the advancement of visual techniques implemented in perinatal medical care enables surgeons and anesthesiologists to perform airway management and to detect , monitor , and surgically treat life - threatening anomalies of the fetal tracheo - bronchial tree [ 111 ] .
thus , the tracheo - bronchial angles in the human fetus are of increasing relevance in perinatal medicine to determine both normal and pathological criteria adapted to anatomical particularities of the fetal tracheo - bronchial tree [ 1113 ] .
the angle of tracheal bifurcation refers to both the interbronchial angle , measured between the central axes of the right and left main bronchi , and the subcarinal angle , measured along the inferior borders of the 2 main bronchi [ 1316 ] .
as presented in helical ct images , the interbronchial angle is somewhat wider than the subcarinal one .
the tracheal bifurcation angle was found to vary from 40 to 99 , on average 6065 [ 1821 ] .
interestingly enough , in indian population the subcarinal angle was considerably smaller , and averaged 53.54 in fetuses with crl of 131200 mm , 62.98 in neonates , 58.58 in children aged 110 years , and 54.56 in teenagers .
since the deviation of the left main bronchus from the vertical is considerably greater than that of the right main bronchus , the left bronchial angle is much larger than the right one . to date , the radiographic and anatomical literature has mainly focused on the tracheal bifurcation angle in neonates , children , and adults [ 12,15,16,1822 ] , and to some extent on the fetus , mainly based on very subjective methods with the use of protractors and goniometers .
another serious limitation of most radiographic studies is the lack of standardization with respect to the phase of respiration , type of projection , degree of rotation , or posture .
in fact , the angle of tracheal bifurcation tends to decrease by up to 9 between inspiration and expiration alone .
the present autopsy study is the first in the medical literature to highlight the fetal tracheo - bronchial angles with the help of objective and repeatable digital image analysis , using an adequate program of nis - elements br 3.0 ( nikon ) . to supplement the missing information on the fetal tracheo - bronchial angles , in this study we aimed to investigate :  values of the right and left bronchial angles , and their sum as the interbronchial angle at varying gestational ages ; and  possible both male - female and age - related differences between the studied parameters
the study sample consisted of 73 human fetuses of both sexes ( 39 males and 34 females ) at the age of 1425 weeks of gestation ( table 1 ) , which had been derived from spontaneous abortions and stillbirths , and never intubated . in no case was the cause of fetal death related to congenital cardiovascular or laryngeo - tracheal anomalies .
the series included fetuses , which were only the outcome of causes of placental insufficiency .
the research was consistent with the rules of the research ethics committee of our university ( kb 189/2011 ) .
after opening the thorax through sternotomy , the heart and its great vessels were dissected with the use of microsurgical instruments , and then removed from the mediastinum .
thus , the trachea and main bronchi with no conspicuous anomalies were visualized in the posterior mediastinum . in every fetus ,
the trachea and 2 main bronchi , in situ with a millimeter scale , were placed perpendicularly to the optical lens axis , recorded using a nikon d200 camera , and digitalized to tiff images .
afterward , digital pictures of the trachea and main bronchi ( figure 1 ) were subjected to digital image analysis , using an adequate computer program ( nis - elements br 3.0 , nikon ) .
first , in each picture , the following 3 borderlines were consistently reproduced , as follows : the central axis of the right main bronchus , the central axis of the left main bronchus , and the vertical passing through the inferior point of tracheal bifurcation .
after that , the 3 variables of tracheal bifurcation angle were precisely defined and assessed .
right bronchial angle ( in degrees ) , computed between the central axis of the right main bronchus and the vertical passing through the inferior point of tracheal bifurcation ;   left bronchial angle ( in degrees ) , computed between the central axis of the left main bronchus and the vertical passing through the inferior point of tracheal bifurcation ; and   interbronchial ( tracheal bifurcation ) angle ( in degrees ) , computed as the angle of intersection between the 2 central axes of the right and left main bronchi .
we did not evaluate the subcarinal angle because of its irregular outlines . in a continuous attempt to minimize measurement and observer bias ,
the 3 repeated semi - automatic measurements were taken for each variable and averaged to minimize intra - observer variation .
the differences between the repeated measurements , as the intra - observer variation , were evaluated by one - way anova test for paired data and post - hoc bonferroni test .
the data obtained were checked for normality of distribution using the kolmogorov - smirnov test and for homogeneity of variance with the use of levene s test .
as the first step in the statistical analysis , the t - test was used to examine the influence of sex on the values of the parameters studied . since the fetuses were not equally distributed with relation to gestational age , to examine sex differences we tested possible differences between the following 9 age groups : 1416 , 17 , 18 , 19 , 20 , 21 , 22 , 23 , and 2425 weeks .
next , we tested possible sex differences for the whole sample , without taking into account age . to examine the relationship between values of every tracheo - bronchial angle and fetal age , the kruskal - wallis test was used .
the values of 3 tracheo - bronchial angles were plotted against fetal age in weeks to establish their growth dynamics .
we attempted to introduce both linear and nonlinear regression analysis to derive the possible curve of best fit for each angle studied against gestational age .
no statistically significant differences were found in the evaluation of intra - observer reproducibility of the 3 tracheo - bronchial angles ( p>0.05 , the one - way anova test for paired data , and post - hoc bonferroni test ) .
obviously , inter - observer variability was not assessed , since all measurements had been done by 1 observer .
the morphometric values obtained were characterized by normality of distribution ( the kolmogorov - smirnov test ) and homogeneity of variance ( levene s test ) . as a consequence , all the quantitative variables were expressed as the mean  standard deviation .
the statistical analysis showed no male - female differences for the 3 tracheo - bronchial angles ( p>0.05 , t - test ) , so the numerical data without regard to sex are presented in table 2 .
the statistical analysis showed that values of the 3 tracheo - bronchial angles evolved independently of fetal age ( p>0.05 , kruskal - wallis test ) .
the right bronchial angle ( figure 2 ) ranged from 11.4 to 41.8 , with its overall mean of 26.97.0. throughout analyzed period , the values of left bronchial angle ( figure 3 ) varied from 24.8 to 64.8 , and averaged 46.28.0. as a consequence , the interbronchial angle totalled from 36.2 to 96.6. the overall mean of the interbronchial angle averaged 73.112.7 ( figure 4 ) .
the regression curves of best fit for every tracheo - bronchial angle against fetal age could not be modelled because of great inter - individual variability of their values at varying gestational ages . instead ,
figure 5 summarizes only the developmental trend of the 3 tracheo - bronchial angles during the analyzed period .
the present study describes a cross - sectional interpretation of the 3 tracheo - bronchial angles based on the evidence from 73 fetuses at ages of 1425 weeks .
thus , it is rather a populational perspective than a true representation of growth in itself .
the main limitation of this study is a relatively narrow gestational age , ranging from 14 to 25 weeks . apart from this ,
our results suffer from lack of inter - observer variability because all numerical data have been controlled by 1 researcher .
fortunately , the adequate digital program of nis - elements br 3.0 ( nikon ) appears to compensate for that disadvantage due to precise semi - automatic evaluation of the 3 tracheo - bronchial angles in digital pictures .
the medical literature on the tracheal bifurcation angle offers many discrepancies between reported values   with relation to sex-   and age - dependent   differences . as reported by karabulut , the 4 tracheo - bronchial angles ( right and left bronchial , interbronchial and subcarinal ) were significantly greater in females than males .
similarly , according to kamel et al . , the subcarinal angle in adults ranged from 36 to 121 in men , and from 47 to 115 in women , with overall means of 7620 , and 8120 respectively , without any correlation with age .
this remains contradictory to most of the previous studies , which reported the tracheal bifurcation angle to be independent of sex [ 12,13,1820,25 ] , and to our findings in the material under examination . to our knowledge , the significantly greater tracheo - bronchial angles in females may partly have resulted from obesity of the examined women , positively correlated with the tracheal bifurcation angle .
most authors have agreed that , irrespective of sex and age , the right bronchial angle was smaller than the left one [ 9,12,13,15,16,1921,25,26 ] . according to harjeet et al .
, in the 3 groups of fetuses with crl of 61130 mm , 131200 mm , and 201270 mm , the left bronchial angles were consequently wider than the right ones , and averaged 32.176.98 vs. 17.377.77 , 33.798.57 vs. 19.758.73 , and 34.743.67 vs. 20.541.86 , respectively . in the material under examination , the right and left bronchial angles correlated with the literature data , ranging from 11.4 to 41.8 , with the mean of 26.97.0 on the right , and from 24.8 to 64.8 with the mean of 46.28.0 on the left , with no effect of age or sex .
as reported by jit and jit , in neonates , infants , and children , the right bronchial angle was always smaller than the left . however , tahir et al .  
noticed the right main bronchus deviates significantly less from the vertical on chest radiograms in children over 3 years when compared to younger children ( 25 vs. 28 ) .
this may have been influenced by a greater proportion of upright chest radiograms in older children and a greater proportion of supine chest radiograms in younger children .
a radiographic study by harjeet et al .   revealed an insignificant increase in the subcarinal angle with age from 49.5312.10 in fetuses with crl of 61130 mm , through 53.5416.17 in fetuses with crl of 131200 mm , up to 55.273.66 in fetuses with crl of 201270 mm .
although in the present study the interbronchial angle attained much larger values than the afore - mentioned subcarinal angle , varying widely from 36.2 to 96.6 to average 73.112.7 , there were no statistical differences with age , in keeping with karabulut s helical ct studies .
of note , in the present study the values of fetal tracheo - bronchial angles were unpredictable in particular individuals .
in fact , regression curves of best fit for every tracheo - bronchial angle against fetal age could not be computed . in patients aged 1785 years
, the mean interbronchial and subcarinal angles averaged 7713 ( range 49109 ) and 7316 ( range 34107 ) .
furthermore , with relation to the subcarinal angle , the interbronchial angle was greater , smaller , or equal in 67% , 28% , and 5% of individuals , respectively . after presenting the morphometric analysis of tracheo - bronchial angles
the subcarinal and interbronchial angles were found to correlate positively with the size of the left atrium of the heart . the tracheal bifurcation angle may be widened due to cardiac disease ( left atrial enlargement , cardiomegaly , and pericardial effusion ) and mediastinal abnormalities ( subcarinal masses ) .
however , the angle of tracheal bifurcation may be reduced after pulmonary lobectomy and lobar collapse .
furthermore , there was a weak inverse correlation between the shape of thorax and the tracheal bifurcation angle .
thus , as the thorax was shorter and wider , the angle of tracheal bifurcation increased . both major anatomical   and clinical textbooks   support that inhaled foreign bodies
are more likely ( 70% ) to enter the right main bronchus than the left because the former is wider and steeper , slightly deviating from the midline of the trachea [ 12,13,16,1921,30 ] .
some authors   emphasized the greater airflow through the right lung , and the left position of the carina .
.   stated that in 66% of children , especially under age 1 year , the carina was positioned to the left of the midline of the trachea .
furthermore , kamel et al .   found the carina to be situated on the left in up to 81.4% of individuals .
the left - sided carina is reported to increase the  catchment area  of the right main bronchus .
the tracheo - bronchial angles change independently of sex and fetal age . the left bronchial angle is wider than the right .
values of the 3 tracheo - bronchial angles are unpredictable because their regression curves of best fit with relation to fetal age can not be modelled .
the 2 bronchial angles and the interbronchial angle are of great relevance in the location of inhaled foreign bodies , and in the diagnosis cardiac diseases and mediastinal abnormalities .
 backgroundboth the advancement of visual techniques and intensive progress in perinatal medicine result in performing airway management in the fetus and neonate affected by life - threatening malformations . 
 this study aimed to examine the 3 tracheo - bronchial angles , including the right and left bronchial angles , and the interbronchial angle , in the fetus at various gestational ages.material/methodsusing methods of anatomical dissection , digital image analysis with an adequate program ( nis - elements br 3.0 , nikon ) , and statistics , values of the two bronchial angles and their sum as the interbronchial angle were semi - automatically measured in 73 human fetuses at the age of 1425 weeks , derived from spontaneous abortions and stillbirths.resultsno male - female differences between the parameters studied were found . 
 the 3 fetal tracheo - bronchial angles were found to be independent of age . 
 the right bronchial angle ranged from 11.4 to 41.8 , and averaged 26.97.0 for the whole analyzed sample . 
 the values of left bronchial angle varied from 24.8 to 64.8 , with the overall mean of 46.28.0. as a consequence , the interbronchial angle totalled 36.296.6 , and averaged 73.112.7.conclusionsthe tracheo - bronchial angles change independently of sex and fetal age . 
 the left bronchial angle is wider than the right one . 
 values of the 3 tracheo - bronchial angles are unpredictable since their regression curves of best fit with relation to fetal age can not be modelled . 
 both of the 2 bronchial angles and the interbronchial angle are of great relevance in the location of inhaled foreign bodies , and in the diagnosis cardiac diseases and mediastinal abnormalities . 
Background
Material and Methods
Results
Discussion
Conclusions