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Upper airway modifications in head extension during development
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ACCPM-172; No. of Pages 6 Anaesth Crit Care Pain Med xxx (2016) xxx–xxx
Original Article
Upper airway modifications in head extension during development§ Antoine Be´cret a, Renaud Vialet b,*, Kathia Chaumoitre c, Anderson Loundou d, Nathalie Lesavre e, Fabrice Michel b a
Department of Anesthesia and Intensive Care, Hoˆpital d’Instruction des Arme´es Laveran, 34, boulevard Laveran, 13013 Marseille, France Department of Anesthesia and Intensive Care, Pediatric and Neonatal Intensive Care Unit, Hoˆpital Nord, Assistance-Publique des Hoˆpitaux de Marseille, chemin des Bourrely, 13015 Marseille, France c Medical Imaging Department, Hoˆpital Nord, Assistance-Publique des Hoˆpitaux de Marseille, chemin des Bourrely, 13015 Marseille, France d Public Health Department, Self-Perceived Health Assessment Research Unit, School of Medicine, 27, boulevard Jean-Moulin, 13005 Marseille, France e Clinical Investigations Center, Hoˆpital Nord, Assistance-Publique des Hoˆpitaux de Marseille, chemin des Bourrely, 13015 Marseille, France b
A R T I C L E I N F O
A B S T R A C T
Article history: Available online xxx
Background: One of the requirements of laryngoscopy is to determine which head position will result in optimal visualization. Our hypothesis was that parameters derived from magnetic resonance imaging (MRI) can help quantify the effect of age on airway modifications due to head extension during development. Method: In children undergoing planned MRI, additional sequences on the upper airways were performed: one in a near-neutral position, the other with the head extended at 358. The axis of the face, the pharynx, the larynx, the trachea, and the line of glottic visualization were determined. The following angles were calculated: the Visu-Lar angle, formed by the line of glottic visualization and the laryngeal axis, and the Phar-Lar angle, formed by the pharyngeal and laryngeal axes. Results: One hundred and fifty-five patients (1 to 222 months of age [25–145] months) were included and 54% were under general anaesthesia. Age had no effect on the variation in the Visu-Lar angle, which diminished as a function of head extension, nor on the variation in the Phar-Lar angle, which was minimal in the neutral position. During extension, anatomical axes rotated similarly, and the visualization axis rotated the most, followed by the pharyngeal and laryngeal axes. These results were not correlated with general anaesthesia. Conclusion: Regardless of age, head extension diminished the Visu-Lar angle, and increased the Phar-Lar angle. This study supports that, as in adults, head extension is probably the key factor for good visualization conditions during laryngoscopy on children, but clinical data is needed to confirm this result. ß 2016 Socie´te´ franc¸aise d’anesthe´sie et de re´animation (Sfar). Published by Elsevier Masson SAS. All rights reserved.
Keywords: Growth variation Anaesthesiology Intubation Magnetic resonance
1. Introduction Since the publication of Bannister’s work in 1944 in The Lancet [1], airway imaging has generated much discussion. Nevertheless, solid evidence elucidating airway configuration during laryngoscopy and intubation in children is rare. Airway access in children is considered to be more complex and difficult than in adults, as paediatric airways and cephalic proportions differ throughout growth, which frequently frustrates
§
Trial registration number: NCT00920673. * Corresponding author. Tel.: +33 4 91 96 96 68; fax: +33 4 91 96 27 51. E-mail address: [email protected] (R. Vialet).
non-experienced or non-specialized healthcare providers. To the best of our knowledge, empirical recommendations for the optimal head position have never been precisely analysed using objective data [4]. One of the key requirements for laryngoscopy is the determination of which head position will enable optimal conditions for glottic visualization. This issue can be approached by anatomical angle measurement via magnetic resonance imaging (MRI) studies of the airway in different head positions. With this method, Adnet et al. concluded that for anatomical angle variations (measured via MRI imaging), the sniffing position does not differ from simple head extension [2]. The clinical potential associated with the MRI study was later confirmed when a subsequently initiated clinical trial could not demonstrate a
http://dx.doi.org/10.1016/j.accpm.2016.04.003 2352-5568/ß 2016 Socie´te´ franc¸aise d’anesthe´sie et de re´animation (Sfar). Published by Elsevier Masson SAS. All rights reserved.
Please cite this article in press as: Be´cret A, et al. Upper airway modifications in head extension during development. Anaesth Crit Care Pain Med (2016), http://dx.doi.org/10.1016/j.accpm.2016.04.003
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ACCPM-172; No. of Pages 6 A. Be´cret et al. / Anaesth Crit Care Pain Med xxx (2016) xxx–xxx
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significant advantage of the sniffing position over simple head extension [3]. Our hypothesis was that MRI imaging data in children would be a first important step before clinical studies by providing objective data describing and quantifying the effect of age on airway configuration during head extension. Our main objective was to determine the effect of age on two clinically significant anatomical angles for laryngoscopy and intubation: the angle between the visualization and laryngeal axes (Visu-Lar angle) and the angle between the pharyngeal and laryngeal axes (Phar-Lar angle). The secondary objective was to describe all the axes of the airway during head extension.
2. Methods From July 2009 to September 2011, all children who had a planned MRI (with our without general anaesthesia) were considered for inclusion in the study. The MRIs were indicated by paediatricians not involved in the study. Indication of general anaesthesia was not protocoled, and followed the routine practice of the physicians involved. General anaesthesia was proposed by the paediatrician or the radiologist (after failure of awake procedures, and indicated by an anaesthesiologist). The children and their parents received oral and written information about the study and the added time for the MRI. Informed written consent was obtained from the parents. Exclusion criteria included the following: obesity (Body mass index [BMI] > 97th percentile) [5], macro- or microcephaly [6], head dysmorphism, tumour or abnormality near the upper airway, need for upper airway control (i.e., laryngeal mask or tracheal tube) if general anaesthesia was chosen, an emergency MRI or the lack of consent from the child or either of the parents. The patients under general anaesthesia did not receive any premedication and were anaesthetised with sevoflurane via a high concentration mask. During the MRI examination, the inspired fraction concentration was between 2 to 3%. For the first head position, the child was in the supine position, directly on the flat MRI table, and the head was positioned in Frankfort’s plane [7], defined as the plane between the external auditory canal and the external corner of the eye, perpendicular to the table. For the second head position (extension), after the first MRI acquisition, the head was extended at an incline of 358 to the Frankfort’s plan. Before each sequence, the head was positioned by the MRI technician and verified with a non-magnetic square from the MRI tube. All studies were performed using a 1.5 Tesla system (Maestro class; Siemens, Erlanger, Germany). The study sequences were completed using an ear-nose-throat antenna. The acquisition technique was a spin echo sequence with a repetition time of 703 ms and an echo time of 13 ms. T1-weighted images were obtained in the sagittal plane. Demographic characteristics for each patient were recorded. For both head positions for each patient, the axes measured included the axis of the face (face), which extends from the brow to the chin; the pharyngeal axis (Phar) (Fig. 1), which extends through the anterior portion of the atlas and C2; the laryngeal axis (Lar), which extends through the centre of the lower (cricoid cartilage) and upper (airway centre at the base of the epiglottis) laryngeal orifices; the tracheal axis (Trac), which extends from the lower laryngeal orifice through the centre of the trachea at the second ring level; and the line of glottic visualization (Visu), which extends from the lower end of the upper incisors or the gum (in edentulous children) to the corniculate cartilage (posterior part of the thyroid cartilage).
The following angles were calculated: the Visu-Lar angle, formed by the line of the glottic visualization and laryngeal axis, and the Phar-Lar angle, formed by the pharyngeal and laryngeal axes. Variations between the resting position and head extension of the Face, Phar, Lar, Visu, Visu-Lar angle and Phar-Lar angle were also calculated. Every MRI study was interpreted independently by two of the authors. When the difference between the measurements was less than 108, the final measurement was the average of the two measurements. When the difference was greater than 108, a third measurement was made by the two authors together. Inclusions were prospectively made following an age-stratified plan (Table 1). The results from a previous study [8] helped determine the number of patients per age group; thus, a total number of 150 patients was needed for an a-risk of 0.05 and a b-risk of 0.80. Then, an age-stratified plan was determined: after verification of linearity, correlation coefficients (parametric and polychoric) were calculated for each age group and their significance tested. Then, coefficients were established for each age group. The demographic data are reported as medians and quartiles. The head extension values in the first and second positions are reported as averages and standard deviations. The number of children in each head extension group is displayed in a histogram (Fig. 2). Multivariate analysis (a general linear model) was used to test for an age effect and included head extension, weight, sex, general anaesthesia (yes or no) and cranial circumference as variables. The relationships between the axis positions and head extension were analysed using non-parametric Spearman correlation. A P-value of less than 0.05 was considered statistically significant. All data analyses were performed using SPSS v17.0 software (SPSS, Inc., Chicago IL).
3. Results Over a period of 27 months, 168 patients were included in the study and a total of 155 examinations were studied. After several months of inclusion, we encountered difficulties in including children over 24 months old (Appendix 1). After approval by the statistician, we accepted a deviation in the scheduled stratification plan (Table 2). Twenty examinations were excluded from the study because of examination interruption at the child’s request or because the images were otherwise unusable (metal-induced artefacts/movement of the child). The patients’ characteristics are shown in Table 1 and the age distribution is reported in Table 2. The study design was expected to describe two levels of head extension in each child (08 and 358). However, continuous degrees of head extension (Fig. 2) were obtained in both groups. In the neutral extension group, there was in fact an average extension of 138 (8). In the extension group, the average extension was 138 (11). The mean amplitude in extension achieved in our population was 268 (9). Multivariate analysis showed that age was not statistically correlated with Visu-Lar or Phar-Lar angle variation during head extension while considering weight, gender, cranial circumference and the presence or absence of general anaesthesia (Table 3). The t-value was 1.66 (P = 0.1) for the Visu-Lar angle and 0.64 (P = 0.52) for the Phar-Lar angle. Age was correlated with movement of the axes: Visu (t = 2.51 [P = 0.01]), Phar (t = 2.35 [P = 0.02]), Lar (t = 2.87 [P < 0.01]) and Trac (t = 2.66 [P < 0.01]) (Fig. 1).
Please cite this article in press as: Be´cret A, et al. Upper airway modifications in head extension during development. Anaesth Crit Care Pain Med (2016), http://dx.doi.org/10.1016/j.accpm.2016.04.003
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Fig. 1. Two examples of angles modifications during extension.
During head extension, all the anatomical axes rotated in the same direction, but not to the same extent. The rho correlation coefficients are shown in Table 4. The visualization axis was more affected by head extension, as on average 93% more magnitude was transferred to this axis during extension. The magnitude change was far lower for the pharyngeal axis. The laryngeal axis had a negligible but statistically significant rotation. On average, changes in the tracheal axis were not associated with head extension. The Visu-Lar angle significantly narrowed during head extension due to the movement of the visualization axis, but the
Table 1 Demographic data presented as medians (lower quartile; upper quartile).
Fig. 2. Observed head extensions histogram.
n = 155 Age (months) Weight (kg) Height (cm) Sex ratio BMI (kg/m2) Cranial perimeter (cm) Head anteroposterior diameter (cm) General anaesthesia
86 (24; 142) 21 (12; 41) 120 (86; 150) 56 16.9 (15.6; 19.1) 52 (48; 54) 17.9 (16.5; 18.6) 50.3%
laryngeal axis did not vary. The Phar-Lar angle significantly decreased during head extension due to the movement of the pharyngeal axis, but the laryngeal axis did not vary (Table 4). Although the values of the two angles decreased when the head was extended, the pattern differed. During head extension, the Visu-Lar angle had positive values and decreased with respect to the neutral position value. However, the Phar-Lar angle initially
Please cite this article in press as: Be´cret A, et al. Upper airway modifications in head extension during development. Anaesth Crit Care Pain Med (2016), http://dx.doi.org/10.1016/j.accpm.2016.04.003
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4 Table 2 Planned stratification plan and actual inclusions. Age group (years)
Planned inclusions
Actual inclusions
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Total
25 20 20 15 15 4/5 4/5 4/5 4/5 4/5 4/5 4/5 4/5 4/5 4/5 150
24 14 12 9 7 9 5 6 11 8 11 10 7 13 9 155
Table 4 Univariate analysis of the effect of head extension and age on anatomic axis positions and angle values.
Visualization Laryngeal axis Pharyngeal axis Trachea Phar-Lar angle Visu-Lar angle
Axis correlation coefficient with extension
R2
0.93 (P < 0.001) 0.14 (P < 0.05) 0.66 (P < 0.001) 0.01 (P = 0.83) 0.61 (P < 0.001) 0.83 (P < 0.001)
0.85 0.03 0.42 < 0.01 0.34 0.66
R2: Rho correlation coefficient (significance) All the anatomic axes rotated in the same direction, but not to the same extent. The visualization axis was the more affected by head extension, as on average 93% of the magnitude of the extension was transferred to this axis. It was far less transferred to the pharyngeal (and vestibule) axis. On average, the larynx and trachea axes are not linked to head extension. The Visu-Lar angle significantly narrowed during head extension due to the movement of the visualization axis, as the larynx axis did not vary. The Phar-Lar angle significantly narrowed during head extension, due to the movement of the pharynx axis, as the larynx axis did not vary.
showed positive values during head flexion, and these values became negative during head extension (Fig. 3). A null value for the Phar-Lar angle found on the face axis was 08, i.e., the head was in a neutral position. 4. Discussion The main result of this study is that modifications of the VisuLar and Phar-Lar angles are not influenced by patient age. To the best of our knowledge, this is the first study describing and analysing the anatomical axes of the upper airway during head extension throughout the entire range of child growth and development. As anatomical modifications are more pronounced in the earlier stages of life [9], the study inclusion criteria were designed to allow stratification by age. Two-dimensional imaging could not represent the actual conditions of laryngoscopy and intubation. However, MRI imaging of the airway provided important evidence with regard to laryngoscopy [2,8,10–12] and had the advantage of being objective. We thus decided to describe the anatomy and its modifications in a manner similar to the methods proposed by Adnet et al. [2]. The Visu-Lar angle (angle o´ in the Adnet study) may be considered as representative of actual glottic visualization during direct laryngoscopy. The ease of insertion of a nasotracheal tube certainly depends on the Phar-Lar angle (i.e., the b-angle in the Adnet study). The exact localization of the axes can be difficult, particularly in the smallest patients, but the independent analyses by two readers should have limited the effects of measurement variability. As concerns statistical analysis, head extension was considered to be a continuous but not normally distributed variable. As weight
was the more discriminant variable (out of gender, height, weight, and BMI), only the results for weight are reported (Table 3). The Spearman correlation test was chosen because head extension was not normally distributed. One of the strengths of this study was the sample size. The absence of an age effect on the Visu-Lar and Phar-Lar angles cannot be due to a lack of power. The study was primarily designed to obtain two groups of distinct and standardized head extensions, but these exact extensions were not achieved. This issue confirms the difficulties encountered in accurate standardization of head position in children, when the head is out of reach. As the head is out of reach during MRI acquisition, such difficulties may be the result of a spontaneous trend toward head flexion in children, particularly in anesthetized children [13,14], of unnoticed slides between positioning (outside the MRI tube) and MRI acquisition, and of difficulties in maintaining a steady position in conscious children. However, the mean degree of extension achieved was 268. This value was greater than in a previous study [8] and was almost equivalent to the extension applied to the head during a direct laryngoscopy in the operating room with a Macintosh laryngoscope [15,16]. In fact, the distribution of the observed head extension (Fig. 2) revealed a fairly homogeneous distribution throughout the entire range. Those patients who underwent general anaesthesia did not have either an endotracheal tube or a laryngeal mask in the upper airways that could have biased the results [17–19]. In this study, anaesthesia did not have significant effects on the observed axes and angles. This finding does not necessarily rule out all effects of anaesthesia on upper airway configuration, but only suggests that anaesthesia does not have a quantitative influence on airway axes compared to the effects of head extension and patient age.
Table 3 t-values from multivariate analysis of the variation of anatomic angles during head extension. The results are expressed as: partial correlation coefficients (level of signification of the correlation). Extension Visu-Lar angle variation Phar-Lar angle variation
10.19 (P < 0.01) 5.88 (P < 0.01)
Age 1.66 (P < 0.1) 0.64 (P = 0.52)
Weight
Gender
2.62 (P = 0.01) 1.23 (P = 0.22)
0.83 (P = 0.41) 0.33 (P = 0.74)
Cranial perimeter 1.02 (P = 0.31) 0.53 (P = 0.59)
General anaesthesia 0.37 (P = 0.71) 0.16 (P = 0.87)
Age was not statistically correlated with either angle variation during head extension while considering weight, gender, cranial perimeter and anaesthesia. The negative correlations coefficients indicate that during head extension the Visu-Lar and Phar-Lar angles were reduced, showing a better alignment of the visualisation axis with the laryngeal axis.
Please cite this article in press as: Be´cret A, et al. Upper airway modifications in head extension during development. Anaesth Crit Care Pain Med (2016), http://dx.doi.org/10.1016/j.accpm.2016.04.003
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Fig. 3. Visu-Lar and Phar-Lar angles vs. head extension plots.
To the best of our knowledge, this is the first study that provides rational data describing the effect of age on airway modifications during head extension. These data strongly support empirical recommendations favouring ‘‘slight extension’’ for laryngoscopy in children and infants [4,13]. The reduction of the Visu-Lar angle during head extension shown by Adnet et al. [2,8] and another previous study was confirmed in our study. To explain the evolution of the Visu-Lar angle, we focused on the axis it formed with: the visualization axis and the laryngeal axis. During head extension, the visualization axis, which was always positive, rotated almost with the same amplitude as the head extension (Table 4), but the laryngeal axis had an amplitude of only 1/8 of that of the extension. This observation explains why the Visu-Lar angle decreased during head extension. The relative macrocephaly of infants and young children induces an anterior neck flexion during head extension [8]. It should be mentioned that this does not mean that the movements of the cervical column and the atlanto occipital joint are the same in children and adults, but only that the relationship between head extension and the Visu-Lar angle does not depend on age. It can be hypothesized that the absence of an age effect on the reduction of the Visu-Lar angle indirectly confirms the results of Adnet et al. [2], in which no superiority was demonstrated for the sniffing position compared with simple head extension for reducing the Visu-Lar angle. Previous studies have found an increase in the Phar-Lar angle during head extension [2,8]. In our study, however, this correlation was negative. This could be due to the use of angle absolute values in previous studies. Another interesting point is that the analysis of the Phar-Lar angle throughout the movement showed that the observed minimum angle was close to that of the neutral position (Fig. 3). The best alignment of the laryngeal and pharyngeal axes was achieved in the neutral position, which seems to be the best position for the introduction of a nasotracheal tube, regardless of age. Analysis of the movements of each axis from head flexion through complete head extension gave information regarding the dynamic configuration of the airway throughout extension. During head extension (i.e., clockwise head rotation), all the axes studied rotated in the same direction. The difference between the axes was the amplitude of the rotation during head extension. The visualization axis was the axis that rotated the most as a result
of head movement. The pharyngeal axis rotated 60% between head flexion to extension. The laryngeal axis had a clinically insignificant rotation, because, with a correlation coefficient of 0.15 and a 358 extension applied to the head, the movement transmitted to the larynx would be only 58, which is close to the precision limit for data interpretation. The upper cervical portion of the trachea did not rotate in this study, as measured by the axis that passed through the lower laryngeal orifice and the centre of the trachea at the second ring. Paediatric airways challenge the confidence of non-specialized healthcare providers. Obviously, anatomical considerations are no substitutes for practice and experience, but just as the principles of airway patency in adults are applicable to children [20], this study supports that the fundamental bases of laryngoscopy are valid in infants and children. 5. Conclusion The main conclusion of this study is that age has no influence on the Visu-Lar or Phar-Lar angles. Regardless of the age of the child, head extension was the only factor that improved glottic visualization and the best alignment of the pharynx and larynx was obtained in the neutral position. Throughout head extension movement, the visualization axis rotated the most, followed by the pharyngeal axis. The movement of the laryngeal axis was clinically insignificant, and the trachea did not rotate. Paediatric airways differ from those of adults. However, this study supports that, like in adults, the degree of head extension is probably the key factor for laryngoscopy and intubation success in infants and children. Funding The local ethics committee (Comite´ de protection des personnes pour la recherche biome´dicale Sud-Me´diterrane´e 2) approved this study, which was registered on clinicaltrials.gov (ClinicalTrials.gov identifier: NCT00920673). This research was carried out without funding. Disclosure of interest The authors declare that they have no competing interest.
Please cite this article in press as: Be´cret A, et al. Upper airway modifications in head extension during development. Anaesth Crit Care Pain Med (2016), http://dx.doi.org/10.1016/j.accpm.2016.04.003
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Appendix 1
12 first months
15 next months
Paents screened: 205
Paents screened: 224
Exclusion criteria: n = 5
Exclusion criteria: 4
Inclusion not considered:
Inclusion not considered:
(age class completed) n= 92
(age class completed) n= 146
Inclusion considered (n = 74)) (age class not complete, or minor age difference from the scheduled straficaon plan)
Inclusion considered: (n= 108)
Parental / paent consent:n = 71
Parental/paent consent:n = 104
Total: n = 175 Exclusion (images not usable): n = 20 RMI examinaons included: n = 155
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Please cite this article in press as: Be´cret A, et al. Upper airway modifications in head extension during development. Anaesth Crit Care Pain Med (2016), http://dx.doi.org/10.1016/j.accpm.2016.04.003
ACCPM-172; No. of Pages 6 Anaesth Crit Care Pain Med xxx (2016) xxx–xxx
Original Article
Upper airway modifications in head extension during development§ Antoine Be´cret a, Renaud Vialet b,*, Kathia Chaumoitre c, Anderson Loundou d, Nathalie Lesavre e, Fabrice Michel b a
Department of Anesthesia and Intensive Care, Hoˆpital d’Instruction des Arme´es Laveran, 34, boulevard Laveran, 13013 Marseille, France Department of Anesthesia and Intensive Care, Pediatric and Neonatal Intensive Care Unit, Hoˆpital Nord, Assistance-Publique des Hoˆpitaux de Marseille, chemin des Bourrely, 13015 Marseille, France c Medical Imaging Department, Hoˆpital Nord, Assistance-Publique des Hoˆpitaux de Marseille, chemin des Bourrely, 13015 Marseille, France d Public Health Department, Self-Perceived Health Assessment Research Unit, School of Medicine, 27, boulevard Jean-Moulin, 13005 Marseille, France e Clinical Investigations Center, Hoˆpital Nord, Assistance-Publique des Hoˆpitaux de Marseille, chemin des Bourrely, 13015 Marseille, France b
A R T I C L E I N F O
A B S T R A C T
Article history: Available online xxx
Background: One of the requirements of laryngoscopy is to determine which head position will result in optimal visualization. Our hypothesis was that parameters derived from magnetic resonance imaging (MRI) can help quantify the effect of age on airway modifications due to head extension during development. Method: In children undergoing planned MRI, additional sequences on the upper airways were performed: one in a near-neutral position, the other with the head extended at 358. The axis of the face, the pharynx, the larynx, the trachea, and the line of glottic visualization were determined. The following angles were calculated: the Visu-Lar angle, formed by the line of glottic visualization and the laryngeal axis, and the Phar-Lar angle, formed by the pharyngeal and laryngeal axes. Results: One hundred and fifty-five patients (1 to 222 months of age [25–145] months) were included and 54% were under general anaesthesia. Age had no effect on the variation in the Visu-Lar angle, which diminished as a function of head extension, nor on the variation in the Phar-Lar angle, which was minimal in the neutral position. During extension, anatomical axes rotated similarly, and the visualization axis rotated the most, followed by the pharyngeal and laryngeal axes. These results were not correlated with general anaesthesia. Conclusion: Regardless of age, head extension diminished the Visu-Lar angle, and increased the Phar-Lar angle. This study supports that, as in adults, head extension is probably the key factor for good visualization conditions during laryngoscopy on children, but clinical data is needed to confirm this result. ß 2016 Socie´te´ franc¸aise d’anesthe´sie et de re´animation (Sfar). Published by Elsevier Masson SAS. All rights reserved.
Keywords: Growth variation Anaesthesiology Intubation Magnetic resonance
1. Introduction Since the publication of Bannister’s work in 1944 in The Lancet [1], airway imaging has generated much discussion. Nevertheless, solid evidence elucidating airway configuration during laryngoscopy and intubation in children is rare. Airway access in children is considered to be more complex and difficult than in adults, as paediatric airways and cephalic proportions differ throughout growth, which frequently frustrates
§
Trial registration number: NCT00920673. * Corresponding author. Tel.: +33 4 91 96 96 68; fax: +33 4 91 96 27 51. E-mail address: [email protected] (R. Vialet).
non-experienced or non-specialized healthcare providers. To the best of our knowledge, empirical recommendations for the optimal head position have never been precisely analysed using objective data [4]. One of the key requirements for laryngoscopy is the determination of which head position will enable optimal conditions for glottic visualization. This issue can be approached by anatomical angle measurement via magnetic resonance imaging (MRI) studies of the airway in different head positions. With this method, Adnet et al. concluded that for anatomical angle variations (measured via MRI imaging), the sniffing position does not differ from simple head extension [2]. The clinical potential associated with the MRI study was later confirmed when a subsequently initiated clinical trial could not demonstrate a
http://dx.doi.org/10.1016/j.accpm.2016.04.003 2352-5568/ß 2016 Socie´te´ franc¸aise d’anesthe´sie et de re´animation (Sfar). Published by Elsevier Masson SAS. All rights reserved.
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significant advantage of the sniffing position over simple head extension [3]. Our hypothesis was that MRI imaging data in children would be a first important step before clinical studies by providing objective data describing and quantifying the effect of age on airway configuration during head extension. Our main objective was to determine the effect of age on two clinically significant anatomical angles for laryngoscopy and intubation: the angle between the visualization and laryngeal axes (Visu-Lar angle) and the angle between the pharyngeal and laryngeal axes (Phar-Lar angle). The secondary objective was to describe all the axes of the airway during head extension.
2. Methods From July 2009 to September 2011, all children who had a planned MRI (with our without general anaesthesia) were considered for inclusion in the study. The MRIs were indicated by paediatricians not involved in the study. Indication of general anaesthesia was not protocoled, and followed the routine practice of the physicians involved. General anaesthesia was proposed by the paediatrician or the radiologist (after failure of awake procedures, and indicated by an anaesthesiologist). The children and their parents received oral and written information about the study and the added time for the MRI. Informed written consent was obtained from the parents. Exclusion criteria included the following: obesity (Body mass index [BMI] > 97th percentile) [5], macro- or microcephaly [6], head dysmorphism, tumour or abnormality near the upper airway, need for upper airway control (i.e., laryngeal mask or tracheal tube) if general anaesthesia was chosen, an emergency MRI or the lack of consent from the child or either of the parents. The patients under general anaesthesia did not receive any premedication and were anaesthetised with sevoflurane via a high concentration mask. During the MRI examination, the inspired fraction concentration was between 2 to 3%. For the first head position, the child was in the supine position, directly on the flat MRI table, and the head was positioned in Frankfort’s plane [7], defined as the plane between the external auditory canal and the external corner of the eye, perpendicular to the table. For the second head position (extension), after the first MRI acquisition, the head was extended at an incline of 358 to the Frankfort’s plan. Before each sequence, the head was positioned by the MRI technician and verified with a non-magnetic square from the MRI tube. All studies were performed using a 1.5 Tesla system (Maestro class; Siemens, Erlanger, Germany). The study sequences were completed using an ear-nose-throat antenna. The acquisition technique was a spin echo sequence with a repetition time of 703 ms and an echo time of 13 ms. T1-weighted images were obtained in the sagittal plane. Demographic characteristics for each patient were recorded. For both head positions for each patient, the axes measured included the axis of the face (face), which extends from the brow to the chin; the pharyngeal axis (Phar) (Fig. 1), which extends through the anterior portion of the atlas and C2; the laryngeal axis (Lar), which extends through the centre of the lower (cricoid cartilage) and upper (airway centre at the base of the epiglottis) laryngeal orifices; the tracheal axis (Trac), which extends from the lower laryngeal orifice through the centre of the trachea at the second ring level; and the line of glottic visualization (Visu), which extends from the lower end of the upper incisors or the gum (in edentulous children) to the corniculate cartilage (posterior part of the thyroid cartilage).
The following angles were calculated: the Visu-Lar angle, formed by the line of the glottic visualization and laryngeal axis, and the Phar-Lar angle, formed by the pharyngeal and laryngeal axes. Variations between the resting position and head extension of the Face, Phar, Lar, Visu, Visu-Lar angle and Phar-Lar angle were also calculated. Every MRI study was interpreted independently by two of the authors. When the difference between the measurements was less than 108, the final measurement was the average of the two measurements. When the difference was greater than 108, a third measurement was made by the two authors together. Inclusions were prospectively made following an age-stratified plan (Table 1). The results from a previous study [8] helped determine the number of patients per age group; thus, a total number of 150 patients was needed for an a-risk of 0.05 and a b-risk of 0.80. Then, an age-stratified plan was determined: after verification of linearity, correlation coefficients (parametric and polychoric) were calculated for each age group and their significance tested. Then, coefficients were established for each age group. The demographic data are reported as medians and quartiles. The head extension values in the first and second positions are reported as averages and standard deviations. The number of children in each head extension group is displayed in a histogram (Fig. 2). Multivariate analysis (a general linear model) was used to test for an age effect and included head extension, weight, sex, general anaesthesia (yes or no) and cranial circumference as variables. The relationships between the axis positions and head extension were analysed using non-parametric Spearman correlation. A P-value of less than 0.05 was considered statistically significant. All data analyses were performed using SPSS v17.0 software (SPSS, Inc., Chicago IL).
3. Results Over a period of 27 months, 168 patients were included in the study and a total of 155 examinations were studied. After several months of inclusion, we encountered difficulties in including children over 24 months old (Appendix 1). After approval by the statistician, we accepted a deviation in the scheduled stratification plan (Table 2). Twenty examinations were excluded from the study because of examination interruption at the child’s request or because the images were otherwise unusable (metal-induced artefacts/movement of the child). The patients’ characteristics are shown in Table 1 and the age distribution is reported in Table 2. The study design was expected to describe two levels of head extension in each child (08 and 358). However, continuous degrees of head extension (Fig. 2) were obtained in both groups. In the neutral extension group, there was in fact an average extension of 138 (8). In the extension group, the average extension was 138 (11). The mean amplitude in extension achieved in our population was 268 (9). Multivariate analysis showed that age was not statistically correlated with Visu-Lar or Phar-Lar angle variation during head extension while considering weight, gender, cranial circumference and the presence or absence of general anaesthesia (Table 3). The t-value was 1.66 (P = 0.1) for the Visu-Lar angle and 0.64 (P = 0.52) for the Phar-Lar angle. Age was correlated with movement of the axes: Visu (t = 2.51 [P = 0.01]), Phar (t = 2.35 [P = 0.02]), Lar (t = 2.87 [P < 0.01]) and Trac (t = 2.66 [P < 0.01]) (Fig. 1).
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Fig. 1. Two examples of angles modifications during extension.
During head extension, all the anatomical axes rotated in the same direction, but not to the same extent. The rho correlation coefficients are shown in Table 4. The visualization axis was more affected by head extension, as on average 93% more magnitude was transferred to this axis during extension. The magnitude change was far lower for the pharyngeal axis. The laryngeal axis had a negligible but statistically significant rotation. On average, changes in the tracheal axis were not associated with head extension. The Visu-Lar angle significantly narrowed during head extension due to the movement of the visualization axis, but the
Table 1 Demographic data presented as medians (lower quartile; upper quartile).
Fig. 2. Observed head extensions histogram.
n = 155 Age (months) Weight (kg) Height (cm) Sex ratio BMI (kg/m2) Cranial perimeter (cm) Head anteroposterior diameter (cm) General anaesthesia
86 (24; 142) 21 (12; 41) 120 (86; 150) 56 16.9 (15.6; 19.1) 52 (48; 54) 17.9 (16.5; 18.6) 50.3%
laryngeal axis did not vary. The Phar-Lar angle significantly decreased during head extension due to the movement of the pharyngeal axis, but the laryngeal axis did not vary (Table 4). Although the values of the two angles decreased when the head was extended, the pattern differed. During head extension, the Visu-Lar angle had positive values and decreased with respect to the neutral position value. However, the Phar-Lar angle initially
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Planned inclusions
Actual inclusions
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Total
25 20 20 15 15 4/5 4/5 4/5 4/5 4/5 4/5 4/5 4/5 4/5 4/5 150
24 14 12 9 7 9 5 6 11 8 11 10 7 13 9 155
Table 4 Univariate analysis of the effect of head extension and age on anatomic axis positions and angle values.
Visualization Laryngeal axis Pharyngeal axis Trachea Phar-Lar angle Visu-Lar angle
Axis correlation coefficient with extension
R2
0.93 (P < 0.001) 0.14 (P < 0.05) 0.66 (P < 0.001) 0.01 (P = 0.83) 0.61 (P < 0.001) 0.83 (P < 0.001)
0.85 0.03 0.42 < 0.01 0.34 0.66
R2: Rho correlation coefficient (significance) All the anatomic axes rotated in the same direction, but not to the same extent. The visualization axis was the more affected by head extension, as on average 93% of the magnitude of the extension was transferred to this axis. It was far less transferred to the pharyngeal (and vestibule) axis. On average, the larynx and trachea axes are not linked to head extension. The Visu-Lar angle significantly narrowed during head extension due to the movement of the visualization axis, as the larynx axis did not vary. The Phar-Lar angle significantly narrowed during head extension, due to the movement of the pharynx axis, as the larynx axis did not vary.
showed positive values during head flexion, and these values became negative during head extension (Fig. 3). A null value for the Phar-Lar angle found on the face axis was 08, i.e., the head was in a neutral position. 4. Discussion The main result of this study is that modifications of the VisuLar and Phar-Lar angles are not influenced by patient age. To the best of our knowledge, this is the first study describing and analysing the anatomical axes of the upper airway during head extension throughout the entire range of child growth and development. As anatomical modifications are more pronounced in the earlier stages of life [9], the study inclusion criteria were designed to allow stratification by age. Two-dimensional imaging could not represent the actual conditions of laryngoscopy and intubation. However, MRI imaging of the airway provided important evidence with regard to laryngoscopy [2,8,10–12] and had the advantage of being objective. We thus decided to describe the anatomy and its modifications in a manner similar to the methods proposed by Adnet et al. [2]. The Visu-Lar angle (angle o´ in the Adnet study) may be considered as representative of actual glottic visualization during direct laryngoscopy. The ease of insertion of a nasotracheal tube certainly depends on the Phar-Lar angle (i.e., the b-angle in the Adnet study). The exact localization of the axes can be difficult, particularly in the smallest patients, but the independent analyses by two readers should have limited the effects of measurement variability. As concerns statistical analysis, head extension was considered to be a continuous but not normally distributed variable. As weight
was the more discriminant variable (out of gender, height, weight, and BMI), only the results for weight are reported (Table 3). The Spearman correlation test was chosen because head extension was not normally distributed. One of the strengths of this study was the sample size. The absence of an age effect on the Visu-Lar and Phar-Lar angles cannot be due to a lack of power. The study was primarily designed to obtain two groups of distinct and standardized head extensions, but these exact extensions were not achieved. This issue confirms the difficulties encountered in accurate standardization of head position in children, when the head is out of reach. As the head is out of reach during MRI acquisition, such difficulties may be the result of a spontaneous trend toward head flexion in children, particularly in anesthetized children [13,14], of unnoticed slides between positioning (outside the MRI tube) and MRI acquisition, and of difficulties in maintaining a steady position in conscious children. However, the mean degree of extension achieved was 268. This value was greater than in a previous study [8] and was almost equivalent to the extension applied to the head during a direct laryngoscopy in the operating room with a Macintosh laryngoscope [15,16]. In fact, the distribution of the observed head extension (Fig. 2) revealed a fairly homogeneous distribution throughout the entire range. Those patients who underwent general anaesthesia did not have either an endotracheal tube or a laryngeal mask in the upper airways that could have biased the results [17–19]. In this study, anaesthesia did not have significant effects on the observed axes and angles. This finding does not necessarily rule out all effects of anaesthesia on upper airway configuration, but only suggests that anaesthesia does not have a quantitative influence on airway axes compared to the effects of head extension and patient age.
Table 3 t-values from multivariate analysis of the variation of anatomic angles during head extension. The results are expressed as: partial correlation coefficients (level of signification of the correlation). Extension Visu-Lar angle variation Phar-Lar angle variation
10.19 (P < 0.01) 5.88 (P < 0.01)
Age 1.66 (P < 0.1) 0.64 (P = 0.52)
Weight
Gender
2.62 (P = 0.01) 1.23 (P = 0.22)
0.83 (P = 0.41) 0.33 (P = 0.74)
Cranial perimeter 1.02 (P = 0.31) 0.53 (P = 0.59)
General anaesthesia 0.37 (P = 0.71) 0.16 (P = 0.87)
Age was not statistically correlated with either angle variation during head extension while considering weight, gender, cranial perimeter and anaesthesia. The negative correlations coefficients indicate that during head extension the Visu-Lar and Phar-Lar angles were reduced, showing a better alignment of the visualisation axis with the laryngeal axis.
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Fig. 3. Visu-Lar and Phar-Lar angles vs. head extension plots.
To the best of our knowledge, this is the first study that provides rational data describing the effect of age on airway modifications during head extension. These data strongly support empirical recommendations favouring ‘‘slight extension’’ for laryngoscopy in children and infants [4,13]. The reduction of the Visu-Lar angle during head extension shown by Adnet et al. [2,8] and another previous study was confirmed in our study. To explain the evolution of the Visu-Lar angle, we focused on the axis it formed with: the visualization axis and the laryngeal axis. During head extension, the visualization axis, which was always positive, rotated almost with the same amplitude as the head extension (Table 4), but the laryngeal axis had an amplitude of only 1/8 of that of the extension. This observation explains why the Visu-Lar angle decreased during head extension. The relative macrocephaly of infants and young children induces an anterior neck flexion during head extension [8]. It should be mentioned that this does not mean that the movements of the cervical column and the atlanto occipital joint are the same in children and adults, but only that the relationship between head extension and the Visu-Lar angle does not depend on age. It can be hypothesized that the absence of an age effect on the reduction of the Visu-Lar angle indirectly confirms the results of Adnet et al. [2], in which no superiority was demonstrated for the sniffing position compared with simple head extension for reducing the Visu-Lar angle. Previous studies have found an increase in the Phar-Lar angle during head extension [2,8]. In our study, however, this correlation was negative. This could be due to the use of angle absolute values in previous studies. Another interesting point is that the analysis of the Phar-Lar angle throughout the movement showed that the observed minimum angle was close to that of the neutral position (Fig. 3). The best alignment of the laryngeal and pharyngeal axes was achieved in the neutral position, which seems to be the best position for the introduction of a nasotracheal tube, regardless of age. Analysis of the movements of each axis from head flexion through complete head extension gave information regarding the dynamic configuration of the airway throughout extension. During head extension (i.e., clockwise head rotation), all the axes studied rotated in the same direction. The difference between the axes was the amplitude of the rotation during head extension. The visualization axis was the axis that rotated the most as a result
of head movement. The pharyngeal axis rotated 60% between head flexion to extension. The laryngeal axis had a clinically insignificant rotation, because, with a correlation coefficient of 0.15 and a 358 extension applied to the head, the movement transmitted to the larynx would be only 58, which is close to the precision limit for data interpretation. The upper cervical portion of the trachea did not rotate in this study, as measured by the axis that passed through the lower laryngeal orifice and the centre of the trachea at the second ring. Paediatric airways challenge the confidence of non-specialized healthcare providers. Obviously, anatomical considerations are no substitutes for practice and experience, but just as the principles of airway patency in adults are applicable to children [20], this study supports that the fundamental bases of laryngoscopy are valid in infants and children. 5. Conclusion The main conclusion of this study is that age has no influence on the Visu-Lar or Phar-Lar angles. Regardless of the age of the child, head extension was the only factor that improved glottic visualization and the best alignment of the pharynx and larynx was obtained in the neutral position. Throughout head extension movement, the visualization axis rotated the most, followed by the pharyngeal axis. The movement of the laryngeal axis was clinically insignificant, and the trachea did not rotate. Paediatric airways differ from those of adults. However, this study supports that, like in adults, the degree of head extension is probably the key factor for laryngoscopy and intubation success in infants and children. Funding The local ethics committee (Comite´ de protection des personnes pour la recherche biome´dicale Sud-Me´diterrane´e 2) approved this study, which was registered on clinicaltrials.gov (ClinicalTrials.gov identifier: NCT00920673). This research was carried out without funding. Disclosure of interest The authors declare that they have no competing interest.
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Appendix 1
12 first months
15 next months
Paents screened: 205
Paents screened: 224
Exclusion criteria: n = 5
Exclusion criteria: 4
Inclusion not considered:
Inclusion not considered:
(age class completed) n= 92
(age class completed) n= 146
Inclusion considered (n = 74)) (age class not complete, or minor age difference from the scheduled straficaon plan)
Inclusion considered: (n= 108)
Parental / paent consent:n = 71
Parental/paent consent:n = 104
Total: n = 175 Exclusion (images not usable): n = 20 RMI examinaons included: n = 155
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Please cite this article in press as: Be´cret A, et al. Upper airway modifications in head extension during development. Anaesth Crit Care Pain Med (2016), http://dx.doi.org/10.1016/j.accpm.2016.04.003