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Developmental anatomy of the airway
PAEDIATRICS
Developmental anatomy of the airway
Learning objectives After reading this article you should be able to: C describe the changes in anatomy of the airway from neonate to older child C understand the differences in airway management associated with changing anatomy from neonate to older child C correctly size paediatric uncuffed and cuffed endotracheal tubes
Corina Lee Edward Doyle
Abstract The airway begins to develop from the primitive foregut at 4 weeks’ gestation. Congenital anomalies may result when this process is abnormal. The anatomy of the airway at birth is uniquely different from older children and adults with a large tongue, long floppy epiglottis, large occiput, cephalad larynx, and narrow cricoid cartilage. These features affect the technique required for endotracheal intubation and facemask ventilation. A neutral head position and straight bladed laryngoscope are usually used. Neonates are also obligate nasal breathers and simultaneously suckle and breathe. Minute volume is rate-dependent and the highly compliant chest easily displays sternal and intercostal recession during respiratory distress, and early onset of fatigue. From the neonatal period onwards the anatomy gradually begins to resemble that of adults. The cricoid descends caudally, the epiglottis becomes firmer and shorter, and the large occiput recedes. By 8e10 years the airway is anatomically adult in most ways other than absolute size. The ‘sniffing the morning air’ and curved laryngoscope become appropriate for endotracheal intubation. Conventionally, uncuffed endotracheal tubes have been used in children; however high volume-low pressure cuffed tubes are now available, allowing monitoring of the cuff pressure intermittently throughout use.
epithelial lining of the respiratory tract. The cranial end of the tube becomes the larynx, mid-section the trachea, and the caudal end bifurcates into two buds e the bronchi and lungs. By Week 6 the rudimentary epiglottis and arytenoids can be identified. If epithelial fusion between the two sides of the larynx fails to regress, complete (atresia) or partial (web) obstruction may occur (Figure 1). Incomplete fusion or abnormality of the tracheo-oesophageal septum may result in laryngeal cleft or tracheo-oesophageal fistula respectively.
The airway at birth The neonate has a large head, short neck, and prominent occiput. The epiglottis is well developed, often described as omega or U shaped, projecting posteriorly at 45 to the vertical. The larynx is funnel shaped and the cricoid cartilage lies at C3/4. The mandible is relatively underdeveloped and, combined with prominent mid-face, results in a degree of micrognathia. To facilitate suckling, the tongue is large in relation to the oral cavity. Neonates must be able to suckle and breathe simultaneously. The apposition of the epiglottis and uvula in the midline effectively separates a midline pathway into the trachea for air
Keywords Airway; anaesthesia; anatomy; larynx; paediatric; trachea endotracheal tube
Embryonic development Development of the human airway starts at Week 4 of gestation. The nasal and oral cavities develop simultaneously, extending to connect with the primitive foregut, ultimately becoming the pharynx. The primary palate begins at Week 5 fusing to become the nasal tip and upper lip, followed by the secondary palate at Week 7 forming the soft and hard palate. Failure of muscles to meet their counterparts at this stage can lead to cleft lip and palate abnormalities. The larynx, trachea and bronchi originate from the median laryngo-tracheal groove e an anterior feature of the primitive pharynx. As the groove deepens, its lips fuse forming a tube lined with endoderm, which becomes the
Corina Lee MRCP FRCA is a locum Consultant Paediatric Anaesthetist at the Royal Hospital for Sick Children, Edinburgh, UK. Conflicts of interest: none declared.
Figure 1 Endoscopic view of the larynx of an infant with stridor, showing an anterior web occupying at least 90% of the glottis. (a) Fused vocal cords; (b) airway. (Reproduced, with permission, from Kubba M, Moores T. Anaesthesia and Intensive Care Medicine 2006; 7: 158e60.)
Edward Doyle MD FRCA is a Consultant Paediatric Anaesthetist at the Royal Hospital for Sick Children, Edinburgh, UK. Conflicts of interest: none declared.
ANAESTHESIA AND INTENSIVE CARE MEDICINE 13:5
217
Ó 2012 Elsevier Ltd. All rights reserved.
PAEDIATRICS
(neonates are nasal breathers) and two lateral channels (piriform fossae) leading into the oesophagus for milk. The large occiput, micrognathia and large tongue can make mask ventilation and endotracheal intubation challenging. Supporting the shoulders and maintaining a neutral head position with neither flexion nor extension of the neck may counter this. The larynx is positioned very cephalad creating an acute angle with the base of tongue, making it appear anterior at direct laryngoscopy. A straight bladed laryngoscope should be inserted lateral to the tongue to counter the extreme angle, and the obliquely angled epiglottis lifted by placing the tip of the blade on its underside. The laryngeal inlet and inferior surface of the epiglottis are innervated by the vagus nerve, which may be stimulated by this technique. Posterior laryngeal pressure may help bring the larynx into view. Newer devices allowing users to ‘look around the corner’ (indirect laryngoscopy) may improve visualization, abolishing the need to displace the tongue into the submandibular space. However training and experience is required before use in neonates and children. Weak intercostal and diaphragmatic muscles (lack of type 1 fibres), horizontal ribs and a protuberant abdomen results in earlier onset of fatigue and less efficient ventilation. Minute volume is rate dependent, and a gas-filled over-distended stomach easily splints the diaphragm. Chest wall-specific compliance is higher, and intercostal or sternal recession is readily visible with increased respiratory effort or airway obstruction.
Postnatal development of the airway The paediatric airway gradually becomes adult-like, with a similar laryngeal view by year 4e5. The larynx descends relative to the cervical spine, the cricoid lying at the level of C5 by 2 years, and C6 by 5 years (Figure 2). The epiglottis adopts a firmer, flatter and more adult orientation, easily lifted by a laryngoscope placed in the vallecula. The tongue size relative to the oral cavity decreases and the prominent occiput recedes. A ‘sniffing the morning air’ position combined with a curved Macintosh laryngoscope is usually appropriate. Adenoidal hypertrophy becomes increasingly evident, peaking at age 5 years and disappearing by adolescence. Airway obstruction may occur secondary to expanding adenoids filling the nasopharynx or tonsillar hyperplasia. This is usually worse during sleep when pharyngeal tone is poor, and may compromise mask ventilation. The cricoid is the only complete ring of cartilage in the respiratory tract, and adequate ventilation with an uncuffed endotracheal tube (ETT) relies on a seal being created between the ETT and cricoid. If the ETT is too large, pressure on the subglottic mucosa may lead to oedema, chondritis of the underlying cricoid cartilage, and eventually subglottic stenosis. A leak test should be performed at a ventilating pressure of 20e25 cmH2O to avoid this. The cricoid is the narrowest part of the airway in children (internal diameter of 4e6 mm at birth, 7e8 mm by 12 months) meaning infants and neonates are more Figure 2 Lateral soft-tissue neck radiographs of children aged (a) 1 month, (b) 3 years and (c) 15 years, showing descent of the larynx with age. The tip of the epiglottis (x), the level of the vocal cords (y) and the level of the cricoid (z) are shown in each case. (Reproduced, with permission, from Kubba M, Moores T. Anaesthesia and Intensive Care Medicine 2006; 7: 158e60.)
ANAESTHESIA AND INTENSIVE CARE MEDICINE 13:5
218
Ó 2012 Elsevier Ltd. All rights reserved.
PAEDIATRICS
Cuffed endotracheal tubes of the MicrocuffÒ type a
MICROCUFFÒ cuffed ETT
Age
Size
Internal diameter (mm)
External diameter (mm)
Cuff resting diameter (mm)
> 3 kge8 months 8 monthse<2 years 2e<4 years
3.0 3.5 4.0
3.0 3.5 4.0
4.3 5.0 5.6
10 12 12
MICROCUFFÒ endotracheal tubes (ETT) are not recommended for use in infants weighing less than 3 kg. There are specific instructions for insertion and the size of ETT chosen for a particular child will differ from other varieties of cuffed ETT. a
Table 1
susceptible to acute airway obstruction than adults. Laminar flow (quiet breathing) is proportional to the fourth power of the radius (HaganePoiseuille equation), and turbulent flow (crying) to the fifth power of radius. A 1 mm circumferential reduction in size due to mucosal oedema would increase airway resistance by 16-fold and 32-fold respectively and decrease cross-sectional area by up to 75%. The neonatal trachea is 5 cm long, growing to 8 cm by 12e18 months of age. There is high risk of inadvertent endobronchial intubation or unplanned extubation. As both main bronchi diverge at the same angle from the carina, endobronchial intubation may be left-or right-sided.
Cuffed ETTs for paediatric use are now available. Advantages include: low-flow anaesthesia; reduced atmospheric pollution; decreased ETT exchanges to achieve correct ‘fit’; higher airway pressures achievable in non-compliant lungs; improved end-tidal carbon dioxide monitoring; possible aspiration prevention during long-term ventilation. The main disadvantage is the need to reduce the diameter of the ETT by 0.5e1 mm, thereby increasing both airway resistance and work of breathing, particularly with smaller diameter tubes. A high-volume low-pressure ETT (MicrocuffÒ PET) with an ultrathin distal cuff seals at lower pressures than conventional cuffs (w11 cmH2O), which is below presumed mucosal perfusion pressure (Table 1). There is no current evidence that they increase post-extubation stridor compared to uncuffed tubes. Cuff pressure can be monitored intermittently during long cases or in the intensive care unit. A
Endotracheal intubation For children over 4 years the formula: Size ¼ ðage=4Þ þ 4:0 mm is used to estimate internal diameter of uncuffed ETTs, but preexisting medical conditions may influence this. A range of tubes of smaller and larger internal diameter should be available and an audible leak check performed. Local guidelines are advised for children below 4 years. For ETT length the formula:
FURTHER READING Adewale L. Anatomy and assessment of the pediatric airway. Pediatr Anesth 2009; 19: 1e8. Creighton RE. The infant airway. Can J Anaesth 1994; 41: 174e6. Doyle E, ed. Oxford specialist handbooks: paediatric anaesthesia. Oxford: Oxford University Press, 2007. Doyle E. Management of the difficult airway in children. CPD Anaesth 2004; 6: 3e12. Wheeler DS, Spaeth JP, Mehta R, et al. Assessment and management of the pediatric airway. In: Wheeler DS, Wong HR, Shanley TP, eds. Resuscitation and stabilization of the critically III child. Springer, 2007.
Length ¼ ðage=2Þ þ ½12 mm ðoralÞ or ½14 mm ðnasalÞ provides a rough estimate, but position should be confirmed by observation of the ETT through the vocal cords and presence of equal bilateral chest movement and breath sounds.
ANAESTHESIA AND INTENSIVE CARE MEDICINE 13:5
219
Ó 2012 Elsevier Ltd. All rights reserved.
Developmental anatomy of the airway
Learning objectives After reading this article you should be able to: C describe the changes in anatomy of the airway from neonate to older child C understand the differences in airway management associated with changing anatomy from neonate to older child C correctly size paediatric uncuffed and cuffed endotracheal tubes
Corina Lee Edward Doyle
Abstract The airway begins to develop from the primitive foregut at 4 weeks’ gestation. Congenital anomalies may result when this process is abnormal. The anatomy of the airway at birth is uniquely different from older children and adults with a large tongue, long floppy epiglottis, large occiput, cephalad larynx, and narrow cricoid cartilage. These features affect the technique required for endotracheal intubation and facemask ventilation. A neutral head position and straight bladed laryngoscope are usually used. Neonates are also obligate nasal breathers and simultaneously suckle and breathe. Minute volume is rate-dependent and the highly compliant chest easily displays sternal and intercostal recession during respiratory distress, and early onset of fatigue. From the neonatal period onwards the anatomy gradually begins to resemble that of adults. The cricoid descends caudally, the epiglottis becomes firmer and shorter, and the large occiput recedes. By 8e10 years the airway is anatomically adult in most ways other than absolute size. The ‘sniffing the morning air’ and curved laryngoscope become appropriate for endotracheal intubation. Conventionally, uncuffed endotracheal tubes have been used in children; however high volume-low pressure cuffed tubes are now available, allowing monitoring of the cuff pressure intermittently throughout use.
epithelial lining of the respiratory tract. The cranial end of the tube becomes the larynx, mid-section the trachea, and the caudal end bifurcates into two buds e the bronchi and lungs. By Week 6 the rudimentary epiglottis and arytenoids can be identified. If epithelial fusion between the two sides of the larynx fails to regress, complete (atresia) or partial (web) obstruction may occur (Figure 1). Incomplete fusion or abnormality of the tracheo-oesophageal septum may result in laryngeal cleft or tracheo-oesophageal fistula respectively.
The airway at birth The neonate has a large head, short neck, and prominent occiput. The epiglottis is well developed, often described as omega or U shaped, projecting posteriorly at 45 to the vertical. The larynx is funnel shaped and the cricoid cartilage lies at C3/4. The mandible is relatively underdeveloped and, combined with prominent mid-face, results in a degree of micrognathia. To facilitate suckling, the tongue is large in relation to the oral cavity. Neonates must be able to suckle and breathe simultaneously. The apposition of the epiglottis and uvula in the midline effectively separates a midline pathway into the trachea for air
Keywords Airway; anaesthesia; anatomy; larynx; paediatric; trachea endotracheal tube
Embryonic development Development of the human airway starts at Week 4 of gestation. The nasal and oral cavities develop simultaneously, extending to connect with the primitive foregut, ultimately becoming the pharynx. The primary palate begins at Week 5 fusing to become the nasal tip and upper lip, followed by the secondary palate at Week 7 forming the soft and hard palate. Failure of muscles to meet their counterparts at this stage can lead to cleft lip and palate abnormalities. The larynx, trachea and bronchi originate from the median laryngo-tracheal groove e an anterior feature of the primitive pharynx. As the groove deepens, its lips fuse forming a tube lined with endoderm, which becomes the
Corina Lee MRCP FRCA is a locum Consultant Paediatric Anaesthetist at the Royal Hospital for Sick Children, Edinburgh, UK. Conflicts of interest: none declared.
Figure 1 Endoscopic view of the larynx of an infant with stridor, showing an anterior web occupying at least 90% of the glottis. (a) Fused vocal cords; (b) airway. (Reproduced, with permission, from Kubba M, Moores T. Anaesthesia and Intensive Care Medicine 2006; 7: 158e60.)
Edward Doyle MD FRCA is a Consultant Paediatric Anaesthetist at the Royal Hospital for Sick Children, Edinburgh, UK. Conflicts of interest: none declared.
ANAESTHESIA AND INTENSIVE CARE MEDICINE 13:5
217
Ó 2012 Elsevier Ltd. All rights reserved.
PAEDIATRICS
(neonates are nasal breathers) and two lateral channels (piriform fossae) leading into the oesophagus for milk. The large occiput, micrognathia and large tongue can make mask ventilation and endotracheal intubation challenging. Supporting the shoulders and maintaining a neutral head position with neither flexion nor extension of the neck may counter this. The larynx is positioned very cephalad creating an acute angle with the base of tongue, making it appear anterior at direct laryngoscopy. A straight bladed laryngoscope should be inserted lateral to the tongue to counter the extreme angle, and the obliquely angled epiglottis lifted by placing the tip of the blade on its underside. The laryngeal inlet and inferior surface of the epiglottis are innervated by the vagus nerve, which may be stimulated by this technique. Posterior laryngeal pressure may help bring the larynx into view. Newer devices allowing users to ‘look around the corner’ (indirect laryngoscopy) may improve visualization, abolishing the need to displace the tongue into the submandibular space. However training and experience is required before use in neonates and children. Weak intercostal and diaphragmatic muscles (lack of type 1 fibres), horizontal ribs and a protuberant abdomen results in earlier onset of fatigue and less efficient ventilation. Minute volume is rate dependent, and a gas-filled over-distended stomach easily splints the diaphragm. Chest wall-specific compliance is higher, and intercostal or sternal recession is readily visible with increased respiratory effort or airway obstruction.
Postnatal development of the airway The paediatric airway gradually becomes adult-like, with a similar laryngeal view by year 4e5. The larynx descends relative to the cervical spine, the cricoid lying at the level of C5 by 2 years, and C6 by 5 years (Figure 2). The epiglottis adopts a firmer, flatter and more adult orientation, easily lifted by a laryngoscope placed in the vallecula. The tongue size relative to the oral cavity decreases and the prominent occiput recedes. A ‘sniffing the morning air’ position combined with a curved Macintosh laryngoscope is usually appropriate. Adenoidal hypertrophy becomes increasingly evident, peaking at age 5 years and disappearing by adolescence. Airway obstruction may occur secondary to expanding adenoids filling the nasopharynx or tonsillar hyperplasia. This is usually worse during sleep when pharyngeal tone is poor, and may compromise mask ventilation. The cricoid is the only complete ring of cartilage in the respiratory tract, and adequate ventilation with an uncuffed endotracheal tube (ETT) relies on a seal being created between the ETT and cricoid. If the ETT is too large, pressure on the subglottic mucosa may lead to oedema, chondritis of the underlying cricoid cartilage, and eventually subglottic stenosis. A leak test should be performed at a ventilating pressure of 20e25 cmH2O to avoid this. The cricoid is the narrowest part of the airway in children (internal diameter of 4e6 mm at birth, 7e8 mm by 12 months) meaning infants and neonates are more Figure 2 Lateral soft-tissue neck radiographs of children aged (a) 1 month, (b) 3 years and (c) 15 years, showing descent of the larynx with age. The tip of the epiglottis (x), the level of the vocal cords (y) and the level of the cricoid (z) are shown in each case. (Reproduced, with permission, from Kubba M, Moores T. Anaesthesia and Intensive Care Medicine 2006; 7: 158e60.)
ANAESTHESIA AND INTENSIVE CARE MEDICINE 13:5
218
Ó 2012 Elsevier Ltd. All rights reserved.
PAEDIATRICS
Cuffed endotracheal tubes of the MicrocuffÒ type a
MICROCUFFÒ cuffed ETT
Age
Size
Internal diameter (mm)
External diameter (mm)
Cuff resting diameter (mm)
> 3 kge8 months 8 monthse<2 years 2e<4 years
3.0 3.5 4.0
3.0 3.5 4.0
4.3 5.0 5.6
10 12 12
MICROCUFFÒ endotracheal tubes (ETT) are not recommended for use in infants weighing less than 3 kg. There are specific instructions for insertion and the size of ETT chosen for a particular child will differ from other varieties of cuffed ETT. a
Table 1
susceptible to acute airway obstruction than adults. Laminar flow (quiet breathing) is proportional to the fourth power of the radius (HaganePoiseuille equation), and turbulent flow (crying) to the fifth power of radius. A 1 mm circumferential reduction in size due to mucosal oedema would increase airway resistance by 16-fold and 32-fold respectively and decrease cross-sectional area by up to 75%. The neonatal trachea is 5 cm long, growing to 8 cm by 12e18 months of age. There is high risk of inadvertent endobronchial intubation or unplanned extubation. As both main bronchi diverge at the same angle from the carina, endobronchial intubation may be left-or right-sided.
Cuffed ETTs for paediatric use are now available. Advantages include: low-flow anaesthesia; reduced atmospheric pollution; decreased ETT exchanges to achieve correct ‘fit’; higher airway pressures achievable in non-compliant lungs; improved end-tidal carbon dioxide monitoring; possible aspiration prevention during long-term ventilation. The main disadvantage is the need to reduce the diameter of the ETT by 0.5e1 mm, thereby increasing both airway resistance and work of breathing, particularly with smaller diameter tubes. A high-volume low-pressure ETT (MicrocuffÒ PET) with an ultrathin distal cuff seals at lower pressures than conventional cuffs (w11 cmH2O), which is below presumed mucosal perfusion pressure (Table 1). There is no current evidence that they increase post-extubation stridor compared to uncuffed tubes. Cuff pressure can be monitored intermittently during long cases or in the intensive care unit. A
Endotracheal intubation For children over 4 years the formula: Size ¼ ðage=4Þ þ 4:0 mm is used to estimate internal diameter of uncuffed ETTs, but preexisting medical conditions may influence this. A range of tubes of smaller and larger internal diameter should be available and an audible leak check performed. Local guidelines are advised for children below 4 years. For ETT length the formula:
FURTHER READING Adewale L. Anatomy and assessment of the pediatric airway. Pediatr Anesth 2009; 19: 1e8. Creighton RE. The infant airway. Can J Anaesth 1994; 41: 174e6. Doyle E, ed. Oxford specialist handbooks: paediatric anaesthesia. Oxford: Oxford University Press, 2007. Doyle E. Management of the difficult airway in children. CPD Anaesth 2004; 6: 3e12. Wheeler DS, Spaeth JP, Mehta R, et al. Assessment and management of the pediatric airway. In: Wheeler DS, Wong HR, Shanley TP, eds. Resuscitation and stabilization of the critically III child. Springer, 2007.
Length ¼ ðage=2Þ þ ½12 mm ðoralÞ or ½14 mm ðnasalÞ provides a rough estimate, but position should be confirmed by observation of the ETT through the vocal cords and presence of equal bilateral chest movement and breath sounds.
ANAESTHESIA AND INTENSIVE CARE MEDICINE 13:5
219
Ó 2012 Elsevier Ltd. All rights reserved.