PulmCrit Wee: MDCalc for the perfect tape-measure intubation

Imagine you went to buy an expensive piece of clothing. Rather than measuring your size, the store owner simply said “well, on average most folks require a medium, so let's try that on, we can always re-size it later.” You would be irritated that they were wasting your time. When you go clothes shopping, you expect measurement to be made so that the clothes fit right the first time. The patients we intubate deserve this same level of consideration.

Accurate tidal volume

Studies continue to support the importance of applying lung-protective ventilation to all patients (e.g. Fuller 2017). The crux of this is setting tidal volume at 6-8 cc/kg ideal body weight. This isn't quite as easy as it sounds, because ideal body weight is based on gender and height (not actual weight). Dangerously high tidal volumes are often estimated for patients who are obese or shorter than average.

The definitive way to determine safe tidal volumes is to perform a calculation using the patient's height and gender. For non-crash intubations, the best approach is to perform these calculations beforehand. If the patient's height isn't known, it can be measured rapidly with a tape-measure. Hence the concept of the tape-measure intubation. This seems to be gaining traction recently, with discussion on EM:RAP this month.

Accurate endotracheal tube depth

In addition to tidal volumes, height may be used to calculate the appropriate ETT depth. The evidence behind this was previously explored on the blog here. This isn't rocket science, but it is a good practice which forces us to use a reasonable ETT depth (a detail that otherwise often goes ignored).

Using a height-based calculation doesn't eliminate the need for other techniques to confirm ETT depth (e.g. chest x-ray, ascultation, ultrasound if that is available). The major advantage is that it will get the ETT into a safe position immediately, rather than requiring repositioning and pulling back from the right mainstem. For patients with critical hypoxemia, delivering the ETT into the mid-trachea immediately may prevent desaturation.

One advantage of a height-based ETT depth strategy is that the intended ETT depth may be calculated before the intubation. This cognitively off-loads the operator during the intubation. There is no need to worry about this during the procedure: just sink the ETT between the cords and secure it at the pre-determined depth. Writing the target ETT depth on a white board before the procedure supports this cognitive off-loading.

Doing the math

The math can be worked out on a table which may be placed on your airway checklist (example below).

If you don't want to carry around a card, I recently collaborated with Dr. Rachel Kwon from the MDCalc team to create an online calculator. Based on the patient's height and gender, this will recommend an ETT depth and tidal volumes. The calculator can be run off the internet or the MDCalc app (which is free).

Team cooperation + consistent practice = higher quality

The task of measuring the patient's height (if unknown) and calculating appropriate depths may be delegated to the respiratory therapist. This makes sense, because the respiratory therapist will be involved in setting up the ventilator and securing the ETT.

There are numerous other ways to determine ETT depth (e.g. being super precise about watching when the balloon passes through the cords). One advantage of a height-based strategy is that this requires no special training, allowing it to be easily protocolized and performed by any team member. This may facilitate department-wide consistency and uniform quality, rather than depending on each operator's skill level.

Emerging evidence continues to support the importance of ventilating patients using lung-protective tidal volumes.
Height-based estimation of ETT depth may allow for a simple, protocolized approach that avoids right mainstem intubation.
A new MDCalc equation may facilitate intubation safety, by simultaneously calculating lung-protective tidal volumes and reasonable ETT depth.