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Airway management

We continue our discussion of airway management this week.

1.  What is the proper way to apply cricoid pressure to reduce the chance of aspiration of gastric contents?
2.  Are there any disadvantages to application of cricoid pressure with respect to aspiration risk?

1.  通过压迫环状软骨以减少胃内容物吸入的方法有何注意事项？
2.  压迫环状软骨以减少误吸的方法存在什么问题？

参考答案：

1.  通过压迫环状软骨以减少胃内容物吸入的方法有何注意事项？

Sellick[1]1961年介绍了通过压迫环状软骨减少全麻诱导时胃内容物吸入的方法后，很快该方法受到临床医生的青睐。过去20年的工作是我们对该方法的生理学的理解更加明确。首先，压力必须作用于环状软骨，而不是甲状软骨或其他咽、喉部结构。甲状软骨压迫可能影响气管插管，因为其结构并非完整的环形，因此并不能对食道进行有效的压迫。其次，必须给于足够的压力。Wraight等[2]对麻醉受试者作用于环状软骨的外力进行了测量，该外力要达到阻止生理盐水流入食道。他们得出结论认为，大多数受试者需要44N（大约10磅）的压力。同时他们进行了另一个研究发现，经验丰富的麻醉科医生使用的压力范围是10-120N。第三，压力必须作用于正中线。如果可导致食道闭塞的足够压力作用于非正中位（侧面）时，将使声门移位，气管插管受影响[2]。第四，必须在意识消失前进行给于压迫。在全麻静脉诱导期间，患者意识消失前食道上段压力减小，因此，推荐必须在诱导时进行环状软骨压迫，并在患者意识消失时增加压迫力量[3]。

2.  压迫环状软骨以减少误吸的方法存在什么问题？

食道下端括约肌压力梯度（食道下段-胃压力差）是抵抗返流的屏障压。最近有研究明确证实，环状软骨压迫降低了食道下端括约肌（LES）张力和屏障压[4]。因为适当的环状软骨压迫常在患者意识消失前使用，此恰当的压力（文献3建议为20N）足以降低LES张力，因此，作者得出结论认为，环状软骨压迫事实上最终增加返流的机会。其机制可能为咽部刺激使LES松弛，如正常吞咽时出现的一样。置入LMA导致LES张力降低，间接支持了该观点[5]。但尚没有临床结果支持放弃环状软骨压迫，需要进行更进一步的研究以明确对于胃高压的患者采取何种策略。

What is the proper way to apply cricoid pressure to reduce the chance of aspiration of gastric contents?

Cricoid pressure as a maneuver to reduce the risk of aspiration of gastric contents during induction of general anesthesia has enjoyed considerable popularity since its introduction into clinical practice by Sellick in 1961 (1). Work performed over the last two decades has refined our understanding of the physiology of this technique. First, pressure must be applied to the cricoid cartilage, not the thyroid cartilage or other pharyngeal or laryngeal structures. Thyroid cartilage pressure may impair endotracheal intubation, and because this structure is not complete posteriorly, effective compression of the esophagus does not occur. Second, sufficient pressure must be applied. Wraight and colleagues (2) measured the external force on the cricoid cartilage required to prevent saline from dripping down the esophagus in anesthetized subjects. They concluded that 44 N (about 10 pounds) was required in most subjects. In a companion study these investigators found that experienced anesthetists used forces ranging from 10 to 120 N. Third, the pressure must be applied in the midline. Forces sufficient to occlude the esophagus, when applied laterally, have been shown to displace the glottis and impair intubation (2). Fourth, pressure must be applied before loss of consciousness. During intravenous induction of general anesthesia, upper esophageal pressure decreases before the patient loses consciousness, and it has been recommended, therefore, that cricoid pressure be applied during induction and then increased as the patient loses consciousness (3).

Are there any disadvantages to application of cricoid pressure with respect to aspiration risk?

The pressure gradient across the lower esophageal sphincter (lower esophageal pressure-gastric pressure) is the "barrier pressure" resisting reflux. It has been convincingly demonstrated recently that application of cricoid pressure decreases lower esophageal sphincter (LES) tone and barrier pressure (4). Since moderate cricoid pressure is sometimes applied prior to the patient's loss of consciousness, and since this moderate force (20 N was suggested by [3]) is sufficient to reduce LES tone, the authors concluded that cricoid pressure could actually increase the chance of regurgitation. The mechanism of this effect may be that pharyngeal stimulation relaxes the LES, as may occur during normal swallowing. Supporting this view is the observation that placement of an LMA causes decreased LES tone (5). No clinical outcome data supports the abandonment of cricoid pressure, and further investigation will be required to determine how best to use the maneuver in patients with elevated gastric pressure.

References:

1.  Sellick BA. Cricoid pressure to control regurgitation of stomach contents during induction of anaesthesia. Lancet 1961; 2:404-6.
2.  Wraight WJ, Chamney AR, Howells TH. The determination of an effective cricoid pressure. Anaesthesia 1983; 38: 461-6.
3.  Vanner RG, Pryle BJ, O'Dwyer JP, Reynolds F. Upper esophageal sphincter pressure and the intravenous induction of anaesthesia. Anaesthesia 1992; 47:371-5.
4.  Tournadre JP, Chassard D, Berrada KR, Bouletreau P. Cricoid cartilage pressure decreases lower esophageal sphincter tone. Anesthesiology 1997; 86:7-9.
5.  Rabey PG, Murphy PJ, Langton JA, et al. Effect of the laryngeal mask airway on lower oesophageal sphincter pressure in patients during general anaesthesia. Br J Anaesth 1992; 69:346-8.


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