What is it?
A rapid sequence or ‘crash’ induction classically involves the rapid administration of an induction agent whilst applying cricoid pressure, immediately followed by a fast acting muscle relaxant with the aim to quickly achieve intubation conditions (traditionally without the need for bag-mask ventilation (BMV) between induction and intubation) and protecting the lungs from aspiration. Rapid sequence induction is therefore indicated in situations with an elevated risk of aspiration.
Physiology of aspiration
While the most common concern is aspiration of stomach contents, anything that is not ordinarily present in the pharynx can be aspirated, such as blood, pus or foreign bodies. The aspiration of stomach contents must logically be a sequence of events:
- The stomach must contain something,
- the esophageal barrier pressure has to be reduced,
- gastric contents have to find their way into the pharynx
- and then down the trachea.
Stomach contents can be due to the patient not being starved or gastric emptying being delayed. Slow gastric emptying can be caused by drugs, particularly opiates, stress response, for example following trauma, or a neuropathy (e.g. post vagotomy, diabetes). Mechanical obstruction of the gastric outlet or further down the digestive tract might lead to reflux of bowel contents into the stomach.
Esophageal barrier pressure is defined as follows: [Esophageal barrier pressure] = [Lower esophageal sphincter pressure] – [Intragastric pressure]
There are multiple reasons why intragastric pressure might be elevated. They include but are not limited to:
- High intra-abdominal pressure transmitted to the stomach, for example due to ascites, in pregnancy or due to abdominal wall fat in obese patients
- Inadvertent gastric insufflation with difficult bag-mask ventilation
Equally there are many reasons why the lower esophageal sphincter pressure might be reduced, such as:
- Administration of anesthetic drugs
- Hiatal hernia, especially when symptoms of GERD are present
Gastric or bowel contents can make their way into the pharynx by either vomiting or regurgitation/ reflux. Vomiting is an active process requiring muscle contraction of the abdominal wall. Therefore vomiting does not occur in adequately anesthetized or paralyzed patients. Regurgitation or reflux on the other hand is passive, requiring a pressure difference between stomach and pharynx as well as impaired lower esophageal sphincter function. Patient positioning, i.e. head-up tilt, can therefore go some way to prevent regurgitation or reflux. Head-down tilt (Trendelenburg position) does not stop reflux of stomach contents, it rather encourages it, but it can prevent aspiration once gastric contents are in the pharynx. Cricoid pressure during rapid sequence induction is applied to replace the function of a lower esophageal sphincter in preventing passive regurgitation.
Cricoid pressure/ Sellick maneuver
Pressure is applied to the cricoid rather than the thyroid cartilage or tracheal rings because the cricoid cartilage is a full, rigid ring-like structure and therefore any force applied to it is completely transmitted to the underlying esophagus. For the same reason cricoid pressure should not lead to airway occlusion.
Giving cricoid pressure involves palpating the thyroid cartilage with the index finger and then applying backward pressure to the cricoid cartilage with thumb and middle finger. The force applied should be around 30N, i.e. equivalent to a 3kg weight. The assistant performing cricoid pressure should gently palpate the relevant anatomy before induction of anesthesia and then apply full force as consciousness is lost.
Note: Cricoid pressure should not be applied or immediately released if a patient is actively vomiting as esophageal rupture can otherwise occur.
The evidence in support of cricoid pressure is not overwhelming. You can read a summary of the pros and cons of applying cricoid pressure during RSI here.
Setup for a rapid sequence induction
Check that you have...
- Prepared your drugs
- Positioned the patient on a guerney or OR table that can be rapidly tilted head-up and down
- Suction within easy reach
- An assistant who can perform cricoid pressure
- A fast-flowing reliable intravenous access
Rapid sequence induction in practice
Traditional teaching of the RSI technique emphasizes not to bag-mask ventilate after the administration of the muscle relaxant in order to avoid gastric insufflation and thereby further increase the risk of regurgitation/ aspiration. There is no evidence base for this recommendation. In fact, the original paper describing the RSI technique (Sellick, 1961) states that BMV can be done without insufflating the stomach. You should decide on a case-by-case basis whether BMV during RSI is safe and necessary, depending on
- the ease with which it can be achieved, i.e. high airway pressures can be avoided. Clearly, using high airway pressures in a patient who is difficult to 'bag' will increase the risk of gastric distension.
- the need for it. As your overriding goal is to prevent hypoxemia, BMV might be necessary in certain patients, e.g. those hypoxemic at baseline, with high oxygen consumption or low FRC, such as obese, pregnant or septic patients.
Failed intubation with rapid sequence induction
A failed intubation is much more likely in the context of rapid sequence compared with standard induction due to time pressure/ stress and cricoid pressure.
The UK's Difficult Airway Society has produced a treatment algorithm for the difficult and failed rapid sequence induction:
Reproduced from: Henderson JJ, Popat MT, Latto IP, Pearce AC. Difficult Airway Society guidelines for management of the unanticipated difficult intubation. Anaesthesia 2004; 59: 675-94, with permission from Blackwell Publishing Ltd
The evidence for rapid sequence induction
Contrary perhaps to the widespread use of RSI in clinical practice, the evidence for the technique to prevent aspiration is virtually non-existent, as concluded in a literature review.
“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.” (Neilipovitz and Crosby, 2007)
Is a rapid sequence induction indicated in anticipated difficult airway management?
There seems to be a persistent belief among some that a rapid sequence induction is a safe technique in patients with a difficult airway. The assumption is that in the event of a ‘can’t intubate- can’t ventilate’ situation the induction agent and Suxamethonium will wear off quickly allowing sufficient spontaneous respiration to resume before the patient develops life-threatening hypoxia.
This is not reliably the case and therefore this approach cannot be recommended as safe in managing the anticipated difficult airway.
Sellick, B.A. (1961), “Cricoid pressure to control regurgitation of stomach contents during induction of anaesthesia”, Lancet, Vol. 2 No. 7199, pp. 404–406.
Neilipovitz, D.T. and Crosby, E.T. (2007), “No evidence for decreased incidence of aspiration after rapid sequence induction”, Canadian Journal of Anesthesia/Journal canadien d'anesthésie, Vol. 54 No. 9, pp. 748–764.