In 2004 the Association of Anaesthetists of Great Britain and Ireland (AAGBI) published updated guidelines for the checking of anaesthetic equipment . Following a critical incident in which capnograph tubing was found to be blocked after the induction of anaesthesia  we noted that the AAGBI guidelines do not give specific advice on how the function of a capnograph should be checked. We undertook a survey of anaesthetists across the UK to establish if and how they check the capnograph before starting an anaesthetic.
Questionnaires were sent by post to the anaesthetic departments of 202 UK hospitals. A covering letter to the anaesthetic department's College Tutor (an educational supervisor who is always of consultant physician grade) asked them either to complete the questionnaire themselves or to pass it to an interested consultant colleague. Eight weeks after the initial mailing, a duplicate questionnaire was sent to non-responders. The responses themselves were anonymized. A total of 163 responses (an 81% response rate) were obtained between June and October 2004. A copy of the questionnaire used is shown in Appendix 1.
Type of capnograph predominantly used
These are shown in Table 1. Most respondents reported using sidestream capnography, in which sampling takes place through a tube attached to the breathing circuit or filter. Fewer respondents use an in-line analyser, in which the carbon dioxide (CO2) sensor sits within the anaesthetic circuit. Most of those who reported using a combination of capnography machines had sidestream capnography in the operating theatre but an in-line device in the anaesthetic room (most UK hospitals have an ‘anaesthetic room’ adjacent to the operating theatre where anaesthesia is induced before the patient is moved into the operating theatre).
Who checks the capnograph?
The responses to this question are shown in Table 2. Over half of respondents said that they check the capnograph themselves; many of these did so in addition to other staff present. The operating department practitioner (ODP) or anaesthetic nurse (hereafter the term ODP is used to represent both) was the only member of staff to check the function of the capnograph preoperatively in 41.7% of responses. Three respondents relied on medical engineers to check the function of the capnograph on a regular basis.
How is the capnograph checked?
Among the 163 responses received the commonest reported methods of checking capnograph function were holding the sampling tube in the tester's expired breath (41.7%), relying on the capnograph or anaesthetic machine's automatic self-test (20.9%), and checking that the machine was switched on (11.0%). These figures may not be truly representative because several respondents replied that they did not know how their ODP checked the equipment. For the 85 consultants who said that they check the capnograph themselves at the start of a list, the commonest techniques used for the check are detailed in Table 3.
Three respondents use CO2 from a cylinder attached to the anaesthetic machine to check capnograph function; one respondent injects air into the sample tubing to ensure it is not blocked; five answers were unintelligible.
The questionnaire gave the opportunity for respondents to comment on the technique they used to check the capnograph. Of the 68 who reported the use of expired CO2 (either their own or that of their ODP) for checking, 17 expressed specific concerns about sterility or infection risk using such a technique. Eight respondents said that their capnograph was tested by blowing into the circuit through a filter, which was then discarded. Twenty respondents reported that they did not use a filter when testing the capnograph with their own breath, but that the subsequent presence of a filter before the circuit was used for a patient made their technique acceptable.
Frequency of changing the capnograph sampling tube
For those respondents who reported using a sampling tube for capnography, the frequency of changing the tubing is summarized in Table 4. One respondent reported changing the sampling tube between each patient.
Thirty respondents (18%) reported having had one or more critical incidents involving capnography in the previous 5 yr, of whom 19 specified what had happened. The commonest events involved a leaking sampling tube leading to entrainment of room air and measurement error (five respondents) and blockage of the sampling tube (four respondents) due either to kinking or moisture. Two respondents reported complete capnograph machine failure, and three described critical incidents caused by accidental disconnection of the sampling tube from the breathing circuit.
The questionnaire included a free text section, which was used by several correspondents to justify or clarify previous answers and by some to make further points. Several comments were received concerning the importance of using clinical methods in addition to capnography to confirm correct endotracheal tube placement.
The importance of capnography in anaesthetic practice is undisputed. As well as confirming correct placement of an endotracheal tube, the capnograph gives early warning of failure of ventilation, anaesthetic circuit faults, gas or fat embolism to the lungs, sudden circulatory collapse and malignant hyperpyrexia. In an analysis of 2000 critical incidents occurring during anaesthesia, 8% were first detected by the use of a capnograph .
Guidelines for checking anaesthetic equipment were produced by the AAGBI in 2004. In the section relating to monitoring equipment, the guidelines recommend that the user should:
‘Check that gas sampling lines are properly attached and free from obstruction or kinks. In particular check that the oxygen analyser, pulse oximeter and capnograph are functioning correctly and that appropriate alarm limits for all monitors are set .’
The guidelines suggest that such checks should be carried out at the beginning of each operating theatre session, and after any alteration to specific monitoring equipment. The checklist published alongside the guidelines goes further in advising that all monitoring devices, including the capnograph, should be checked before each new patient during a session. There is therefore some confusion in published policy here; our experience is that in practice checks are performed at the beginning of the session and after changes in equipment, but not between individual patients. However, whereas there is detailed advice on how to check equipment such as flowmeters and vaporizers, there are no specific instructions on how to test the capnograph. We received several comments on returned questionnaires suggesting that participating anaesthetists were interested in finding out how such tests were conducted in other hospitals.
The fact that over half of the consultant anaesthetists who returned our questionnaire test capnograph function by blowing into the sampling tube probably indicates consensus regarding a practical safety method. A comprehensive review article on capnometry published in 1992 endorsed the use of the anaesthetist's own breath to check correct function  and the editor of the European Journal of Anaesthesiology, commenting on a letter reporting a critical incident in which a CO2 sampling line became occluded, noted that ‘a useful and quick test is to disconnect the gas sampling tube and breathe over it, checking a waveform appears on the monitor’ . It appears that ODPs or anaesthetic nurses are less likely to blow into the sample tube than are consultants and are more likely to rely on the machine self-test or the fact that the machine is turned on: this may reflect the greater assumption of responsibility that physician anaesthetists accept for their patients in the UK.
A quarter of those consultant respondents who use expired breath to test the capnograph volunteered concerns about sterility when using the technique. It may be argued that the only way to be certain that the technique is sterile is to always blow into the sampling tube through a breathing filter, which is subsequently discarded; this has implications in terms of organization and cost. Several respondents argued that the use of a new filter for each patient protects that patient from contamination which may be present in the sampling tube or anaesthetic machine; this rationale for the use of such a testing technique relies on the not unreasonable expectation of a filter being present between the patient and the anaesthetic machine at all times.
One alternative to the above technique, used by one in six consultant respondents, is to allow the machine to perform a self-test. While this may involve a zero calibration, calibration to a known concentration of CO2 provided by a medical technician, and even self-purging procedures , it is unlikely that such tests can be performed at the beginning of each operating list, whenever there is a change of equipment, or before each new patient as suggested by the AAGBI. Indeed, when considering recent case reports of critical incidents involving capnography it is apparent that the individual causes were not, and could not have been, detected by such a self-test, but might well have been detected had the presence of a normal response to CO2 been sought immediately before the start of anaesthesia [2,5,6].
A total of 7.1% of respondents described using the expired CO2 in patients' breath to confirm capnograph function, although only two respondents (2.4%) specifically mentioned doing this for each patient during preoxygenation. The usefulness of preoxygenation in testing capnograph function and circuit continuity before anaesthetizing an individual patient has been identified in two recent editorials in British journals. The Chairman of the Safety Committee of the AAGBI, reporting on behalf of the Expert Group on Blocked Anaesthetic Tubing (EGBAT) , advocated capnography through the assembled breathing circuit and face mask before induction of anaesthesia to demonstrate that expired air can pass retrogradely into the circuit. This was in the context of identifying blockages in the anaesthetic circuit itself. An editorial last autumn identified the usefulness of preoxygenation in confirming capnograph function and providing a baseline capnograph measurement against which to evaluate subsequent changes ; this of course requires proper approximation of the face mask to the patient's face.
Our survey has shown that the most common method for testing capnograph function among consultant anaesthetists and their assistants in the UK is the direct measurement of exhaled breath. We have identified a concern among colleagues about the implications for sterility of that practice, and an interest in what techniques others use.
1. Checking Anaesthetic Equipment
3. 2004. London: Association of Anaesthetists of Great Britain and Ireland.
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5. Brownlow H, Wallace S. A difficult intubation made more difficult. Eur J Anaesth
6. Morris E. Do we need Luer ‘storage ports’ on breathing system filters? Eur J Anaesth
7. Bell MD. Routine preoxygenation - a new ‘minimum standard’ of care? Anaesthesia
8. Carter JA. Checking anaesthetic equipment
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Appendix 1: Copy of Questionnaire
We have kept this as simple as possible - please give as much information as you think we need
- Does the capnograph you use mainly, actually sit in the circuit (main stream) or does it sample via a sample tube attached to the circuit or filter?
- Who, in your practice, physically checks the function of the capnograph?
- How is it tested (both in terms of checking it is turned on, etc., and making sure it reacts to CO2)?
- Do you have any comments on your above answer (e.g. implications for sterility, frequency of testing, etc.)?
- How frequently is the capnograph sampling tubing changed in your hospital?
- Have you had any critical incidents involving capnography in the past five years?
- Any other comments?
Thanks very much for returning the questionnaire in the enclosed prepaid envelope.