Resident Eagle: Are All Supraglottic Airways the Same?

Resident Eagle is a new column authored by top EMS physicians and medical directors from the U.S. Metropolitan Municipalities EMS Medical Directors Consortium (the "Eagles"), who represent America’s largest and key international cities. Each month they will discuss the latest cutting-edge issues and findings in emergency care. 

The use of supraglottic airways has been a game-changer for EMS. The addition of these devices to our arsenal has given practitioners new options and more tools to manage the airway than ever before. 

Now if we can’t get the time-honored endotracheal tube in place quickly or don’t have the right-level provider on scene, we simply place a supraglottic airway and proceed with our care. In patients who need acute airway management, there’s lots of other things that need our attention, and a tube is a tube, right?

Of course the answer is no, it isn’t. As we more closely examine the role of SGAs and their impact on disease states like cardiac arrest, the more confusing things get. And the more important details become.

Significant Variability

Supraglottic airways have been considered as substitutions for an endotracheal tube with the assumption there is little difference among the SGAs on the market. But a recently completed study suggests there are differences in the performance of different supraglottic airways, and that the differences may have a significant impact in some disease states, such as cardiac arrest. Cardiac arrest care is changing rapidly, and the literature is now replete with adjuncts, enhancements, and techniques that improve blood flow, especially to the brain, and significantly improve neurologically intact survival. 

High-quality CPR underlies many of these enhancements and is clearly necessary for many current adjuncts to effectively work. The addition of the impedance threshold device (ITD) can improve the efficiency of CPR but requires sealing the airway to produce a small vacuum in the thoracic cavity. When an inadequate seal is present, leaks occur, effectively eliminating the vacuum and thereby negating the effect of the ITD. Said another way, if you cannot maintain a good airway seal, the ITD will show no benefit.

To gain more insight into the ability of SGAs to seal the airway, this study compared endotracheal intubation to a variety of SGAs during a variety of types of CPR. The study was performed in an instrumented cadaver model that provided precise measurement of intrathoracic and vascular pressures.

Seven cadavers of mixed gender, size, and anatomic variability were intubated, underwent manual CPR with and without the ITD, automated CPR with the ITD, and active compression-decompression (ACD) CPR while the intrathoracic pressures were measured. Subsequently the cadavers had a random order of five different SGAs placed and similar variations of CPR performed. Again intrathoracic pressures were measured for each SGA.

The ET was then replaced, CPR performed, and the intrathoracic pressure obtained again in order to confirm nothing had changed from the initial reading. The goal of these measurements was to determine if the SGAs were able to seal adequately to reduce intrathoracic pressure equal to an ET.

The results revealed significant variability across the different SGAs. Supraglottic airways with large posterior pharyngeal balloons performed poorly, leaked significantly, and struggled to create a seal adequate to generate negative intrathoracic pressure. SGAs designed to seat around the laryngeal opening performed nearly as well as an ET and created seals that allowed for intrathoracic vacuum as well as the endotracheal tube.

This is a vitally important finding and should prompt EMS providers to carefully consider which SGA they use. SGAs that don’t seal well are not equivalent to endotracheal tubes and essentially negate any benefit CPR adjuncts may provide. In addition, the inadequacy of this seal and its negation of the effect of the ITD may well have influenced many past studies comparing airway management in cardiac arrest.

How we secure airway devices may also have an impact on cerebral flow in patients in low-flow states such as cardiac arrest. The abovementioned cadaver model has shown that endotracheal tube securement devices tied tightly around the neck, especially if they overlay the vascular areas of the anterior neck, can inadvertently provide a tourniquet effect, reducing flow both into and out of the brain. In addition, the use of cervical collars to reduce unintentional head movement during cardiac arrest can have a similar effect if they are placed too tightly.

And while we’re on the subject of airways, let’s not forget breathing. Our ability to secure a patient’s airway can be lifesaving but may inadvertently make it much easier for EMS practitioners to overventilate patients, resulting in elevated intrathoracic pressures, variable end-tidal CO₂ readings, and intracranial autoregulation that can reduce brain blood flow and actually make patients worse.

We must pay close attention to how we ventilate and realize the pressure swings for positive-pressure ventilation should be smooth and gentle. We must use tidal volumes of about 5 cc/kg and understand the current adult bag can easily provide levels twice that. And we must be sure the person operating the bag understands how dramatic a negative effect overventilation has on the physiology of patients in cardiac arrest.

Conclusion

Supraglottic airway choice may have a substantial impact on the effectiveness of CPR enhancements and may even alter patient outcomes. Patients in low-flow states are likely much more impacted by this issue than those with more normal blood pressures. And those undergoing CPR may have cerebral blood flow affected by how and where we secure our airways or try to prevent unintended movement, resulting in airway loss.

Finally, inattention to the subtleties of ventilation can quickly result in overventilation, unintended autoregulation, and potentially a worse outcome. We must use all the tools and experience available to ensure these factors are addressed if we are to maintain real improvement in cardiac arrest outcomes.  

Joe Holley, MD, FACEP, FAEMS, is EMS medical director for the state of Tennessee EMS, Memphis Fire Department, and Shelby County Fire Department.