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Canadian Journal of Anaesthesia

, Volume 40, Issue 3, pp 262–270 | Cite as

An analysis of laryngoscope blade shape and design: new criteria for laryngoscope evaluation

  • Rex R. D. Marks
  • Richard Hancock
  • Peter Charters
Equipment

Abstract

Laryngoscope blade design has tended to be relatively arbitrary and so far scientific analysis has not allowed useful comparisons between blade shapes. A new theoretical method of analysing laryngoscope blades is introduced and uses the depth of insertion profiles of two angular measurements. One represents eyeline displacement and the other the forward space that the blade occupies at the level of the mandible. Photographs of straight and curved blades were studied on Cartesian graphs with the tip T, at the origin and handle fittings parallel to the x-axis of the graph. Then, IT is any line from the origin to the incisor surface and represents a point of contact with the upper incisors for a given depth of blade insertion. Angle EIT (eyeline displacement) is to a tangent from I along the lower lingual surface of the blade. Point M is on the upper lingual surface of the blade, at right angles to IT, 1/3 of the distance from I along IT. Angle MIT (forward space) may be positive or negative depending on whether M is in front of or behind IT. The angles EIT, MIT and their additive combination are used in blade analysis. Negative MIT compensates for eyeline displacement as Macintosh size 3 and 4 blades have better combination scores than Miller size 3. All three are superior to the straight Soper size 3 blade. The Macintosh size 1 and 2 blades are quite different from the larger Macintosh blades. This theoretical basis for blade analysis is consistent with commonly expressed clinical opinions and may influence blade design in the future.

Key words

Anatomy: airway, larynx Equipment: laryngoscopes Intubation: tracheal 

Résumé

La conception des lames de laryngoscope tient de l’arbitraire et a jusqu’à maintenant a défié toute analyse scientifique comparative. Les auteurs présentent une nouvelle méthode analytique basée sur le profil de la lame vue sous deux angles différents après son insertion. Le premier montre la déviation de la ligne de vision et l’autre l’espace rétromandibulaire que la lame occupe. Des photographies de lames droites et courbes sont transposées sur des coordonnées cartésiennes avec la pointe T à l’origine et le point d’attache du manche parallèle à l’abscisse du graphique. IT devient la ligne qui naît de la surface des incisives et représente le point de contact avec les incisives supérieures pour une profondeur donnée d’insertion. L’angle EIT (déplacement de la ligne de vision) est la tangente de I avec la surface linguale inférieure de la lame. Le point M se situe sur la surface linguale supérieure de la lame, à angle droit avec IT, au tiers de la distance entre I et IT. L’angle MIT (espace antérieur) peut être positif ou négatif selon que M est en avant ou en arrière de IT. Les angles EIT et MIT et leur combinaisons additives sont utilisés pour l’analyse des lames. Un angle MIT négatif compense pour le déplacement de la ligne de vision: les lames Macintosh 3 et 4 ont une meilleur cotte que la Miller 3. Les trois sont supérieures à la lame droite Soper 3. Les Macintosh 1 et 2 sont tout à fait différentes des Macintosh plus longues. Cette analyse théorique est consistante avec les impressions cliniques courantes et pourrait dans l’avenir influencer la conception des lames.

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Copyright information

© Canadian Anesthesiologists 1993

Authors and Affiliations

  • Rex R. D. Marks
    • 1
  • Richard Hancock
    • 2
  • Peter Charters
    • 1
  1. 1.Department of AnaesthesiaWalton HospitalLiverpoolEngland UK
  2. 2.Department of Medical IllustrationWalton HospitalLiverpoolEngland UK

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