General information of patients and airway assessment results
373 patients were selected to be intubated under general anesthesia. Laryngeal mask airway management was performed in 58 patients, and surgery was temporarily canceled in 11 patients. Therefore, a total of 304 patients were successfully included in this study, including 137 male patients and 39 patients with difficult laryngoscopy. All patients were successfully intubated within 3 attempts. After group analysis of all patients according to whether they had difficult laryngoscopy, the differences of Mallampati classification, ULBT classification, IID, and the C-TMD between the two groups were statistically significant, while the differences of body mass index were not. Descriptive data of the patients and the airway assessment results are shown in Table 1.
Table 1. Comparison between the difficult and the non-difficult laryngoscopy group
variable
|
Difficult laryngoscopy
|
Non-difficult
laryngoscopy
|
P
value
|
n=39
|
n=265
|
Sex (male/female, n)
|
26/13
|
111/154
|
0.003
|
Age (y)
|
57±13
|
49±16
|
<0.001
|
Height (cm)
|
163±7
|
165±8
|
0.32
|
weight (kg)
|
62±11
|
64±12
|
0.52
|
Body mass index (kg/m2)
|
23.4±3.7
|
23.4±3.5
|
0.94
|
Mallampati classification
(<3/>2grade, n)
|
5/34
|
128/137
|
<0.001
|
ULBT (>1/1grade, n)
|
33/6
|
148/117
|
<0.001
|
IID<3 finger width
(yes/no,n)
|
31/8
|
92/173
|
<0.001
|
C-TMD < 1 finger width
(yes/no,n)
|
35/4
|
81/184
|
<0.001
|
Data are shown as the means±standard deviation or numbers. Difficult laryngoscopy was defined as a Cormack and Lehane grade >2.
Abbreviations: ULBT, Upper lip bite test; IID, interincisor distance; C-TMD, condyle-tragus maximal distance.
All patient characteristics were compared using Mann-Whitney U test for continuous variables and c2or Fisher exact test for categorical variables.
The time of intubation and the number of intubation attempts of all predictors
The intubation time was shorter with the C-TMD >1 finger group 46.8±7.3s, compared to the C-TMD <1 finger group 50.8±8.6s (P<0.01). The intubation time differences of IID and Mallampati classification were statistically significant, while the ULBT were not. First attempt success rate was higher in the C-TMD >1 finger group 98.9% than in the C-TMD <1 finger group 87.1% (P<0.01). The intubation attempts differences of ULBT and Mallampati classification group were not statistically significant (see Table 2 and Table 3).
Table 2.The number of intubation attempts of each predictor
|
|
The number of
intubation attempts
1/2/3(n)
|
P value
|
C-TMD
|
>1 finger
|
186/2/0
|
|
|
<1finger
|
101/9/6
|
<0.001
|
IID
|
>3finger
|
175/5/1
|
|
|
<3finger
|
112/6/5
|
0.023
|
ULBT
|
1 grade
|
120/2/1
|
|
|
>1 grade
|
167/9/5
|
0.143
|
Mallampati Test
|
<3 grade
|
124/9/0
|
|
|
>2 grade
|
163/2/6
|
0.064
|
Abbreviations: ULBT, Upper lip bite test; IID, interincisor distance; C-TMD, condyle-tragus maximal distance.
The number of intubation attempts were compared using c2or Fisher exact test for categorical variables.
Table 3. The time of tracheal intubation of each predictor
|
|
The time of intubation
|
P value
|
C-TMD
|
>1 finger
|
46.8±7.3s
|
|
|
<1finger
|
50.8±8.6s
|
<0.01
|
IID
|
>3finger
|
47.1±7.3s
|
|
|
<3finger
|
50.1±8.7s
|
<0.01
|
ULBT
|
1 grade
|
47.5±8.3s
|
|
|
>1 grade
|
48.9±7.8s
|
0.146
|
Mallampati Test
|
<3 grade
|
47.0±7.9s
|
|
|
>2 grade
|
49.3±8.0s
|
0.013
|
Abbreviations: ULBT, Upper lip bite test; IID, interincisor distance; C-TMD, condyle-tragus maximal distance.
The time of intubation were compared using Mann-Whitney U test for continuous variables.
Correlation of all predictors with intubation time and intubation attempts
The r values for all the predictors, such as the C-TMD, IID, ULBT, Mallampati test in comparison with the intubation time were 0.226 (P<0.001), 0.173 (P<0.01), 0.099 (P=0.11), 0.171 (P<0.01), respectively, with the intubation attempts were 0.252 (P<0.001), 0.151 (P<0.01), 0.135 (P<0.05), 0.203 (P<0.01),respectively.(see Table 4).
Table 4. Correlation analysis of each predictor (n = 304)
|
C-L Levels
(r /P value)
|
The number of intubation attempts
(r /P value)
|
The time of
intubation
(r /P value)
|
C-TMD
|
0.317/<0.001
|
0.252/<0.001
|
0.226/<0.001
|
IID
|
0.261/<0.001
|
0.151/<0.001
|
0.173/<0.01
|
ULBT
|
0.266/<0.001
|
0.135/<0.05
|
0.099/0.11
|
Mallampati classification
|
0.213/0.002
|
0.203/<0.001
|
0.171/<0.01
|
Spearman correlation analysis was used for all correlations.
Abbreviations: ULBT, Upper lip bite test; IID, interincisor distance; C-TMD, condyle-tragus maximal distance; C-L, Cormack-Lehane.
Comparison of preoperative predictors and the Cormack-Lehane Levels
The r values of correlation between C-TMD, IID, ULBT, Mallampati classification and Cormack-Lehane Levels was 0.317,0.261,0.266 and 0.213 respectively (all P values were less than 0.01). Paired chi-square and agreement test showed that the C-TMD < 1 finger width had a significant k value (0.485) (see Table 4 and Table 5 for details).
Table 5. Agreement test between each predictor and difficulty laryngoscopy (n = 304)
predictors
|
|
Difficult laryngoscopy
|
k value
|
95% CI
|
NO
|
YES
|
C-TMD
|
>1 finger
|
184
|
4
|
|
|
|
<1finger
|
81
|
35
|
0.485
|
0.286 - 0.612
|
IID
|
>3finger
|
173
|
8
|
|
|
|
<3finger
|
92
|
31
|
0.382
|
0.127-0.534
|
ULBT
|
1 grade
|
117
|
6
|
|
|
|
>1 grade
|
148
|
33
|
0.127
|
0.035 - 0.216
|
Mallampati Test
|
<3 grade
|
128
|
5
|
|
|
|
>2 grade
|
137
|
34
|
0.138
|
0.17 - 0.255
|
Abbreviations: ULBT, Upper lip bite test; IID, interincisor distance; C-TMD, condyle-tragus maximal distance; CI, confidence interval.
The predictive value of each predictor to predict difficult laryngoscopy
The receiver operating characteristic (ROC) curve analysis showed that the AUC of the C-TMD, IID, Mallampati classification and ULBT classification for predicting difficult laryngoscopy were 0.699, 0.637, 0.613 and 0.648 respectively (all P values were less than 0.001,See Figure 3 for details).
The OR value, sensitivity and specificity, positive predictive value and negative predictive value of each predictor were calculated. Among them, the predictive value of the C-TMD was the highest: OR value was 10.09 (95%CI: 4.19-24.28), sensitivity was 0.469 (95%CI:0.325-0.617), specificity was 0.929 (95%CI: 0.877-0.964), positive predictive value was 0.676 (95%CI: 0.484-0.745), negative predictive value was 0.847 (95%CI :0.825-0.865) (see Table 6 for details).
Table 6. Value of each predictor in predicting difficult laryngoscopy (n = 304)
predictors
|
Odds ratio
(95% CI)
|
Sensitivity
(95% CI)
|
Specificity
(95% CI)
|
PPV
(95% CI)
|
NPV
(95% CI)
|
C-TMD<1finger
|
10.09
(4.19-24.28)
|
0.469
(0.325-0.617)
|
0.929
(0.877-0.964)
|
0.676
(0.484-0.745)
|
0.847
(0.825-0.865)
|
IID<3finger
|
3.54
(1.55-8.12)
|
0.429
(0.288-0.578)
|
0.845
(0.778-0.898)
|
0.467
(0.418-0.493)
|
0.824
(0.746-0.887)
|
ULBT>1grade
|
2.48
(1.02-6.02)
|
0.796
(0.657-0.898)
|
0.400
(0.322-0.482)
|
0.295
(0.174-0.324)
|
0.861
(0.735-0.923)
|
Mallampati classification
>2grade
|
1.77
(0.78-3.91)
|
0.691
(0.546-0.817)
|
0.503
(0.422-0.584)
|
0.306
(0.276-0.371)
|
0.839
(0.813-0.908)
|
Abbreviations: CI, confidence interval; ULBT, Upper lip bite test; IID, interincisor distance; C-TMD, condyle-tragus maximal distance; NPV, negative predictive value; PPV, positive predictive value.
Results of reliability test
Twenty volunteers (14 males and 6 females) were successfully enrolled in the trial. There was no difference between the incidence rates of C-TMD <1 finger width estimated by the two anesthesiologists (both were 5%). The maximum movement distance of the condyle measured by ultrasound was 12.6 ± 2.3 mm. Analysis of the correlation between whether the C-TMD <1 finger width and the maximum movement distance of the condyle measured by ultrasound showed r = 0.91, and P<0.001 (Spearman correlation analysis).