INTRODUCTION

 

The expanding use of supraglottic airways (SGAs) in the airway management of both in-hospital and out-of-hospital cardiac arrest has expanded in recent years, with some prehospital organizations abandoning the use of tracheal intubation completely.1 In addition resuscitation guidelines have been altered to prioritize chest compression over airway management and have strongly advocated the placement of an SGA instead of an endotracheal tube (ETT) in cardiac arrest.2 An article recently published by Segal, et. al.3 in the journal Resuscitation has addressed the potential for SGAs of various designs to impair carotid blood flow in a model of swine cardiac arrest.  The anatomic relationship of carotid artery position with respect to pharyngeal anatomy is thought to be similar in human and swine, thus the Segal, et al. study of the use of SGAs in a model of cardiac resuscitation is important for understanding the potential adverse effects of SGA utilization during airway management in cardiac arrest.   A description of SGA position with respect to the carotid arteries using advanced imaging methods would also provide important information for understanding the potential adverse effects of SGA utilization in advanced cardiac care.

 

We wish to describe the actual anatomic position of an Air-Q SGA during routine MRI examination of a patient's cervical spine and brain.  Specific anatomic relationships are identified; in particular, the relationships between SGA position and tracheal tube position with respect to the vasculature of the head and neck are highlighted.  To our knowledge, this is the first report of the anatomic relations of an SGA to vascular structures in the neck in a live patient. The Aurora Institutional Review Board considered this study exempt from IRB oversight.

 

Case:  A middle-aged male presented for MRI of brain and cervical spine related to neurologic degenerative changes related to cervical radiculopathy.  A previous attempt at MRI scan was unsuccessful due to the patient's history of chronic pain and claustrophobia, thus the patient presented for repeat MRI study with general anesthetic care.   Due to the patient's history of cervical radiculopathy, an airway management strategy was selected that would allow the patient's head and neck to remain completely neutral during the process of the induction of anesthesia, mask ventilation both with facemask and SGA and finally, tracheal intubation.

 

METHODS

 

In addition to the standard informed consent for administration of general anesthesia, consent was obtained from the patient for the use of his MRI scans for quality management and teaching purposes, with all protected patient information removed.  The patient was preoxygenated with a facemask applied with tight fitting mask straps and anesthetized with a combination of propofol and rocuronium.  Following loss of consciousness, an Air-Q 4.5 SGA (Fig 1.) was inserted with the patient's head and neck completely neutral, and the patient was ventilated through the SGA in anticipation of tracheal intubation.  Adequacy of ventilation was assessed via exhaled capnography prior to intubation.

 

Approximately 3 minutes following induction, the patient's trachea was intubated through the Air-Q SGA with a properly bent Levitan Optical Stylet, which served as a fiber optic guide for laryngeal exposure and tube insertion.  The use of the Levitan Optical Stylet allowed tracheal intubation while maintaining the patient's head and neck in a completely neutral position.  Tracheal tube placement was confirmed both by capnography as well as auscultation of the bilateral lung fields.

 

Immediately prior to placement of the patient into the MRI tube, the air was removed from the cuff of both the Air-Q SGA and the tracheal tube, and these cuffs were reinflated to the appropriate volumes with saline into which a small amount of Gadolinium was added (20 ml for the SGA, 7 ml for the tracheal tube).  To create the saline solution into which the contrast media was added, Gadolinium 1 ml (Multihance, 529 mg) was diluted into 60 ml of sterile saline.  MRI images were collected without incident, and the patient was allowed to emerge from anesthesia in the MRI suite without incident at the conclusion of the procedure.   T1 weighted images were acquired (with 1.5mm slices) and viewed with the Osirix image processing application for viewing and analysis.

 

 

RESULTS:

 

The vertical extent of the cuff of the SGA ranged from the inferior endplate of the second cervical vertebra to the level of the C6/7 intervertebral disc. The maximal horizontal width of the LMA was at the level of the C4 and was 6 cm in diameter.

 

The cuff of the tracheal tube was also visible with appropriate placement in the trachea below the level of the vocal cords.

 

Figures 2 and 3 demonstrate images at the level of maximum width of the SGA (C4) showing the position of the cuff relative to the carotid arteries and their bifurcations. There are differences in distance from cuff to artery on each side varying between 0.4 and 1 cm.

 

On review of all the images available there appears to be no focal loss in caliber in the dimensions or in the shape of the common carotid artery and its major terminal branches.

 

The images also reveal asymmetry in placement of the cuff with the right cuff placed just lateral to the laryngeal inlet and medial to the attachment of the pharyngeal constrictors to the greater cornu of the hyoid. The left side of the SGA cuff lies lateral to the greater cornu of the hyoid and the pharyngeal constrictors have been distended laterally.

 

Of note the carotid sheath lies posterolateral to the cuff at all times.

 

There were no images of the neck performed prior to SGA placement and therefore it is difficult to know if there was displacement of the carotids from their normal position. However it is apparent from these images that there is no anatomic distortion to the shape and size of the vessel.

 

The rigid, domed posterior part of the SGA can be seen faintly lying in the midline just anterior to the vertebral bodies on the sagittal and axial images.

 

LIMITATIONS:

 

While we have shown that the Air-Q SGA does not cause significant anatomic distortion of the neck vasculature, it is important to note that this is no guarantee of adequate flow within the carotid arteries.

 

Our patient was in a stable, anaesthetized condition and presumably pressure within the carotids was normal and unlikely to be compressed by the pressure generated by an SGA. This is very different from the work of Segal et al which was a cardiac arrest model where pressure in the carotid was dependant on CPR.

 

We have only discussed one particular brand of SGA and while similar, other devices may result in different placements and anatomical implications.

 

In addition, we have only described positioning in a single patient and while placement in this case was appropriate it is unknown how slight differences in position may affect the anatomical relations in the neck.

 

 

DISCUSSION:

 

We have been able to demonstrate the anatomic positioning of the Air-Q SGA in a stable, anaesthetized human subject. While there appears to be some displacement of the soft tissues of the pharynx, there does not seem to be compression of the vasculature in the neck.

 

This is in contrast to the pig cardiac arrest model of Segal et al which showed complete occlusion of both the internal and external carotids when post-mortem angiograms were performed. This occlusion of the carotid vasculature was present with 3 different types of supraglottic device. They also showed reduce flow within the carotids during CPR.

 

In our case it is clear that the carotids lie posterior and lateral to the cuff of the SGA. Given the positioning of the SGA it seems that the pressure of the cuff is directed anteriorly and laterally and is likely to have little direct pressure on the carotid sheath. This is in keeping with a previous cadaveric study which showed anterior displacement of the laryngeal skeleton with inflation of the cuff of a SGA.4

 

The only fixed part of the neck at the level of the larynx is the cervical vertebral column. The rigid dorsal part of the SGA was seen to be lying anterior to the deep neck flexors and vertebral bodies of the mid cervical spine. As the cuff is inflated the dorsal part will remain in place against the cervical spine and the laryngeal structures will be pushed anteriorly – again sparing the carotids.

 

The report by Segal, et. al, if applicable to humans, has significant implications for resuscitation guidelines. More work is needed to determine the effect of the SGAs in human subjects, both in healthy and critically ill subjects.

 

CONCLUSION:

 

At recommended cuff pressure and positioning, the Air-Q SGA caused significant displacement of the normal position of the pharyngeal constrictors but appeared to have no impact on the caliber and anatomic position of the carotid arteries.

REFERENCES:

1.        Deakin CD, Clarke T, Nolan J, et al. A critical reassessment of ambulance service airway management in prehospital care: Joint Royal Colleges Ambulance Liaison Committee Airway Working Group, June 2008. Emerg Med J. 2010;19;27(3):226–33.

2.        Nolan JP, Soar J, Zideman DA, et al. European Resuscitation Council Guidelines for Resuscitation 2010 Section 1. Executive summary. Resuscitation. 2010;81(10):1219–76.

3.        Segal N, Yannopoulos D, Mahoney BD, et al. Impairment of carotid artery blood flow by supraglottic airway use in a swine model of cardiac arrest. Resuscitation. 2012;83(8):1025-30.

4.        Nandi PR, Nunn JF, Charlesworth CH, Taylor SJ. Radiological study of the Laryngeal Mask. Eur J Anaesthesiol Suppl. 1991;4:33–9.