We have read and been taught that sniffing position is the foundation for the difficult airway as placing the patient in the “sniffing position” facilitates aligning the three axes to visualize the larynx.
Do you know that the THREE AXIS ALIGNMENT THEORY was proposed by Bannister and MacBeth in 1944. What they did was to take tissue x-rays when patient was in different position, e.g, neutral, hyperextension, head elevation. They drew the lines of the 3 axis based on the schematic of the x-rays. The theory was never truly validated. In 2001, Adnet’s group demonstrated by MRI images that ‘sniffing’ position would not align the three axes anatomically. Several similar studies have substantiated this finding that the three axis theory is not accurate. Greenland proposed two curves theory which is a more accurate representative of the axes.
Greenland proposed two curve theory as below in a nutshell
- primary curve is the oropharyngeal curve
- secondary curve is the pharyngo-glotto-tracheal curve
- point of inflection is the base of the epiglottis, the tangent of this point is the laryngeal vestibule axis
The maneuvers we do change the curve as such:
- Head lift flattens the secondary curve
- neck extension flattens the primary curve
- sniffing position does both
- the laryngoscope completes flattening of the primary curve
Successful laryngoscopy and tracheal intubation requires alignment of both curves with the line of sight and the trachea.
This theory explains why we have difficulty in intubating some patients and what strategies we can do to improve our chance of intubation. For instance, we all had this happened to us that we can see the vocal cords with glide, but we can’t pass the tube to the vocal cord. The curvature of the glide conforms to the primary curve. With the videoscope at the tip of the glide, you have great success at visualizing the larynx. There is an upslope at the secondary curve. If the upslope is more steep which is what we know as aka anterior airway, you will need to negotiate the angle by manipulating the angle of the ET tube with 35-45 degree hockey bend or by giving external cricoid pressure.
Essentially, all the difficult airways are due to unable to flatten primary and secondary curves due to pathological disease processes or injuries. Therefore, it is difficult to bring the line of sight of trachea. For example, for patient who has cervical fracture, the head is in neutral position. You can’t extend the neck nor lifting the head. Therefore, you usually run into difficult airway situation because you can’t visualize the trachea by aligning primary and secondary curves if you are using just regular MAC or Miller. Besides cervical issues indicated as red dot above, most of difficult airways are dealing with orophargeal issues, such as TMJ, large tongue etc. All these pathological issues essentially reduce the volume of the blue triangle as shown in blue lines above known as anterior column. When you have reduced volume in the orophargeal cavity, it is difficult to flatten primary curve and consequently difficult to see vocal cords and pass the tube. Once we are able to systematically identify the cause of the difficult airways, we can come up with the solution to the identified problems.
I hope this post brings you a different perspective of assessing difficult airways. Let me know what you think of this.
- Adnet F, Borron SW, Lapostolle F, Lapandry C. The three axis alignment theory and the “sniffing position”: perpetuation of an anatomic myth? Anesthesiology. 1999 Dec;91(6):1964-5. PubMed PMID: 10598648.
- Greenland KB. A proposed model for direct laryngoscopy and tracheal intubation. Anaesthesia. 63(2):156-61. 2008.
- Greenland KB, Eley V, Edwards MJ, Allen P, Irwin MG. The origins of the sniffing position and the Three Axes Alignment Theory for direct laryngoscopy. Anaesth Intensive Care. 2008 Jul;36 Suppl 1:23-7. Review. PubMed PMID:
- Greenland K. The ramped position and its relationship to the 2-curve theory. Anesth Analg. 2011 Dec;113(6):1524-5; author reply 1525. PubMed PMID: