Pulmonary aspiration of gastric contents following vomiting or regurgitation during induction of anaesthesia is a life-threatening complication.1 Although a lot of effort has been made to prevent this complication, many problems remain unsolved.2–4 The head-up position may decrease the likelihood of regurgitation.5–7 However, assuming the oesophagus is 30 cm long, the difference in vertical height between the larynx and stomach is only 8 and 15 cm with 15° and 30° head-up tilts, respectively. When the driving pressure of gastric contents during vomiting or regurgitation exceeds the gravitational effect of the head-up position, gastric contents may enter the trachea and bronchi. In contrast, the head-down position helps to prevent pulmonary aspiration but may increase the likelihood of regurgitation.2,8–10 The head-down position is recommended to prevent gastric contents entering the trachea and bronchi.11 However, there are no studies regarding the ideal extent of head-down tilt.
In the supine position, most of the oesophagus is located behind the larynx. The pharynx is bow-shaped and concave posteriorly. Thus, if the mouth is level with the inferior border of the larynx (the arytenoid cartilages) gastric contents may not enter the trachea, even when vomiting or regurgitation occurs. The extent of the head-down tilt should be equal to the angle between the horizontal plane and the line connecting the corner of the mouth and the arytenoid cartilage, which is defined as the mouth–arytenoid angle. If the inferior border of the bifurcation of the trachea (the carina) is higher than the mouth, gastric contents may enter the trachea but should not enter the bronchi or lungs.
The sniffing and simple extension positions are considered the optimal head–neck positions for laryngoscopy and intubation.12–14 Sellick2 suggested the use of the full cervical spine extension position for tracheal intubation in patients at a risk of aspiration, defined as the Sellick position. The head–neck position for tracheal intubation can influence the height of the mouth, larynx and tracheal bifurcation. For example, when the patient is placed in the sniffing position, the corner of the mouth is elevated as a result of neck flexion. In contrast, in the Sellick position, the height of the corner of the mouth may be close to that of the larynx and tracheal bifurcation.
The aims of this study were to examine the head-down tilt angle required to prevent aspiration in the neutral, simple extension, sniffing and Sellick positions in a manikin, to compare this angle with the mouth–arytenoid and mouth–carina angles and to determine the optimal head-down tilt and head–neck positions for preventing aspiration.
Many manikins are designed for practising face-mask ventilation and tracheal intubation. However, it is not known whether the position of the upper airway structures in the manikin is suitable for use as a patient simulator. To examine the suitability of the manikin for a pulmonary aspiration model during vomiting or regurgitation, we measured the mouth–arytenoid angle in human volunteers in the four head–neck positions and compared them with that in a manikin.
Ethical approval for this study (Ethical Committee No. 10–65) was provided by the 28th Ethical Committee of Nippon Steel Yawata Memorial Hospital, Kitakyushu, Japan (Chairperson Dr T. Ishitsuka) on 6th August 2010. Written informed consent to participate in the study was obtained from all participants.
We used the Airway Trainer Manikin (Laerdal Ltd, Stavanger, Norway) because of the life-like and watertight structure of the upper airway and separation of the oesophagus and trachea. The manikin was placed supine without a pillow and its head was neither extended nor flexed (neutral position; Fig. 1a).12–14 With radiograph guidance, two marks were placed on the surface of the manikin; one was in line with the arytenoid cartilage and the other with the posterior border of the carina (Fig. 2a). The mouth–arytenoid (θ) and mouth–carina (ϕ) angles were defined as the angle between the horizontal plane and the line connecting the mouth corner and the arytenoid cartilage and that between the horizontal plane and the line connecting the mouth corner and the carina, respectively.
The oesophagus and bronchi were connected to flexible tubes of 2.5 and 1.5 cm in diameter, respectively (Fig. 2a). The distal end of the oesophageal tube was attached to a reservoir over the head, and the tube was clamped. The reservoir was filled with 1.2 l of crystalloid fluid coloured with povidone iodine. The height of the fluid was maintained at 130 cm above the arytenoid cartilage. After removing the clamp, the fluid that entered the bronchial tube was collected in a beaker. Aspiration of fluid into the trachea was confirmed using a fibreoptic bronchoscope passed through the tracheal bifurcation into the trachea after removing one of the bronchial tubes (Fig. 2b). We also observed for expulsion of coloured fluid from the mouth of the manikin. After removing the residual fluid in the upper airway, trachea and oesophageal and bronchial tubes, the table was tilted to an angle of between 5° and 50° head-down in 5° increments using a goniometer. At each increment of head-down tilt, the release of fluid from the oesophagus was repeated. The head–neck position was then changed from the neutral to simple extension, sniffing and Sellick positions, defined as a position in which the head was maximally extended without a pillow, with a pillow 7 cm in height and by extending the head-of-table under the occiput, respectively (Fig. 1b–d).12–14 When changing the head–neck position, we corrected the marks on the surface of the manikin indicating the arytenoid cartilage and the carina. The release of coloured fluid from the oesophageal tube at the head-down tilt angle of between 0° and 50° in 5° increments was repeated in the same fashion as in the neutral position.
Measurements were repeated 10 times at each head-down tilt angle in each head–neck position. Aspiration into the trachea was defined as positive when the fluid was found at any of the 10 measurements at each head-down tilt angle in each head–neck position. The researcher who measured the volume of fluid that entered the bronchial tube and the mouth–arytenoid and mouth–carina angles on the photograph in the four positions did not know the head-down tilt angle and the head–neck position being used.
Suitability of the manikin for pulmonary aspiration model
Thirty healthy adult volunteers (14 men, 16 women; mean age 56 years, range 20–81 years) were studied. All volunteers were of normal mean (SD) height [160 (8) cm] and weight [56 (8) kg]. Exclusion criteria included cervical spine disorder (instability, severe stenosis, severe immobility), history of cervical spine injury and previous neck surgery. Volunteers were also excluded if their arytenoid cartilage was difficult to identify externally (e.g. due to a short neck or obesity). Each individual lay supine on the table without a pillow, looking straight with neither head extension nor flexion (neutral position). A mark was placed on the skin of the neck over the arytenoid cartilage (the posterior border of the thyroid cartilage at the level of the laryngeal prominence). The volunteer was asked to extend the head maximally without a pillow (simple extension), with a pillow 7 cm in height (sniffing) and by bending the head-of-table under the occiput until the neck was extended (Sellick). When changing the head–neck position, we reassessed the mark on the skin. A photograph was taken in each head–neck position and the mouth–arytenoid angle determined.
The data of the human volunteers were checked for normal distribution using the Kolmogorov–Smirnov test. To test for a significant difference, a one-way analysis of variance with a Bonferroni posthoc correction for multiple comparisons was performed. A P value less than 0.05 was considered significant.
The aspiration of coloured fluid into the manikin bronchi and the trachea at each head-down tilt angle in each head–neck position is shown in Tables 1 and 2, respectively. The head-down tilt angles for protecting the trachea from aspiration were 45°, 35° and 10° in the neutral, simple extension and Sellick positions, respectively (Table 2), which coincided with the mouth–arytenoid angle (Table 3). The positions used in this study were not adequate to prevent aspiration into the trachea in the sniffing position. The head-down tilt for protecting the bronchi from aspiration were 25°, 25°, 40° and 0° in the neutral, simple extension, sniffing and Sellick positions, respectively (Table 1), which coincided with the mouth–carina angle (Table 3). Fluid was not expelled from the mouth at 10° or less, 5° or less and 20° or less head-down tilt angles in the neutral, simple extension and sniffing positions, respectively (Table 1).
Suitability of the manikin for pulmonary aspiration model
The mean mouth–arytenoid angles in the human volunteers were 39° [95% confidence interval (CI), 25° to 53°], 27° (95% CI, 14° to 41°), 52° (95% CI, 36° to 68°) and 11° (95% CI, 4° to 17°) in the neutral, simple extension, sniffing and Sellick positions, respectively. The mouth–arytenoid angle in the Sellick position was smaller than in the other head–neck positions (P < 0.001). These were similar to those in the manikin, indicating suitability of the manikin for a patient simulator in this research.
This study showed that aspiration depended on the differences in the vertical height of the mouth, larynx and tracheal bifurcation. When the corner of the mouth was made to be level with the carina by the use of head-down tilt (mouth–carina angle), gastric contents did not enter the bronchi but did enter the trachea. Pulmonary aspiration was partially prevented in this head-down position. In order to completely prevent aspiration, levelling the mouth corner with the arytenoid cartilage (mouth–arytenoid angle) was needed. More than 45° and 35° of head-down tilt was necessary for complete prevention of aspiration in the neutral and simple extension positions, respectively. In the sniffing position, pulmonary aspiration could not be prevented within the head-down tilt angles that we assessed. There is a risk of the patient sliding with excessive head-down tilt. In this study, the manikin slid from the table at 15° head-down tilt when it was not fixed on the table. Dixon et al.15 described that means for preventing the patient from sliding were needed when the table was tilted to 25°. Thus, 15° to 20° is a reasonable upper limit when means for preventing sliding are not used. In our manikin, head-down tilt within clinically relevant ranges could not prevent pulmonary aspiration in the neutral, simple extension and sniffing positions that are commonly used during induction of general anaesthesia and tracheal intubation. In contrast, when placed in the Sellick position, both the trachea and the bronchi were protected from aspiration at 10° of head-down tilt. In our volunteers, the mouth–arytenoid angle was less than 15° in 87% in this position, which supported results of the manikin study. These results indicate the potential for preventing aspiration completely by combining the Sellick position and head-down tilt within clinically relevant ranges in most of patients with a normal cervical spine. Because the mouth–arytenoid angle can be measured with external landmarks, the extent of the head-down tilt angle for complete prevention of aspiration can be estimated prior to induction of general anaesthesia.
At 10° or less, 5° or less and 20° or less of head-down tilt in the neutral, simple extension and sniffing positions, respectively, vomiting and regurgitation may not be appreciated because fluid was not be expelled from the mouth. The head-down position is recommended to prevent aspiration after vomiting and regurgitation.11 However, in the head–neck positions we assessed, it should be noted that the diagnosis of aspiration may be difficult when placing the table horizontal. Although some anaesthetists argue in favour of the head-up position during induction of general anaesthesia in patients at risk of aspiration,5–7 vomiting and regurgitation may not be detected in any position.
Among the head-neck positions we studied, the sniffing and simple extension positions are advantageous for laryngoscopy and intubation.12–14 The mouth–arytenoid angle in the simple extension position was smaller than that in the sniffing position. However, to prevent aspiration, the simple extension position may not be better than the sniffing position because the mouth–arytenoid angles in both positions exceeded 15°. In contrast, the Sellick position may be effective in prevention of aspiration. However, prolonged attempts at intubation in this position could increase the risk of pulmonary aspiration, as it is not always the best position for laryngoscopy. When choosing the head–neck position for induction of anaesthesia, a careful analysis of the risks of pulmonary aspiration and the degree of airway difficulty predicted should be performed on an individual basis.
To prevent aspiration, there are other methods described. Rotation of the head should be effective because of lowering the mouth corner, although we did not examine this because of the inability to rotate the head in most manikins. Placing a pillow under the shoulders will also be useful because the larynx is elevated without changing the position of the mouth. Placing the patient in the lateral position has been recommended for tracheal intubation in patients at risk of aspiration.16 This position will be effective because the height of the mouth is level with that of the larynx and the tracheal bifurcation. However, many, even experienced, anaesthetists may be unfamiliar with laryngoscopy and intubation in patients lying in the lateral position.17 Previous studies have demonstrated that direct laryngoscopy and intubation in this position are more difficult than in the supine position.17,18
There are some limitations to our experimental design. First, we studied Japanese volunteers with normal cervical spines. The results may not be applicable to patients with limited cervical spine motion. In addition, the Sellick position is contraindicated in patients with cervical spine instability or suspected cervical spine injury. Also, as the volunteers in this study were relatively small, the measurements may not be applicable to other ethnic populations who are taller and with different facial features. Second, the configuration of the upper airway of the manikin that we used and the changes with different head–neck positions may differ from those of patients. However, human study for pulmonary aspiration by vomiting and regurgitation was not feasible. We examined the similarity to the airway of human of a number of manikins, their watertight properties and the ease to place them in the four head–neck positions. We found the Airway Trainer Manikin was the best at meeting these demands. We did not measure the mouth–carina angle in human volunteers. Finally, there are many factors involved in pulmonary aspiration during vomiting or regurgitation, for example, the type of the aspirate and the oesophageal pressure. We did not examine aspiration of particulate matter. Oesophageal pressure during vomiting or regurgitation has been postulated to be 60 to 105 cmH2O, although it has not been extensively studied.19,20 We used an oesophageal pressure of 130 cmH2O, which was equal to that used by Bercker et al.21
In summary, we have shown that a difference in the vertical height between the mouth, larynx and tracheal bifurcation was one of the important factors in preventing pulmonary aspiration. When the corner of the mouth was level with the inferior border of the larynx (the mouth–arytenoid angle) by the use of head-down tilt, pulmonary aspiration was completely prevented. Excessive head-down tilt was necessary to prevent aspiration in the head–neck positions commonly used for tracheal intubation. Only in the Sellick position was the degree of head-down tilt to prevent aspiration within a clinically relevant range. These findings indicated a potential for preventing pulmonary aspiration during induction of general anaesthesia by combining the Sellick position and head-down tilt, although this position may not be the best for laryngoscopy. The extent of the head-down tilt required can be estimated prior to induction of anaesthesia by measuring the mouth–arytenoid angle on the skin surface.
Financial support for this study was provided solely from institutional or departmental sources. No conflicts of interest or external funding declared.
1. Warner MA, Warner ME, Weber JG. Clinical significance of pulmonary aspiration
during the perioperative period. Anesthesiology
2. Sellick BA. Cricoid pressure to control regurgitation of stomach contents during induction of anaesthesia. Lancet
3. Reynolds SF, Heffner J. Airway
management of the critically ill patients: rapid-sequence intubation. Chest
4. El-Orbany M, Connolly LA. Rapid sequence induction and intubation: current controversy. Anesth Analg
5. Snow RG, Nunn JF. Induction of anaesthesia in the foot-down position
for patients with a full stomach. Br J Anaesth
6. Hodges RJ, Bennett JR, Tunstall ME, Knight RF. General anaesthesia for operative obstetrics: with special reference to the use of thiopentone and suxamethonium. Br J Anaesth
7. Stept WJ, Safar P. Rapid induction/intubation for prevention of gastric content aspiration
. Anesth Analg
8. Cassorla L, Lee JW. Patient positioning and anesthesia.
In: Miller RD, editor. Anesthesia; 7th ed. Philadelphia: Churchill Livingston Elsevier; 2010. pp. 1151–1170.
9. Inkster JS. The induction of anaesthesia in patients likely to vomit with special reference to intestinal obstruction. Br J Anaesth
10. Cameron JL, Zuidema GD. Aspiration
pneumonia. Magnitude and frequency of the problem. JAMA
11. Cucchiara RF, Faust RJ. Patient positioning.
In: Miller RD, editor. Anesthesia; 5th ed. Philadelphia: Churchill Livingston; 2000. pp. 1017–1032.
12. Adnet F, Baillard C, Borron SW, et al. Randomized study comparing the ‘sniffing position
’ with simple head extension for laryngoscopic view in elective surgery patients. Anesthesiology
13. Takenaka I, Aoyama K, Iwagaki T, et al. The sniffing position
provides greater occipito–atlanto–axial angulation than simple head extension: a radiological study. Can J Anesth
14. Berry JM. Conventional (laryngoscopic) orotracheal and nasotracheal intubation (single lumen tube). In: Hagberg CA, editor. Benumof's airway management: principles and practice
, 2nd ed. Philadelphia: Mosby Elsevier; 2007. pp. 379–392.
15. Dixon BJ, Dixon JB, Carden JR, et al. Preoxygenation is more effective in the 25 degrees head-up position
than in the supine position
in severely obese patients: a randomized controlled study. Anesthesiology
16. Kirchner E. Notfälle und Aspirationsgefahr. Ist eine Weiterentwicklung gängiger Methoden zur Prophylaxe der Aspiration
möglich? [Emergencies and aspiration
. Can the usual methods for the prophylaxis of aspiration
be further developed?]. Anaesthesist
17. McCaul CL, Harney D, Ryan M, et al. Airway
management in the lateral position
: a randomized controlled trial. Anesth Analg
18. Takenaka I, Aoyama K, Iwagaki T, Kadoya T. Efficacy of the Airway
Scope on tracheal intubation in the lateral position
: comparison with the Macintosh laryngoscope. Eur J Anaesthesiol
19. Fanning GL. The efficacy of cricoid pressure in preventing regurgitation of gastric contents. Anesthesiology
20. Brimacombe J, Keller C. Hypopharyngeal seal pressure during projectile vomiting with the ProSeal laryngeal mask airway
: a case report and laboratory study. Can J Anaesth
21. Bercker S, Schmidbauer W, Volk T, et al. A comparison of seal in seven supraglottic airway
devices using a cadaver model of elevated esophageal pressure. Anesth Analg