though the CobraPLA functions quite well as a supraglottic airway, with some unique advantages (i.e. ability to pass a standard ET fully into the trachea) for both elective and emergency situations.  The final place for the CobraPLA in airway management will be determined after additional experience with its use and additional studies are reported.

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