Author: Tucker Matthews, MD

Edition 3: We’re back to look at more new airway papers! 

Paper 1: Cm A, Rao SS, K R, R V, Kn V, T S D. The Ratio of Height to Thyromental Distance (RHTMD) and Height to Sternomental Distance (RHSMD) as the Predictive Tests for Difficult Tracheal Intubation. Cureus. 2022 Sep 3;14(9):e28734. doi: 10.7759/cureus.28734. PMID: 36211089; PMCID: PMC9528921. 

Methods/Background: There isn’t a great predictor of anatomic difficult intubation, and so these authors from India wanted to evaluate the potential for using the ratio of height to thyromental distance (RHTMD) or the ratio of height to sternomental distance (RHSMD). They used 400 consecutive patients getting general anesthesia for surgery, and defined a difficult airway as having a Cormack and Lehane grade of 3 or 4. Currently, people typically use thyromental distance, Mallampatti scores, or the Wilson scoring system to evaluate for difficult airways, but these all have high false positive rates. Some have proposed using the RHTMD or RHSMD instead, but there aren’t studies that compare them to each other. 

Notable results: The text is written in a really odd way that isn’t particularly useful, but basically, they found that both ratios are about 100% specific (RHSMD only incorrectly identified 2 of 320 easy C-L grade airways as difficult). Interestingly though, the RHSMD is only 37.5% sensitive (which is pretty terrible obviously), in comparison to the RHTMD, which was at 62.5% (not amazing, but a whole lot better). So RHSMD winds up winning the award for simultaneously being less sensitive and specific, which doesn’t exactly inspire confidence. One other interesting tidbit is that their estimated incidence of difficult glottic visualization was at 3.3%. 

Thoughts: One limitation is that these are using pre-determined cutoffs for both scores, but they aren’t providing the information as to why those specific numerical values were chosen. So the reader is just supposed to assume that the original papers provide strong evidence. But the main limitation is that for some wild reason, they didn’t correlate these scores to any other of the scoring systems. So we don’t have the ability to compare with the tests that are actually used most commonly. That just feels like something that would have been very reasonable to do. Overall, this is sort of interesting, but all of these pre-intubation scoring systems are so fast and easy to do, that I just feel like we should be doing each one really quickly to try and predict. Unfortunately, it seems from my experience that ED providers don’t typically conduct much of a pre-procedural assessment of this, other than maybe a quick three-finger thyromental assessment. Maybe we should start to be more systematic about that. 

Summary: There are many proposed systems to assess for possible anatomically difficult airway. It’s probably best at this point to stick with the quick and easy finger-breadth assessments that we do already. 

Paper 2: Cesarano M, Grieco DL, Michi T, Munshi L, Menga LS, Delle Cese L, Ruggiero E, Rosà T, Natalini D, Sklar MC, Cutuli SL, Bongiovanni F, De Pascale G, Ferreyro BL, Goligher EC, Antonelli M. Helmet noninvasive support for acute hypoxemic respiratory failure: rationale, mechanism of action and bedside application. Ann Intensive Care. 2022 Oct 14;12(1):94. doi: 10.1186/s13613-022-01069-7. PMID: 36241926; PMCID: PMC9568634. 

Methods: This is a review about helmet noninvasive respiratory support, which investigated a total of 100 studies, of which 8 were RCTs. 

What they say: Non-invasive mechanical ventilation obviously has a role in the setting of acute hypoxic respiratory failure, but there is also evidence that in certain patients, it simply delays necessary intubation and likely perpetuates self-inflicted lung injuries. These self-inflicted injuries generally happen because of the crazy inspiratory efforts, which lead to a variety of complications, like diaphragm injury, but also cause issues within the lung itself wherein atalectatic lung regions problematically get aerated first because of the extremely high relative alveolar pressure gradients. The PEEP from NIMV obviously helps with this, but the risks are still present. Nonetheless, avoiding passive ventilation helps avoid sedation complications, immobility, and via controlled spontaneous respiration, there is improved aeration of dependent lung regions. 

While most of the studies investigating non-invasive forms of ventilation have used mask delivery systems, recently some pilot clinical trials have suggested the potential for helmet-based circuits, which may be able to mitigate some of the risks of self-inflicted lung injury, in particular by allowing higher levels of PEEP (10-15 cmH2O). Additionally, the helmet winds up having a slight inspiratory desynchronization (there’s basically a delay because of the increased circuit size) which actually protects against those massive transpulmonary pressure inspiratory changes. Overall, these authors suggest that helmet-based systems are most likely helpful in patients with high inspiratory effort hypoxia, who require significant PEEP and inspiratory pressure gradients, as opposed to those simple hypoxia patients who could just use simple CPAP. 

One obvious factor of the helmet systems is that the patient has a greater chance of rebreathing their expired air that sits in the helmet, and so the manufacturers have had to develop some fancy techniques of recycling this air prior to inspiration. But it seems like they have pretty effectively sorted this out. Additionally, there isn’t any clear way of measuring tidal or minute ventilation due to the helmet itself taking up such a substantial amount of the tidal volume (and because there is so much compliance in the helmet and the patient’s skin. 

Another huge area of benefit is patient comfort. It’s not just that they avoid the tactile feeling of a tight-fitting mask, but they also are able to see much more easily. There’s also the ability to drink through a special straw. And patient’s with unique facial contours, or facial hair, are still easy candidates. 

So what does the data show? Not surprisingly, helmets are better than normal nasal cannula. What is more interesting though, is that helmets seem to perform better than HFNC in certain areas, even though previous evidence has shown to potential of HFNC to outperform facemask NIMV in acute hypoxic respiratory failure. In direct comparisons between the helmet and facemask, there is likely benefit to helmet use as opposed to prolonged mask use, though it’s less clear if there are benefits with shorter duration of therapy. 

Thoughts: If you haven’t seen these, they are awesome looking. It makes the patient look like an astronaut. Or an old-fashioned scuba diver. So just for aesthetics, I am completely in favor. Practically, I’m not sure there is much of a reason to use these in the Emergency Department setting just yet, until there is widespread use within intensive care units. Most of our ED patients are placed onto NIMV for immediate stabilization, and so the mask should be ok, as long as we can effectively stabilize and then get them up to the ICU. But if our ICUs were to start using these instead of a mask (which at this point the data doesn’t support), then I’d be all for changing in the ED as well. But I’m optimistic based on the data on patient tolerability, in addition to the likely physiologic benefits, that these could be the way forward if cost is not a barrier. 

Summary: Keep your eye out for these helmet non-invasive delivery systems. They seem to be pretty effective, well-tolerated, and super cool looking. That being said, this will take some time to get institutions on board. 

Paper 3: Nakagawa K, Sagisaka R, Morioka D, Tanaka S, Takyu H, Tanaka H. The association of delayed advanced airway management and neurological outcome after out-of-hospital cardiac arrest in Japan. Am J Emerg Med. 2022 Oct 13;62:89-95. doi: 10.1016/j.ajem.2022.10.010. Epub ahead of print. PMID: 36279683. 

Methods: There is fairly inconclusive evidence on the benefits of implementing advanced airway management (endotracheal tube or supraglottic airway; AAM) in pre-hospital cardiac arrest. This is a retrospective cohort study from Japan, looking at the association between neurologic outcomes and the time from patient contact to AAM in out of hospital cardiac arrest. 

Notable results: 

-This study attempted to use the fact that different Japanese regions have different ALS protocols, leading to different times of advanced airway management during OHCA cases. 

-After exclusions, 182,913 patients were eligible for analysis. 

-They noted that delay in AAM time after patient contact was associated with poor neurologic outcomes as measured by the Glasgow-Pittsburg cerebral performance category (CPC). 

-They attempted to account for regional differences with a multi-level model 

Thoughts: There’s the obvious massive limitation that this paper is supposing that the differences in CPC scores are primarily explained by this one variable, as opposed to the other effects of having a different ALS protocol. And they don’t really talk about what differentiates different region’s unique protocols. Some of these cases are having AAM after 25 minutes of time on scene, which just sounds crazy to me. What could possibly happen in an OHCA case to delay AAM for so long? Everything I can think of would also probably increase the likelihood of having an eventual poorer neurologic prognosis. Also, it’s possible that the different regions have different qualities of their ICU and ED doctors, which would affect the continued care on these patients, and thus, their eventual CPC scores. So overall, it’s probably some legitimate further evidence in the need for rapid AAM in OHCA (as opposed to BVM), but it’s not the strongest evidence. 

Summary: It’s probably bad to delay advanced airway management once initial patient contact is made in out of hospital cardiac arrest. 

Paper 4: Arthur J, Caro D, Topp S, Chadwick S, Driver B, Henson M, Norse A, Spencer H, Godwin SA, Guirgis F. Clinical predictors of endotracheal intubation in patients presenting to the emergency department with angioedema. Am J Emerg Med. 2022 Oct 19;63:44-49. doi: 10.1016/j.ajem.2022.10.017. Epub ahead of print. PMID: 36327748. 

Methods: There aren’t decision-making tools to help ED physicians with disposition for patients presenting with angioedema. This is a retrospective chart review investigating factors associated with intubation in patients presenting to an emergency department with angioedema. 

Notable results: 

-This is at a single tertiary hospital, and they identified 594 cases that met their criteria. 

-Interestingly, there are less cases overall in the fall and winter (potentially notable also that this would be in a Florida climate). 

-A total of 14.9% of the patients were intubated. 

-The factors which they identified as most predictive of intubation (and so the ones they used in the scoring system which they developed) were a history of hypertension, anterior tongue edema, pharyngeal edema, drooling, and shortness of breath. 

-Isolated lip edema was actually protective against intubation. 

Thoughts: One obvious difficulty here is that this is a pathology where people intubate in anticipation of a potential problem. So we can’t easily use whether or not a patient is intubated as our dependent variable, because that’s already dependent on the confounder of physician subjective reasoning. So if anything, this study seems to indicate the factors that predict whether a physician will decide to intubate, but not whether a patient necessarily should be intubated. In the authors defense, they had an awesome attempt to handle this, by looking at the duration of mechanical ventilation for the patients (with the idea that those who are quickly extubated likely didn’t need to be intubated so much in the first place). I personally don’t actually think this is a correct interpretation though, given that the time until extubation is probably more explained by either the rapidity of their response to a steroid course, or by their comorbidities, two things which I’m not entirely convinced are actually related to the patient needing the intubation in the first place. That being said, the low-risk patients were extubated faster on average. They also tried to make cutoffs for low, intermediate, and high risk, but their low risk patients still were intubated 5.1% of the time, which is pretty darn high if you ask me. 

Summary: Overall I really like this paper. The authors were very aware of the limitations, and did some interesting analysis to try and provide more information. This is a really great little step towards having a decision-making tool for ED patients with angioedema. 

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