endotracheal intubation is one of the most important skills for anesthesiologists in securing the airway during general anesthesia and resuscitation .
failure to secure the airway can cause anesthesia - related life - threatening morbidity and mortality .
however , the difficult laryngoscopy and tracheal intubation rate still remains at 1.513% due to poor reliability of traditional protocols , algorithms , and combinations of screening tools in identifying a potentially difficult airway .
due to the portable , noninvasive characteristics , point - of - care ultrasound ( us ) technique has been widely used in the operating room for ultrasound - guided nerve block , central venous access , and pneumothorax diagnosis . with improved visualization of airway structures ,
prasad et al . first found that us can reliably image all of the structures visualized by ct , and the infrahyoid airway structure parameters measured by ultrasound agree well with the parameters measured by ct .
adhikari et al . further demonstrated that the anterior neck soft tissue thickness measured by ultrasound at hyoid bone and thyrohyoid membrane levels can be used as an index to predict difficult laryngoscopy , but only the anterior neck soft tissue thickness at thyrohyoid membrane levels can be used as an independent predictor of difficult laryngoscopy .
interestingly , they did not find a correlation between us measurements and clinical screening tests .
.   also found that an abundance of fat tissue at the anterior neck region as detected by ultrasound in israeli obesity patients was a very good independent predictor of difficult laryngoscopy , being a much better predictor than body mass index ( bmi ) per se .
however , a similar study in the unites states showed that us quantification of anterior soft tissue and general bedside screening tests failed to predict difficult laryngoscopy in american obese patients , suggesting that racial or body shape differences might exist . to define the role of airway us in predicting difficult laryngoscopy
, we evaluated the feasibility of ultrasound in predicting difficult laryngoscopy in a chinese han population .
after approval of the research protocol by the local hospital ethics committee for human studies and obtaining personal informed consent , 203 american society of anesthesiologists ( asa ) grade i and ii adult patients undergoing elective surgeries and receiving general anesthesia were included in this study .
exclusion criteria included patients who had facial , cervical , pharyngeal and epiglottis surgical or trauma history , patients with most teeth lost , and patients with arthritis .
the modified mallampati score ( mms ) was specified according to the visibility of pharyngeal structures with the patient in an upright sitting position , head in neutral position , mouth wide open , and tongue protruding to its maximum without phonation ( figure 1a ) .
class i is visualization of the hard palate , soft palate , fauces , uvula , and pillars .
class ii is visualization of the hard palate , soft palate , fauces , and base of uvula .
thyromental distance ( tmd ) was measured from the mental prominence to the thyroid cartilage with the patient s neck extended fully .
interincisor gap ( iig ) was measured from the upper central incisors to the lower central incisors while the patient s mouth was fully opened .
ultrasound measurements were performed by the primary investigator with the patient supine and the head and neck in neutral position .
the thicknesses of anterior neck soft tissue at hyoid bone , thyrohyoid membrane , and anterior commissure levels were obtained transversely across the anterior surface of the neck with a 136 mhz hfl38 linear array ultrasound probe attached to a sonosite s - nerve machine ( sonosite inc . ,
the minimal distance from the hyoid bone to skin surface ( dshb ) was measured ( figure 2a , 2b ) . at thyrohyoid membrane level ,
the distance from skin to epiglottis midway ( dsem ) between the hyoid bone and thyroid cartilage was measured ( figure 2c , 2d ) . at anterior commissure level ,
the minimal distance from skin to anterior commissure ( dsac ) was obtained ( figure 2e , 2f ) .
after anesthesia induction with midazolam 0.04 mg / kg , propofol 22.5 mg / kg , fentanyl 24 g / kg , and succinylcholine 2 mg / kg , endotracheal intubation was carried out by anesthesia providers with a minimum of 2 years of endotracheal intubation experience .
the macintosh blades were used to expose the target larynx , and no external laryngeal pressure was used to facilitate this process .
classification of laryngoscopic views was based on the method described by cormack and lehane ( figure 1b ) .
continuous data are expressed as means ( standard deviation [ sd ] ) ; categorical data are expressed as numbers of occurrences ( percents ) .
comparison analysis was performed using the t - test for continuous variables and chi - square or fisher exact tests , as appropriate , for non - continuous variables .
the best fit lines of dshb , dsem , dsac , mms , iig , and tmd were determined using a least - squares regression technique .
receiver operating characteristic ( roc ) analyses were used to calculate the comparable threshold values of dshb , dsem , dsac , tmd , iig , and mms .
optimal cutoff values were calculated using the youden index ( calculated as : sensitivity + specificity  100 ) .
statistical analyses were performed using medcalc for windows , version 12.6.1.0 ( medcalc software , ostend , belgium ) .
the level of statistical significance was p<0.05 , and p<0.0001 was considered to be very statistically significant .
continuous data are expressed as means ( standard deviation [ sd ] ) ; categorical data are expressed as numbers of occurrences ( percents ) .
comparison analysis was performed using the t - test for continuous variables and chi - square or fisher exact tests , as appropriate , for non - continuous variables .
the best fit lines of dshb , dsem , dsac , mms , iig , and tmd were determined using a least - squares regression technique .
receiver operating characteristic ( roc ) analyses were used to calculate the comparable threshold values of dshb , dsem , dsac , tmd , iig , and mms .
optimal cutoff values were calculated using the youden index ( calculated as : sensitivity + specificity  100 ) .
statistical analyses were performed using medcalc for windows , version 12.6.1.0 ( medcalc software , ostend , belgium ) .
the level of statistical significance was p<0.05 , and p<0.0001 was considered to be very statistically significant .
a total of 203 eligible patients ( 120 females , 83 males ) were included in this study .
there were 28 of 203 patients ( 13.8% ) categorized as difficult laryngoscopy , and 50% of patients with difficult laryngoscopy in our study were males .
no differences were noted in sex , age , and height , but bw and bmi values in the difficult laryngoscopy group were higher . the bmi value was 25.632.80 kg / m for the difficult laryngoscopy group , and 23.613.43 kg / m for the easy laryngoscopy group ( p<0.05 ) .
airway evaluation parameters , including mms , tmd , iig , dshb , dsem , and dsac , are shown in table 2 .
the thicknesses of anterior neck soft tissue measured by us were greater in the difficult laryngoscopy group compared to the easy laryngoscopy group at the level of the hyoid bone , thyrohyoid membrane , and anterior commissure .
however , no statistically significant difference was found between easy and difficult laryngoscopy groups for tmd and iig .
a strong positive correlation existed between dsem and dshb ; moderate positive correlations between dsem and dsac , dshb and dsac ; small positive correlations between mms and dshb , mms and dsem , mms and dsac ; and very small positive correlations between tmd and iig .
the correlation coefficients along with 95% confidence interval ( ci ) between dsem and dshb , dsem and dsac , dshb and dsac , mms and dshb , mms and dsem , mms and dsac , and between tmd and iig were 0.74 ( 0.670.80 , p<0.0001 ) , 0.60 ( 0.500.68 , p<0.0001 ) , 0.69 ( 0.610.75 , p<0.0001 ) , 0.32 ( 0.190.44 , p<0.0001 ) , 0.27 ( 0.140.40 , p=0.0001 ) , 0.32 ( 0.190.44 , p<0.0001 ) , 0.18 ( 0.05 to 0.31 , p=0.0089 ) , respectively ( figure 3a3 g ) .
small negative correlation were found between iig and mms ( r=0.27 , p=0.0001 ) ; and very small negative correlations were found between mms and tmd ( r=0.20 , p=0.0042 ) , iig and dsac ( r=0.18 , p=0.0113 ) , and iig and dshb ( r=0.15 , p=0.0344 ) ( figure 3h3k ) .
no correlations were found between tmd and dsem , tmd and dshb , and tmd and dsac ( data not shown ) . to further assess the roles of mms , iig , tmd , dshb , dsem , and dsac in predicting difficult laryngoscopy ,
the roc curves ( figure 4 ) were drawn using medcalc software with laryngoscopy grade over ii as the threshold of difficult laryngoscopy . as determined by the youden index , the optimal cutoff values ( with sensitivity and specificity in parentheses ) for mms , iig , tmd , dshb , dsem , and dsac to predict difficult laryngoscopy were over 2 ( 50.0% , 82.3% ) , 3.8 cm ( 75.0% , 48.0% ) , 5.9 cm ( 21.4% , 93.7% ) , 1.28 cm ( 85.7% , 85.1% ) , 1.78 cm ( 100.0% , 66.3% ) , and 1.1 cm ( 75.0% , 80.6% ) , respectively .
the areas under the roc curve ( aucs ) were significantly different from the area under the reference line ( area=0.5 ) for mms , dshb , dsem , and dsac ; and the p values of mms , dshb , dsem , and dsac were all less than 0.0001 .
no difference between the aucs of tmd , iig , and the area under the reference line were identified , suggesting that mms , dshb , dsem , and dsac might be better than tmd and iig in predicting difficult laryngoscopy ( table 3 ) .
with the rapid expansion of portable us in anesthesia for guiding the placement of central venous catheters and the performance of regional block [ 1113 ] , some preliminary results exploring the feasibility of portable us in airway evaluation and management have been reported , even though the air attenuates us transmission and thereby creates artifacts [ 35,14,15 ] . in the present study , we compared the role of the us measurements and traditional screening tests , including iig , tmd , and mms , in predicting difficult laryngoscopy in a chinese han population .
we found that strong positive linear correlations existed among the thicknesses of anterior neck soft tissue measured by us at hyoid bone , thyrohyoid membrane , and anterior commissure levels .
the aucs of mms , dshb , dsem , and dsac are all over 0.7 , indicating they are all good parameters in predicting difficult laryngoscopy .
the aucs of tmd and iig were less than 0.7 , suggesting that tmd and iig were poor parameters in predicating difficult laryngoscopy . to clearly visualize the glottis and smoothly finish the intubation procedure , anesthesiologists need to place the laryngoscope blade to the base of the epiglottis , and then lift it up
therefore , in addition to the performer s skills and experience , many factors , including dental status , mouth opening , oropharyngeal space , mandibular space , and neck motility , are all involved this complicated procedure .
mms mainly reflects the size of the tongue relative to the oral cavity , or the oropharyngeal space [ 1820 ] , iig means the mouth opening , and tmd can reflect the length of the neck .
the mallapati score was first introduced by mallampati et al . in 1985 , and then modified by samsoon and young in 1987 . due to the simple and clear evaluation standards , mms has become one of the most commonly used tools to predict a difficult airway .
however , mms mainly depends on patient cooperation and body position , which can greatly affect the accuracy of airway evaluation .
therefore , mms has been reported to be a good predictor by many , but was found to be of limited value by others . a recent meta - analysis involving 177 088 patients
demonstrate that mms is inadequate as a stand - alone test to predict difficult laryngoscopy or tracheal intubation , but it may well be a part of a multifactorial model for the prediction of a difficult tracheal intubation .
iig has been repeatedly reported to be associated with difficult intubation , but our results showed that there was no significant difference in mean iig between difficult and easy laryngoscopy groups ( 3.740.43 vs. 3.860.47 cm ) .
furthermore , the auc of iig was 0.580.06 , which was far less than 0.7 , suggesting that iig might be not a good screening parameter for predicting difficult laryngoscopy in our study .
our iig conclusion was similar to the previous report by savva , but different from results of other studies .
whether this difference was really caused by racial factors requires further investigation because the measurements of iig need patients to fully cooperate .
tmd is also used as a screening test to predict difficult airway , but our results suggest that tmd is not a useful screening test .
however , we could not determine whether this was because of limited patient numbers or because tmd was not useful in predicting difficult airway due to the small sample size .
ct , mri , and other imaging techniques can accurately measure the thickness of anterior neck soft tissue , but they are expensive and unavailable in many operating rooms .
portable us is inexpensive , rapid , and convenient to perform in the operating room , and most importantly , it can quantify the neck fat thickness as accurately as mi .
however , the results of 2 prior studies focusing on soft tissue thickness at and below the level of the vocal cords in a specific group of obese patients are inconsistent .
further measured the anterior neck soft tissue at the hyoid bone and thyrohyoid membrane levels , which is important to displace the glottis by the laryngoscopic blade , and found that us measurements of anterior neck soft tissue thickness at the level of hyoid bone and thyrohyoid membrane can be used to predict difficult laryngoscopies , and a 2.8-cm us measurement at the thyrohyoid membrane was a good independent predictor of difficult laryngoscopy .
our results show that the thicknesses of anterior neck soft tissue at the level of the hyoid bone ( 1.51 [ 95% ci=1.401.61 ] cm vs. 0.98 [ 95% ci=0.941.02 ] cm ) , the thyrohyoid membrane ( 2.39 [ 95% ci=2.172.62 ] cm vs. 1.49 [ 95% ci=1.431.55 ] cm ) , and the anterior commissure ( 1.30 [ 95% ci=1.181.42 ] cm vs. 0.92 [ 95% ci=0.870.94 ] cm ) were greater in the difficult laryngoscopy group and were significantly correlated .
furthermore , the ranges of anterior neck soft tissue for those with difficult laryngoscopy were mutually exclusive from those patients with an easy laryngoscopy , indicating that they are independent predictors of difficult laryngoscopy , even though the ranges of these 3 parameters were smaller than in a previous report .
glottis exposure by place laryngoscope is a very complicated procedure , and many subjective and objective factors such as the provider s skills and experience , airway secretions , and abnormalities of anatomical structures are involved in this procedure .
the investigators were not totally blinded to the study purpose , and some clinical signs might indicate the possibility of difficult laryngoscopy , which can cause some bias during us measurements . due to the low incidence of difficult laryngoscopy , it is impossible for us to randomly assign an equal number of patients to both groups , thus there were 28 patients in the difficult group and 175 patients in the easy group .
anterior neck soft tissue thicknesses measured by us at the hyoid bone , thyrohyoid membrane , and anterior commissure levels are independent predictors of difficult laryngoscopy . the commonly used screening tests for difficult intubation have only poor - to - moderate predictive power when used alone .
combinations of these screening tests or risk factors with us measurements might increase the ability to predict difficult laryngoscopy .
 backgroundthe aim of this study was to determine if ultrasound ( us ) measurements of anterior neck soft tissue thickness at hyoid bone ( dshb ) , thyrohyoid membrane ( dsem ) , and anterior commissure ( dsac ) levels can be used to predict difficult laryngoscopy.material/methodswe included 203 patients age 2065 years scheduled to undergo general anesthesia in this prospective observational study . 
 correlation analysis and receiver operating characteristic curve ( roc ) analysis were used to determine the roles of screening tests [ interincisor gap ( iig ) , thyromental distance ( tmd ) , modified mallampati score ( mms ) ] and us measurements ( dshb , dsem , dsac ) in predicting difficult laryngoscopy.resultsthere were 28 out of 203 patients categorized as difficult laryngoscopy . 
 dshb , dsem , dsac , and mms were greater in the difficult laryngoscopy group ( p<0.0001 ) . 
 there was a strong positive correlation between dsem and dshb ( r=0.74 ) ; moderate positive correlations between dsem and dsac ( r=0.60 ) , dshb and dsac ( r=0.69 ) ; small positive correlations between mms and dshb ( r=0.32 ) , mms and dsem ( r=0.27 ) , mms and dsac ( r=0.32 ) , all p values 0.0001 ; very small positive correlation between tmd and iig ( r=0.18 , p=0.0089 ) ; small negative correlation between iig and mms ( r=0.27 , p=0.0001 ) ; and very small negative correlations between mms and tmd ( r=0.20 , p=0.004 ) , iig and dsac ( r=0.18 , p=0.011 ) , iig and dshb ( r=0.15 , p=0.034 ) . 
 the areas under the roc curve ( aucs ) of mms , dshb , dsem , and dsac were significantly larger compared with the reference line ( p<0.0001).conclusionsanterior neck soft tissue thicknesses measured by us at hyoid bone , thyrohyoid membrane , and anterior commissure levels are independent predictors of difficult laryngoscopy . 
 combinations of those screening tests or risk factors with us measurements might increase the ability to predict difficult laryngoscopy . 
Background
Material and Methods
Statistical analysis
Results
Discussion
Conclusions