Also posted over at Paramedicine 101 (now at EMS Blogs) and at Research Blogging. Go check out the excellent material at these sites.
One of the reasons we use RSI (Rapid Sequence Induction/Intubation) is to protect the airway from aspiration of stomach contents, blood, debris, and other things that might make their way into the lungs and make the patient’s already very bad day, very much worse.
Does RSI protect against aspiration of stomach contents?
We are presented with a patient who appears to need airway management.
You believe that tracheal intubation to isolate the respiratory from the gastrointestinal tract is considered to be the optimum method to prevent aspiration in at-risk patients. Limiting the time that the airway is unprotected during the induction of anesthesia is intuitively advisable and the practice of rapid sequence induction (RSI) with cricoid pressure is widely accepted as the standard of care in this setting.1 
When the word intuitively is used in a medical journal, that is a bad sign. The concerns about protecting the airway for anesthesia are minor concerns compared to those faced by EMS in the much less controlled prehospital environment.
As you contemplate the intervention, you wonder what evidence is available to measure the impact of RSI on the incidence of aspiration, how it should best be performed, and what is its risk-to-benefit profile.
Certainly, we should have considered this before beginning RSI, but this is a way of involving us in the care of a patient. I imagine Theodoric of York pausing during an intubation to ponder this. Naaaah!
Does this –
protect against this?
A search of the available research (2007) was performed and –
It was readily apparent that any conclusions addressing the primary question would be inadequately supported due to the limited number of studies, most of which were retrospective in nature. As well, the working definition of RSI used by researchers was variable and many of its component parts were of unproven or questionable merits.
This is not a review of whether EMS should use RSI, but of the evidence that RSI works in the ideal environment of the OR (Operating Room).
For the purpose of our review and discussion, we defined RSI as it would be conventionally carried out by practicing anesthesiologists. The technique evaluated includes preoxygenation, rapid administration of predetermined doses of both induction and paralytic drugs, concurrent application of cricoid pressure, avoidance of bag and mask ventilation, and direct laryngoscopy followed by tracheal intubation.
How many of us avoid the use of BVM (Bag Valve Mask) ventilation for preoxygenation?
If we have paralyzed the patient’s muscles to prevent stomach contents from being propelled out of the stomach, haven’t we also paralyzed the muscles that may prevent oxygen from entering the stomach?
If we are using BVM ventilation before giving paralytics, and some of that oxygen is forced into the stomach by BVM, aren’t we providing more pressure to propel stomach contents into the airway?
Can cricoid pressure decrease the amount of oxygen that enters the stomach by positive pressure ventilation?
However, a number of factors make it difficult to employ aspiration as the outcome variable in studies assessing the impact of RSI. Aspiration is rare and very large numbers of patients would need to be studied to assess the impact of RSI on its occurrence.
Is aspiration rare because RSI works to protect against aspiration?
Is aspiration rare regardless of RSI?
For practical reasons, surrogate outcomes, such as ease or success of intubation with RSI, are the most commonly reported, with successful tracheal intubation being the single most common outcome reported in clinical evaluations of RSI protocols.
Surrogate endpoints are great for the initial assessment of a treatment, but do not tell us what we need to know about whether what we are doing is actually helping patients, is of no benefit to patients, or is harmful to patients.
We need to do better than just following some old wives’ tales from a time when far less was known about patient care.
Further, many of the reports assessing RSI outcomes are simulations of RSI conducted in healthy elective populations who may not be representative of the cohorts of patients typically subjected to RSI.
In EMS, we should not be treating many healthy patients.
EMS is supposed to be providing not elective airway management, but necessary airway management.
Following our analysis of the literature it was apparent that there was no evidence available that would allow the following question to be answered: “Does RSI reduce either the incidence or the adverse consequences of aspiration during emergency airway management?” In fact, there is no study, randomized, controlled, blinded, or otherwise, that measures the impact of any intervention on the incidence of aspiration, nor is there likely to be a statistically meaningful study conducted on this issue.
This seems to prevent the study of RSI for aspiration prevention by anesthesiologists, but maybe it is still something that EMS can examine.
We are fortunate in that our patients tend to be much more nauseated by us. At least they tend to vomit on us, or around us, much more often than they do around others (maybe oncologists or gastroenterologists see more vomit than EMS).
Can we show that the attempts to prevent aspiration are more than just placebo?
How rare is aspiration in EMS?
How many patients might benefit from RSI to prevent aspiration?
Do we want to know if we are harming our patients?
 No evidence for decreased incidence of aspiration after rapid sequence induction.
Neilipovitz DT, Crosby ET.
Can J Anaesth. 2007 Sep;54(9):748-64. Review.
PMID: 17766743 [PubMed - indexed for MEDLINE]
Assuming that the incidence of aspiration during emergency surgery is 0.15%,13 a strategy that would simply reduce the incidence by 50% would require a study of approximately 50,000 patients to confirm that benefit (one-tailed hypothesis for improved outcome, α = 0.05, β = 0.20). Thus, the strength of any recommendation favouring the use of RSI for the prevention of aspiration would be Grade D.
All we need to understand about the evidence grading system is that D is bad. The grades do not go any lower than D. D includes expert opinion, which is the least reliable evidence that should ever be considered. Expert opinion is what is behind one of the worst abuses of patients – the Standard Of Care – I’m doing it because everyone else is doing it, not because there is any good reason to believe it is good for the patient.
Science alone of all the subjects contains within itself the lesson of the danger of belief in the infallibility of the greatest teachers in the preceding generation … Learn from science that you must doubt the experts. As a matter of fact, I can also define science another way:
Science is the belief in the ignorance of experts. – Richard Feynman.
Neilipovitz DT, & Crosby ET (2007). No evidence for decreased incidence of aspiration after rapid sequence induction. Canadian journal of anaesthesia = Journal canadien d’anesthesie, 54 (9), 748-64 PMID: 17766743