View of the larynx obtained using the Miller blade and paraglossal approach, compared to that with the Macintosh blade

Anaesthesia and Intensive Care, Sept, 2008 by B. Achen, O. C. Terblanche, B.T. Finucane

There were no serious complications observed during this study.

DISCUSSION

Modern laryngoscopy in anaesthesia evolved from direct laryngoscopy performed by ENT surgeons. Magill wrote a report about "An Improved Laryngoscope for Anaesthetists" in the "New Inventions" section of the Lancet in 1926 (4). That device retained many of the features of the direct laryngoscope used by ENT surgeons, however the lighting was much improved and the distal end of the spatula was flat, wide and tilted slightly upwards, which was an advantage in elevating and controlling the epiglottis when passing catheters into the larynx. The Miller laryngoscope was introduced in 1941 at a time when endotracheal intubation was being performed more frequently. Miller did a comparison with other laryngoscope blades available at the time and cited a number of advantages. It came in just one size and was not designed for infants (5). Macintosh introduced his laryngoscope in 1943. Information about this device was published in the "New Inventions" section of the Lancet (6). Macintosh expressed concerns about the use of "the long straight blade...Occasionally this manoeuvre jeopardises the patients' upper teeth or takes a minor divot out of the posterior pharyngeal wall". He stated the following: "The new laryngoscope is designed so that when its short curved blade is in position the tip will fit into the angle made by the epiglottis with the base of the tongue". He goes on to say "I can expose the larynx more easily and at a lighter plane of anesthesia than with any of the standard laryngoscopes". Did Macintosh have some purpose in mind in designing a curve for the Macintosh blade? Perhaps the curve was designed to conform with the curved shape of the tongue? Macintosh himself made the following statement about the curve: "I find now that users place too much emphasis on the curve... For two years I have used blades ranging from those with a well marked curve to the perfectly straight...I had some difficulty in deciding which was best for all round use". This quotation is found in a remote correspondence section of the Lancet in 1944 (7). Therefore it would appear that when designing the Macintosh blade, Macintosh had no particular purpose for using a curve at all. The main idea was to design a blade so that the tip slotted into the vallecula and with some pressure on the hyoid bone the epiglottis rotated upwards thereby exposing the vocal cords. Macintosh observed that the vallecula was innervated by the glossopharyngeal nerve whereas the under surface of the epiglottis was innervated by the superior laryngeal nerve which was a branch of the vagus. He indicated that patients reacted far more vigorously physiologically to vagal than glossopharyngeal nerve stimulation. This probably was very relevant when inhalation agents were solely used to anaesthetise patients for endotracheal intubation.

The results of this study demonstrate that the Miller blade provides a better view of the larynx than the Macintosh blade and this observation concurs with those of other investigators Bellhouse8 and Henderson (9). The results contradict the findings from Arino et al (10) who observed better intubating conditions with the Miller blade but had difficulty placing the ETT. This difference may be explained on that basis that we used the paraglossal approach, which was not used in Arino's study. The paraglossal approach does require diligence to master but is certainly worth the effort and the introduction of improvements in the Miller blade in recent years by Henderson facilitate the paraglossal approach because of the width and overall design of the blade.