The 3:3:2 Rule for predicting The Difficult Airway from on Vimeo. is coming in late APRIL 2010 – videos of lectures on all of acute medicine.

The L.E.M.O.N. approach is just that. An approach to predicting the potentially difficult airway. It is a way of adding some measurable parameters to what should become a ‘gestalt’ approach, where you know or have a ‘vibe’, a feeling, call it what you will, about the relative ease or difficulty of any airway.

This gestalt is usually gained following years of experience BUT it can be taught.

The LEMON rule  allows us to remember to look externally and to look at those parameters that will make the intubation simple or difficult.

LEMON stands for:

L – Look externally – Is the patient obese, do they have a high arched palate, a short neck, facial or neck trauma?

E – Evaluate the 3:3:2 rule – 3cm mouth opening, 3cm thyromental distance, 2cm between hyoid bone and thyroid notch. If unsure as to how much a cm is, just use the 3 fingers or 2 fingers approach

M – Mallampati Score – remember a Mallampati 4 is associated with a >10% chance of difficult airway

O – Obstruction – Is there a tumour, epiglottitis, recent neck surgery?

N – Neck mobility – Is the patient in a cervical collar, are they elderly?

OK, so let’s look at what the LEMON Rule really tells us. The most important aspects of it are: can you open your mouth wide enough, can you move your neck to allow better positioning and is there adequate space in the floor of the mouth into which to displace the tongue?

Remember in airway, the tongue is the enemy, the patient’s enemy and yours. The tongue falls backwards into the hypopharynx and is responsible for obstructing the airway in most cases. The goal is to clear the tongue out of the way, or at least to lift it, so that we can ventilate. This is what procedures like chin lift and jaw thrust do. Movement of the mandible, to which the tongue is attached, moves the tongue.

When intubating, we need to have adequate room, to get the laryngoscope blade in, as well as the endotracheal tube and be able to see. We then need to displace the tongue with the laryngoscope blade. The only real place to displace it into is the floor of the mouth. There has to be adequate volume at the base of the mouth to push the tongue into. This is why patients with micrognathia can be so difficult to intubate, they have an inadequately sized lower jaw, and so almost no place to displace the tongue into.

How useful is predicting really? The reality is that it doesn’t change the fact that you need to secure the airway stat. These patients are not elective patients, but come in in a decompensated state and need immediate assistance.

The ability to predict the potentially difficult airway is useful in that it puts you in the right frame of mind, you start thinking about alternatives should you not be able to intubate.

One of the greatest dangers in any life saving procedure is to get fixated. “I’ve got to get the tube, I’ve got to get the tube, I’ve got to get the tube.” You try again and again and the patient starts to desaturate and now you are playing catch-up. With the ability to predict a potentially difficult airway, you may say to yourself, I’ll try a tube and it should be in if I use all those techniques that I’ve learnt, like using an introducer in the endotracheal tube and bending the end at 35-40 degrees, getting correct positioning and even getting an assistant to open the mouth further for me by pulling the cheek further open. If I can’t tube, I may try again, or go straight to a bougie. It’s this thought process that allows you to see the big picture and not fixate.