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Laryngopharyngeal pH measurement
  1. E J Spurrier1,
  2. M J Clancy2,
  3. C D Deakin3
  1. 1School of Medicine, Southampton University School of Medicine, Southampton General Hospital, Southampton, UK
  2. 2Emergency Department, Southampton General Hospital
  3. 3Department of Anaesthesia, Southampton General Hospital
  1. Correspondence to:
 Mr E J Spurrier
 School of Medicine, Southampton University School of Medicine, Southampton General Hospital, Tremona Road, Southampton SO16 6YD, UK;


Objectives: Most emergency department (ED) intubations are to prevent gastric contents aspiration. The incidence of aspiration is unknown and intubation has complications. Balancing these risks requires an idea of the incidence of aspiration. This study assessed one technique for investigating the aspiration risk in ED patients. Cricoid pressure is used to reduce this risk and the technique may also examine this manoeuvre.

Methods: Cohorts of unconscious adult ED and elective surgical patients were recruited. The posterior pharyngeal wall pH was measured immediately before and after intubation. Pharyngeal pH was used to indicate risk of aspiration, and pH change to assess the efficacy of cricoid pressure.

Results: Eight ED and 48 control patients were recruited. In the ED cohort, pH ranged from 6.0 to 8.0 before intubation and 4.7 to 8.0 after intubation: a mean decrease of 0.3 (95% CI 1.5 decrease to 0.9 increase). In the control cohort pH ranged from 5.8 to 8.0 before intubation and 6.0 to 8.0 after intubation: a mean increase of 0.4 (95% CI 0.1 to 0.6 increase).

Conclusions: This is a simple, cheap, and repeatable technique for assessing aspiration risk in emergency intubations. A larger study is required to assess the efficacy of cricoid pressure.

  • airway management
  • aspiration
  • intubation
  • cricoid pressure

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Most intubations undertaken in the emergency department (ED) are for protection of the airway from potential aspiration of vomit.1,2 Most studies assessing the incidence and mortality of aspiration are based on elective patients. Such patients are typically starved, pre-medicated, and healthier than emergency patients. The incidence of pulmonary aspiration is extremely low with a low mortality. Previous studies have found that in elective patients who are intubated for surgery, the incidence of aspiration varies from 1 in 1116 cases3 to 1 in 14 129 cases,4 with a maximum mortality attributed to aspiration of 1 in 45 454.5 In contrast very few papers deal with patients intubated in the ED, who may have full stomachs, no pre-medication, and often no definitive diagnosis. The mortality rate associated with aspiration might be expected to be higher for these patients. The incidence of aspiration in patients intubated for airway protection has been reported as 3.5% (n = 133) with no patients suffering complication from aspiration,6 but may be as high as 45%.7 These papers do not, however, give a definitive indication of the risks associated with aspiration in these patients, as their results and methods vary widely.

We were interested in measurement of the laryngopharyngeal pH as this would:

  • Identify the patient population and number of patients with acid at the level of the laryngopharynx at the time of intubation and thus who may have aspirated or be at risk of aspiration;

  • establish that cricoid pressure (the Sellick manoeuvre) is effective. If this manoeuvre is effective in protecting the airway from aspiration by stopping gastric contents leaking into the laryngopharynx during intubation, then it might be expected that some patients would have a decrease in their laryngeal pH when the pressure on the cricoid is released after intubation. Cricoid pressure may not be a benign intervention and can cause significant complications if there is an undiagnosed or unknown laryngeal injury.8

We wish to report a pilot study of our experience of the method of measuring the laryngopharyngeal pH.


Gastric contents in the laryngopharynx can be identified by measurement of pepsin (using complex methods that are unsuitable for the ED) or by pH measurement. Electronic meters were considered but rejected because they are expensive, require a large fluid volume, and were not disposable. We decided to use universal indicator paper (RS components, precision no 114, 261-8803) to measure the pH as it is readily available, cheap and disposable, building on the method used by Carlsson and Islander9 reported in 1981.


This was an observational study using two cohorts; one of ED patients being intubated for airway protection and one of elective surgical patients.


The Emergency Department and Cardiothoracic Department at Southampton General Hospital.


  • Adult patients undergoing rapid sequence intubation within the ED. Patients who had undergone cardiopulmonary resuscitation were excluded.

  • Adult patients undergoing elective cardiothoracic or vascular surgery requiring endotracheal intubation with no known history of gastro-oesophageal reflux.


A pH strip was mounted on Magill forceps and secured with an elastic band. Immediately before intubation, while the supine patient had cricoid pressure applied, the pH strip was immersed in the fluid lying on the posterior pharyngeal wall at the level of the larynx. The patient was then intubated, the tube secured, and cricoid pressure released. The pH measurement was then repeated. Both pH strips were then read.


Ethical approval was granted by the School of Medicine ethics committee. For the elective patients, consent was obtained before operation. For the ED cohort, deferred consent was obtained from the patient’s relatives.


All ED staff, cardiac surgeons, and cardiac anaesthetists were informed of this study and no objections were raised.


Table 1 shows the results.

Table 1

 Pre-intubation and post-intubation oropharyngeal pH, and patient characteristics

Control group

Forty eight control patients were recruited. In 24 patients pH increased during intubation; pH decreased in seven patients. The increase in average pH during intubation has not been explained. It was thought that the tube cuff lubricant or soap used to wash laryngoscopes might be alkaline and cause a pH rise, but measurement of these proved this was not the case.

Emergency department group

Eight ED patients were recruited. Two pH measurements were discarded because of blood on the test strip obscuring the pH measurement. There was a definite pH increase during intubation in two patients and a decrease in one patient. There was a decrease in mean pH during intubation.


The technique was simple, cheap, and without complication. The recruitment of patients in the ED setting was difficult and disappointingly low. This was probably because of the stressful nature of rapid sequence induction and staff forgetting to undertake pH measurements, the first of which needed to be taken immediately before tube placement. This technique shows promise as a research tool in investigating the incidence of aspiration in unconscious patients and the efficacy of the Sellick manoeuvre.


We would like to thank staff in the Southampton General Hospital Emergency Department, and surgeons and anaesthetists working in the cardiac service, for their kind help in recruiting patients for this study.

 E J Spurrier, study completed as fourth year project. Study design, literature review, ethics approval, recruitment, and paper authorship. Mr M J Clancy, project supervisor; assisted with study design, literature review, recruitment in the emergency department, and paper authorship. Professor C D Deakin, study design and recruitment assistance, in particular publicising the study among cardiac surgeons and anaesthetists