anesth analg 1995 tse 254 8
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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
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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.
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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
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ANESTHETIC ACTIONS AND OUTCOMES TSE ET AL.
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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.
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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.
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