Artículos de Reflexión

Introduction. The Sellick maneuver or cricoid pressure is a procedure that is routinely performed in the prophylaxis for pulmonary aspiration as part of a rapid sequence intubation. It has been considered very controversial specifically on its usefulness as a standard safety practice in the emergent airway management. It usually has been considered a pressure maneuver, however it usually has seen assessed measuring Newtons (N) without considering the area of the cricoid cartilage. For this reason, it must be understood in terms of cricoid force instead of cricoid pressure.

Objetive. To highlight the controversial issues this maneuver has had over the 50 years of its use in airway management and pulmonary aspiration prevention. To explain the common error of describing it in terms of force and not pressure.

Methods. Literature review about the controversial aspects of the Sellick maneuver in emergent scenarios in anesthesia.

Conclusion. Since the initial description by Dr. Brian Arthur Sellick in 1961 of the maneuver, using cricoid pressure to prevent them regurgitation of gastric contents to the pharynx used as part of the rapid sequence induction/ intubation, there have been multiple criticisms worthwhile to review, regarding the safety it provides in protecting the airway. We recommend that the terms pressure and force be differentiated and be used more appropriately when describing the maneuver, as the area of the cartilage should be considered in these measurements.

Keywords: Cricoid cartilage, pressure, cervical vertebrae, acid aspiration syndrome (Source: MeSH, NLM).

The Sellick maneuver or cricoid pressure (CP) was described by the English anesthesiologist, Dr. Brian Arthur Sellick in 1961 (1). It consists in applying backward pressure over the cricoid cartilage to compress and occlude the superior esophagus against the cervical vertebrae and thus prevent regurgitation of gastric content to the pharynx. This maneuver was later included in the rapid sequence induction/intubation described by Safar in 1970 (2), and ever since has become a standard in the airway management of patients with a full stomach (3, 4). Currently, there has been much controversy whether this maneuver really decreases the incidence of pulmonary aspiration. To answer this question with a clinical trial, it would have to overcome two major hurdles. First it would require a very large sample, as the incidence of pulmonary aspiration is very low (1.4 /10.000 anesthetics); and second it would need to overcome the ethical aspects of such research, so as there are no such trials, the current recommendation for its use is evidence level grade D (5).

Neelakanta, in 2003, published a case report in which the patient had the cricoid pressure released after intubation, and who did not aspirate even when abundant liquid content was observed in the mouth (6). This case has been considered one of the best evidences of the effectiveness of cricoid pressure in preventing pulmonary aspiration. However, many other findings have been described since the introduction of CP in clinical practice: for example in a prospective study 9 out of 12 patients at high risk of pulmonary aspiration had pulmonary infiltrates even though CP was performed (7).

Two cases of fatal pulmonary aspiration syndrome occurred even after CP and antiacid prophylaxis (8). In a maternal mortality report in the United Kingdom between 1972 and 1975, there were 13 cases of pulmonary aspiration of which five had CP performed incorrectly (9). Williamson reported three adverse outcomes after CP, one in whom the maneuver produced hiccups before induction which led to pulmonary aspiration and death; a second patient with a significant liquid content in the oropharynx even though CP was performed correctly; and third a patient in whom CP prevented visualization of the vocal chords (10).

Other studies have intended to determine the usefulness of CP examining secondary outcomes like successful intubation, laryngeal visibility during laryngoscopy, inferior esophageal sphincter tone, gastric insufflation, and esophageal occlusion assessed with diagnostic imaging.

Turgeon et al compared CP to no CP and reported that there were no differences in the rate of failed intubations, or in the visibility during laryngoscopy (11). Other results assessing laryngeal view are to contradictory; some report CP is useful for tracheal intubation (12), others describe no impact similar to the results of Turgeon et al (11,13), whereas others like Snider et al found major problems when CP is combined with the BURP maneuver to facilitate intubation (14). Moynihan et al found that CP decreased gastric insufflation during mask ventilation in children (15).

Regarding the airway anatomy, it has been assumed that the esophagus is located behind a cricoid cartilage, and that this in turn, is located in front of the cervical spine in an axial plane, however, when computed axial tomography has been performed, controversial findings have been obtained. In a study with magnetic resonance imaging, Smith found that in half of the patients the esophagus is not located behind the cricoid cartilage and that when CP was practiced the esophagus was displaced laterally in 90 % of them (16). On the other hand, Rice also using MRI observed that CP occludes the hypopharynx and not the esophagus, so that the position of the esophagus is not relevant (17).

All these suggest that the Sellick maneuver described more than 50 years ago, involves many variables that include the patients and the operator (18). The effect of cricoid pressure on the larynx will depend on the individual characteristics, the method of applying force, the relationship between the esophagus, the cricoid and the cervical spine, the ventilation technique, etc. There is no strong evidence its benefits regarding a decrease in gastric distention and decreasing the risk of pulmonary aspiration (19,20).

As in any therapeutic alternative in medicine, a risk-benefit assessment must be made as stated by Ellis et al (18), so a patient with higher risk of pulmonary aspiration and low risk of desaturation (ie patients with appendicitis) can benefit of this maneuver; whereas a patient with low risk of aspiration and high risk of desaturation (a hypoxic and tachypneic patient with severe pneumonia) should not have CP recommended.

Another topic of controversy has been the way cricoid pressure has been measured in most studies. Sellick did not propose any quantitative method to assess the force applied during CP, he only described the maneuver: "when anaesthesia is started, a pressure is exerted over the cricoid cartilage", "a conscious patient can only tolerate a moderate pressure without being uncomfortable", "as soon as consciousness is lost a firm pressure can be applied" (1,20).

In 1983, Wraight et al measured the force over the cricoid (using a cricoid yoke) to prevent the reflux of normal saline through a modified orotracheal tube inserted in the esophagus in 24 patients under elective anaesthesia (21). From their data and previous measurements of gastric pressures in awake patients, they estimated that the cricoid pressure needed to prevent pulmonary aspiration in patients who required emergent anaesthesia was 44 N (9,81 N = 1 kg = 2,2 lb).

Clayton and Vanner (22) suggested that applying a backward force over the cricoid cartilage, alters the measurement of the weight of the person applying the force, in other words if the person who applies the force over the cricoid stands on a weighing scale he will have an apparent decrease of 1 kg of weight which is equivalent to 9.81 N (Newtons). With these data the weight applied in most studies (2.5 to 3 kg) would correspond to of force between 25 N and 30 N which is the recommended force when performing CP.

From this another issue arises: why are force units used to measure pressure? According to the international weights and measurement standards, pressure is defined as the force exerted over an area and its unit of measurement is the pascal which is equivalent to the force of 1 Newton exerted over an area of 1 square meter (23). For this reason cricoid pressure should be reported in pascals or other units of pressure like mmHg or cmH₂0, and not in units of force or mass because an important component would be ignored which is the area of the cricoid cartilage.

When considering these parameters, 30 N of force exerted over an area of 6.25 cm² (the area of the cricoid plate is between 4 and 9 cm² to) produce a pressure of 48 kPa (kilopascals) whereas the same force applied over an area of 4 cm² exerts a pressure of 74 kPa, which is much higher than the first one and could affect the laryngeal view. This could be an important factor in the variability of the results of studies of CP.

There are two solutions for this problem: either change the units with which cricoid pressure is measured (pascals or mmHg) or change the name of the maneuver to cricoid force. Although it may seem trivial, it is improper that in medicine a verifiable science, some magnitude would not be measured appropriately in its studies.

Future research on cricoid pressure should measure pressure with the appropriate units including the consideration of the area of the cricoid cartilage which could be another factor explaining the variability in the effectiveness of the maneuver.

In conclusion, Sellick's maneuver is a procedure that has been used in the prophylaxis of pulmonary aspiration as part of the rapid sequence induction/intubation, however there is much controversy about it and still it cannot be considered a standard safety practice in evidence based airway management.

We highlight the error of considering the maneuver as a pressure, measuring it in Newtons without taking into account the area of the cricoid cartilage. Therefore it would be improper to refer to it in terms of cricoid pressure instead of cricoid force.

REFERENCIAS

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