Cricoid pressure is used in anaesthetic practice to prevent the regurgitation of gastric contents into the oropharynx during the induction of anaesthesia. This prevents primary adverse events, such as a soiled airway, as well as secondary adverse outcomes such as aspiration pneumonia. The practice was proposed by Brian Arthur Sellick in 1961 (Sellick, 1961).
To apply cricoid pressure, approximately 30N of force should be applied anteriorly to the cricoid cartilage. The cricoid cartilage is the only complete ring of cartilage in the larynx and applying this force causes compression to the posterior structures. This occludes the oesophagus and thus prevents regurgitation.
Some research has suggested that cricoid pressure may be ineffective, even when carried out correctly. Cheney (2000) reports that in a cohort of 67 anaesthetic deaths, 17 experienced aspiration despite cricoid pressure being applied. A recent multicentre, transnational study found a higher rate of regurgitation when cricoid pressure was applied in children at 1.9% versus 1.2% (Kojima et al, 2018).
Despite UK paramedics not providing rapid sequence induction (RSI), a national survey of UK paramedics reported that 91% use cricoid pressure during all intubations. This survey had 1506 respondents, which reflected 7.3% of UK paramedics at the time the study was conducted (Younger et al, 2016).
Paramedics are often criticised for low first-pass intubation rates of around 80% (Dyson et al, 2017) and should therefore aim to facilitate a laryngeal view that maximises the probability of safe intubation (Harris et al, 2010). Moreover, given the poor levels of continuing professional development and insufficient ongoing training from ambulance trusts (Deakin et al, 2009), cricoid pressure may be inappropriate if it makes intubation more difficult.
Multiple authors have raised concerns that cricoid pressure may harm patients through higher rates of failed intubation and other adverse outcomes (Cook 2016; Gwinnutt and Gwinnutt, 2016; Scober 2017).
This literature review examines the evidence pertaining to the impact of cricoid pressure upon laryngoscopy and intubation, using data from both prehospital and in-hospital studies to discuss how it may influence paramedic practice.
Methodology
The PubMed and Cochrane Library databases were searched with the following search terms: (((cricoid pressure) OR sellick)) AND (((((rapid sequence induction) OR RSI) OR intubation) OR laryngeal) OR success).
Inclusion criteria were: cricoid pressure application during tracheal intubation; direct laryngoscopy; human participants; and primary or secondary endpoints pertaining to impact upon intubation or laryngoscopy.
Exclusion criteria were: studies not reported in English; video laryngoscopy; cricoid application for purposes other than intubation (e.g. bag-valve-mask ventilation); articles before 2003; animal and cadaver studies; and the full article being unavailable to the author.
A time frame filter from 1 January 2003–31 December 2018 was applied. Studies were assessed for eligibility first by title, then abstract, then with full manuscript examination, with full inclusion and exclusion criteria applied; PRISMA methodology was used (Moher et al, 2009) (Figure 1).
Results
Study characteristics
Seven studies were included for discussion; three were from prehospital settings (n=2040) and four were from hospitals (n=4307). Two of the in-hospital studies had small population sizes of less than 100 patients each (Turgeon, 2005; Bohman et al, 2018). Four studies were randomised controlled trials (RCTs) and three were prospective observational studies. All RCTs were carried out in hospitals. Studies were conducted in a variety of countries: France (2), UK (2), Australia (1) Canada (1) and USA (1) (Table 1).
| Author, date | Number of patients | Country | Setting | Population | Patient characteristics |
|---|---|---|---|---|---|
| Haslam et al, 2005 | 40 | UK | In hospital | Elective patients undergoing intubation | Indications unknown |
| Turgeon et al, 2005 | 700 | Canada | In hospital | Elective patients undergoing intubation | Indication unknown |
| Harris et al, 2010 | 402 | UK | Prehospital | Patients receiving intubation by physician-led helicopter emergency medical services | Cardiac arrest: 42 |
| Burns et al, 2016 | 443 | Australia | Prehospital | Patients receiving intubation by physician-led helicopter emergency medical services | Traumatic cardiac arrest: 37 |
| Caruana et al, 2017 | 1195 | France | Prehospital | Patients receiving intubation by physician-led ground emergency medical services | Cardiac arrest: 665 |
| Birenbaum et al, 2018 | 3472 | France | In hospital | Patients with at least 1 risk factor for regurgitation | 2287 emergency rapid sequence induction (indications unknown) |
| Bohman et al, 2018 | 95 | United States | In hospital | Patients with at least 1 risk factor for micro-regurgitation | All planned elective surgeries (Indications unknown) |
Intubation operator
Of the 6347 patients enrolled in the seven studies, only 290 in one study were intubated by a paramedic (Burns, 2016). In all other prehospital studies, patients were intubated by physicians or nurses. All prehospital studies were conducted in specialist critical care teams.
For in-hospital studies, intubation was undertaken by physicians or nurses. In addition, intubation took place in the operating department environment. No intubations took place in settings such as intensive care units or emergency departments.
Impact upon laryngoscopy
Investigators from each study used a variety of endpoints to measure the impact upon laryngoscopy. This was done using familiar reporting techniques, such as a Cormack-Lehane Grade (Cormack and Lehane, 1984) or through the rate of difficult intubations experienced by the operator (Table 2).
| Study, year, number | Method | Measure of impact upon laryngoscopy and intubation | With cricoid pressure | Without cricoid pressure | P value | 95% confidence interval | ||||
|---|---|---|---|---|---|---|---|---|---|---|
| Haslam et al, 2005 n=40 | Prospective data collection | Measurement of antero-posterior length of the rima of glottis with different amounts of CP applied | Improved | No change | Worsened | N/R | N/R | |||
| 0–10N | 19 | 9 | 12 | |||||||
| 10–20N | 15 | 9 | 16 | |||||||
| 20–30N | 13 | 8 | 19 | |||||||
|
Turgeon et al, 2005
|
Randomised to receive CP or sham | Failure to Intubate in 30s | 4.4% | 3.7% | 0.70 | N/R | ||||
|
Harris et al, 2010
|
Prospective data collection only. All patients received CP as firstline | Impact of removing CP when a difficult intubation was encountered* | Improved laryngeal view | No change | Worsened laryngeal view | N/R | N/R | |||
| 50% | 40.9% | 0% | ||||||||
|
Burns et al, 2016
|
Prospective data collection. No randomisation | Incidence of CP application in difficult intubations | Difficult with CP | Difficult without CP | 0.2235 | N/R | ||||
| 44% | 36% | |||||||||
|
Caruana et al, 2017
|
Prospective data collection. No randomisation | After propensity cohort matching, risk difference of: |
Risk difference of difficult laryngoscopy and intubation when cricoid pressure applied | N/R | −0.07 to 0.08 |
|||||
| -0.001 |
||||||||||
| Birenbaum et al, 2018 |
Randomised to receive CP or sham | CLG |
90% CLG I or II |
95% CLG I or II |
<0.001 |
N/R | ||||
|
Bohman et al, 2018
|
Randomised to CP or nil | Retrospective notes review to look for difficult intubation | 10.8% | 7.3% | 0.665 | N/R | ||||
CP: cricoid pressure; CLG: Cormack-Lehane Grade; N/R: not reported.
Discussion
Birenbaum et al's (2018) study was the only one to produce statistically significant results to prove that cricoid pressure has a detrimental impact upon laryngoscopy and successful intubation. No prehospital studies had any statistically significant results.
Two studies concluded that while cricoid pressure does make intubation more difficult, it does not increase the incidence of failed intubation or adverse events (Turgeon et al, 2005; Birenbaum et al, 2018).
Other studies were either unable to draw a definitive conclusion or had a primary outcome other than the impact of cricoid pressure upon laryngoscopy and successful intubation.
The results of the present literature review produce many discussion points for further consideration in order to assess their implications for practice.
Operator experience
All intubation operators in all studies were highly experienced in airway management. Consequently, their experience may allow a degree of compensation for difficulties caused by cricoid pressure; for example, Birenbaum et al (2018) found no increase in the number of failed intubations or patients requiring mask ventilation, or with more than three attempts at intubation.
Paramedics worldwide have varied scopes of practice and work in a range of clinical settings, with different amounts of exposure to intubation and therefore varied proficiency at intubation. If cricoid pressure creates more adverse intubation conditions, paramedics who are less proficient at intubation may be disproportionately affected, although it may possible for this to be compensated for by more proficient paramedics.
Worldwide variation in practice
Even within physician-led prehospital critical care teams, consensus about the role of cricoid pressure in airway management is lacking.
The London helicopter emergency medical service (HEMS) mandates cricoid pressure in all prehospital RSIs, whereas Greater Sydney HEMS states that cricoid pressure should ‘no longer be routinely applied to patients undergoing prehospital RSI’ (Harris et al, 2010; Habig et al, 2011). The Scandinavian emergency anaesthesia guidelines are more intermediate and leave the application of cricoid pressure to the discretion of the clinician (Jensen et al, 2010).
With a large variation in practice worldwide, there is potential for these HEMS services to report the incidence of difficult intubation and their use of cricoid pressure to address the knowledge gap highlighted by this literature review.
For paramedics working for UK NHS ambulance services, the Association of Ambulance Chief Executives (2019) states that cricoid pressure should not routinely be used during intubation of a patient receiving cardiopulmonary resuscitation (CPR). However, there is no guidance for other patient groups.
Paramedic knowledge
A study investigating knowledge of cricoid pressure in emergency medical technicians and paramedics found that 64.4% believed its purpose was to improve laryngoscopy; only 26.7% correctly answered that it was intended to reduce the risk of regurgitation. Moreover, 57.8% of respondents were unsure how much force to apply and 78.3% reported never having had any refresher training in it (Black et al, 2012).
If understanding of cricoid pressure is poor, it is likely to be used inappropriately, such as during cardiac arrest. Moreover, many respondents were not sure how much pressure to apply. The effect of applying incorrect levels of cricoid pressure has not been investigated, although all clinicians across the seven studies had extensive training and practice at cricoid pressure. One study reported daily refresher training and testing (Turgeon, 2005).
Difficult to perform
Noll et al (2019) found that anaesthetic physicians and assistants who perform cricoid pressure frequently are poor at performing cricoid pressure. Only 1.3% of participants could achieve the 30±5N for the duration of time it would take to complete intubation. As mentioned, participants in studies had extensive training in performing cricoid pressure correctly; however, this level of training does not seem pragmatic to most paramedics, especially those working outside critical care teams.
In addition to the poor knowledge of the purpose of cricoid pressure, the effect of poorly applied cricoid pressure has not been investigated. However, the probable variation in how it is applied is likely to result in poor laryngoscopy and intubation success for some paramedics.
Limitations
The principal limitation of this literature review is the scarcity of prehospital studies, compounded by there being no prehospital RCTs. This means that research conducted in the environment most representative of prehospital paramedics is solely observational and open to self-reporting biases of reporting organisations.
Furthermore, there was a huge amount of heterogeneity between studies, in both methodology and endpoints, so the author did not undertake any pooled analysis of the results.
Finally, many studies met the inclusion criteria for this literature review but did not have the impact of cricoid pressure upon laryngoscopy and intubation as their primary endpoint; instead, they were often investigating the relationship of cricoid pressure with regurgitation. This issue also led to limited data reporting, resulting in large amounts of incomplete data, such as that around patient characteristics, as well as a lack of statistical analysis of intubation-orientated endpoints. Unfortunately, this reduces confidence in the results upon which this literature review is constructed.
Conclusion
Cricoid pressure is likely to have a detrimental effect upon laryngoscopy and successful intubation. The incidence of failed intubation and other adverse events is likely to be affected by the experience of the operator and their ability to overcome challenging intubation conditions. Less experienced operators may benefit from removing cricoid pressure from their practice and certainly it should not be used during cardiac arrest.
However, pragmatic research in the prehospital environment by paramedics is needed to further assess the hypothesis that cricoid pressure has a detrimental effect on intubation and laryngoscopy. As further research is conducted to understand the effectiveness of cricoid pressure at preventing regurgitation, the use of cricoid pressure in clinical practice by paramedics and other health professionals may disappear.