Body posture during simulated tracheal intubation

A. J. Matthews, C. J. H. Johnson and N. W. Goodman

Department of Anaesthesia, Southmead Hospital, Bristol BS10 5NB, UK

SummarySeventeen experienced anaesthetists and 15 novices were filmed intubating the trachea of atraining manikin. Measurements were made of the distance from manikin’s chin to subject’s noseand of the angles at the elbow, the shoulder and of the forearm with the horizontal. Trainedsubjects stood further back (trained: median 43 cm, interquartile range 41–56 cm; novices 35 cm,26–38 cm; Mann–Whitney U, p< 0.01), with a straighter arm (trained elbow angle: 1088,99–1218; novices’: 928, 88–1028; Mann–Whitney U, p< 0.01). Trained subjects tended to holdthe laryngoscope closer to the hinge, with a pincer grip; novices were more likely to use a fullgrip of the handle. Trainers should consider giving novices explicit intructions on how to standand how to hold the laryngoscope.

Keywords Intubation, tracheal; training.

......................................................................................Correspondence to: Dr N. W. GoodmanAccepted: 20 September 1997

Tracheal intubation is difficult to learn and difficult toteach. Textbooks have descriptions and pictures [1, 2] ofhow best to position the head and neck of the patient andhow to insert the laryngoscope into the mouth, but wecould find no reference in the literature to the best posturefor the subject intubating to adopt.

During the teaching of medical students we havenoticed that novices and experienced people tend tostand differently when attempting tracheal intubation.Novices tend to crouch more and to peer closely intothe mouth. We think that this is not the best position forintubation. It is more difficult to use binocular visionwhen closer to the mouth. There are also mechanicaldisadvantages. Once the laryngoscope is in the vallecula,the movement needed is lifting and pushing. This move-ment is easier with a straighter arm and the crouched,closer position encourages levering of the laryngoscope byrotation at the wrist rather than the correct lifting motion.We decided to analyse the body posture of trained subjectsand novices during simulated intubation of the trachea.

At first we used a Polaroid camera, but it was difficult totake pictures at the required moment in the intubationsequence. The pictures did give us ideas of what measure-ments would be useful and we did the study eventually bymaking these measurements from film taken with acamcorder.

There has been a preliminary report of our findings [3].

Methods

We asked subjects unaware of the object of the study tointubate a Laerdal intubation manikin using a Macintoshlaryngoscope (no. 3 blade) and a 9-mm Portex trachealtube. The manikin was placed at the head of a fixed-heighttrolley. The occiput was raised on one pillow in a con-ventional intubation position. Two intubation sequenceswere recorded from each subject: the first from the sideand the second from the front. The subject was asked totell us when the tracheal tube had passed through the vocalcords. All filming was done by one investigator (C.J.) usinga Canon E70 camcorder placed on a tripod in a standardposition. For some subjects, a close-up was taken of thehandgrip on the laryngoscope; the tripod was not used forthis view.

Video images were captured at the moment of intuba-tion and stored as digital files (Simple Text v2.3.3, Macin-tosh Performa 5300). These images were anonymised andcoded by N.G. (Graphic Converter v2.5, qThorstenLemke).

From the Polaroid photographs, which were not other-wise used in the study, we decided to measure threeprimary variables: the distance from the nose of the subject

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331Q 1998 Blackwell Science Ltd

to the chin of the manikin (nose–chin distance), the anglefrom wrist to elbow to shoulder (elbow angle) and theangle from subject’s chin to shoulder to elbow (shoulderangle). After analysing these variables, we made a post hocmeasurement of the angle of the forearm to the horizontal.

Measurements were made by A.M.: distances by mea-suring directly on printed images; angles on the computermonitor (NIH Image v1.52). The nose–chin distance wasconverted to true distance. Statistical analysis of differencesbetween novices and skilled subjects was by Mann–Whit-ney U-test (p< 0.05) using StatViewSE þ Graphics (v1.04).

Handgrips on the laryngoscope were describedqualitatively.

Results

Angles and distancesThe study was made up of two groups. The trained groupwere all anaesthetists: eight consultants and nine traineeswith more than 2 years’ training. The novice groupconsisted of nine medical students, four surgical traineesand two trainee operating department staff. All the noviceshad previously attempted a few intubations, either on amanikin or on patients.

All subjects were told that we wished to film them aspart of a study and that it was not part of any assessment ofthem. They were not told the purpose of the study.

Trained subjects had a significantly greater nose–chindistance (median 43 cm cf. 35 cm; p < 0.01) and elbowangle (1088 cf. 928; p < 0.01) than novices. Only twotrained subjects had a nose–chin distance less than themedian for the novices. Shoulder angle was not signifi-cantly different (p ¼ 0.19). The angle of the forearm to the

horizontal was less in the trained subjects (188 cf. 348;p < 0.01 – post hoc) (Fig. 1). For only one novice was thisangle less than the median for trained subjects, three ofwhom held the forearm above the horizontal. Fournovices, but no trained subject, rested the left elbow onthe pillow during intubation.

We did not use the front-on views for any formalmeasurements. These views (not shown) particularlyexaggerated the crouched position of the novices, whosefaces were often obscured behind the hand holding thelaryngoscope.

RepeatabilityN.W.G. repeated the forearm angle measurements: meandifference ¹1 6 78 (2 standard deviations).

HandgripsHandgrips were variable and we were unable to devisesuitable measurements. All the data are presented in Fig. 2.Trained subjects (Fig. 2, left panel) tended to hold thelaryngoscope closer to the hinge (T1, T2, T9, T10, T11),with a pincer grip between thumb and index and middlefingers (T2, T3, T8); novices (Fig. 2, right panel) tendedto hold further along the handle (N3, N7, N9), with a fullgrip using all the fingers (N1, N5, N9, N12). A grip wesaw only used by novices had the thumb almost parallel tothe long axis of the laryngoscope handle (N2, N12).

Discussion

There is no doubt that our trained and novice subjectsadopted different postures during tracheal intubation. Thenovices tended to crouch, head closer to the mouth, elbow

A. J. Matthews et al. • Posture and tracheal intubation Anaesthesia, 1998, 53, pages 331–334................................................................................................................................................................................................................................................

332 Q 1998 Blackwell Science Ltd

Figure 1 Box plots (median, 25th–75thcentiles, 10th–90th centiles and outliers) ofmeasurements of posture during trachealintubation of a manikin. Measurementsfrom trained subjects (n ¼ 17; clear boxes)and novices (n ¼ 15; stippled boxes) asshown.

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more flexed and forearm further from the horizontal. Thetrained subjects tended to stand back, elbow less flexed andforearm close to or even below the horizontal. Thesepostures were more or less what we had expected and weimagine that many other anaesthetists would have madethe same predictions. We do not think that our precon-ceived ideas influenced our findings importantly.

The manikin was on a trolley of fixed height and there isbound to be some variation in posture between tall andshort individuals during intubation. However, operatorheight is unlikely to be a confounding factor. Althoughnot easy to measure consistently, it was obvious thatsubjects moved their upper bodies and bent their knees(which were not visible on the video) to compensate forheight. This affected the ‘roundness’ of the upper back andthe angle from the hip to the shoulder (which we did notmeasure), but we believe the primary variables to beindependent of these. Two of the tallest skilled subjectshad elbow angles and nose–chin distances less than themedian for the group.

Obviously, the skilled subjects were older than thenovices, but presbyopia is an unlikely explanation fortheir increased nose–chin distance. Four skilled subjectswere aged more than 45 years, but all subjects wore glassesas appropriate.

Although we are confident in concluding that ourtrained subjects took up a different posture, we do notknow whether novices crouch because they find intuba-tion difficult, or find intubation difficult because theycrouch. We do not know exactly how novices findintubation difficult. It may be because they cannot seewhat they are doing, which makes it understandable thatthey try to get closer. It may just be a response to any not-yet-learned manipulative skill. Trained subjects, faced witha difficult intubation, may also crouch, although C.J., whomanages and teaches difficult intubations in his routinepractice, believes the first response of the trained subject isto drop the eyeline to improve alignment between the eye

and the tip of the laryngoscope. This posture is differentfrom the novices’ posture in that the elbow angle is stillopen and the distance between subject’s nose and patient’schin not greatly decreased.

We now teach students explicitly to try to stand upand stand back when attempting intubation. They tell usit does give a better view (which is perhaps because ofbinocular vision, but we have not tested this), but thatimpression is obviously biased by our beliefs and doesnot mean that intubation becomes easier. The only wayto be certain would be from a randomised trial, withgroups of students randomly allocated to receivingexplicit or just general instructions and their successmeasured at learning intubation. Such a trial would bedifficult. We are encouraged in our conclusion byinformal discussions with other anaesthetists; some hadnoticed the tendency of novices to crouch and somealready teach novices to stand back. One anaesthetistcommented that he tells students that they need only tolook in the mouth, not get into it.

Standard teaching of intubation includes avoiding lever-ing the laryngoscope blade. Levering is certainly morelikely if crouching and is encouraged by holding thelaryngoscope hard by the handle. The exaggerated,close, crouched position, in which four of the novicesrested the elbow on the trolley, makes lifting the laryngo-scope very difficult and levering very easy. For all thesereasons, we suggest explicit instructions are helpful.

References

1 Latto JP, Vaughan RS, eds. Difficulties in Tracheal Intubation,2nd edn. London: WB Saunders, 1997.

2 Benumof JL. Airway Management: Principles and Practice. StLouis, Missouri: Mosby Year Book Inc., 1996.

3 Matthews AJ, Johnson CJH, Goodman NW. Body postureduring simulated tracheal intubation. British Journal ofAnaesthesia 1997; 79: 137P.

A. J. Matthews et al. • Posture and tracheal intubation Anaesthesia, 1998, 53, pages 331–334................................................................................................................................................................................................................................................

334 Q 1998 Blackwell Science Ltd