Anaesthesia, 1992, Volume 47, pages 829-83 I
Does the laryngeal mask airway compromise cricoid pressure?
T. I. STRANG
Summary The laryngeal mask airway was inserted in 10 cadavers. At postmortem the chest was opened and an infusion set primed with a dilute barium solution was inserted into the oesophagus and ligated in place. A cricoid force of 43 N was then applied and the infusion set was positioned so that when the clamp was opened it generated a pressure of 7.8 kPa within the oesophagus. The cricoid pressure was able to stop the p o w of @id into the oesophagus. This demonstrates that cricoid pressure is effective in preventing reJrux at intragastric pressures which are encountered clinically and the presence of the laryngeal mask airway does not compromise this.
Key words Larynx; cricoid pressure. Equipment; laryngeal mask airway. Complications; regurgitation.
The laryngeal mask airway (LMA), which was first introduced in 1988 [I], has been used extensively to provide a safe airway in spontaneously breathing patients who are not at risk from aspiration of gastric contents. Recently, Chadwick and Vohra reported its use in conjunction with cricoid pressure, in a patient who was at risk from pulmonary aspiration, when failed intubation occurred at emergency Caesarean section (21. However, the effectiveness of cricoid pressure in the presence of a correctly positioned LMA has not yet been established. Method Consent to undertake this investigation was obtained from the coroner. Postmortem examination began with removal of the anterior chest wall to expose the lungs and mediastinum. An LMA of appropriate size was inserted according to the manufacturer’s instructions [3]. Correct positioning was confirmed by (i) fibreoptic endoscopy, (ii) observing inflation of the lungs and (iii) palpation of the mask in the neck. The oesophagus was then incised 3 cm below the level of the cricoid cartilage. A standard infusion set was modified by cutting off the distal 5cm of tubing and creating an additional 2 mm lateral orifice, 0.5 cm from the distal end.
The infusion was then primed with a dilute salinebarium mixture and the modified end was inserted into the proximal oesophagus which was then ligated around it. The cadaver was positioned with the lower cervical vertebrae flexed and the atlanto-occipital joint extended; the classical ‘sniffing the morning air’ position used for tracheal intubation. A cricoid force of 43 N was applied using a latex mould of the author’s neck, which overlay the cricoid cartilage of the cadaver and supported a 4.4 kg lead mass. The apparatus was held in place, being careful that no additional force was transmitted onto the cricoid cartilage by the author’s hands. The infusion was then positioned so that when the clamp was opened it generated a pressure of 7.8 kPa up the oesophagus. The accuracy of this pressure had previously been verified with a water manometer. The infusion was started whilst cricoid pressure was maintained and the inspection chamber was observed for evidence of flow. After cricoid pressure had been released, evidence of flow was once again sought (Fig. I).
Results Insertion of the LMA was possible in all four male and six female cadavers. In all 10 cadavers the lungs could be
Dr T.I. Strang, MB, ChB, DCH, FRCAnaes, Registrar, Department of Anaesthetics, Royal Albert Edward Infirmary, Wigan Lane, Wigan WNI 2MN. This paper was awarded the President’s prize at the Group of Anaesthetists in Training Meeting, Bristol, April 1992. Accepted 6 February 1992. 0003-2409/92/010829
+ 3 %08.00/0
@ 1992 The Association of Anaesthetists of Gt Britain and Ireland
829
830
T.I. Strang In three cases lateral X rays of the neck were taken, which demonstrated the effectiveness of the cricoid pressure (Fig. 2(a) and (b)). In all 10 cases, when the clamp was opened, approximately 10-15 drops of fluid dripped into the oesophagus, after which all flow ceased. When cricoid pressure was released, in all cases the dilute barium solution immediately gushed through the drip set into the oesophagus and was subsequently observed in the pharynx when the LMA was removed. In six of the cases, the pathologist commented on the presence of white staining within the trachea.
7.8kPa n Fig. 1. Lateral view of the laryngeal mask airway in place and the infusion set ligated in the oesophagus. Horizontal arrow indicates a pressure of 7.8 kPa within the oesophagus. Vertical arrow shows a cricoid force of 43N applied in the neck.
inflated though the LMA and the upper part of the cuff could be palpated through the thyrohyoid membrane. In nine cases a bronchoscope was available and views of the larynx further confirmed correct positioning of the mask.
Discussion Intragastric pressure may increase up to 7.84 kPa in patients with an abdominal mass (such as a gravid uterus) after the administration of suxamethonium [4]. In 1961, Sellick described his method of cricoid pressure which has been adopted widely to protect the patient against passive reflux of gastric contents [5] and Wraight et al. subsequently showed that a force of 44 N applied to the cricoid cartilage was sufficient to protect most patients from regurgitation [6].
Fig. 2. Lateral X rays of the neck showing (a) cricoid pressure preventing passive reflux of barium; (b) reflux of barium when cricoid pressure was released.
Does the LMA compromise cricoid pressure? The effect that the presence of the LMA has on cricoid pressure has never been examined, despite the fact that it has been used in patients at risk of regurgitation [2]. In this study, a fibreoptic bronchoscope was used to confirm that the LMA was correctly positioned, as has recently been suggested by Payne [7]. The infusion set was modified at its distal end to produce an additional lateral orifice so that flow was not obstructed by oesophageal tissue. Thus, if the flow of dilute barium into the oesophagus was prevented, it must have been as a result of the cricoid pressure alone. A weak solution of barium was used, so that regurgitated fluid in the pharynx would be clearly visible. When X rays were taken, a more concentrated solution was used. The initial 10-15 drops of fluid that entered the oesophagus before flow ceased represented the 1 ml dead space volume in the oesophagus which had to be filled before intra-oesophageal pressures reached a level of 7.8 kPa. The force exerted by a 4.4 kg mass in the direction of the gravitational field is 43 N, although any error in the direction of the cricoid pressure would have resulted in a cricoid force of less than 43 N. This study therefore confirmed that a cricoid force of 43 N is protective against a distal oesophageal pressure of up to 7.8 kPa and the presence of the LMA did not prevent the cricoid pressure from being effective. However, the appearance of dilute barium in the trachea of some cadavers after release of cricoid pressure also indicates that the LMA alone does not prevent passive regurgitation. The compliance of cadaveric tissue is reduced as a result of rigor mortis, therefore, in the more compliant tissues of live patients, it is probable that lower forces would be required to produce effective cricoid pressure.
83 1
A recent clinical study has shown that cricoid pressure does not reduce the ease of insertion or positioning of the LMA [8] and this cadaveric study suggests that the presence of the LMA does not compromise the effectiveness of cricoid pressure in preventing reflux. However, since dilute barium appeared in the tracheas of some cadavers after release of cricoid pressure, the exact r61e of the LMA in the management of failed intubation has yet to be determined.
Acknowledgments The author thanks Dr A. Brain for the donation of two laryngeal mask airways, G. Millner (Mortuary Technician) for his assistance and co-operation, A. Lang and A. Boardman for radiographic services and Ms A. Shirley for typing this manuscript.
References [I] BRAINAIJ. The Laryngeal Mask-a new concept in airway management. British Journal of Anaesthesia 1983; 55: 801-5. [2] CHADWICKIS, VOHRA A. Anaesthesia for emergency Caesarean section using the Brain laryngeal airway. Anaesthesia 1989; 44: 261-2. [3] BRAIN AIJ. The lntervent Laryngeal Mask. Instruction Manual, 1990. [4] ANDERSEN N. Changes in intra-gastric pressure following the administration of suxamethonium. Preliminary report. British Journal of Anaesthesia 1962; 34: 363-7. [5] SELLICKBA. Cricoid pressure to control regurgitation of stomach contents during induction of anaesthesia. Lancet 1961; 2 404-6. [6] WRAIGHT WJ, CHAMNEY AR, HOWELLS TH. The determination of an effectivecricoid pressure. Anaesthesia 1983; 3 8 461-6. [7] PAYNE J. The use of the fibreoptic laryngoscope to confirm the position of the laryngeal mask. Anaesthesia 1989; 44: 865.
Does the laryngeal mask airway compromise cricoid pressure?
T. I. STRANG
Summary The laryngeal mask airway was inserted in 10 cadavers. At postmortem the chest was opened and an infusion set primed with a dilute barium solution was inserted into the oesophagus and ligated in place. A cricoid force of 43 N was then applied and the infusion set was positioned so that when the clamp was opened it generated a pressure of 7.8 kPa within the oesophagus. The cricoid pressure was able to stop the p o w of @id into the oesophagus. This demonstrates that cricoid pressure is effective in preventing reJrux at intragastric pressures which are encountered clinically and the presence of the laryngeal mask airway does not compromise this.
Key words Larynx; cricoid pressure. Equipment; laryngeal mask airway. Complications; regurgitation.
The laryngeal mask airway (LMA), which was first introduced in 1988 [I], has been used extensively to provide a safe airway in spontaneously breathing patients who are not at risk from aspiration of gastric contents. Recently, Chadwick and Vohra reported its use in conjunction with cricoid pressure, in a patient who was at risk from pulmonary aspiration, when failed intubation occurred at emergency Caesarean section (21. However, the effectiveness of cricoid pressure in the presence of a correctly positioned LMA has not yet been established. Method Consent to undertake this investigation was obtained from the coroner. Postmortem examination began with removal of the anterior chest wall to expose the lungs and mediastinum. An LMA of appropriate size was inserted according to the manufacturer’s instructions [3]. Correct positioning was confirmed by (i) fibreoptic endoscopy, (ii) observing inflation of the lungs and (iii) palpation of the mask in the neck. The oesophagus was then incised 3 cm below the level of the cricoid cartilage. A standard infusion set was modified by cutting off the distal 5cm of tubing and creating an additional 2 mm lateral orifice, 0.5 cm from the distal end.
The infusion was then primed with a dilute salinebarium mixture and the modified end was inserted into the proximal oesophagus which was then ligated around it. The cadaver was positioned with the lower cervical vertebrae flexed and the atlanto-occipital joint extended; the classical ‘sniffing the morning air’ position used for tracheal intubation. A cricoid force of 43 N was applied using a latex mould of the author’s neck, which overlay the cricoid cartilage of the cadaver and supported a 4.4 kg lead mass. The apparatus was held in place, being careful that no additional force was transmitted onto the cricoid cartilage by the author’s hands. The infusion was then positioned so that when the clamp was opened it generated a pressure of 7.8 kPa up the oesophagus. The accuracy of this pressure had previously been verified with a water manometer. The infusion was started whilst cricoid pressure was maintained and the inspection chamber was observed for evidence of flow. After cricoid pressure had been released, evidence of flow was once again sought (Fig. I).
Results Insertion of the LMA was possible in all four male and six female cadavers. In all 10 cadavers the lungs could be
Dr T.I. Strang, MB, ChB, DCH, FRCAnaes, Registrar, Department of Anaesthetics, Royal Albert Edward Infirmary, Wigan Lane, Wigan WNI 2MN. This paper was awarded the President’s prize at the Group of Anaesthetists in Training Meeting, Bristol, April 1992. Accepted 6 February 1992. 0003-2409/92/010829
+ 3 %08.00/0
@ 1992 The Association of Anaesthetists of Gt Britain and Ireland
829
830
T.I. Strang In three cases lateral X rays of the neck were taken, which demonstrated the effectiveness of the cricoid pressure (Fig. 2(a) and (b)). In all 10 cases, when the clamp was opened, approximately 10-15 drops of fluid dripped into the oesophagus, after which all flow ceased. When cricoid pressure was released, in all cases the dilute barium solution immediately gushed through the drip set into the oesophagus and was subsequently observed in the pharynx when the LMA was removed. In six of the cases, the pathologist commented on the presence of white staining within the trachea.
7.8kPa n Fig. 1. Lateral view of the laryngeal mask airway in place and the infusion set ligated in the oesophagus. Horizontal arrow indicates a pressure of 7.8 kPa within the oesophagus. Vertical arrow shows a cricoid force of 43N applied in the neck.
inflated though the LMA and the upper part of the cuff could be palpated through the thyrohyoid membrane. In nine cases a bronchoscope was available and views of the larynx further confirmed correct positioning of the mask.
Discussion Intragastric pressure may increase up to 7.84 kPa in patients with an abdominal mass (such as a gravid uterus) after the administration of suxamethonium [4]. In 1961, Sellick described his method of cricoid pressure which has been adopted widely to protect the patient against passive reflux of gastric contents [5] and Wraight et al. subsequently showed that a force of 44 N applied to the cricoid cartilage was sufficient to protect most patients from regurgitation [6].
Fig. 2. Lateral X rays of the neck showing (a) cricoid pressure preventing passive reflux of barium; (b) reflux of barium when cricoid pressure was released.
Does the LMA compromise cricoid pressure? The effect that the presence of the LMA has on cricoid pressure has never been examined, despite the fact that it has been used in patients at risk of regurgitation [2]. In this study, a fibreoptic bronchoscope was used to confirm that the LMA was correctly positioned, as has recently been suggested by Payne [7]. The infusion set was modified at its distal end to produce an additional lateral orifice so that flow was not obstructed by oesophageal tissue. Thus, if the flow of dilute barium into the oesophagus was prevented, it must have been as a result of the cricoid pressure alone. A weak solution of barium was used, so that regurgitated fluid in the pharynx would be clearly visible. When X rays were taken, a more concentrated solution was used. The initial 10-15 drops of fluid that entered the oesophagus before flow ceased represented the 1 ml dead space volume in the oesophagus which had to be filled before intra-oesophageal pressures reached a level of 7.8 kPa. The force exerted by a 4.4 kg mass in the direction of the gravitational field is 43 N, although any error in the direction of the cricoid pressure would have resulted in a cricoid force of less than 43 N. This study therefore confirmed that a cricoid force of 43 N is protective against a distal oesophageal pressure of up to 7.8 kPa and the presence of the LMA did not prevent the cricoid pressure from being effective. However, the appearance of dilute barium in the trachea of some cadavers after release of cricoid pressure also indicates that the LMA alone does not prevent passive regurgitation. The compliance of cadaveric tissue is reduced as a result of rigor mortis, therefore, in the more compliant tissues of live patients, it is probable that lower forces would be required to produce effective cricoid pressure.
83 1
A recent clinical study has shown that cricoid pressure does not reduce the ease of insertion or positioning of the LMA [8] and this cadaveric study suggests that the presence of the LMA does not compromise the effectiveness of cricoid pressure in preventing reflux. However, since dilute barium appeared in the tracheas of some cadavers after release of cricoid pressure, the exact r61e of the LMA in the management of failed intubation has yet to be determined.
Acknowledgments The author thanks Dr A. Brain for the donation of two laryngeal mask airways, G. Millner (Mortuary Technician) for his assistance and co-operation, A. Lang and A. Boardman for radiographic services and Ms A. Shirley for typing this manuscript.
References [I] BRAINAIJ. The Laryngeal Mask-a new concept in airway management. British Journal of Anaesthesia 1983; 55: 801-5. [2] CHADWICKIS, VOHRA A. Anaesthesia for emergency Caesarean section using the Brain laryngeal airway. Anaesthesia 1989; 44: 261-2. [3] BRAIN AIJ. The lntervent Laryngeal Mask. Instruction Manual, 1990. [4] ANDERSEN N. Changes in intra-gastric pressure following the administration of suxamethonium. Preliminary report. British Journal of Anaesthesia 1962; 34: 363-7. [5] SELLICKBA. Cricoid pressure to control regurgitation of stomach contents during induction of anaesthesia. Lancet 1961; 2 404-6. [6] WRAIGHT WJ, CHAMNEY AR, HOWELLS TH. The determination of an effectivecricoid pressure. Anaesthesia 1983; 3 8 461-6. [7] PAYNE J. The use of the fibreoptic laryngoscope to confirm the position of the laryngeal mask. Anaesthesia 1989; 44: 865.
