Cricoid pressure in contemporary anaesthesia
Niranjan J Sathianathen
Aim: To evaluate the role of cricoid pressure in modern day anaesthetics. Methods: A literature review was conducted using the following databases: The Cochrane Database of Systematic Reviews, PubMed, Scopus, Ovid MEDLINE and EMBASE. Articles were found using following terms: cricoid pressure, aspiration, laryngoscopy, airway obstruction, anaesthesia, anaesthesiology, airway management, Sellick’s and rapid sequence intubation. Results: The literature review revealed a lack of high-level evidence supporting the use of cricoid pressure, however, observational studies have suggested a benefit in preventing gastric aspiration. The application of cricoid pressure is inconsistent and generally variable amongst clinicians. Sellick’s manoeuvre is occasionally associated with airway obstruction. Conclusion: When applied correctly, cricoid pressure may still have a role in preventing pulmonary aspiration of gastric contents. There is however a risk of airway obstruction and given the inconsistencies in technique, cricoid pressure should only be employed by trained individuals.
The first description of cricoid pressure (CP) was by Monro in 1774 when he used it to prevent gastric insufflation in near-drowned victims. But it was not until 1961 when Sellick utilised the manoeuvre to prevent regurgitation of gastric contents during the induction of anaesthesia that it came to the forefront of clinical anaesthesiology.  Consequently, over the last 50 years, CP has become an integral part of the rapid sequence induction (RSI) – an airway management technique employed in patients at high-risk of aspiration where administration of a sedative and muscle relaxant occurs virtually simultaneously. [2,3] However, more recently, there has been much dispute regarding the efficacy and evidence behind Sellick’s manoeuvre leading some anesthetists to even completely abandon it from their practice. 
This paper examines the evidence base for the use of cricoid pressure and whether it still has a role in the clinical milieu.
The incidence of pulmonary aspiration of gastric contents has historically been reported as being low overall but it still remains a critical issue in modern anaesthetic practice, as shown by a survey in which 71% of respondents reported encountering at least once case during their careers. [5,6] Importantly, aspiration is associated with severe consequences in regards to morbidity and morality, with a 1999 Australian study reporting a death rate of 3.8%. [6,7]
The purpose of Sellick’s method is to decrease the likelihood of Mendelson’s syndrome, that is, aspiration pneumonitis – one of the recognised complications of general anaesthesia. The pathophysiology behind Mendelson’s syndrome lies in the concept that the unconscious patient has diminished protective airway reflexes placing them at an increased risk of aspiration.  This is supported by Vanner and Pryle who displayed an immediate loss of upper oesophageal tone after losing consciousness.  Consequently, physical compression of the oesophagus by placing pressure on the cricoid cartilage is expected to prevent the regurgitation of gastric contents.
To determine whether the application of cricoid pressure decreases the likelihood of gastric aspiration. To evaluate whether the benefits of cricoid pressure outweigh its adverse sequelae.
A search was undertaken of the medical literature using the following keywords and their alternative spellings: cricoid pressure, aspiration, laryngoscopy, airway obstruction, anaesthesia, anaesthesiology, airway management, Sellick’s and rapid sequence intubation. Results from all searches were refined with Boolean operators. The search was conducted in the following databases: The Cochrane Database of Systematic Reviews, PubMed, Scopus, Ovid MEDLINE and EMBASE. The search retrieved 571 papers. The reference lists of included studies were also manually reviewed to identify additional relevant literature. Papers including both human and cadaveric studies were included. The author determined which of the retrieved articles were to be included in the review and there were no explicit exclusion criteria.
Evidence for efficacy
Sellick’s original articles supporting the use of CP in preventing aspiration are observational studies consisting of 26 and one patient(s) respectively. [1,10] His first article details a successful trial of his technique amongst cadavers by filling their stomach and placing them in a head down position while applying cricoid pressure. It was not until nearly a decade later that four more cadaver-based studies validated Sellick’s original findings. [9,11-13] Recently, a case report by Neelakanta further supported the use of cricoid pressure in preventing aspiration.  No randomised controlled trials have been conducted to assess the efficacy of CP in the prevention of pulmonary aspiration.
In regards to gastric insufflation, four historical studies have suggested that CP has a positive clinical outcome on decreasing the amount of gas in the stomach amongst patients being ventilated by a facemask. [15-18] It is important to note that that most recent of these studies was conducted in 1993 and consequently clinical protocols used at the time of the cited studies have changed compared to the present day and are inherently more conducive to decreasing aspiration risk, hence potentially undermining this suggested advantage. [19,20]
Anatomical and physiological debate
The crux of the cricoid pressure technique relies on the principle that the cricoid cartilage, oesophagus and vertebral bodies lie in a single axial plane. Subsequently, in theory, backward pressure on the cricoid cartilage against the posterior vertebral bodies should occlude the oesophagus. Studies of both computed tomography and magnetic resonance imaging have disputed this concept and demonstrated that the oesophagus is naturally displaced laterally in relation to the midline of vertebral bodies in 49% and 53% of individuals respectively. [21,22] With the application of CP, this lateral displacement is further exacerbated thus questioning the primary foundation of Sellick’s technique.  This phenomenon is not limited to just adults, but also seen in the paediatric population with younger children impacted more than those older.  Moreover, Rice et al. demonstrated that it is the hypopharynx and not the oesophagus that is situated behind the cricoid cartilage, and further, that the oesopahgus is inferior to the level of the cricoid ring; but interestingly concluded that the alimentary tract is still compressed adequately. 
Similar to the debate surrounding the anatomic foundations of CP, there have also been contrasting views regarding its physiological basis. Published papers have suggested that the application of cricoid pressure actually opposes its intended effect of preventing regurgitation by decreasing lower oesophageal sphincter tone.  While this has been shown to cause an increase in gastric distention during bag-mask ventilation, Skinner et al. suggested that the loss of tone has no significant impact on the risk of gastric reflux amongst healthy individuals. [19,27]
Despite the case reports of CP preventing aspiration, the technique is far from perfect. Surveys revealed that up to 14% of anaesthetists had witnessed aspiration in the presence of cricoid pressure being applied.  A potential explanation for this may be the inconsistencies in CP technique between individuals. In his original report, Sellick did not specifically quantify the amount of force necessary to achieve an adequate compression of the alimentary tract other than describing it as “firm”.  Consequently, the exact amount of force required to achieve the primary aim of preventing aspiration without compromising the ease of laryngoscopy and other complications has been greatly debated. A group of researchers first suggested that a force of 44 Newtons (N) needs to be applied, before Vanner recommended that 20 N be applied in the conscious patient before increasing to 40 N after the onset of anaesthesia. [29,30] In a more recent paper, the optimal amount of force was modified to 10 N and 30 N in the conscious and unconscious patient respectively.  Despite these rather precise theoretical recommendations, numerous studies have discovered that the CP forces applied by health professionals are discordant. [32-34] However, additional training was shown to improve staff technique. [33-37]
Adverse sequelae of CP
Whether stemming from the pitfalls in individual technique or not, CP has been associated with airway obstruction. Hartsilver & Vanner found that cricoid pressure applied according to the initial recommendations of Wraight et al. resulted in complete airway obstruction in 35% of patients. [29,38] This vastly declined to 2% when the recently suggested 30 N of force was applied; however, if this force is applied in an upward and backward direction as suggested by Vanner et al.  obstruction was seen in 56% of the population.  Another study reported 35% of their cohort as having airway obstruction evidenced by decrease in tidal volume on application of CP, of which 31% had complete obstruction.  However, it should be noted that the clinical utility of a decrease in tidal volume as a measure is questionable due to the application of CP during apnoea. Two further randomised controlled studies supported the association between CP and airway obstruction. [41,42] Additionally, the application of CP has been shown to impede laryngeal mask placement in patients who have failed RSI and require an airway resulting in a life-threatening situation as the patient cannot be adequately ventilated. [43,44] The experience is similar when attempting to place a laryngeal tube and laryngeal tube-suction II.  However, these difficulties were not encountered when using a cuffed oropharyngeal airway where no significant difference in tidal volume or peak inspiratory pressure was demonstrated when comparing “post-manoeuvre” measurements to baseline, regardless of whether CP was applied or not.  It has been reiterated that CP should be removed if any difficulty is encountered during intubation. 
The effect of Sellick’s manoeuvre on laryngoscopy has been the subject of much research and findings have varied. Vanner et al. propose that CP in either the classical form or in the modified upward and backward direction improves views at laryngoscopy compared to no CP, with the best views seen in the modified upward and backward group.  Furthermore, CP shows a greater improvement in views in the left lateral than supine position.  Nevertheless it should be noted that pressure on the thyroid cartilage resulted in superior views in 88% of patients compared to only 11% when pressure is applied to the cricoid cartilage.  In juxtaposition, Haslam et al. concluded that the relationship between CP and laryngoscopic views is complex in which low levels of force (< 30 N) may result in improved views but as the pressure increases there is an analogous increased risk of complete obstruction.  Similarly, Noguchi et al. determined that views are worsened with application of CP, however it is important to note that this was an observational study. 
Additionally, the application of cricoid pressure in the conscious patient has been shown to induce vomiting and cause oesophageal injury.  In their cadaveric studies, Vanner & Pryle reported oesophageal rupture occurring in 30% of the cadavers studied.  Another case report details experiencing this complication in a living patient.  Furthermore, by decreasing lower oesophageal sphincter tone, CP increases the likelihood of vomiting. [53,54] In contrast, Khan & ul Haq demonstrated a trend of decreased rates of nausea and vomiting in the postoperative period amid patients that had CP applied compared to the no CP group, but the results were not statistically significant. 
Although there are studies that support the use of CP in preventing aspiration, it is important to consider that except for Neelankanta et al., they are relatively historical and cadaveric-based which questions its applicability in the modern clinical landscape and in living patients.  On the other hand, adverse consequences of CP have been described in both case reports and randomised trials. Furthermore, the debate surrounding the theoretical foundations of Sellick’s manoeuvre has cast doubts on its efficacy.
Cricoid pressure was once described as the “lynchpin of physical prevention [of aspiration]” and as a minimum of standard of care, thus implying that any randomised trials evaluating its efficacy would be unethical.  However, in light of recent evidence that has suggested several adverse sequelae of utilising Sellick’s manoeuvre, it may now be ethically acceptable to conduct a randomised study in order to better evaluate the value of CP. The impetus to perform further research is greater given that evidence supporting the use of cricoid pressure is of poor quality. The results of such a trial would be pivotal in either reaffirming or eradicating CP from the modern anaesthetic landscape.
Although many anaesthetists have already discontinued the use of CP from their practice, this may prove to have been somewhat hasty. While the evidence base exhibiting the benefits of cricoid pressure is scarce, it is important to consider the extremely low levels of aspiration-related death from anaesthesia since its introduction into the clinical domain. It is acknowledged that anaesthetic practice over that time period has undergone considerable change and whilst there are likely to be other factors also responsible for the low mortality rates, the true impact of Sellick’s manoeuvre cannot be quantified with any certainty.
Nevertheless, the body of evidence suggesting the cessation of cricoid pressure application needs to be considered carefully. This is especially the case when continued application of pressure results in airway obstruction and, subsequently, difficulty with ventilating the patient. Consequently, the risks and benefits need to be considered on an individual basis and CP should not be applied unless clinical judgment suggests otherwise.
Importantly, this paper has exhibited the substantial inconsistencies in cricoid pressure technique, which demands for better training of health professionals. The importance of this should not be underestimated as rectification of technique may result in both superior efficacy in preventing aspiration as well as a reduction in some of the reported adverse effects.
While there have been a number of reports of the adverse sequelae of applying cricoid pressure, of which complete airway obstruction is the most severe, it may still have a role in preventing aspiration and improving laryngoscopic views – especially at low forces, that is, below 30 N. Subsequently, its use in clinical practice should be evaluated on an individual basis by a risk-benefit analysis.
Overall, cricoid pressure is a pseudoaxiom that has been adopted as part of standard anaesthetic practice without solid evidence supporting its efficacy. In considering the currently available literature, its status as being universally accepted and applied during anaesthetic induction appears to be under threat. Further research is needed in order to more definitively determine whether cricoid pressure has a role to play in clinical practice.
Conflict of interest declaration
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