7/25/2019 Anesth Analg 1995 Tse 254 8

1/5

ANESTHETIC ACTIONS AND OUTCOMES

SECTION EDITOR

JOHN H. TINKER

Predicting Difficult Endotracheal Intubation in Surgical

Patients Scheduled for General Anesthesia: A Prospective

Blind Study

Jimson C. Tse, MD, PhD*, Eric B. Rimm, sat, and Ayyaz Hussain, FFARCS (En@*

* Department of Anesthesiology, St. Elizabeths Medical Center of Boston, Tufts Universi ty School of Medicine, and

t Departments of Nutrition and Epidemiology, Harvard School of Public Health, Boston, Massachusetts

We conducted a prospective, blind study to determine

whether a dif ficu lt endotracheal intubation could be

predicted preoperatively by evaluation of one or more

anatomic features of the head. In 471 adults presenting

for elective surgery, the size of the tongue relative to the

oral cav ity was assessed according to the Mallampati

classification (oropharyngeal class), and the distance

between the chin and thyroid cartilage (thyromental

distance) and the angle at full extension of the head

(head extension) were measured. At laryngoscopy, the

dif ficult y in visualizing the larynx was determined by a

diffe rent observer. Assignment to oropharyngeal Class

3, a thyromental distance 57 cm, and a head extension

~80, considered either alone or in various combina-

tions, had low sensitivity and positive predictive values

in identifying patients with airways that were dif ficu lt

to intubate, but high specificity and negative predictive

values. We conclude that these three tests are of little

value in predicting diff icu lt intubation in adults, al-

though the likelihood of an easy endotracheal intuba-

tion is high when they yield negative results.

(Anesth Analg 1995;81:254-8)

B

cause failed endotracheal intubation is a prin-

cipal cause of morbidity and mortality in anes-

thetized patients (l), there is a need for accurate

tests to predict difficult intubation. When a difficult

intubation occurs unexpectedly in a patient after gen-

eral anesthesia has been induced, there might be an

unfavorable outcome if the patients lungs cannot be

adequately ventilated by mask or an endotracheal

tube cannot be properly inserted with use of other

techniques. Unexpected difficult intubations are prob-

ably the result of a lack of accurate predictive tests for

difficult intubation and inadequate preoperative ex-

aminations of the airway.

During direct laryngoscopy, the vocal cords are vi-

sualized by placing the head in the sniffing position

(extension of the head at the atlantooccipital joint and

upper part of the cervical spine, with flexion of the

neck at the lower cervical spine). Three preoperative

tests for assessinga patients airway for difficult intu-

bation have been proposed, and it has been suggested

Presented in part at the 68th Clinica l and Scien tific Congress of

the International Anesth esia Research Society, Orlando, FL, March

1994.

Accep ted for publication February 24, 1995.

Address correspondence and reprint requests to Ayyaz H ussain,

FFARCS (Eng), Department of Anesthes iology, St. Elizabeths Med-

ical Center of Boston, 736 Cambridge St., Boston, MA 02135.

254

An&h Analg 1995;81:254-8

that the most accurate results are obtained when find-

ings from these evaluations are combined (2). The tests

are assignment to oropharyngeal class, an assessment

of the size of the tongue in relation to the size of the

oral cavity (3); measurement of the thyromental dis-

tance, an indicator of the mandibular space anterior to

the larynx (4); and measurement of the head (atlan-

tooccipital) extension (5). No study has examined the

usefulness of these tests when used together. We

therefore conducted a prospective, blind study of their

accuracy, used alone and in various combinations, in

predicting difficult endotracheal intubation.

Methods

Approval for the study was obtained from our insti-

tutions human subjects committee, which did not

require informed patient consent to be obtained be-

cause the measurements performed were noninvasive

and had no monetary cost. Consecutive male and

female patients aged 18 yr and older scheduled to

undergo elective surgery under general anesthesia in

our general community hospital between December

1992 and June 1993 were considered for enrollment.

Patients with obvious malformations of the airway,

edentulous patients, and patients who required cri-

coid pressure for rapid-sequence intubation were

excluded from the study. Edentulous patients were

01995 by the International Anesth esia Research Society

0003.2999/95/ 5.00

7/25/2019 Anesth Analg 1995 Tse 254 8

2/5

ANESTH ANALG

1995;81:254-8

ANESTHE TIC ACTIONS AND OUTCOMES TSE ET AL. 255

PREDICTING DIFFICULT TRACHEAL INTUBATION

,?

-----L 1,

/____:

\

\

Figure 1. Angle measured with bubble goniome ter to obtain values

for degrees of head extension. The angle asses sed was that between

a line joining the angle of the mouth and tragus of the ear with the

horizontal.

excluded to avoid introduction of a variable that may

independently affect the predictability of difficult

intubation.

The following measurements were obtained preop-

eratively by two members of the anesthesiology de-

partment not subsequently involved in intubating the

airways of the patients they evaluated. In most pa-

tients, the measurements of thyromental distance and

head extension angle were performed twice and the

results averaged for the data analysis. The Mal lampat i

sign was assessed once in each patient, except those in

whom two or more evaluations were required to con-

firm the classification assignment.

1. Classification of the oropharyngeal view was

done according to the Mallampati criteria (31,

with slight modifications to avoid ambiguity.

Thus, assignment to oropharyngeal Class 1 in-

dicated that the faucial pillars, soft palate, and

uvula could be visualized; assignment to Class 2

indicated that the uvula was only partly visible;

and assignment to Class 3 indicated that the

uvula was completely masked by the base of the

tongue and that the posterior pharyngeal wall

was not visible. The examination to determine

oropharyngeal class was done with the aid of a

flashl ight. The patients were in a sitting position

with the tongue fully protruding; they were not

asked to say ah.

2. The distance in centimeters between the thyroid

prominence and the most anterior part of the

chin, with the head fully extended, was meas-

ured with a ruler.

3. The maximum extension of the head was as-

sessed as the size of the angle between a line

joining the angle of the mouth and tragus of the

ear with the horizontal line (Figure 1). A bubble

goniometer was used for this assessment. The

patients were in a supine position on a flat bed

without a pillow, and care was taken to ensure

that they did not lif t their shoulders while ex-

tending the head.

Al l measurements were recorded on a form not seen

by the anesthesiologist who subsequently performed

the intubation.

Intubation was done with the patient adequately

anesthetized and fully relaxed on the operating room

table. A peripheral nerve stimulator was used in cases

in which there was doubt about the relaxation. The

head was placed in the sniffing position, and laryn-

goscopy was performed with a Macintosh No. 3 blade

by the anesthesiologist assigned to the case. The la-

ryngeal view was graded according to the method

described by Cormack and Lehane (6) as Grade I (ful l

view of the glottis), Grade II (glottis partly exposed,

anterior commissure not seen), Grade III (only epiglot-

tis seen), or Grade IV (epiglottis not seen). A grade of

I or II was considered to represent easy intubation and

a grade of III or IV to represent difficult intubation.

Intubations were performed by anesthesiology resi-

dents with at least 6 mo of experience or by staff

anesthesiologists.

The preoperative assessment data and the intuba-

tion findings were used to determine the accuracy of

the three tests in predicting difficult intubation. The

sensitivity, specificity, and positive and negative pre-

dictive values of each of the evaluations used alone

and together in various combinations were calcula ted

(Appendix).

Results

A total of 471 patients (220 men and 251 women aged

18-89 yr) were enrolled in the study. Sixty-two of

them were found at laryngoscopy to have airways that

were diff icul t to intubate (laryngoscopy Grade III or

IV). There were no failed intubations. Assignment to

oropharyngeal Class 3, a thyromental distance 57 cm,

and a head extension 580 were selected as indicators

of difficult intubation. Information on the accuracy of

the tests used alone and together is given in Table 1

and Figures 2 and 3.

We found that al l the tests had low sensitivity,

although each test used alone had a higher sensitiv-

ity than the combination tests. The combination us-

ing al l three tests had the lowest sensitivity. Al l the

tests and combinat ions also had low positive pre-

dictive values (18 -38 ). Only one patient among

the 50 with a thyromental distance ~10 cm had a

difficult intubation, as did only one of the 54 pa-

tients with a head extension angle >lOO. All the

tests had high negat ive predictive values; some

were highly specific.

7/25/2019 Anesth Analg 1995 Tse 254 8

3/5

256 ANEST HETIC ACTIONS AND OUTCOMES TSE ET AL. ANESTH ANALG

PREDICTING DIFFICULT TRACHEAL INTUBATION 1995;81:254-8

Table

1. Tes ts for Difficult Intubation

Test TP FP TN J?N Sens (%) Spec (%) PPV (%) NPV (%)

OK 3

TMDs7cm

HE 5 80

OPC 3, TMD 5 7 cm

OK 3,

HE 5 80

TMD 5 7 HE

m,

I 80

OK 3, TMD I 7 cm,

and HE 5 80

41 145 264 21 66 65 22 93

20 82 327 42 32 80 20 89

6 27 382 56 10 93 18 87

13

33 376 49 21 92 28 88

4

10

399 58 6 98 29 87

3

11

398 59 5 97 21 87

3 5 404 59 5 99 38 87

TP = true positive; FP = false positive; TN = true negative; FN = false negative; Sens = sensitivity; Spec = spec ificity; PPV = positive predictive value; NPV

= negative predictive value; OK = oropharyngeal class ; TMD = thyromental distance : HE = head extension.

Figure 2. Sensitivity, spec ificity, and positive predictive value of

Figure 3. Sensitivity, spec ificity, and positive predictive value of

the thyromental distance used alone to predict difficult intubation.

the head extension angle used alone to predict difficult intubation.

If, for example, a thyromental distance of 9 cm is used as the

If, for example, a head extension angle of 85 is used as the thresh-

threshold value for identifying patients whose airways will be dif-

old value for identifying patients whose airways will be difficult to

ficult to intubate, that screening measure would have a sensitivity of

intubate, that screening measure would have a sensitivity of 32%, a

97%, a spec ificity of 18%, and a positive predictive value o f 16%.

spec ificity of 72%, and a positive predictive value of 19%.

Discussion

The incidence of difficult intubation is reported to

be 10/o-18 (7-ll), depending on the criteria used to

define it; that of failure to intubate is 0.05 -0.35

(6,9). In our study, the rate of difficult intubation

was 13 , and there were no failures to intubate the

trachea.

A test to predict difficult intubation should have

high sensitivity, so that it will identify most patients

in whom intubation will truly be difficult. It should

also have a high positive predictive value, so that

only a few patients with airways actually easy to

intubate are subjected to the protocol for manage-

ment of a difficult airway. In this study, we found

that the commonly used tests for forecasting intu-

bation type had inadequate sensitivity and positive

predictive values in predicting difficult intubation,

used either alone or together. The assessments did

have high negative predictive values, and some

were highly specific. Our findings indicate that

these screening evaluations have little value in pre-

dicting difficult intubation, although when their re-

sults are negative there is a high probability that

intubation will be easy.

The oropharyngeal Class 3 test had a sensitivity of

66 , that is, it preoperatively identified 41 of the 62

patients who later had a difficult intubation. The tests

positive predictive value was 22 ; it identified 186

patients who would have a difficult intubation, but, in

fact, 145 of them had an easy intubation. Thus, if one

were dependent on the results of this test, and all

patients in whom difficult intubation was predicted

were considered for awake intubation, many patients

with easy-to-intubate airways would be unnecessarily

subjected to that procedure.

The oropharyngeal Class 3 test was useful when its

results were negative: of the 285 intubations predicted

to be easy, 264 actually were so. Therefore, if all the

patients identified by this test to have airways easy to

7/25/2019 Anesth Analg 1995 Tse 254 8

4/5

ANESTH ANALG

ANESTHETIC ACTIONS AND OUTCOMES TSE ET AL.

257

1995;81:254--8

PREDICTING DIFFICULT TRACHEAL INTU BATION

intubate were anesthetized, only a few would unex-

pectedly be found to have difficult-to-intubate air-

ways after the induction of anesthesia.

Our findings contradict those of Mallampati et al.

(31, who reported that assignment to oropharyngeal

Class 3, as they defined it, was a good predictor of

difficult intubation, with a sensitivity of 50 and a

positive predictive value of 93 . The discrepancy be-

tween their results and ours has three possible

sources.

First, in the study by Mal lampat i et al. (3), the same

person who did the preoperative evaluation also

graded the laryngoscopy view, thereby introducing

the possibility of bias into the assessment. In our in-

vestigation, a patients assignment to an oropharyn-

geal class and the laryngoscopic examination were

always performed by a different anesthesiologist.

Second, the description of the three oropharyngeal

classes by Mal lampat i et al. (3) is imprecise in that it is

unclear whether Class 3 is defined by an inabili ty to

see the fauc ial pillars or by masking of the uvula. We

placed patients who had complete masking of the

uvula and no visualization of the posterior pharyngeal

wall in Class 3 and those with incomplete masking of

the uvula in Class 2, but Mallampati et al. (3) may

have classified such patients differently.

Third, the uncertainty created by the ambiguous

def init ion of oropharyngeal Class 3 increases with the

number of evaluators in a study as a result of interin-

dividual variations in interpretation (12). The investi-

gation by Mallampati et al. (3) used 22 evaluators for

the preoperative assessment; we had only two.

A find ing of a thyromental distance 17 cm was also

not a good predictor of difficult intubation in our

study, in which its sensitivity was 32 and its positive

predictive value was 20 . When the oropharyngeal

Class 3 and thyromental distance 57 cm assessments

were used together, the sensitivity and positive pre-

dictive value were 21 and 28 , respectively. These

findings do not support those of Frerk (lo), who re-

ported that assignment to oropharyngeal Class 3 or 4

had a sensitivity of 81.2 and a specificity of 81.5 in

predicting difficult intubation. In his investigation, the

sensitivity and specificity of a thyromental distance

~7 cm were 90.9 and 81.5 , respectively. When

Frerk used both tests, the sensitivity and specificity

were 81.2 and 97.8 , respectively. The discrepancy

between our findings and those of Frerk (10) can be

explained partly by the different definitions used for

diff icul t intubation in the two studies. Frerk defined

difficult intubation as a need to use a gum elastic

bougie.

Bellhouse and Dare (51, in a .radiologic study of

assessments of cervical and facia l characteristics in

predicting difficult intubation, calculated head exten-

sion by estimating the angle traversed by the occlusal

surface of the maxillary teeth when the head is ex-

tended from the neutral position. They found that

patients with a limitation in extension who were in

oropharyngeal Class 4 had a 95 likelihood of having

a difficult intubation. In our study, head extension

was measured with a bubble goniometer to ensure

that the patients were complete ly horizontal during

the assessment. We found that use of a head extension

angle ~80 to predict difficult intubation had a sensi-

tivity of 8 and a positive predictive value of 21 .

The results of our study and that of Bellhouse and

Dare cannot be compared directly because those au-

thors included an oropharyngeal Class 4 in their in-

vestigation and we did not.

The anatomic features of the head and neck used in

the tests we evaluated generally affected the laryngeal

view independently of each other. As a result, the

combination tests had a lower sensitivity and a higher

positive predictive value than some of the tests used

alone.

Designing a good predictive test for difficult intu-

bation is problematic because many factors may affect

visualization of the larynx at intubation, such as the

maximum mouth-opening distance, the circumference

and length of the neck, and several characteristics that

cannot be quantified accurately. These include the

compressibility of the tongue and soft tissues of the

floor of the mouth and the extent of subluxation of the

temporomandibular joint during laryngoscopy. In ad-

dition, the ability of the person performing the intu-

bation cannot be easily incorporated into a standard-

ized assessment.

We did not find that tests using an oropharyngeal

Class 3, a thyromental distance 57 cm, a head exten-

sion angle 580, or any combination of these factors

predicted difficult intubation reliably. Their sensitivi-

ties and positive predictive values were too low for

them to be clinical ly useful. However, the tests had

high specificities and negat ive predictive values, thus

providing reassurance that negat ive results indicate

truly easy endotracheal intubation. We therefore do

not recommend that all patients in whom difficult

intubation is predicted with use of these tests have

awake intubation. Instead, awake intubation should

be done only in patients in whom ventilation might be

difficult , those at risk of regurg itation of stomach con-

tents, those with an obvious abnormality predisposing

them to diff icult intubation, or those with a history of

difficult intubation. Most patients in whom a difficult

intubation is suspected can have their airways intu-

bated while asleep by means of any of the several

methods available.

The authors thank Mrs. Mary DiGiovanni for techn ical support and

Ms. Ren6e J. Robillard for editorial assista nce.

7/25/2019 Anesth Analg 1995 Tse 254 8

5/5

258 ANESTHET IC ACTIONS AND OUTCOMES TSE ET AL.

PREDICTING D IFFICULT TRACHEAL INTUB ATION

ANESTH ANALG

1995;81:254-8

Appendix

Definition of Terms

True positive = a difficult intubation that had been

predicted to be difficult.

False positive

= an easy intubation that had been

predicted to be difficult.

True negative

= an easy intubation that had been

predicted to be easy.

False negative = a difficult intubation that had been

predicted to be easy.

Sensitivity = the percentage of correctly predicted

difficult intubations as a proportion of all intuba-

tions that were truly difficult, i.e. ,

true positives

true positives+false negatives.

Specificity = the percentage of correctly predicted

easy intubations as a proportion of al l intubations

that were truly easy, i.e.,

true negatives

true negatives-tfalse positives.

Positive predictive value = the percentage of correctly

predicted difficult intubations as a proportion of all

predicted difficult intubations, i.e.,

true positives

true positives+false positives.

Negative predict ive value = the percentage of cor-

rectly predicted easy intubations as a proportion of

all predicted easy intubations, i.e.,

true negatives

true negatives-tfalse negatives.

References

1.

2.

3.

4.

5.

6.

7.

8.

9.

10.

11.

12.

Caplan RA, Posner KL, Ward RJ, Cheney EW. Adverse respira-

tory events in anesthe sia: a closed cla ims analysis. Anesthe siol-

ogy 1990;72:828-33.

Benumof JL. Management of the difficult adult airway, with

spe cial emp hasis on awake tracheal intubation. Anesthesiology

1991;75:1087-1110.

Mallampati SR, Gatt St, Gugino LD, et al. A clinic al sign to

predict difficult trache al intubation: a prospective study. Can

Anaesth Sot J 1985;32:429-34.

Patil VU, Stehling LC, Zaunder HL. Fiberoptic endoscopy in

anesthe sia. Chicago: Ye ar Book Medical Publishe rs, 1983.

Bellhous e Cl?, Dare C. Criteria for estimating likelihood of dif-

ficulty of endotracheal intubation with the Macintosh laryngo-

scope . Anaesth Intensive Care 1988;16:329-37.

Cormack RS, Lehane J. Difficult tracheal intubation in obstetrics.

Anaesthesia 1984;39:1105-11.

Oates JDL, MacLeod AD, Oates ID, et al. Comparison of two

methods for predicting difficult intubation. Br J Anaesth 1991;

66:305-9.

Rocke DA, Murray WB, Rout CC, Gouws E. Relative risk anal-

ysis of factors assoc iated with difficult intubation in obstetric

anesthe sia. Anesthesiology 1992;77:67-73.

Samsoo n GLT, Young JRB. Difficult tracheal intubation: a ret-

rospective study. Anae sthesia 1987;42:487-90.

Frerk CM. Predicting difficult intuba tion. Anaes thesia 1991;46:

1005-8.

Finucaine BT, Santora AH. Difficult intubation: principles of

airway managem ent. Philadelph ia: Davis, 1988.

Wilso n ME, John R. Problems with the Mallampati sign [letter].

Anaesthesia 1990;45:486-7.