Should smaller endotracheal tubes be used for elective surgery?

Welcome back to Depth of Anesthesia. This is a podcast that critically explores our clinical practices. I'm David Hao and I'm an anesthesia resident at the Massachusetts General Hospital. This week's episode will be featured on Anesthesiology News, one of the go-to resources for the latest in our specialty. In the past few months, we've built a wonderful team here at Depth of Anesthesia that is committed to tackling dogmas by exploring the evidence.

If you'd like to support our mission, consider contributing to our new Patreon. Your donation will help offset the increasing costs of hosting and producing this material. I'll include a link in our episode description And a big shout out to Dr. Joyce who tweeted at Depth of Anesthesia to share a recent narrative review on tracheal tube size in adults undergoing elective surgery. Dr. Joyce commented that this was another example of quote, that's the way I've always seen it done reasoning. And I thought that was really spot on.

It's now my pleasure to welcome to the show the co-authors of this review that was recently published in Anesthesia. Dr. Carmale is a recent fellow in the Department of Anesthesiology at Vancouver General Hospital, who will be returning as a staff anesthesiologist in the near future. Dr. Carmale, welcome to the show. It's great to be here, David. Thanks for having us.

Thank you for being here. My second guest is Dr. Peter Rose, who's a staff anesthesiologist in the Department of Anesthesiology at Vancouver General Hospital. Dr. Rose, welcome to the show. Thanks a lot for having us here, David, and thank you to Sham too for initiating this review on the topic. So our case today is that of a 35-year-old woman who's presenting for a laparoscopic appendectomy.

The anesthesia resident who's setting up for the case picks out a Miller tube blade and a seven millimeter ET tube. The attending observes this and asks the resident how the ET tube size was selected. The resident responds, Don't we always use a seven oh tube for women and a seven five tube for men? When asked whether a smaller E T tube would be appropriate, the resident questions whether a smaller tube would be as effective for ventilation and as safe in protecting against aspiration. A claim is a practice decision that we either believe is true or is something we default to.

What are some of the claims in this case? So I think this is a great question from the resident. Where does the equipoise lie here? The utility of a tracheal tube as a conduit to the airway has to be balanced against the risks. In general, the internal diameter is how good an airway conduit the tube is, whilst the outer diameter is how big it actually is.

And it is that factor we should consider with regards to trauma and damage. With that being said, the main claims or dogmas that are voiced when considering smaller tracheal tubes are that they are less likely to maintain a secure patient airway. less reliable in facilitating positive pressure ventilation? And are they less reliable in terms of sealing the trachea and protecting the lungs from aspiration? So here now are two questions for our listeners to consider. What is the level of your agreement with the claims that Dr. Carmali has presented?

And what is the level of evidence for what you believe? I think it's fair to say that selecting an appropriately sized tracheal tube requires a general understanding of tracheal dimensions. What's the inherent challenge with this? I mean pinning down some concrete numbers on this is difficult because it really depends on who you are measuring, where you are measuring, and how you are measuring. Let's take an example.

There's significant variation, even depending on which ethnic population is being studied. Even within a discrete population of the same ethnicity, We might find quite a bit of inconsistency regardless of which methodology is used. Taking it a step further, where we measure and how we do so also matters. For example, are you measuring at the cricoid? Are you measuring at the grotic or the subgrottic portion?

And then using post-mortem, endoscopic, or cross-sectional imaging will all influence the measurements. Keep in mind, living tissue has elastic properties, so we may also expect some error with cadaveric studies when preserved specimens are used Nevertheless, in general, the average diameter in the coronal plane at the level of the cricoid cartilage is about 15 mm in men and 11.6 mm in women But when we look further to the distance between the cricoritinoid articular facets, the distance could be anywhere from 8 to 18 millimeters in men and 7.4 to 13 millimeters in women. So knowing that the outer diameter of an ET tube is usually at least one to one and a half centimet uh millimeters rather greater than the internal diameter.

That means that a standard 7.5 millimeter or 7 millimeter tube for women has the potential to exceed the lower range we see in terms of the distance between the cricoarritoid articular facets. Is that correct? This is an important consideration because the site is commonly identified as an area prone to injury. The tracheal tube rests, so to speak, on the retinoid cartilage mounted on these facets. So it sounds like there's a ton of factors that could impact our ability to get accurate and concrete numbers on trivial dimensions.

Is there anything that we know of like height or weight that might correlate? That's right. We haven't even touched on how the slice or cut plane of cross-sectional imaging can impact measurements, as well as technical factors and overall quality of the image. But even if there is not perfect agreement, there are definitely trends For example, recent studies using multiplanar CT imaging demonstrate that height is positively correlated with tracheal size. Patient sex has an influence, but body weight has not consistently been found to sh shown to have good correlation It is unclear if body mass index has any effect, but if it does, it seems to demonstrate an inverse correlation with tracheal size.

And of note, interestingly is an independent patient risk factor for subglutic stenosis following tracheostomy in critical care patients. Do we have a sense of what the range of an average adult tracheal diameter is? So the often quoted normative range is thirteen to twenty seven millimeters in men and about ten to twenty one millimeters in women. So looking at a ballpark of ten to thirty millimeters in men and ten to twenty millimeters in women. But this means that a typical eight oh Tracheal tube may exceed the dimensions of the subglottic portion or um region in patients, often females, at the lower end of the range, as the outer diameter would exceed ten millimeters.

And as was mentioned earlier, a tracheal tube is both an internal diameter and an external diameter. So when we say an 8-0 -sized ET tube, that number refers only to the internal diameter. But sizing is a bit more complicated than just using the internal diameter, right? Absolutely. The internal diameter is all about how good the tube is as an airway conduit, but the outer diameter is more relevant to trauma The ability of a tube to resist aspiration and air leak with positive pressure ventilation is a bit more complicated and something we'll discuss the evidence for later.

A further factor to consider is that the physical characteristics of the tube change with size, manufacture, and the type of tube used For example, a reinforced tube and something like an evac tube or suction above the cuff tracheal tubes have a greater outer diameter for a given internal diameter. The internal diameter of a tube is also related to its length with many manufacturers. A good example are preformed tubes such as an oral or nasotracheal ray tube. Here you may want a smaller tube, like a 6mm tube, to pass through the nose, but you may be hampered because of how short it is. Conversely, looking at an 8mm tube, you may find this too long for the same size.

Additionally, an overlooked aspect of tube sizing is the cuff and potential cuff-induced airway injury. Larger tubes have a proportionally longer cuff Therefore, at a given depth, this cuff will lie more proximally in the trachea, closer to the vocal cords, and be associated with a longer segment of contact with the tracheal mucosa. In the review, you guys also discussed a couple of core functions that an endotracheal tube must fulfill. The claims we identified from the case tended to focus on the ability of smaller tubes to fulfill these functions What exactly are these core functions that you guys describe? Well in the review we go through the main reasons you might want to insert an adcagracheal tube.

These include maintaining a secure patent airway. Permitting access to the airway , for example, for intubation aids like bougies or stalets , for suction catheters, if you want to do fibroxtobronchoscopy, and so on. And then we need to facilitate positive pressure ventilation. And lastly, we need to be able to seal the trachea and protect the lungs from aspiration. Let's start then with the first function you've mentioned of maintaining a secure and patent airway.

The claim here is that small tracheal tubes are less likely to maintain patency due to either intraluminal or external compression. What evidence do we have that explores this claim? It goes without saying it takes less to block a smaller hole. So intraluminal obstruction of the tracheal tube from common causes such as biofilm and patient secretions may be problematic. This can result in increased tracheal tube resistance and airway obstruction, and is more common with heat and moisture exchanger devices we use in anesthesia than when compared with other forms of humidification Whilst it certainly is more of an issue in prolonged intubation, it has been demonstrated to occur within hours.

A study conducted by Wilson and colleagues, published in Chest in 2009. tested new endotracheal tubes with internal diameters of seven through to eight point five as controls against endotracheal tubes collected randomly from patients in a mixed medical surgical ICU after extirpation. The authors aimed to establish the resistance of each size group as measured by pressure drops. Within hours of intubation, the resulting pressure drop was consistent with an effective reduction in the tube internal diameter of 1 mm in 50% of cases. So let's simplify it and put it another way. The pressure drop of 50% of excavated tubes was equivalent to the next smaller size of control endotracheal tubes.

The study also interestingly demonstrated that it is not easy to risk predict which patients and endotracheal tubes might be most affected The degree of change was highly variable, occurred in all sizes of tubes, and was unrelated to the duration of intubation So this is something we could quite easily see in a long surgical case, for example. And just to clarify, the concept of pressure drop is a physics measurement. that was used by the authors to quantify the change in pressure across the system, essentially reflecting the degree of resistance to flow within the endotracheal tubes. That's right. It kind of goes back to Ohm's law in that sense, V equals IR. And in up to 80% of exturbative tubes, the measure, the measured pressure drop, exceeded three standard deviations of that in size matched controls.

But on a separate note, we also want to be able to insert things through our tracheal tube, as I said before, including intubation aids like stalets or bougies, such as and bronchoscopes. This is all part and passive anesthesia. So a reliable tube has to be able to perform all of these functions Got it. So to summarize, we have evidence of organized secretions increasing resistance in a somewhat unpredictable manner, even within hours of intubation. And this all seems to suggest that once a tube is placed in a patient, really regardless of starting internal diameter, it seems that the true internal diameter changes and it's tough to predict the function accurately afterwards based on the study.

And we also need to consider the two for other functions as a conduit, for example, for things like suctioning and bronchoscopy. What about external forces causing collapse or kinking of the tracheal tube? So this is not commonly reported in the literature, but there are a number of studies suggesting that smaller tubes are more prone to kinking, and it's something that probably stands to reason Here's a laboratory study published by Fudegawa in twenty seventeen. It looked at the role of tube size and internasal compression and respirator respiratory pressure loss during nasotracheal intubation. What they found was that the diameter of the tube was the largest factor in nasotracheal tube pressure loss.

A 6O tube had more than three times the pressure loss of an eight oh tube. The authors attributed this to multiple factors, including partial compression of the nasotracheal tube being more significant in smaller tubes. So overall with respect to the first claim that smaller tubes are less able to maintain a paint airway We do have some evidence that suggests increased intraluminal resistance is seen across all tube sizes in a fairly variable and unpredictable manner. And we also see evidence suggesting that smaller tubes may be more prone to pressure loss, at least with nasotracheal intubation from external compression. Our second claim is that smaller tracheal tubes are less able to facilitate positive pressure ventilation.

What evidence do we have related to this claim? I think this is where people really start to get worried. When I first started anesthesia, I was asked to breathe through a 6-0 tube as an example of how difficult it is. and what we are actually asking our patients to do. I think it is fair to say that breathing through a straw is more difficult than when breathing through our open mouths And this is supported by the Posseur equation. Smaller endotracheal tubes have a smaller intraluminal radius and a greater resistance to airflow Looking at mechanical models, you can predict that the work of breathing increases by two -thirds to almost 100% for every one millimeter reduction in internal diameter for seven, eight, and nine millimeter tubes This seems valid in the context of spontaneous ventilation, but controlled or assisted ventilation modes are available now on most modern anesthetic machines.

What happens when the ventilator is introduced to compensate for the work of breathing? I'm glad you asked this. Applying positive pressure support or ventilation removes or overcomes the additional work of breathing due to the endotracheal tube. It overcomes resistance and splints the airway open to optimize pulmonary mechanics. The smaller the tube, the more support is needed, however.

For example, you'd need an added pressure support of about 8 centimeters of water for a 7 tube compared to only four centimeters water for a nine tube. So for spontaneous negative pressure breathing, absolutely, a smaller tube will make that more difficult. Even if we look at the ICU population, in which spontaneous breeding trials are conducted to assess readiness for extirbation. Oftentimes, a small degree of pressure support or CPAP is administered to overcome the resistance of the introphrachial tube So with positive pressure and assistance support modes, we can easily overcome this. And there is very little time or indication that we anyway have our patients breathing spontaneously in the periopta period Right.

So in general, in spite of the additional work of breathing, you might expect spontaneous ventilation is generally limited to pretty short periods before extubation. And so those concerns might be less relevant. Now, there's also the concern of possible barrow trauma with smaller tubes. Positive pressure ventilation can cause higher peak ventilator pressures, but does this translate to increased intratracheal or alveolar pressures? The peak pressure may absolutely be higher, but we don't have evidence that suggests that this equates to barotrauma induced lung injury.

Lung injury can span the entire gamut from volutrauma, adelectrauma, biotrauma, and barotrauma. For barotrauma, we are really interested in the plateau and intratracheal pressures specifically. In relation to endotracheal tube size, this was studied by Stenquist and colleagues back in the 70s In ten patients with no known airway disease undergoing minor ENT procedures. Each patient was sequentially intubated with 5, 6, 7, and 8 millimeter low-pressure cuffed endotracheal tubes As expected, peak pressures increased with decreasing endotracheal tube size. While keeping a constant minute ventilation of thirteen liters a minute, a maximum of over sixty centimeters of water was seen for a five millimeter endotracheal tube.

End expiratory tracheal pressure also increased with decreasing endotracheal tube size. End expiratory tracheal pressure was zero with all volumes using the 8mm tube and was greatest at six centimeters of water for the five millimeter tube at 12 and 13 liters per minute of ventilation volumes For six millimeter tubes, maximum endexpiratory tracheal pressure was only three centimeters of water. In general, maximum intotracheal pressures and pause ventilator pressures were comparable between endotracheal tube sizes. So taking all this, it seems to suggest that a high minute ventilation between two to three times normal, it is still possible that intratracheal pressures may increase, presumably due to airflow limitation and gas trapping But this is really minimal until we start using tubes that are smaller than a 6-0.

It sounds like this was a fairly robust characterization that seems to demonstrate that tracheal pressures are only marginally greater in smaller tubes. than those obtained with larger tubes and really is only seen more significantly when tubes are less than six millimeters in size. I do want to point out that this experiment was performed in the airways of healthy patients. You're right, David. That's a key observation. Studies that measure these pressures in human subjects tend to exclude patients with airway disease, particularly those with existing expiratory airflow limitation.

Another concern that you guys have raised is the possibility of this increased resistance to expiratory airflow. Could this compromise ventilation and cause dynamic hyperinflation and intrinsic PEEP? That's a great question. Mechanical models using a short expiratory time of two seconds demonstrated an intrinsic peep of only two centimeters of water. in the context of hyperventilation through a six millimeter tube. But models have fixed compliance and they don't accurately reflect the real world we live in where conditions are changing and there's dynamic physiology So a rather reassuring prospective observational study was conducted by Cohen colleagues and published in Anesthesia.

They looked at 30 patients. Intubated with a size 6.0 or 6.5 tube, undergoing elective neuro and spinal surgery. So this would include prone patients who may be more susceptible to secretions accumulating in the tube. They found that patients' lungs can be ventilated to normal capnia with rates of 7 to 8 liters per minute of ventilation without any increase in distal tracheal inflation pressures or gas trapping. It is important to recognize that this is a small study and it did include information about comorbidities, but it didn't actually include patients with aircraft. instruction, those patients were actively excluded.

So we can't really take these findings and apply it to that patient cohort. Got it. So to summarize the discussion here, it seems like both mechanical models and studies in healthy patients show that Despite increased peak ventilator pressures, this does not appear to translate to higher tracheal pressures, except when tubes are less than 6 mm in size It also seems that clinically significant expiratory flow limitations causing gas trapping is not seen, at least in this prospective study, with 6.0 and 6.5 millimeter tubes. That's absolutely right. We don't have the evidence showing that peak pressures translate to elevated intratracheal pressures unless airflow limitation is an issue.

This wouldn't be expected in healthy patients or or unless minute volumes are excessively high, as we described previously. We do have to use our clinical judgment, however, with regards to margins of safety and where we feel the benefit risk balance lies. We may have to approach the healthy appendectomy from our case quite differently than a patient, say, with a high BMI who is undergoing a prolonged laparoscopic procedure in and say steep Trun Chandellenberg position. That's a very fair point. Now moving on to our third claim, the claim being that smaller tracheal tubes are less able to seal the trachea and protect from aspiration.

Smaller tracheal tubes occupy less of the cross-sectional area in the trachea, so there's a concern that these tubes will be less able to seal the trachea, or that an extensive cuff pressure may be necessary to do so What evidence do we have exploring this claim? Now this is where cuff diameter is key. is a bit historical. It comes back from the fact that the first cuffed tracheal tubes were manufactured with low-volume high-pressure cuffs. So you wanted a larger tube to include most of the lumen so you could limit how much cuff inflation and therefore pressure was needed exerted on the tracheal walls. Modern tracheal tubes, however, are all high-volume, low-pressure cuffs to conform to the trachea, and they exert less pressure on the tracheal mucosa.

A lot of the data-supported concerns about the ability to seal the trachea with smaller tubes is based on studies in pediatric populations. This data is tough to reliably extrapolate because when you test these pediatric tubes, they have lower compliance and actually behave more like the old low-volume, high-pressure cut tubes. When we look at some animal models, an X vivo canine model from 1975 nicely demonstrated that if a tube with a high-volume, low-pressure cuff is used to approximate the size of the trachea For example, a 9-0 tube, the cuff is unable to be fully inflated. And you end up with these ridges in the cuff, which don't actually exert complete uniform pressure on the trachea At a constant pressure of 20 centimetres of water, therefore, when they checked with heparonized blood, this leaked through invaginations of a cuff. at about five mils per minute with a size 90 tube, but was negligible with a six oh tube because these ridges were not demonstrated.

So it is these the presence of these longitudinal ridges with the larger tubes that are potentially associated with risks of microaspiration of above the cuff debris or even an air cuff leak with higher ventilator pressures. So the canine model actually seems to suggest that when the tube is larger, it was less effective in creating an adequate seal due to the fact that ridges were now forming in the cuff. something that was not seen with smaller tubes. Do we have any studies that explore this in in real patients? Appropriate study done in 2011 by Huang and colleagues looking at this very issue. The study observed thirty patients scheduled for elective surgery under general anesthesia.

After induction of anesthesia, the trachea was intubated with two sizes of tracheal tubes in a random sequence. For males, these were tubes with internal diameters of 7.5 and 8 millimeters In females the tube's internal diameters were seven and seven point five millimeters. After intubation, with each tube, air leak pressures were evaluated at intercuff pressures of twenty, twenty five, and thirty centimeters of water. What they found was that uh when smaller tubes were used, there was actually no need for higher cuff pressures to achieve a tracheal seal. And in fact, use of a smaller tube. allowed for greater interthoracic pressures to be achieved before an air leak past the cuff was even detected.

That's super interesting because one might think that a larger tube would be more effective in terms of sealing the trachea. Could the findings in this study be explained by vaginations in the cuff? That's right. The authors also performed a bench top study and found that more longitudinal folds developed as the tube size became larger. It was thought that when the cuffs are inflated within the trachea, fewer invaginations develop in smaller cuffs, and that might reduce air leak.

And this comes down to the residual volume of the cuff. Back when tubes had high pressure, low volume cuffs, the tube had to approximate the diameter of the trachea. But nowadays, with low pressure, high volume cuffs If we use the tube to approximate the trachea, the cuff contacts the tracheal wall and exerts pressure before it is fully inflated. So we end up reaching a cuff pressure of 20 to 30 centimeters of water, which is usually our ballpark. But there's still a significant residual volume in the cuff that because it's not fully inflated, and therefore we end up getting these ridges where air can leak and debris above the cuff can leak.

Alright, so for the listeners out there interested in a relatively simple teaching model, a way to demonstrate this is to use a 20 milliliter syringe with the plunger removed to represent the trachea. Then you can insert endotracheal tubes of different sizes and inflate the cuff to show that larger ridges that form or invaginations that develop in the cuff when you're using a larger tube Of course, this is not a true representation of tracheal anatomy, but the definitely the dripping saline above the cuff and then seeing it trickle through below the cuff is a vivid demonstration of the mechanism that Cham was described. Great. So if I'm understanding these results correctly, these studies seem to suggest that as a whole, smaller tubes do not necessarily need increased cuff pressures to achieve tracheal seal with smaller tubes.

And perhaps even more interestingly, upsizing the tube might actually be less effective for preventing leak across the cuff due to these cuff folds or invaginations that form. Definitely. In terms of sealing the trachea, the evidence really suggests from models and small studies that smaller tubes may actually demonstrate greater efficacy But as you stated before, this has to be balanced with the narrower lumen and how large a conduit you need your airway to be. There's always going to be a risk versus benefit trade-off. I think this is super fascinating, and the syringe technique is definitely something I'm going to be trying out for myself.

I'd like to now introduce Ashwini Joshi. She's a fourth-year medical student, part of the Depth of Anesthesia team Uh and she's currently at Harvard Medical School applying into anesthesiology this cycle. She helps significantly with putting this show together, and she's gonna lead the last bit of our discussion here. Thank you so much for that introduction, David, and thanks for having me on the show. We've talked quite a bit about the safety concerns with smaller endotracheal tubes today.

And so I'd like to shift our focus briefly now to larger tubes. What are the concerns with using a larger endotracheal tube? Well, the main concern is airway injury, both due to direct trauma Or abrasion, as well as pressure injury to the mucosa resulting in ischemia. Fortunately, overall serious injury is rare. In the ASA closed claims analysis 87 % of airway related claims were classified as temporary or non-disabling.

Um and do we have any evidence that larger endotracheal tubes are in fact associated with a greater degree of airway injury? Speaking experimentally, yes. This work is largely demonstrated in animal models, but what we see repeatedly in these models is that larger tubes Are more likely to impinge on laryngeal structures, resulting in damage to the vocal cord apparatus and subglottic mucosa, resulting in lacerations, erosions, or even tracheal rupture. These often manifest as two common symptoms of intubation-related injury, either sore throat or hoarseness, or a combination of the two. It is a bit harder to study injuries in humans because serious airway injuries like eretnoid subluxation or subglottic stenosis are thankfully relatively uncommon.

And often only manifest really in patients months to years after surgery, often with symptoms of breathlessness or hoarseness And so considering these two common symptoms of intubation-related injury, what evidence do we have that smaller tube sizes reduce the incidence of sore throat specifically? Sore throat is really common, often seen in up to half of our female patients, and is increasingly becoming an indicator of the quality of anesthesia as part of studies collecting patient-reported outcomes In a well-conducted, blinded and randomized study of 100 healthy patients undergoing plastic surgery or ENT surgery, the occurrence of post-operative sore throat was almost halved. in women intubated with a six oh tube, about twenty-seven percent, compared with a seven oh tube, about fifty per cent, up to two hours postoperatively.

This finding has been supported by a recent meta-analysis and one large observational perspective trial that found a pool risk reduction of 0.56 when smaller tubes were used. That gives a number needed to treat of about seven, which is actually a great number. Yeah, so it seems to me that the evidence suggests that smaller tube sizes reduce the incidence of sore throat in the studied populations. Hoarseness, on the other hand, might suggest a level of injury at the larynx. What evidence do we have that smaller tube sizes reduce the incidence of hoarseness?

Yeah, so the the evidence here is less robust, but still suggestive. In a recent meta -analysis, uh this found that smaller tubes were associated with a pooled risk reduction at 24 hours of 0.63 But I must stress that this was largely based on one randomized controlled trial in patients undergoing thyroid surgery In this particular RCT, uh four groups were studied, and they were endotracheal tube size uh six oh. And then one with 7-0 and then two other groups, both with uh pre-intubation boluses of intravenous lidocaine at 1.5 milligrams per kilogram, again with a six-o tube and a seven oh tube uh the incidence of hoarseness was significantly different between the four groups, uh, at six hours after intubation.

At this time, uh the incidence of hoarseness was significantly higher in the group that had this the seven O endotracheal tube with the placebo saline injection rather than the lidocaine. And the rate was 28 % compared with all other groups being no more than 13%. So quite a significant reduction here. Yeah, and and one other interesting thing that I've noticed is that a lot of the clinical investigations we've discussed so far have primarily included healthy and oftentimes female patients. And so how do we put this in context for patients with comorbidities or who are intubated in the ICU?

That's a good point. Throughout this episode. We've seen that the evidence around the claims regarding risks and benefits of using smaller tubes applies predominantly to healthier patients undergoing elective surgery This might not have a place in those high-risk patients, all those patients who will end up going to YCU, because the priorities of your tracheal tube may be different in those contexts. To take an example, in the ICU, a larger diameter airway tube may be necessary to mitigate against the risk of obstruction from biofilms, secretions, mucus plugs In high-risk patients who are already physiologically compromised. The safety margin there is much more delicate There may also be a greater need to insert things through the tube, such as large bore suction catheters and you know larger diameter bronchoscopes for therapeutic purposes In this context, sore throat is not a pressing concern and a safe airway must come first.

Of course, the corollary to all of that is that these are actually the very patients who are at greater risk of developing the serious and chronic airway complications down the line. As you mentioned, patients with airway disease have not been robustly studied for airflow limitation using smaller tubes. It is difficult to extrapolate the data we have just discussed to this population, and so I would not do that there is a much more careful balance in these patients of the functions of our endotracheal tubes, i. e. maintaining a secure airway, permitting positive pressure ventilation, preventing aspiration, and providing access to the airway. balanced against the prevention of airway injury. Ashwini, thanks so much for leading that portion of the discussion.

And Dr. Kalmali, that makes a lot of sense and I think it really speaks to needing to understand your patient population and the patient population that the studies were actually conducted in. Dr. Carmali and Rose, would you be able to summarize some of the evidence we've discussed today? Yes, uh absolutely. So overall the evidence seems to suggest that smaller tracheal tubes may be more prone to external compression. Increased intraluminal resistance may also be seen, but the degree of change seems highly variable. and causes increased resistance across all sizes in an unpredictable manner.

In general, most devices, such as slimline bronchoscopes and intubation aids, will safely go through a minimum internal diameter of six millimeters. It also appears safe to use these smaller tubes for positive pressure ventilation without increasing factors associated with lung injury Also, minute ventilations as high as two to three times normal may be achieved with minimal, if any, gas trapping. Once we get below a size Six millimeter two, we start to have more concerns in the literature of issues with airway patency and safety of invasive ventilation, specifically gas trapping. That's also the minimum diameter needed to smoothly permit passage of devices like suction catheters without compromising the patient's physiology. And finally, smaller tubes may actually be more effective in preventing aspiration due to a better cuff to tracheal seal and less vaginations in the cuff.

Just going on from what um Dr. Rose said there, there is understandably concern about using small tracheal tubes in lective surgery. But much of the concerns about these tubes originate from other contexts of care, such as the ICU. We conducted this review because we deliberately wanted to look at this lower-risk elective population Because they comprise the bulk of our elective surgery. ASA one and two patients having daycase or short case short stay surgeries. As with all things in medicine, there's a number needed to treat and a number needed to harm from our interventions.

We have to use our clinical judgment and the best evidence available to find where the benefit and risk lies for our individual patient. Safety, of course, has to be the overall priority, but that also means considering the avoidable harm that we do. Intubation-related injury in the form of sore throat and hoarseness is fairly common, and the literature is suggestive of a relationship with longitude sizes. Ultimately, we concluded in the paper that clinicians should use the smallest tube which permits the safe conduct of anesthesia in their judgment. Thanks so much for this summary.

Let's return now to the two questions at the top of our show for our listeners to consider. What is the level of your agreement with the claims that smaller tracheal tubes are less likely to maintain a secure paint and airway, less reliable in facilitating positive pressure ventilation? and less able to seal the trachea and protect the lungs from aspiration. And what now is the level of evidence for what you believe? Dr. Carmale and Dr. Rose, thank you so much for taking the time to join me on the show today. Thanks for having us, David.

Thanks a lot, David. Thanks for joining us on the show today, and be sure to read the excellent review paper which inspired this episode. We're always looking for writers for our blog and for future episodes. Happy New Year and thanks for being a listener. Stay hungry and keep asking questions.

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